Oral presentations and abstracts

In some of the images taken by OSIRIS, pieces of debris can be seen as bright tracks instead of points sources as result of the combination of movements of both particles and spacecraft. The properties of those tracks, such as orientation, length and total brightness, depend on various comets parameters, including the activity on the nucleus surface, capable of lifting and accelerating the particles, and the characteristics of dust grains, as grain sizes, spatial distribution, velocity, density and sensitivity to radiation pressure. Previous works have focused on retrieving some of these grain properties from the mentioned images, but since the images show the 2D projection of the 3D dust motion, they rely on different methods to obtain the distance between the camera and the debris.

In this work, a new method to bypass this distance determination requirement is proposed. The main steps involved are (i) analyze a large set of images containing tracks generated by moving dust grains, and obtain distribution for selected track parameters (orientation, length, total brightness, etc.) using an algorithm based on the Hough transform method; (ii) compare these results with the ones obtained from artificial images, generated by modeling the three dimensional motion of the debris in the gas flow field of the comet, under the influence of gravity, radiation pressure and gaseous drag; (iii) iterate this process in order to refine the parameters characterizing the physical properties of the dust emission used by the model.

How to cite: Lemos, P.: Statistical analysis of dust grain tracks in Rosetta/OSIRIS images, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-38, https://doi.org/10.5194/epsc2020-38, 2020.

In August 2019, 2I/Borisov, the second interstellar object and first visibly active interstellar comet, was discovered on a trajectory nearly perpendicular to the ecliptic. Observations of planet forming disks and debris disks serve as probes of the ensemble properties of extrasolar planetesimals, but the passage of an active interstellar comet through our Solar System provides a rare opportunity to individually study these small bodies up close in the same ways in which we investigate objects originating from our own Outer Solar System. Ground-based observations of short period comet 67P/Churyumov–Gerasimenko revealed a coma dust composition indistinguishable from what was measured on its nucleus by the orbiting Rosetta spacecraft. Therefore when 2/I Borisov had a dust dominated tail, we attempted to study its composition with near-simultaneous griJ photometry with the Gemini North Telescope. We obtained two epochs of GMOS-N and NIRI observations in November 2019, separated by two weeks. We will report on the inferred optical-near-IR colors of 2I/I Borisov’s dust coma/tail and nucleus. We will compare our measurements to other observations of 2I/Borisov and place the interstellar comet in context with the Col-OSSOS (Colours of the Outer Solar System Survey) sample of small KBOs and interstellar object ʻOumuamua observed in grJ with Gemini North, using the same setup.

How to cite: Schwamb, M., Bannister, M., Marsset, M., Fraser, W., Pike, R., Snodgrass, C., Kavelaars, J., Benecchi, S., Lehner, M., Peixinho, N., and Fitzimmons, A.: Near-Simultaneous Optical + NIR Photometry of Interstellar Comet 2I/Borisov, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-46, https://doi.org/10.5194/epsc2020-46, 2020.

Abstract

Institute of Astronomy (University of Latvia) with Ventspils International Radio Astronomy Centre (Ventspils University of Applied Sciences) participation is implementing the scientific project “Complex investigations of the small bodies in the Solar system” related to the research of the small bodies in the Solar system (mainly, focusing on asteroids and comets) using methods of radio astronomy and signal processing. One of the research activities is hydroxyl radical (OH) observation in the radio range - single antenna observations and VLBI (Very Long Baseline Interferometry)  observation. To detect weak (0.1 Jy) OH masers of astronomical objects using radio methods, a research group in Ventspils adapted the Irbene RT-32 radio telescope working at 1665.402 and 1667.359 MHz frequencies. Novel data processing methods were used to acquire weak signals. Spectral analysis using Fourier transform and continuous wavelet transform were applied to radio astronomical data from multiple observations related to weak OH maser detection. Multiple comets (Comet C/2017 T2 (PANSTARRS), Comet C/2019 Y4 (ATLAS), Comet C/2020 F8 (SWAN)) observations were carried out in 2019-2020.

Introduction

There are four known (1612.231,  1665.402, 1667.359 and 1720.530 MHz) hyperfine transitions of OH at 18 cm wavelength which have been used for 40 years, historically to observe comets. In 1973, the molecule OH in comet Kahoutek [1] was observed from the Nancay 30 meter telescope.  The 18 cm line is the result of an excitation from resonance fluorescence, whereby molecules absorb solar radiation and then reradiate the energy. The OH molecule absorbs the UV solar photons and cascades back to the ground state Lambda doublet, where the relative populations of the upper and lower levels strongly depend upon the heliocentric radial velocity (the “Swings effect”) [2]. The result of comets observations in 1.6GHz frequency band made by other astronomy groups [3],[4],[5],[6] and others - show that the typical peak source flux densities of the comet are in the range of 4 to 40 mJy. Weakness of the radio signal is the main challenging factor. Assuming that the detection threshold is 3*σ, at least 1.3 to 13 mJy noise floor  is required. Significant work was invested to prepare the instrumentation of Irbene 32-meter antenna for spectral line observation at L band. This includes improvement of receiver system sensitivity at 1.665 and 1.667 GHz, by building and installing new secondary focus front-end [7].

Observations and data processing

To detect OH masers of the comets, multiple observation sessions were performed using Irbene radio telescope RT32 at 1665.402 and 1667.359 MHz frequencies. Comet Atlas C/2019 Y4 was observed 133 hours, Panstarrs C/2017 - 149 hours, Swan C/2020 F8  - 110 hours. Data calibration and processing methods were necessary to filter out weak OH maser signals from radio astronomical data sets. A programmed USRP X300/310+TwinRX spectrometer is used to record data using 16bit+16bit (real + imag part) per sample. For spectral data calibration, the frequency switching method [8] was integrated in the observation process and data processing was implemented to collect data using long integration time, consequently to perform the compensation of the Doppler shift. For data filtering Fourier transforms, Blackman-Harris window function, Butterworth Low Pass, Locally Weighted Scatterplot Smoothing functions and wavelet transforms were used. Observations of small bodies are possible with the best available accuracy when optical (using the optical Schmidt telescope of Institute of Astronomy) and radio methods are combined [9]. Data processing from two independent simultaneous measurements (using specific Kalman filters) allows one to reduce human errors in sporadic sources. 

Summary and Conclusions

Observations of OH masers of comets can be a very challenging task. The upgrade of the L frequency band receiver was performed in Irbene, Latvia to observe OH masers of comets. Multiple data processing methods were developed to acquire a weak signal. OH masers of the comets (Comet C/2017 T2 (PANSTARRS), Comet C/2019 Y4 (ATLAS), Comet C/2020 F8 (SWAN)) were observed, and the observation process of Comet C/2019 U6, Comet 2P/Encke and Comet C/2020 F3 (NEOWISE) are ongoing in summer 2020.

Acknowledgements

This research is funded by the Latvian Council of Science, project„Complex investigations of the small bodies in the Solar system”, project No. lzp-2018/1-0401.

References

[1] Crovisier, J, et al., Comets at radio wavelengths, C. R. Physique 17 (2016) 985–994 

[2] Despois, D., et al “The OH Radical in Comets: Observation and Analysis of the Hyperfine Microwave Transitions at 1667 MHz and 1665 MHz, Astronomy and Astrophysics, vol. 99, no. 2, June 1981, p. 320-340.

[3] J. Crovisier et al., “Observations of the 18-cm OH lines of comet 103P/Hartley 2 at Nançay in support to the EPOXI and Herschel missions”, Icarus, Volume 222, Issue 2, February 2013, Pages 679-683

[4] B.E.Turner, “Detection of OH at-18-centimeter wavelength in comet KOHOUTEK”, Astrophysical Journal, vol. 189, p.L137-L139

[5] A.J.Lovell et a.l “Arecibo observation of the 18 cm OH lines of six comets”,ESA Publications Division, ISBN 92-9092-810-7, 2002, p. 681 - 684

[6]A. E. Volvach et al. ,”Observations of OH Maser Lines at an 18cm Wavelength in 9P/Temper1 and Lulin C/2007 N3 Comets with RT22 at the Crimean Astrophysical Observatory”, Bulletin of the Crimean Astrophysical Observatory June 2011, Volume 107, Issue 1, pp 122–124

[7] M. Bleiders, A. Berzins., N. Jekabsons, V. Bezrukovs, K. Skirmante, Low-Cost L-band Receiving System Front-End for Irbene RT-32 Cassegrain Radio Telescope, Latvian Journal of Physics and Technical Sciences, 2019, Vol.56, No.3, 50.-61.lpp.

[8] B. Winkel, A. Kraus, and U. Bach. Unbiased flux calibration methods for spectral-line radio observations. Astronomy and Astrophysics, 540:A140, Apr 2012.

[9] K. Skirmante, I. Eglitis, N. Jekabsons, V. Bezrukovs, M. Bleiders, M. Nechaeva and G. Jasmonts, Observations of astronomical objects using radio (Irbene RT-32 telescope) and optical (Baldone Schmidt) methods, Astronomical and Astrophysical Transactions, vol.31, issue 4, 2020

How to cite: Skirmante, K., Bleiders, M., Jekabsons, N., Bezrukovs, V., and Jasmonts, G.: Observations of OH masers of comets in 1.6GHz frequency band using the Irbene RT32 radio telescope, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-171, https://doi.org/10.5194/epsc2020-171, 2020.

