Oral presentations and abstracts
Our knowledge of the physical and dynamical properties of the small body populations in the solar system is constantly improving, thanks to new Earth- and space-based observations, space missions as well as theoretical advances, and the appearance of the first interstellar objects. The goal of this session is to highlight recent results that are providing fundamental clues about the early stages of the solar and extrasolar systems.
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.
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.
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  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”) . The result of comets observations in 1.6GHz frequency band made by other astronomy groups ,,, 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 .
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  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 . 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.
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.
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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<qKBO<-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.
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 rh, total production rate Q, radius of the nucleus Rn, surface temperature Tn, mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side an. 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 ai are tabulated for a given mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side an. 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 FG, the gas drag FA, and the solar radiation pressure FS. 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 bi are tabulated for a given mechanism of gas production (surface sublimation or diffusion from interior) and level of activity of the night side an. This approximation gives the relative deviation from numerical solution less than 5% for n=4 and 17% for n=3.
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 ).
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.
Braga-Ribas et al., 2014, Nature 508, 72
Ortiz et al., 2017, Nature 550, 219
Sicardy et al., 2019, Nature Astronomy 3, 146
The work leading to these results has received funding from the European Research Council under the European Community's H2020 2014-2020 ERC Grant Agreement n°669416 "Lucky Star"
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.
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 .
We analyze in-situ density data of the two sensors COPS and DFMS (see ) 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 rh 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  and .
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 rh-2-line specifies a radiation model assuming linear relation between solar irradiation and gas production. The horizontal bars indicate two time intervals Ia and Ic 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 ∼rh−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 Ic from 190 d to 380 d after perihelion, the productions for H2O, O2, CH3OH, H2CO, and NH3 follow power laws rhα 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 Ic. 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 Ia 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 Ia.  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 . Each triangular surface element of a shape model with 3996 elements (see Fig. 2) holds a single gas source described by . 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.
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 rh-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.
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.
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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  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  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 . 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 . 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  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 . Pluto’s solar phase curve is also relatively flat between phase angles of 0.5-1.5° , 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 . 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.