In this work we combine several constraints provided by the crater records on Arrokoth and the worlds of the Pluto system to compute the size-frequency distribution (SFD) of the crater production function for craters with diameter D≤ 10km. For this purpose, we use a Kuiper belt objects (KBO) population model calibrated on telescopic surveys, that describes also the evolution of the KBO population during the early Solar System. We further calibrate this model using the crater record on Pluto, Charon and Nix.  Using this model, we compute the impact probability of bodies with diameter d>2km on Arrokoth, integrated over the age of the Solar System, that we compare with the corresponding impact probability on Charon. Our result, together with the observed density of sub-km craters on Arrokoth's imaged surface, constrains the power law slope of the crater production function. Other constraints come from the absence of craters with 1<D<7km on Arrokoth, the existence of a single crater with D>7km and the relationship between the spatial density of sub-km craters on Arrokoth and of D ~ 20km craters on Charon. Together, these data suggest the crater production function on these worlds has a cumulative power law slope of -1.5<q<-1.2. Converted into a projectile SFD slope, we find -1.2<q_KBO<-1.0. These values are close to the cumulative slope of main belt asteroids in the 0.2-2km range, a population in collisional equilibrium (Bottke et al. 2020). For KBOs, however, this slope appears to extend down to objects a few tens of meters in diameter, as inferred from sub-km craters on Arrokoth. From the measurement of the dust density in the Kuiper belt made by the New Horizons mission, we predict that the SFD of the KBOs become steep again below approximately 30m. All these considerations strongly indicate that the size distribution of the KBO population is in collisional equilibrium.

How to cite: Morbidelli, A., Nesvorny, D., Bottke, W., and Marchi, S.: A re-assessment of the Kuiper belt size distribution for sub-kilometer objects, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-15, https://doi.org/10.5194/epsc2020-15, 2020.

The Colours of the Outer Solar System Origins Survey (Col-OSSOS, Schwamb et al., 2019) has examined the surface compositions of Kuiper Belt Objects (KBOs) by way of broadband g-, r- and J-band photometry, using the Gemini North Hawaii Telescope. This survey showed a bimodal distribution in the colours of the objects surveyed, consistent with previous colour surveys (Tegler et al., 2016). These broadband surface colours can be considered a proxy for surface composition of these KBOs, so this survey allows the frequency of different surface compositions within the outer Solar System to be explored. The bimodality of the observed colours suggests the presence of some sort of surface transition within the Kuiper belt, perhaps due to a volatile ice-line transition in the pristine planetesimal disk that existed before Neptune’s migration. The Outer Solar System Origins Survey (OSSOS, Bannister et al., 2018), from which Col-OSSOS selected objects brighter than 23.6 r-band magnitude, has well characterised and quantified biases, so allowing for comparisons between the observations and numerical models of the Kuiper belt.

By applying different colour transitions to the primordial planetesimal disk, in this work we explore the possible positions for ice line/colour transitions within the planetesimal disk that existed before Neptune’s migration. Within Schwamb et al. (2019), a simplified toy model was used to investigate the possible position of this transition. Nesvorny et al. (2020) has investigated the primordial colour fraction, in particular how it can create the inclination distribution that we see in the colours of KBOs today. In this work we use a full dynamical model of the Kuiper belt to more precisely pinpoint the possible location of this transition. We make use of the model by Nesvorny & Vokrouhlicky (2016) of Neptune’s migration from 23 au to 30 au, and the consequent perturbation of the Kuiper belt into its current form. This model allows precise tracking of the objects from their pre-Neptune migration to post-Neptune migration positions, allowing various colour transition positions in the initial disk, an example of which is shown in Figure 1, to be compared with the Col-OSSOS observations of the modern day disk.

Figure 1: An example red/neutral transition at 27 au. The left plots show the objects in the primordial disk, while the right plots show the objects post-Neptune migration from the model of Nesvorny & Vokrouhlicky (2016).

The OSSOS survey simulator (Lawler et al., 2018) can then be used to calculate which of the simulated objects could have been observed by OSSOS, and so selected by Col-OSSOS for surface colour observations. The colour transition within the initial disk, shown in Figure 1, is moved radially outwards through the disk and the corresponding outputs are compared with the Col-OSSOS colour observations to see which initial disk colour transition positions are consistent with the modern day Kuiper belt. We will present results combing an accurate dynamical model of the Kuiper Belt’s evolution by Nesvorny & Vokrouhlicky (2016) with Col-OSSOS photometry. We will explore multiple radial colour distributions in the primordial planetesimal disk and implications for the the positions of ice line/colour transitions within the Kuiper Belt’s progenitor populations.

References

Bannister, M. T., Gladman, B. J., Kavelaars, J. J., et al. 2018, ApJS, 236, 18
Lawler, S. M., Kavelaars, J. J., Alexandersen, M., et al. 2018, Front. Astron. Space Sci., 5, 14
Nesvorny, D., Vokrouhlicky, D., Alexandersen, M., et al. 2020, AJ, in press
Nesvorny, D., & Vokrouhlicky, D. 2016, ApJ, 825
Schwamb, M. E., Bannister, M. T., Marsset, M., et al. 2019, ApJS, 243, 12
Tegler, S. C., Romanishin, W., Consolmagno, G. J., & J., S. 2016, AJ, 152, 210

How to cite: Buchanan, L., Schwamb, M., Fraser, W., Bannister, M., Marsset, M., Pike, R., Nesvorny, D., Kavelaars, J., Benecchi, S., Lehner, M., Wang, S.-Y., Thirouin, A., Peixinho, N., Volk, K., Alexandersen, M., Chen, Y.-T., Delsanti, A., Gladman, B., Gwyn, S., and Petit, J.-M.: Col-OSSOS: Probing Ice Line/Colour Transitions within the Kuiper Belt's Progenitor Populations, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-185, https://doi.org/10.5194/epsc2020-185, 2020.

To ensure the safety of a spacecraft and efficiency of the instrument operations it is indispensable to have simple (i.e. with minimal number of parameters and which does not require long time simulations) models for the assessments of the dusty-gas coma parameters. The dusty-gas flow preserves typical general features regardless the particular coma model and the characteristics of the real cometary coma. Therefore, elementary models which account only for the main factors (due to the absence of reliable information on the surface and the interior) affecting the dusty-gas motion could be used for rough estimations of integral characteristics and asymptotic behavior of dusty-gas motion (e.g. terminal velocity, and distance and time when it is reached). At the same time, over-simplification of coma representation is undesirable also. For example, the assumption of spherical symmetry of the flow makes no difference between day and night sides of the coma.

We propose a simple approximation of the gas coma parameters based on the numerical solutions of axisymmetric coma with different activity of the night side (see for example Crifo et al. 2002). This model is given by heliocentric distance r_h, total production rate Q, radius of the nucleus R_n, surface temperature T_n, mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side a_n. This model is able to reproduce typical anisotropy of gas density distribution in real coma and it better conform the physics of real coma than the frequently used model of spherical expansion. In the polar frame with axis directed to the Sun and polar angle φ (solar zenith angle), the spatial distribution of gas density is approximated by third order polynomial of cos(φ):

where coefficients a_i are tabulated for a given mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side a_n. This approximation gives the relative deviation from numerical solution less than 10% in the most part of the coma.

For the dust environment it is assumed that dust grains are spherical homogeneous isothermal particles, non-rotating, with invariable mass (i.e. non-sublimating and noncondensing). The grains released from the surface are submitted to the nucleus gravity F_G, the gas drag F_A, and the solar radiation pressure F_S. We do not allow for solar tidal effects, nor for mutual dust collisions, even though these effects are not always negligible. The theoretical basis of such approach is given in Crifo et al. 2005.

In order to cover a broad range of physical conditions we use dimensionless description of dust dynamics proposed in Zakharov et al. 2018. In this case it is possible to limit the parameter space to three general dimensionless factors Iv, Fu, Ro. The factor Iv characterizes the efficiency of entrainment of the particle within the gas flow; Fu characterizes the efficiency of gravitational attraction; Ro characterizes the contribution of solar radiation pressure. In contrast to the gas environment, the structure of the dust environment can change drastically depending on the particular combination of Iv, Fu, Ro. Therefore, the spatial distribution of dust density can be approximated only for separate combinations of Iv, Fu, Ro within a certain range. The present study covers the range of 5·10^-6 < Iv < 0.1, 10^-7 < Fu < 3·10^-7, 1.5·10^-10 <Ro < 6·10^-6 (i.e. 10^-3 < Ro/Fu < 40.0). For the dust density along sunward direction we propose the approximation:

where coefficients b_i are tabulated for a given mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side a_n. This approximation gives the relative deviation from numerical solution less than 5% for n=4 and 17% for n=3.

Acknowledgements:

This research was also supported by the Italian Space Agency (ASI) within the ''Partecipazione italiana alla fase 0 della missione ESA Comet Interceptor'' (ASIINAF agreement n.Accordo Attuativo" numero 2020-4-HH.0 ).

References:

Crifo,J.F., Lukianov,G.A.,  Rodionov,A.V.,  Khanlarov,G.O., Zakharov, V.V., Comparison between Navier–Stokes and Direct Monte–Carlo Simulations of the Circumnuclear Coma. I. Homogeneous, Spherical Source, Icarus 156, 249–268, 2002.

Crifo, J.-F., Loukianov, G.A., Rodionov, A.V., Zakharov, V.V., Direct Monte Carlo and multifluid modeling of the circumnuclear dust coma Spherical grain dynamics revisited. Icarus 176, 192–219, 2005.

Zakharov, V.V., Ivanovski, S.L., Crifo, J.-F., Della Corte, V., Rotundi, A. , Fulle, M., Asymptotics for spherical particle motion in a spherically expanding flow, Icarus, Volume 312, p. 121-127, 2018.

How to cite: Zakharov, V., Rodionov, A., Fulle, M., Ivanovski, S., Bykov, N., Della Corte, V., and Rotundi, A.: Practical relations for assessments of the dusty-gas coma parameters, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-223, https://doi.org/10.5194/epsc2020-223, 2020.

Narrow and dense rings have been detected around the small Centaur body Chariklo (Braga-Ribas et al. 2014), as well as around the dwarf planet Haumea (Ortiz et al. 2017).

Both objects have non-axisymmetric shapes that induce strong resonant effects between the rotating central body with spin rate Ω and the radial epicyclic motion of the ring particles, κ. These resonances include the classical Eccentric Lindblad Resonances (ELR), where κ = m(n-Ω), with m integer, n being the particle mean motion. These resonances create an exchange of angular momentum between the body and the collisional ring, clearing the corotation zone, pushing the inner disk onto the body and repelling the outer part outside of the outermost 1/2 ELR, where the particles complete one orbital revolution while the body executes two rotations, i.e. n/Ω ~ 1/2 (Sicardy et al. 2019)

Here I will focus on higher-order resonances. They may appear either by considering other resonances such as n/Ω ~ 1/3, or the same resonance as above (n/Ω ~ 1/2), but with a body that has a triaxial shape. In this case, the invariance of the potential under a rotation of π radians transforms the 1st-order 1/2 Lindblad Resonance into a 2nd order 2/4 resonance.

Second-order resonances are of particular interest because they force a strong response of the particles near the resonance radius, in spite of the intrinsically small strength of their forcing terms. This stems from the topography of the associated resonant Hamiltonian, which possesses an unstable hyperbolic point at its origin.

The width of the region where this strong response is expected will be discussed for both Chariklo's and Haumea's rings. The special case of the second-order 1/3 resonance will be discussed, as it appears that both ring systems are close to that resonance.

This work is intended, among others, to pave the way for future collisional simulations of rings around non-axisymmetric bodies.

How to cite: Sicardy, B., Renner, S., and El Moutamid, M.: Ring dynamics around the Centaur Chariklo and the dwarf planet Haumea: effects of high-order resonances , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-295, https://doi.org/10.5194/epsc2020-295, 2020.

Abstract

During a two year period between 2014 and 2016 the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) has been probed by the Rosetta spacecraft. Density data for 14 gas species was recorded with the COmet Pressure Sensor (COPS) and the Double Focusing Mass Spectrometer (DFMS) being two sensors of the ROSINA instrument. The combination with an inverse gas model yields emission rates on each of 3996 surface elements of a surface shape for the cometary nucleus.

The temporal evolution of gas production, of relative abundances, and peak productions weeks after perihelion are investigated. Solar irradiation and gas production are in a complex relation revealing features differing for gas species, for mission time, and for the hemispheres of the comet. This characterization of gas composition allows one to correlate 67P/C-G to other solar and interstellar comets, their formation conditions and nucleus properties, see [3].

Gas production

We analyze in-situ density data of the two sensors COPS and DFMS (see [1]) for the 14 major and minor gas species H₂O, CO₂, CO, H₂S, O₂, C₂H₆, CH₃OH, H₂CO, CH₄, NH₃, HCN, C₂H₅OH, OCS, and CS₂ between August 1st 2014 and September 5th 2016 and heliocentric distances r_h between 3.5 au and 1.24 au. Based on the inverse gas model (section below) the temporal evolution of the cometary gas production is evaluated for all 14 gas species, see [5] and [6].

Figure 1: Temporal evolution of the production rates for comet 67P/C-G during the apparition in 2015. The boxes indicate the systematic uncertainties with respect to the limited surface coverage only. The r_h^-2-line specifies a radiation model assuming linear relation between solar irradiation and gas production. The horizontal bars indicate two time intervals I_a and I_c before and after perihelion.

Fig. 1 shows the production rates for the three species H₂O, CO, and HCN complemented by the line for an idealized production assuming a gas production ∼r_h⁻². The H₂O fraction is more than 80 % of the cometary production at peak gas activity during the interval 17 d to 27 d after perihelion. During the time interval I_c from 190 d to 380 d after perihelion, the productions for H₂O, O₂, CH₃OH, H₂CO, and NH₃ follow power laws r_h^α with α ≤ −4.5. A linear relation between solar irradiation and gas production at that time can be excluded for these gases. The group of gases containing CO₂, CO, H₂S, CH₄, HCN, C₂H₅OH, OCS, and CS₂ holds higher exponents −3 ≤ α (see CO and HCN in Fig. 1) in interval I_c. Restricted to the southern hemisphere, the exponents α further approach −2 such that a linear relation between solar irradiation and gas production can be assumed.
As shown in Fig. 1 during the time interval I_a from -290 d to -180 d before perihelion the gases CO and HCN show a significant production decrease although solar irradiation increases at the same time. CO₂, H₂S, O₂, and C₂H₆ remain nearly constant during I_a. [2] report production decrease for CO and stagnation for HCN on comet C/1995 O1 Hale-Bopp which might be explained by interacting sublimations of two different gas species.

Figure 2: Nucleus of comet 67P/C-G approximated with 3996 triangular elements. Colors indicate source strength of H₂O two weeks after perihelion August 2015.

Model and data analysis

Gas density data of the sensors COPS and DFMS are applied to a simplified inverse gas model for collisionless gas expansion around the nucleus of 67P/C-G, see [4]. Each triangular surface element of a shape model with 3996 elements (see Fig. 2) holds a single gas source described by [7]. Within each of 50 time intervals lasting 9 d to 29 d, each source holds a constant emission rate. Numerical efficiency allows to fit all emission rates on the elements, in all intervals, and for all gases on the HLRN-IV supercomputer.

Summary

The temporal evolution of the gas production of comet 67P/C-G for 14 gas species for a two year period during the apparition 2015 is evaluated. Solar irradiation and production are in a complex relation and show different phenomena. For a number of gases, including CO₂, production is close to a r_h^-2 law, during parts of the outbound mission. For other gases, including H₂O, steeper gradients hold. Inbound CO and HCN hold decreasing production with increasing irradiation at the same time.

Acknowledgements

The work was supported by the North-German Supercomputing Alliance (HLRN). Rosetta is an European Space Agency (ESA) mission with contributions from its member states and NASA. We acknowledge herewith the work of the whole ESA Rosetta team. Work on ROSINA at the University of Bern was funded by the State of Bern and the Swiss National Science Foundation.

References

[1] Balsiger H., et al., 2007, Space Science Reviews, 128, 745
[2] Biver N., et al., 2002, Earth, Moon, and Planets, 90, 5
[3] Bodewits D., et al., 2020, Nature Astronomy
[4] Kramer T., Läuter M., Rubin M., Altwegg K., 2017, MNRAS, 469, S20
[5] Läuter M., Kramer T., Rubin M., Altwegg K., 2019, MNRAS, 483, 852
[6] Läuter M., Kramer T., Rubin M., Altwegg K., 2020, MNRAS, submitted
[7] Narasimha R., 1962, J. Fluid Mech., 12, 294

How to cite: Läuter, M., Kramer, T., Rubin, M., and Altwegg, K.: Gas production for 14 species on comet 67P/Churyumov-Gerasimenko from 2014-2016, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-319, https://doi.org/10.5194/epsc2020-319, 2020.

Introduction. Pluto was first identified in 1930 and since that time has completed less than 40% of its orbit (248 Earth-years). Studies of Pluto's surface composition have been ongoing for only a small subset of this period, beginning with the first evidence for CH₄ (methane) ice on the surface [1] only a few years before Pluto reached equinox in 1988 and perihelion in 1989. Therefore, the majority of spectroscopic studies have taken place during northern hemisphere spring, as Pluto recedes from the Sun. Since Pluto has a ~122° obliquity and an eccentric (e=0.25) orbit, these seasonal transitions ought to be extreme [2] and potentially observable over time. Simulations of Pluto's surface evolution suggest that the entire northern hemisphere, except for Sputnik Planitia, will be devoid of volatile ices (N₂, CO, CH₄) by 2030 [3]. Given that in 2015 New Horizons saw extensive deposits of volatile ices in the northern hemisphere [4,5] the removal process must occur relatively rapidly, if the models are correct. The duration of the New Horizons flyby was too brief to observe large-scale changes in surface composition or ice distribution.

Observations. One method for evaluating changes on Pluto on timescales of a few years while accounting for rotational variability is to obtain spectra at the same sub-observer latitude and longitude roughly a year apart [6]. This cadence is made possible by the inclination of Earth's orbit with respect to the ecliptic, which presents a limited range of sub-observer latitudes on Pluto repeating ~14 months later. In order to quantify Pluto's short-term surface changes, while correcting for its rotational variability, we designed a spectroscopic observing program specifically to make use of these “matched pairs.” Pluto was observed on 13 nights between June 2014 and August 2017 using TripleSpec, a cross-dispersed spectrograph [7] at the Apache Point Observatory’s Astrophysical Research Consortium 3.5-meter telescope. These spectra were obtained at an average resolving power of ~3500 from 0.91 to 2.47 μm. Matched pairs typically corresponded to spectra obtained in June of one year and August of the next year, at roughly the same (±10°) sub-observer longitude, avoiding opposition where the viewing geometry at small phase angles affects band depth and width [8]. Pluto’s solar phase curve is also relatively flat between phase angles of 0.5-1.5° [9], the range over which the matched pair components were acquired. Therefore, any differences in viewing geometry do not significantly affect the spectra.

Analysis and Results. To evaluate changes in surface composition over time, we computed integrated band areas for the 1.16, 1.19, 1.33, 1.66, and 1.72 μm CH₄ absorption features in each of the corrected nightly spectra. We also calculated shifts in the band centers for the same features as a proxy for the amount of N₂ in solution with CH₄ [10]. The changes in CH₄ band depth and band center position for each matched pair (later date minus newer date) are presented in Figures 1 and 2, respectively. Only those changes detected at ±5-σ for band depth and ±3-σ for band center shift were considered statistically significant. The only significant changes were detected between 2014-06-17 and 2015-08-19, centered on a sub-observer longitude of ~280°, which showed an increase in CH₄ band areas as well as a blueshift in the band centers. No other matched pair showed a significant change over the corresponding time period.

Discussion. Due to scheduling and weather, the majority of the matched pairs were obtained of the anti-Charon hemisphere, home to the bright, volatile-rich Sputnik Planitia. However, the only sub-observer hemisphere with a statistically significant band area increase included only a small portion of Sputnik but a large portion of the low-albedo, volatile-poor Cthulhu Macula [4,5]. The sub-observer hemisphere for the 2015-08-19/2014-06-17 matched pair was unique and contained the largest fraction of Cthulhu Macula.

The increase in CH₄ band areas and the blueshifting of the band centers in the spectra of one unique sub-observer hemisphere between June 2014 and August 2015, and the lack of strong evidence for any decrease in band areas on any sub-observer hemisphere, points to real short-term changes in Pluto's surface composition over this time frame. The lack of significant detections on other hemispheres centered on Sputnik does not necessarily indicate a lack of changes on those hemispheres. Sputnik is a large reservoir of volatile ices that models suggest undergoes little change over a Pluto orbit [3], so the spectra of these sub-observer hemispheres should be dominated by Sputnik’s contribution, drowning out smaller changes on other areas of the surface. Conversely, the 2015-08-19/2014-06-17 sub-observer hemisphere contains a large portion of the volatile-depleted Cthulhu Macula, which would amplify the same changes in surface composition in the spectra.

On the sub-observer hemispheres not dominated by Sputnik, volatile N₂ and CH₄ ices are present primarily in the north polar region, with alternating latitudinal bands of CH₄ diluted with N₂ (55-90° N and 20-35° N) and N₂ diluted with CH₄ (35-55° N), as measured by New Horizons in 2015, about one month prior to the second half of the 2015-08-19/2014-06-17 matched pair [5]. The observed changes in the spectra of this sub-observer hemisphere indicate both an increase in CH₄ concentration and an increase in N₂ concentration in the north polar regions. While this sounds contradictory, it can be achieved by preferential sublimation of more-volatile N₂ from latitudes northward of 55° as Pluto approaches northern hemisphere summer, resulting in an increase in CH₄ concentration in those regions, combined with deposition of that N₂ onto the latitudinal band from 35-55°.

References.

[1] Cruikshank, D.P., et al., 1976. Science 194, 835-837. [2] Binzel, R.P., et al., 2017. Icarus 287, 30-36. [3] Bertrand, T., Forget, F., 2016. Nature 540, 86-89. [4] Grundy, W.M., et al., 2016. Science 351, aad9189. [5] Protopapa, S., et al., 2017. Icarus 287, 218-228. [6] Grundy, W.M., et al., 2013. Icarus 223, 710-721. [7] Wilson, J.C., et al., 2004. SPIE 5492, 1295-1305. [8] Pitman, K., et al., 2017. P&SS 149, 23-31. [9] Verbiscer, A., et al., 2019. EPSC-DPS2019-1261. [10] Protopapa, S., et al., 2015. Icarus 253, 179-188.

How to cite: Holler, B., Yanez, M., Young, L., Protopapa, S., Verbiscer, A., and Chanover, N.: Evaluating Temporal Evolution of N2 and CH4 Ices on Pluto with APO/TripleSpec from 2014-2017, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-336, https://doi.org/10.5194/epsc2020-336, 2020.

A fraction of near-Earth asteroids has the orbital elements similar to those of comets, but a visual aspect as any other point-like source. These are potentially dormant comets nuclei who entered in a period of inactivity. Their study can provide a new understanding of the final state in which volatile-rich objects reside and of the existing organic material or water content distribution from the early Solar System.

Dynamically, cometary orbits can be filtered by their Tisserand parameter with respect to Jupiter (T_Jup). With few exceptions, comets have T_Jup < 3 while asteroids displays T_Jup > 3. Although the value of T_Jup can indicate whether or not the asteroid crosses the Jupiter's orbit, this is not enough to outline a cometary orbit. Tancredi (2014) had developed a method to classify asteroids on cometary orbits (ACOs), based only on orbital elements, which doesn't require any numerical time integration. Beside Tisserand criterion, this algorithm rejects all samples in mean-resonant motion, with large orbital uncertainties and with large minimum orbital intersection distances (MOID) among giant planets.

We seek to make a statistical analysis of the potentially dormant (extinct) comets from near-Earth objects population (NEACOs), using the spectral observations over the visible and near-infrared wavelength interval. The aim of this work is to constraint the fraction of dormant comets orbiting in the near-Earth space. For this study, we’ve compiled a catalog with 149 spectra of near-Earth asteroids (NEAs) with T_Jup < 3. This sample represents 10% out of all known asteroids which obey the T_Jup criterium (Fig. 1).

Fig 1. Absolute magnitude cumulative distributions of all NEACOs from Tancredi's list of ACOs with respect to all with known taxonomy, to those with T_Jup < 3.1 and to all known NEAs (as of January 30, 2020).

The data include new observations of 26 NEAs and 123 spectra retrieved from the literature. The new measurements were obtained with the 2.5 m Issac Newton Telescope and the Nordic Optical Telescope for the visible region, and with the 3.0 m NASA Infrared Facility Telescope for the NIR interval.

For a simplified analysis we’ve grouped all classes from Bus-DeMeo system into four compositional groups. The silicate-like spectra group is mainly consists of objects from Q / S complex (S-, Sr-, Sq-, Sv- types) and some end-members like O-, R- and A-types. In the carbonaceus-like spectra group we've gathered together C-, X- complexes and B-, D-, T- types. The last two groups consist of basaltic asteroids, corresponding to V-type and of relatively rare spectra (miscellaneous) like K- and L-type.

Figure 2. Taxonomic distribution of NEAs with T_Jup < 3.1

The dominant group is of bodies with carbonaceus-like composition, representing 47.5% (71 / 149): 26 C-complex, 22 X-complex, 17 D-type, 6 B-type. The silicate group represents 47% (70 / 149), with an effective of 66 Q/S-complex, 3 R-type and one pure olivine A-type. We report 3 extreme cases with silicate composition: 2 R-types (466130) 2012 FZ23, (394130) 2006 HY51 and one Sr-type (465402) 2008 HW1. Their T_Jup of 2.3, 2.39 and 2.4 respectively are too small for this compositional group. For (466130) we obtained a NIR spectrum. Also, (394130) have a low recorded albedo (0.071), reaching 980 K to perihelion. Taxonomic distribution of entire catalogue (149 samples), presented in Figure 2, displays a strong variation of compositional ratio between silicate and comet-like objects relative to T_Jup.

Within our sample, we could gather data only for 7 asteroids which obey the criteria of Tancredi (2014). All of them are in the Jupiter Familly Comets orbital class. Their spectra classifies them in the carboanceus-like group: 4 D-type (3552, 85490, 248590, 2001 UU92), 1 C-type (475665), 1 T-type (485652) and 1 Xc-type (506437). We conclude that these 7 bodies are dormant / extinct comets. It is important to note that no object with other taxonomy than carbonaceous chondrite, had passed this enhanced criterion, in agreement with the results of Licandro et al. 2018 (who included part of these objects).

References

[1] Tancredi G., 2014, Icarus, 234, 66
[2] Licandro J., et al. 2018, A&A, 618, A170

Acknowledgements

Part of the data utilized in this publication were obtained and made available by the MITHNEOS MIT-Hawaii Near-Earth Object Spectroscopic Survey. 
This work was developed in the framework of EURONEAR collaboration and of ESA P3NEOI projects. The work of M.P. was supported by a grant of the Romanian National Authority for Scientific Research - UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0371. M.P., J.dL. and J.L. acknowledge support from the AYA2015-67772-R (MINECO, Spain), and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870403 (project NEOROCKS).

How to cite: Simion, G., Popescu, M., Licandro, J., Vaduvescu, O., and de León, J.: Spectral characterization of near-Earth asteroids on cometary orbits, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-351, https://doi.org/10.5194/epsc2020-351, 2020.

Introduction

20000 Varuna is one of the most interesting TNOs due to its peculiar physical properties. It rotates relatively fast with a period of 6.3435674 h Santos-Sanz et al. (2013), producing a double-peaked rotational light-curve with a very large amplitude, ~ 0.45 mag, dominated by the body shape (e.g., Jewitt & Sheppard 2002, Lellouch et al. 2002, Hicks et al. 2005, Belskaya et al. 2006). Varuna's area-equivalent diameter is ~700 km (Lellouch et al. 2013) and its estimated density, under the assumption of hydrostatic equilibrium (Chandrasekhar et al. 1987), has a value of ~ 1000 kgm^-3, which is somewhat high compared to other TNOs of similar sizes (Ortiz et al. 2012). 

Observations

We present here a collection of new rotational light-curves from 2005 to 2019, taken from three different observatories in Spain (Sierra Nevada Observatory, Calar Alto Observatory and Roque de los Muchachos Observatory), using telescopes of 1.5-m, 1.23-m and 2.2-m, and 3.6-m diameter, respectively. Additionally, we have included three rotational light-curves from the literature (Jewitt & Sheppard 2002, Lellouch et al. 2002, Hicks et al. 2005) to our study. This data set provides different rotational light-curves in a time span of 19 years. 

Results 

We have detected that the amplitude of the rotational light-curve has evolved over time, producing a considerable change along these 19 years, increasing ~ 0.13 mag. We think that this change is due to a variation in Varuna's aspect angle. We model this variation assuming a simple triaxial shape for Varuna's body. The best fit to the data corresponds to a family of solutions with axial ratios b/a between 0.56 and 0.60, which constrains the pole orientation in two different ranges of solutions (see figure 1).

Figure 1. χ² map of possible values for the axis ratio b/a = 0.60. λ_P and β_P are the ecliptic longitude and latitude of the pole orientation (Schroll et al. 1976). The best solution is given by the range λ_P∈[43, 63]º and β_P∈[-70, -58]º (with its complementary direction also possible for the same values of χ²_PDF). Figure from Fernandez-Valenzuela et al. (2019).

Apart from the remarkable variation of the rotational light-curve amplitude along the 19-year time span, we have detected changes in the overall shape of the rotational light-curve in shorter time scales. After the analysis of the periodogram of the residuals to a 6.3435674 h double-peaked rotational light-curve fit, we found a clear additional periodicity (see figure 2). We propose that these changes in the rotational light-curve shape are due to a large and close-in satellite whose rotation induces the additional periodicity. The peak-to-valley amplitude of this oscillation is in the order of 0.06 mag (see figure 3). We estimate that the proposed satellite orbits Varuna with a period of 11.9897 h (or 23.9794 h), assuming that the satellite is tidally locked at a distance of 1300 km (or 2000 km) from Varuna. For more detailed information see Fernandez-Valenzuela et al. (2019).

Figure 2. Lomb periodogram spectral power derived from the residuals of the Fourier function fit to Varuna's photometric data. A maximum spectral power of 45 is obtained at the frequency of 2.003 cycles/day (11.9819 h). This periodicity corresponds to the satellite's revolution period. Figure from Fernandez-Valenzuela et al. (2019).

Figure 3. Residuals of Varuna's observational data folded to the 11.9819 h period detected using the Lomb periodogram technique. The blue line represents a one-order Fourier function fit to the points. Figure from Fernandez-Valenzuela et al. (2019).

Acknowlegements: EFV acknowledges UCF 2017 Preeminent Postdoctoral Program (P³). Part of the research leading to these results has received funding from the European Unions Horizon 2020 Research and Innovation Programme, under Grant Agreement No. 687378 (SBNAF). P.S-S. acknowledges financial support by the Spanish grant AYA-RTI2018-098657-J-I00 "LEO-SBNAF" (MCIU/AEI/FEDER, UE). We would like to acknowledge financial support by the Spanish grant AYA-2017-84637-R and the financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofísica de Andalucía (SEV- 2017-0709).

How to cite: Fernández-Valenzuela, E., Ortiz, J. L., Morales, N., Santos-Sanz, P., Duffard, R., and Lellouch, E.: Evidence for a close in satellite in the large TNO Varuna, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-393, https://doi.org/10.5194/epsc2020-393, 2020.

For a long time it was thought that the cyano (CN) radical, observed remotely many times in various stellar and interstellar environments, is exclusively a photodissociation product of hydrogen cyanide (HCN). Bockelée-Morvan et al. (1984) first questioned this notion based on remote observations of comet IRAS-Araki-Alcock. They reported an upper limit for the HCN production rate which was smaller than the CN production rate previously derived by A’Hearn et al. (1983). Even today, this discrepancy observed for some comets is not resolved although many alternative parents have been suggested. Among the volatile candidates, cyanogen (NCCN), cyanoacetylene (HC₃N) and acetonitrile (CH₃CN), according to Fray et al. (2005), are the most promising ones. While cyanoacetylene and acetonitrile are known to be present in trace amounts in comets, as reported for comet Hale-Bopp by Bockelée-Morvan et al. (2000) and for comet 67P/Churyumov-Gerasimenko by Le Roy et al. (2015) and Rubin et al. (2019), the abundance of cyanogen in comets is unknown. Altwegg et al. (2019) were the first to mention its detection in the inner coma of comet 67P/Churyumov-Gerasimenko, target of ESA’s Rosetta mission.

In this work, we track the signatures of cyanogen in the ROSINA/DFMS (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/ Double Focusing Mass Spectrometer; Balsiger et al. (2007)) data, collected during the Rosetta mission phase. We derive abundances relative to water for three distinct periods, indicating that cyanogen is not abundant enough to explain the CN production in comet 67P together with HCN. Our findings are consistent with the non-detection of cyanogen in the interstellar medium.

A’Hearn M.F., Millis R.L., 1983, IAU Circ., 3802

Altwegg K., Balsiger H., Fuselier S.A., 2019, Annu. Rev. Astron. Astrophys., 57, 113–55

Balsiger H. et al., 2007, Space Science Reviews, 128, 745-801

Bockelée-Morvan D., Crovisier J., Baudry A., Despois D., Perault M., Irvine W.M., Schloerb F.P., Swade D., 1984, Astron. Astrophys., 141, 411-418

Bockelée-Morvan et al., 2000, Astron. Astrophys., 353, 1101–1114.

Fray N., Bénilan Y., Cottin H., Gazeau M.-C., Crovisier J., 2005, Planetary and Space Science, 53, 1243-1262

Le Roy L. et al., 2015, Astron. Astrophys., 583, A1

Rubin M. et al., 2019, MNRAS, 489, 594-607

How to cite: Hänni, N., Altwegg, K., Pestoni, B., Rubin, M., Schroeder, I., Schuhmann, M., and Wampfler, S.: In-situ detection of cometary cyanogen (NCCN), Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-397, https://doi.org/10.5194/epsc2020-397, 2020.

The determination of non-gravitational forces based on precise astrometry is one of the main tools to establish the cometary character of interstellar and solar-system objects. The Rosetta mission to comet 67P/C-G provided the unique opportunity to benchmark Earth-bound estimates of non-gravitational forces with in-situ data. We determine the accuracy of the standard Marsden and Sekanina parametrization of non-gravitational forces with respect to the observed dynamics. Additionally we analyse the rotation-axis changes (orientation and period) of 67P/C-G. This comparison provides a reference case for future cometary missions and sublimation models for non-gravitational forces.

Orbit changes by non-gravitational forces
Among several thousand candidate orbits we have conducted an exhaustive search for the best-fit trajectory ofcomet 67P/C-G to pin down the magnitude and direction of the non-gravitational. Starting from estimates of the non-gravitational Marsden parameters A₁, A₂, A₃ from [1] and [2] (derived from Earth-bound observations of several apparitions of 67P/C-G), we determined an improved solution compatible with Rosetta telemetry [3]
.
Figure 1: Error for non-gravitational force models for comet 67P/C-G [7]. The label r_ESOC denotes the (noisy) ESA provided orbit, the pink curve the best fit Marsden values, the blue curve is computed without non-gravitational forces (r₀). The reference orbit r_obs shows our best fit to the ESOC data.

Fig. 1 shows that the improved solution reduces the residual error from several hundred kilometers (Marsden parametrization) to about 20 km. This allows us to extract the magnitude anddirection of the non-gravitational acceleration (Fig. 2).

Figure 2: Total magnitude of the non-gravitational acceleration observed for 67P/C-G [7] and water production rate [8]

Comet 67P/C-G displayed a very regular activity pattern with diurnally repeating dust and gas emission [4, 5,6]. This in turn suggests a very uniform non-gravitational acceleration and smooth changes of the rotation axis.

Rotation-state changes
The rotation of comet 67P/C-G shows almost no wobbling motion and changes only by 0.5 DEG over the 2015 apparition [9]. This change is smaller than what a homogeneous emission model (i.e. Keller A model) predicts. In addition it indicates a very close alignment of the axis of inertia with the rotation axis. This requires a slightly inhomogeneous mass distribution with an increased density in the larger lobe.

Summary and Conclusions
We followed the state vector changes of the nucleus of 67P/C-G (momentum and angular momentum) in terms of a Fourier decomposition of the diurnal outgassing in the nucleus-fixed frame. Our analysis constrains the gas release and the inhomogeneity of the near surface ices. For 67P/C-G we find that the standard Marsden parametrization of the non-gravitational forces [10] can be improved considerably. No evidence for a forced precession is seen for 67P/C-G. Our methodology can be applied to other small-bodies with outgassing activity, provided that a shape and initial rotation state is known.

Acknowledgements
The work was supported by the North-German Supercomputing Alliance (HLRN). We acknowledge helpful discussions and joint work concerning the rotational state of 67P/C-G with E. Kührt, H.U. Keller, L. Jorda, andS. Hviid [9].

References
[1] Krolikowska, M. 67P/Churyumov-Gerasimenko - Potential Target for the Rosetta Mission.Acta Astronomica53, 195–209 (2003).
[2] Horizons. Asteroid & Comet SPK File Generation Request. https://ssd.jpl.nasa.gov/x/spk.html (2019).
[3] Godard, B., Budnik, F., Muñoz, P., Morley, T. & Janarthanan, V. Orbit Determination of Rosetta AroundComet 67P/Churyumov-Gerasimenko.Proceedings 25th International Symposium on Space Flight Dynam-ics–25th ISSFD, Munich, Germany(2015).
[4] Kramer, T. & Noack, M. On the origin of inner coma structures observed by rosetta during a diurnalrotation of comet 6P/Churyumov–Gerasimenko.The Astrophysical Journal823, L11–L11 (2016). DOI10.3847/2041-8205/823/1/L11.
[5] Kramer, T., Läuter, M., Rubin, M. & Altwegg, K. Seasonal changes of the volatile density in the coma andon the surface of comet 67P/Churyumov-Gerasimenko.Monthly Notices of the Royal Astronomical Society469, S20–S28 (2017). DOI 10.1093/mnras/stx866.
[6] Kramer, T., Noack, M., Baum, D., Hege, H.-C. & Heller, E. J. Dust and gas emission from cometary nuclei:The case of comet 67P/Churyumov–Gerasimenko.Advances in Physics: X3, 1404436–1404436 (2018).DOI 10.1080/23746149.2017.1404436.
[7] Kramer, T. & Läuter, M. Outgassing-induced acceleration of comet 67P/Churyumov-Gerasimenko.Astron-omy & Astrophysics630, A4 (2019). DOI 10.1051/0004-6361/201935229.
[8] Läuter, M., Kramer, T., Rubin, M. & Altwegg, K.Surface localization of gas sources on comet67P/Churyumov-Gerasimenko based on DFMS/COPS data.Monthly Notices of the Royal AstronomicalSociety483, 852–861 (2019). DOI 10.1093/mnras/sty3103.
[9] Kramer, T.et al.Comet 67P/Churyumov-Gerasimenko rotation changes derived from sublimation-inducedtorques.Astronomy & Astrophysics630, A3 (2019). DOI 10.1051/0004-6361/201834349.
[10] Marsden, B. G. Comets and Nongravitational Forces.The Astronomical Journal73, 367 (1968). DOI10.1086/110640.

How to cite: Kramer, T. and Läuter, M.: Non-gravitational force model vs observation: the trajectory and rotation-axis of comet 67P/Churyumov-Gerasimenko, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-403, https://doi.org/10.5194/epsc2020-403, 2020.

The most pristine remnants of the Solar system's planet formation epoch orbit the Sun beyond Neptune, the small bodies of the trans-Neptunian object populations.  The bulk of the mass is in ~100 km objects, but objects at smaller sizes have undergone minimal collisional processing, with "New Horizons" recently revealing that ~20 km (486958) Arrokoth appears to be a primordial body, not a collisional fragment.  This indicates bodies at these sizes (and perhaps smaller) retain a record of how they were formed.  However, such bodies are impractical to find by optical surveys due to their very low brightnesses.  Their presence can be inferred from the observed cratering record of Pluto and Charon, and directly measured by serendipitous stellar occultations.  These two methods produce conflicting results, with occultations measuring roughly ten times the number of ~km bodies inferred from the cratering record.  We apply MCMC sampling to explore numerical evolutionary models of the outer Solar system to understand what formation conditions can reconcile the occultations and cratering observations.  We find that models where the initial size of bodies decreases with their semimajor axis of formation, and models where the surface density of bodies increases beyond the 2:1 mean-motion resonance with Neptune can produce both sets of observations.  We discuss the astrophysical plausibility of these solutions, and possible future observations tests of them.

How to cite: Shannon, A., Doressoundiram, A., Roques, F., and Sicardy, B.: Understanding the trans-Neptunian Solar system; Reconciling the results of serendipitous stellar occultations and the inferences from the cratering record., Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-277, https://doi.org/10.5194/epsc2020-277, 2020.

The low-inclination component of the classical Kuiper Belt is thought to be the only population of trans-Neptunian bodies that formed in-situ (Parker et al., 2010). This population, often referred to as the cold classical objects, exhibits a  ~30% observed binary fraction, much higher than for other trans-Neptunian objects (TNOs; Noll et al., 2008). The majority of cold classicals belong to the Very Red (VR) class of the bimodal TNO compositional taxonomy (Fraser and Brown, 2012). Though recently, a population of Less Red (LR) members has been identified, exhibiting a 100% binary fraction (Fraser et al., 2017). These so-called blue binaries are thought to be survivors of a push-out process that occurred during a smooth phase of Neptune’s outward migration. 

Here we report 20 new (g-r) and (r-J) colours of cold classical objects gathered as part of the Colours of the Outer Solar System Origins Survey (Col-OSSOS; Schwamb et al., 2019), bringing the total sample of cold classicals with measured colours to 21 with simultaneous optical and NIR colours, and 103 cold classical TNOs with optical colours alone. In this sample, 29 objects have been identified as binary (Parker, A., personal communication).

Cold classical colours span the full range of optical-NIR colours exhibited by the dynamically excited TNO populations, though they strongly favour red objects;  the VR:LR ratio is ~12 compared to ~3 for the excited TNOs. Moreover, the VR cold classicals have a redder colour distribution than the VR excited TNOs, with the former exhibiting a mean (g-r)~0.95 and the latter, a mean (g-r)~0.8.

The optical colour distribution of binary cold classicals is significantly different than that of the single (or unresolved) cold classical systems (see Figure 1), with the binary sample exhibiting a tail of lower spectral slopes than is found in the sample of singles. The Kolmogorov-Smirnov test comparing the optical colour distributions of the single and binary samples says that there is a only a 0.3% chance the two samples share the same colour distribution. The Col-OSSOS sample on its own shows a similar result, with a 2% probability of the null hypothesis. This argues for a different origin of some or all of the binary cold classicals over the unresolved or single objects population, and is compatible with the hypothesis that the blue binaries are contaminants having been pushed out from regions closer to the Sun.

Figure 1: cumulative optical colour distributions of the single (or unresolved; solid) and binary (dashed) cold classical TNOs. The vertical line demarks the division between less red and very red compositional classes. Spectral slope is reported in percent reddening per 100 nm normalized in the V-band.

How to cite: Fraser, W., Kavelaars, J., Bannister, M., Marsset, M., Schwamb, M., Buchanan, L., Smith, R., Pike, R., Benecchi, S., Lehner, M., Wang, S., and Peixinho, N.: The Colour Distribution Of The Low Inclination Trans-Neptunian Objects, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-434, https://doi.org/10.5194/epsc2020-434, 2020.

We combine our deep imaging of interstellar comet 2I/Borisov taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019 December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and after its perihelion passage in combination with HST/WFC3 images taken on 2019 October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its outburst and fragmentation of March 2020, thus observing the comet in a relatively undisrupted state. We locate 1-2” long (2,000 - 3,000 km projected length) jet-like structures near the optocenter of 2I that appear to change position angles from epoch to epoch (Fig. 2, left panel). With the assumption that the jet is located near the rotational pole, we determine that 2I's pole points near RA = 322 deg, dec = 37 deg, ecliptic longitude = 341 deg, ecliptic latitude = 48 deg (Fig 2., right panel). We find evidence for periodicity in the time-series lightcurve explained by a nucleus rotation period of ~10.6 h and small amplitude of ~0.05 implying a b/a axial ratio of ~1.5 when combined with our pole solution, unlike the b/a of 4 to 10 found for 1I/`Oumuamua (Bolin et al. 2018). Unless 2I's nucleus was <200 m in size and was spun up rapidly by a pronounced jet after our observations, the March 2020 outburst (Drahus et al. 2020) and fragmentation (Bolin et al. 2020b, Jewitt et al. 2020) was most likely due to calving caused by thermal effects.

References:
Boe et al. 2019. Icarus, Volume 333, p. 252-272., Bolin et al. 2018. ApJL, 852, L2., Bolin et al. 2020a, AJ, 160, 16pp., Bolin et al. 2020b, ATel, 13613., Drahus et al. 2020, ATel, 13549., Fitzsimmons et al. 2019. ApJL, 885, L9., Jewitt et al. 2020, arxiv:2006.01242.

Figure 1. Left panel: r magnitude of 2I as a function of the true anomaly using photometry taken between 2019 March 17 and 2019 November 29 UTC. The blue and orange lines are the predicted brightness as a function of true anomaly angle for H₂O and CO-dominated activity comet from the out-gassing model of 2I from Fitzsimmons et al. 2019. Right panel: the size distribution of ISOs within 3 au of the Sun estimated from the detection of 1I with D~250 m and 2I with D~1.4 km. The solid grey line is fit to data with the function y = ax^b and is based on the estimated size of 1I from the literature and the average of the upper limits on the diameter of 2I assuming 0.04 and 0.1 albedo where a = 0.12 +/- 0.05 and b = 3.38 +/- 1.18.

Figure 2. Left panel: Mosaic of radial profile normalized composite images of 2I taken with HST on 2019 October 12 UTC and 2019 November 16 UTC  using the F350LP filter (HST GO/DD 16009, PI Jewitt) and on 2019 December 8 UTC and 2020 January 27 UTC using the F689M and F845M filters (HST GO 16040, PI Bolin) A south-facing jet is observed in the near-nucleus coma in the images taken on 2019 October 12 and 2019 November 16 and a south-west facing jet is observed in the December 8 UTC and 2020 January 27 images (outlined by the dotted purple wedges).  Right panel: Pole/line-of-sight planes from our 4 observing dates. The intersection zone of around RA = 322 deg, dec = 37 deg in the grey square defines the rotation axis with an uncertainty of ~10 degs.

How to cite: Bolin, B., Lisse, C., Kasliwal, M., Quimby, R., Bodewits, D., Morbidelli, A., and Helou, G.: VISIR Characterization of the Nucleus, Morphology, Activity, Spin-Pole Orientation & Rotation of Interstellar Comet 2I/Borisov by Earth- and Space-based Facilities, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-479, https://doi.org/10.5194/epsc2020-479, 2020.

We present an updated statistical analysis on molecular abundances retrieved from infrared spectra of 20 comets, observed with NIRSPEC-KECK since 1999. Using these results, we investigate the chemical diversity among comets, and we try to correlate them to the chemical and physical processes present during the formation of our planetary system.

Introduction: Comets are considered the remnants of the solar system formation. According to recent dynamical models [1], objects that formed between 5 and 17 AU likely scattered into the Oort cloud (OC), the primary source of long period and dynamically-new comets, while those that formed in the outer proto-planetary disk (beyond 17 AU) entered both the Oort cloud and Kuiper belt (KB) reservoirs. Investigating the chemical diversity in comets may unveil the physical and chemical conditions present during the formation and early evolution of our planetary system (e.g. hydrogenation on dust grains in cold environments or photo-dissociation processes due to UV/X-rays/Cosmic-rays radiation), as well as the processes that may have changed the nucleus composition after its formation (e.g. cosmic rays impacting the outer layer of the nucleus or successive surface warming on repeated passages through the inner solar system).

Since 1985, more than 60 comets have been investigated using ground based high resolution infrared spectrometers, and many efforts have been made to create a classification of these bodies [2,3]. However, some infrared results published before 2011 may contain systematic inaccuracies related to then-incomplete molecular models used to interpret the fluorescence excitation in comets, to a non-properly described atmospheric transmittance models and/or to the use of immature reduction algorithms. These inaccuracies impact mostly comets that were observed before 2011, and they need to be removed [4]. Here, we present a statistical analysis on our revised data for twenty comets and we investigate their possible connections to processes in the proto-planetary disks and/or the natal cloud.

Results and discussion: We have examined the distribution of molecular species among the comet population making use of boxplots (Figure 1). The amount of dispersion that we observe for individual species is expected and could be partially related to the temperature gradient in the proto-planetary disk and/or to the intensity of the radiation field, if present. For instance, the high dispersion range for CO may be related to efficient formation of CH₃OH through hydrogenation processes on dust grains at low temperatures (T < 20 K), or formation of CO₂ at higher temperatures (T > 20 K) [5]. The differential loss of highly volatile species (e.g. CO and CH₄) after the comet formation may also be relevant.

In Figure 2, we show two selected scatterplots for some of the analyzed molecules: these trends should reflect, at least in part, the conditions that were present during the formation of comets. We notice for example that CO shows a high and positive correlation with CH₄, and a much lower but still positive correlation with CH₃OH (an anti-correlation is expected if hydrogenation converted significant CO to CH₃OH). Aspects of the interpretation of the scatterplots will be discussed.

We compared our results with recent disk models [6,7,8], where the relative amounts of CH₃OH, CO, CH₄ and C₂H₆, are expected to depend on what chemical processes and how much radiation field (UV/X/CR) were present at different heliocentric distances from the proto-sun. Considering the combination of these four species (Figure 3), we notice that most of the sampled comets fall in the third and first quadrants and follow an (almost) linear relationship. Depending on the position of the comet with respect to this line, we can define a factor K that can be associated with an increasing processing of the cometary material, and try to reconstruct the chemical and physical history of each analyzed comet.

Conclusions: We report an updated statistical analysis on molecular abundances observed in 20 comets and their possible connections with protoplanetary disk models. The results reveal a diversity in comet composition, and different correlations among the observed chemical species, ultimately giving important hints about the physical and chemical condition present during the formation and evolution of our Solar System.

Figure 1. Boxplot statistic for the chemical species that we observed in 20 comets; for each box we report the interquartile range (IQR), the median (Med), the skewness (Skw), and the whiskers (error bars of the boxplots). Comets characterized by outlier values are highlighted.

Figure 2. Selected scatterplots showing the measured mixing ratios (% with respect to water). For each graphic, the correlation factor is indicated in the upper left corner. Jupiter family and Oort Clouds comets are shown with squares and circles, respectively.

Figure 3: Relationship between relative abundances of selected species and possible connections with the formation and evolution of comets in protoplanetary disks. Jupiter family and Oort Clouds comets are shown with squares and circles, respectively. Colors indicate a possible degree of evolution of the cometary material (violet =  low degree to red = high degree) due to different processes at different life stages of the comet.

This work is supported by the NASA Emerging Worlds Program EW15-57 and the NASA Astrobiology Program 13-13NAI7-0032

References: [1] Morbidelli, A. et al. 2007, Astron. J., 134, 1790; [2] Mumma & Charnley, Ann. Rev A&A, 49, 471 2011; [3] Dello Russo, N. et al. 2016, Icarus, 278, 301; [4] Lippi et al 2020, Astron. J., 159, 157; [5] Tielens, A. G. G. M., Rev. Of Modern Physics, 85, 1021, 2013; [6] Bosman et al. 2018, 618, A182; [7] Walsh 2010 ApJ, 722, 1607; [8] Eistrup et al. 2018, A&A, 613, A14.

How to cite: Lippi, M., Villanueva, G. L., Mumma, M. J., and Faggi, S.: Investigation on the origins of comets as revealed through IR high resolution spectroscopy, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-495, https://doi.org/10.5194/epsc2020-495, 2020.

Centaurs – planet-crossing bodies in the region of the giant planets that mainly originate in the Kuiper Belt/Scattered Disk [1, 2] – are thought to be the primary impactors on the giant planets and their satellites [3-8]. As part of an effort to interpret the cratering records of the saturnian satellites, we are developing a dynamical-physical model for the size distribution of potential impactors on the moons.

Most models of the orbital distribution of "observable" comets^[1] assume that the size of the nucleus does not change with time. These models treat physical evolution only by assuming a lifetime, after which comets are considered inactive or "faded". These models do not specify a fading mechanism, but assume an expression for the probability that a comet remains active after some amount of time [10-14]. Fading can result from loss of all volatiles, formation of a nonvolatile mantle on the surface of the nucleus, or splitting [15, 16].

A model of the erosion of 67P/Churyumov-Gerasimenko and 46P/Wirtanen due to sublimation of water ice throughout their orbital evolution estimates that 67P’s nucleus has shrunk from a radius of 2.5 km to 2 km, while 46P’s has decreased from 1 km to 0.6 km [17]. This calculation assumes that 10% of the nucleus is active and that its density is 500 kg/m³. These estimates are uncertain because comets follow chaotic orbits, but in general, erosion has a bigger effect on smaller nuclei.

Some comets are active well beyond the water-ice sublimation zone within 3 au. Eighteen active Centaurs are currently known [18, 19]. 29P/Schwassmann-Wachmann, which follows a near-circular orbit at 6 au, is a copious source of dust and CO [20-22] and undergoes significant dust outbursts 7 or more times a year [23]. 174P/Echeclus underwent an outburst 13 au from the Sun that released ≈ 300 kg/s of dust for about two months [24], a rate comparable to the 530 kg/s of dust released by 67P at its peak near 1.3 au [25]. Echeclus also underwent several more outbursts near perihelion (≈6 au) with CO outgassing at ≈ 10% the rate of 29P at the same heliocentric distance and dust mass loss rates of 10 - 700 kg/s [20]. 2060 Chiron is another Centaur that is sporadically active in gas and dust, consistent with a more depleted state, like Echeclus [20].

Di Sisto et al. (2009) constructed a model of the orbital distributions of Jupiter-family comets (JFCs) that incorporated planetary perturbations, nongravitational forces, sublimation, and splitting. They considered nuclei with initial radii of 10, 5, and 1 km [26]. Di Sisto et al. found that 5- and 10-km comets usually evolved onto Centaur orbits, while 1-km comets were most likely to shrink below 100 m. Inspired by their work, we are developing a model for the dynamical-physical evolution of JFCs and Centaurs. We will use the orbital distribution found by Nesvorny et al. as our baseline model [14, 27].

We will first focus on modeling the evolution of the size distribution of JFCs. The model will eventually account for mass loss by both JFCs and Centaurs, with activity driven by H₂O, CO, or other volatiles. The fraction of the nucleus that is active will be allowed to vary with size, since smaller nuclei are typically more active [28-30]. We will then implement a model for cometary splitting with these inputs: the frequency of splitting as a function of perihelion distance; the fraction of the comet’s mass released as fragments; the size distribution of the fragments; and the velocity imparted to the fragments by the splitting event. We will present preliminary results of our simulations.

We thank Raphael Marschall for discussions and the Cassini Data Analysis Program for support.

References

[1] Volk, K.; Malhotra, R. ApJ 687, 714–725, 2008.

[2] Di Sisto, R. P.; Rossignoli, N. L. CMDA, in press (arXiv:2006.09657), 2020.

[3] Zahnle, K.; Dones, L.; Levison, H. F. Icarus 136, 202–222, 1998.

[4] Zahnle, K.; Schenk, P.; Levison, H.; Dones, L. Icarus 163, 263–289, 2003.

[5] Di Sisto, R. P.; Zanardi, M. A&A 553, id. A79, 2013.

[6] Di Sisto, R. P.; Zanardi, M. Icarus 264, 90–101, 2016.

[7] Rossignoli, N. L.; Di Sisto, R. P.; Zanardi, M.; Dugaro, A. A&A 627, id. A12, 2019.

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[13] Brasser, R.; Wang, J.-H. A&A 573, id. A102, 2015.

[14] Nesvorný, D. et al. ApJ 845, id. 27, 2017.

[15] Weissman, P. R.; Bottke, W. F., Jr.; Levison, H. F. In Asteroids III, Univ. Arizona Press, pp. 669–686, 2002.

[16] Jewitt, D. C. In Comets II, Univ. Arizona Press, pp. 659–676, 2004.

[17] Groussin, O. et al. MNRAS 376, 1399–1406, 2007.

[18] Jewitt, D. AJ 137, 4296–4312, 2009.

[19] Chandler, C. O. et al. ApJLett 892, id. L38, 2020.

[20] Womack, M.; Sarid, G.; Wierzchos, K. PASP 129, 031001, 2017.

[21] Sarid, G. et al. ApJLett 883, id. L25, 2019.

[22] Wierzchos, K.; Womack, M. AJ 159, id. 136, 2020.

[23] Trigo-Rodríguez, J. M. et al. MNRAS 409, 1682–1690, 2010.

[24] Bauer, J. M. et al. PASP 120, 393–404, 2008.

[25] Marschall, R. et al. Frontiers in Physics, doi: 10.3389/fphy.2020.00227 (arXiv:2005.13700), 2020.

[26] Di Sisto, R. P.; Fernandez, J. A.; Brunini, A. Icarus 203, 140–154, 2009.

[27] Nesvorný, D. et al. AJ 158, id. 132, 2019.

[28] A’Hearn, M. F. et al. Science 332, 1396–1400, 2011.

[29] Tancredi, G.; Fernandez, J. A.; Rickman, H.; Licandro, J. Icarus 182, 527–549, 2006.

[30] Schleicher, D. G.; Knight, Matthew M. AJ 152, id. 89, 2016.

How to cite: Dones, L. and Womack, M.: Physical Evolution Model for Jupiter-Family Comets and Centaurs, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-515, https://doi.org/10.5194/epsc2020-515, 2020.

(90482) Orcus is one of the largest Kuiper belt objects, with one known, relatively large satellite, Vanth. There have been several ~10-20h rotation periods reported in the literature for Orcus, with considerable uncertainty. Here we report on recent measurements of Orcus with the TESS Space Telescope providing a light curve period of 7 h, the fastest rotation among those large trans-Neptunian objects for which the rotation is not expected to cause a distorted, triaxial ellipsoid shape, like in the case of Haumea. While moons of large Kuiper belt objects are usually assumed to be formed from an original large body via collisions, the fast rotation may point to a scenario in which Vanth was captured from a nearby heliocentric orbit early in the history of the Solar system, and subsequent tidal evolution led to the present, nearly circular orbit. In this sense the Orcus-Vanth system is peculiar, as the present rotational characteristics and satellite orbits of all other large Kuiper belt objects are consistent with a collisional origin. 

How to cite: Kiss, C., Pál, A., Szakáts, R., Marton, G., and Müller, T.: The fast rotation of Orcus obtained from TESS measurements, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-516, https://doi.org/10.5194/epsc2020-516, 2020.

Near-opposition photometry employs remote sensing observations to reveal the microphysical properties of regolith-covered surfaces over a wide range of solar system bodies. When aligned directly opposite the Sun, objects exhibit an opposition effect, or surge, a dramatic, non-linear increase in reflectance seen with decreasing solar phase angle (the Sun-target-observer angle). This phenomenon is a consequence of both interparticle shadow hiding and a constructive interference phenomenon known as coherent backscatter [1-3]. While the size of the Earth’s orbit restricts observations of Pluto and its moons to solar phase angles no larger than α = 1.9°, the opposition surge, which occurs largely at α < 1°, can discriminate surface properties [4-6].

The smallest solar phase angles are attainable at node crossings when the Earth transits the solar disk as viewed from the object. In this configuration, a solar system body is at “true” opposition. When combined with observations acquired at larger phase angles, the resulting reflectance measurement can be related to the optical, structural, and thermal properties of the regolith and its inferred collisional history. The Pluto system was at true opposition when it crossed the line of nodes (Fig. 1) for the first time in 87 years in July 2018 [7], and, owing to the eccentricity of its orbit, it won’t be at true opposition again for another 161 years, in 2179. At the subsequent opposition in July 2019, Pluto was still observable at small phase angles and presented the opposite hemisphere to view.

In 2018, the sunlit portion of Pluto’s Charon-facing hemisphere, centered at longitude 30° E, was observable from Earth at a minimum phase angle α = 0.0049° (Fig. 2). In 2019, its anti-Charon hemisphere was visible at phase angles as small as α = 0.0131°, centered at longitude 210° E, the same hemisphere viewed at high spatial resolution by the New Horizons spacecraft [8]. Since the sub-Earth latitudes were 55° and 56° N in 2018 and 2019, respectively, observations in both years include much of Lowell Regio, Pluto’s bright northern polar cap. Here we present and compare Hubble Space Telescope (HST) near-opposition solar phase curves (Fig. 3) of each of these hemispheres acquired between 2015 and 2019 with the Wide Field Planetary Camera 3 (WFC3) using the UVIS F555W filter (0.53 μm). Since Lowell Regio dominates 60% of the anti-Charon hemisphere and 80% of the Charon-facing hemisphere, the shapes of the two solar phase curves are similar; however, any differences in their shapes reveal differences in the scattering properties and physical surface characteristics of the terrains unique to each hemisphere. The New Horizons encounter (anti-Charon) hemisphere includes Tombaugh Regio and Sputnik Planitia and is ~0.1 magnitudes brighter than the Charon-facing hemisphere in the F555W filter. No part of Sputnik Planitia is visible in the Charon-facing hemisphere, so any differences between its solar phase curve and that of the anti-Charon hemisphere are mainly due to the scattering properties of Pluto’s darker equatorial regions. Preliminary results indicate that the slope of the anti-Charon hemisphere’s phase curve between phase angles 0.3° and 1° (0.025 mag/°) is shallower than that of the Charon-facing hemisphere (0.035 mag/°). When combined with New Horizons LORRI images acquired at higher phase angles, the quantitative analyses of the resulting solar phase curves reveal differences in the structure and texture of regolith particles on Tombaugh Regio and Sputnik Planitia and those on Pluto’s dark equatorial regions, including Cthulhu Macula. 

Acknowledgements: Based on observations made with the NASA/ESA Hubble Space Telescope which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555. These observations are associated with HST Programs 15505, 15261 (Verbiscer, PI) and 13667 (Buie, PI).

References:

[1] Hapke, B. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, 2012. [2] Shkuratov, Yu. & Helfenstein, P. 2001. Icarus 152, 96-116. [3] Hapke, B. 2002. Icarus 157, 523-524. [4] Nelson, R. et al. 2015. LPSC 46, 2584. [5] Verbiscer, A. et al. 2018. In Enceladus and the Icy moons of Saturn. P. Schenk et al., Eds. Univ. Arizona Press, Tucson, p. 323-341. [6] Helfenstein, P. and Shepard, M. 2011. Icarus 215, 83-100. [7] Stern, S. A. et al. 2015. Science 350, aad1815.  [8] Verbiscer, A. et al. 2019. EPSC-DPS 2019 1261.

How to cite: Verbiscer, A., Helfenstein, P., Showalter, M., and Buie, M.: A Tale of Two Sides: Pluto's Opposition Surge in 2018 and 2019, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-546, https://doi.org/10.5194/epsc2020-546, 2020.

Introduction: The New Horizons flyby of Arrokoth revealed an ancient, contact binary planetesimal [1,2]. Arrokoth’s both lobes’ respective principal axes are aligned within a few degrees [2], and such configuration suggests a co-orbiting Arrokoth before the coalescence of its two lobes [3]. One mechanism proposed for Kuiper belt object (KBO) binaries to merge into a bilobate body is a random walk due to collisions with other heliocentric bodies [3,4]. For Arrokoth, one thing that remains resolved is that its present-day spin period (15.92 hr) is slower than that predicted from both lobes’ mutual gravitational pull (11.26 hr), assuming a comet-nucleus-like density of 500 kg m^-3 [3], implying ~30% angular momentum loss. While Arrokoth may simply be less dense, it is worth exploring whether collisions with other KBOs could have substantially altered its spin state over time. Here we adapt our Monte Carlo impact simulation for Ceres and Vesta [5] and investigate Arrokoth’s possible spindown (or spinup) by impacts.

Triaxial Arrokoth and model results: We previously carried out Monte Carlo impact simulations with random impacts onto an Arrokoth modeled as an oblate spheroid [6]. Here we model Arrokoth as a triaxial ellipsoid by matching its cross-sectional areas along the principal axes with that from its shape model. We also scale this triaxial body’s density for its moment-of-inertia (MOI) to match that of Arrokoth. As a result, this model geometry approximates what the actual Arrokoth would undergo, regarding its dynamical evolution by a given flux of impactors. In this way we avoid the unnecessary complications for trying to track the ejecta interactions on a truly bilobate object.

We base our range of impactor sizes from Arrokoth’s measured craters [2] along with impactor-crater scaling laws [7,8]. While the lower bound is well-determined near 10-m, the upper bound is less well constrained. Both scalings [7,8] predict that the largest crater “Maryland” (~7-km wide, [2]) could have been created by an impactor ~1-to-2-km wide with a typical impact speed ~300 m/s. The total number of impactors is extrapolated from the crater counts [2] where 40-50 craters or pits were recognized during the flyby on one side of Arrokoth. Hence, we allow for 100 impacts between 10 and 1000 m in our simulations (we vary the upper limit later), assuming dN/dD ~ D^-1.75  [2,7], where N(>D) is the number of impactors with diameters greater than D. Implicit in our modeling is the assumption that Arrokoth's craters postdate its (plausibly very early) merger [3]. We also track a disruption energy threshold for porous asteroids [9] to test for potential catastrophic breakup.

Figure 1 consists the results after 5000 Monte Carlo simulations starting at 11.26 hr, with impactors from cold classical KBOs (CCKBOs). No disruption is predicted among all our simulations. About 80% of the simulations end within 11.2 ± 0.2 hr (1σ). Only 2% of the runs have a final spin increase or decrease by more than half an hour. The total, integrated impactor mass is only 0.01 ± 0.01% (1σ) of M, while the total mass loss ejected is 3.8 ± 1.6 times (1σ) larger [10], indicating that Arrokoth is losing mass almost all the time after these 100 impacts. This finding is opposite to what we have found on Ceres or Vesta [5] where a porous surface tends to retain mass rather than losing mass overall; this is expected because the escape velocity of Arrokoth is ~5 m/s, much lower than a typical impact velocity among KBOs [7].

Even though the majority of the impactor flux onto Arrokoth comes from CCKBOs [7], other subpopulations of KBOs also contribute to its overall spin history. Based on pre-encounter model [7], we randomly select ~11% of the impactors to be hot classicals (with a higher impact speed, see Fig. 2); an extra set of 5000 simulations is implemented (Fig. 3). The incorporation of hot classicals delivers notable differences. Due to slightly increased overall impact speed, ejecta loss is further enhanced (about 6.2 ± 5.2 times (1σ) larger than total impactor mass), albeit the net mass change is still ~0.1% M. Whereas the majority of the simulation runs clusters within 1σ from the average, more than 6% experience net spin changes greater than 0.5 hr; indeed, a few runs even achieve the required spindown from 11.26 hr to 15.92 hr. Therefore, Arrokoth’s potential spindown by impacts has a higher likelihood under this condition, although this possibility is still quite low from a statistical point of view.