Oral presentations and abstracts
Mercury Science in the post-MESSENGER area, open to all Mercury relating subjects, and covering e.g.
- Mercury's interior, surface, environment, and interactions thereof
- BepiColombo Mercury Science
- BepiColombo Flyby (Earth, Venus, Mercury) and Cruise (Radio Science, Solar Wind, Gamma Rays): results and expectations
- Ground-based Mercury Observations: results and plans
- laboratory activities in support of Mercury Science
The variability of Na exosphere of Mercury shows time scales from less than one hour to seasonal variations. While the faster variations, accounting of about 10-20% of fluctuations are probably linked to the planetary response to solar wind and Interplanetary Magnetic Field variability, the seasonal variations (up to about 80%) should be explained by complex mechanisms involving different surface release processes, loss, source and migrations of the exospheric Na atoms. Eventually, a Na annual cycle can be identified. In the past, ground-based observations and equatorial density from MESSENGER data have been analysed. In this study, for a better investigation of the exospheric Na features, we have studied the local time and latitudinal distributions of the exospheric Na column density as a function of the True Anomaly Angle (TAA) of Mercury by means of the extended dataset of images, collected from 2009 to 2013, by the THEMIS solar telescope. Our results show that the THEMIS images, in agreement with previous results, registered a strong general increase at aphelion and a dawn ward emission predominance with respect to dusk ward and subsolar region between 90° and 150° TAA. Unlikely other analyses, ours evidences a predominance of subsolar column density along the rest of Mercury’s orbit. An unexpected relationship between Northward or Southward peak emission and both TAA and local time is also evidenced by our analysis. This result seems to contradict previous results obtained from different data sets and it is not easily explained, thus it requires further investigations.
How to cite: Milillo, A., Mangano, V., Massetti, S., Mura, A., Plainaki, C., Alberti, T., Aronica, A., De Angelis, E., Ippolito, A., Kazakov, A., Noschese, R., Orsini, S., Rispoli, R., Sordini, R., and Vertolli, N.: Exospheric Na distributions along the Mercury orbit with the THEMIS telescope, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-220, https://doi.org/10.5194/epsc2020-220, 2020.
Leblanc, F. et al., 2009. Short-term variations of Mercury's Na exosphere observed with very high spectral resolution. Geophys. Res. Lett. 36, 7.
Mangano, V. et al., 2015. THEMIS Na exosphere observations of Mercury and their correlation with in-situ magnetic field measurements by MESSENGER, Planet. Space Sci. 115, 102-109.
Potter, A. E., R. M. Killen and M. Sarantos. 2006. Spatial distribution of sodium on Mercury. Icarus 181, 1 -12.
Potter, A. E., R. M. Killen and T. H. Morgan, 1999. Rapid changes in the sodium exosphere of Mercury. Planet. Space Sci. 47, 1441-1448.
Potter, A. E. and T. H. Morgan, 1985. Discovery of sodium in the atmosphere of Mercury. Geophys. Res. Lett. 22, 3289-3292.
Sprague, A. L. et al., 1998. Mercury sodium atmosphere enhancements, Radar bright spots, and visible surface features. Icarus 136, 60-68.
How to cite: Killen, R.: Seeing Effects on Ground-Based Images of Mercury's Sodium Exosphere, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-5, https://doi.org/10.5194/epsc2020-5, 2020.
Flux transfer events (FTEs) are magnetic flux ropes formed by multiple reconnection X-lines on the magnetopause between the planetary magnetic field and the interplanetary magnetic field (IMF). During FTE shower intervals, FTEs appear extremely frequently with typical rates of ~ 10 FTEs per minute. This study shows that FTE showers are observed during half of the dayside magnetopause crossings over the entire MESSENGER mission. The occurrence and physical properties of FTE showers depend on the magnetic shear angle across the magnetopause and the magnetosheath plasma (ratio of the plasma thermal pressure to the magnetic pressure). Modeling of these flux ropes in FTE shower intervals are determined to carry most (i.e., 60% - 85%) of the magnetic flux supplying Mercury’s Dungey cycle. Furthermore, sodium ions (Na+) are substantially enhanced during FTE shower intervals as compared with non-shower intervals. FTE showers lead to much larger precipitation rates on the cusp’s surface and would sputter large numbers of planetary atoms into the exosphere, which then be photon-ionized resulting in the enhancement of Na+.
Interactions between Mercury’s global intrinsic magnetic field and supersonic solar wind form a miniature magnetosphere. Without a significant neutral atmosphere, Mercury’s magnetosphere is able to prevent most of the planet’s surface from being directly hit by the streaming solar wind most of the times . Magnetic reconnection on the dayside magnetopause generates FTEs, in which the magnetic field lines have one end connected to the solar wind and the other end connected to the planet. Magnetic flux ropes concentrate magnetic flux in the FTEs (FTE-type FRs), which transport magnetic flux from the dayside magnetosphere into the nightside following the convection of FTEs. Solar wind plasma can be transported along the magnetic field lines and precipitate to the surface at the planet’s cusps causing sputtering . MESSENGER observations have shown that FTE showers correspond to a very high occurrence of FTEs (~ 10 FTEs per minute) [1, 3].
In this work, we investigate how the low Alfvénic Mach number (MA = VSW/VA) solar wind near Mercury’s orbit influences the occurrence and physical properties of FTE showers, and how FTE showers contribute to the Dungey cycle and influence sodium ions dynamics in the dayside magnetosphere.
2. FTE Shower at Mercury
Figure 1. Overview of MESSENGER observations of a dayside magnetosphere crossing on 10 April 2011. (a) and (b) Differential and integrated particle flux for protons. (c) Densities of alpha particle (He++) and sodium group (Na+, m/q = 21 to 30). (d) to (g) Magnetic field measurements. The black arrows in (a) annotate the injected solar wind particles. FTEs and plasma filaments are marked in (g).
Figure 1 shows a FTE shower event observed by MESSENGER on 10 April 2011. Thirty-four FTE-type FRs appeared in two and half minutes surrounding the dayside magnetopause (~ 14 FTEs/minute). The occurrence of FTEs at Mercury is much higher than those at Earth, Jupiter and Saturn, which are commonly separated by several minutes . The magnetic field intensity in the magnetosheath is even slightly larger than the magnetic field intensity in the magnetosphere, implying a thick plasma depletion layer with a low plasma ( 0.1) adjacent to the magnetopause. Magnetic shear angle across the dayside magnetopause is around 120.
The injected solar wind particles are marked in Figure 1a. In the low altitude region, especially at the cusp, the injected particles diamagnetically reduce the field strength and form the plasma filaments (Figure 1g, see also ref. ). The injected solar wind particles could precipitate to the surface and sputter planetary atoms refilling the exosphere. Planetary Na+ appear in a density of ~ 1 cm-3, which is around an order of magnitude higher than the average Na+ densities in the subsolar region .
3. Statistical Results
This study conducts a survey over the entire MESSENGER mission. In a total number of 3748 magnetopause crossings, around half (52%) of them were found to be accompanied by FTE showers. Further detailed investigations show that FTE showers occur more frequently when the magnetic shear angle is large and the magnetosheath plasma is small. The repetition time, i.e., the separation between neighboring FRs, and core magnetic field of FRs are negatively correlated with the shear angle and the plasma . The duration of FRs is positively correlated with the shear angle and the plasma . FTE showers could carry 60% to 85% of the magnetic flux needed to supply Mercury’s Dungey cycle.
The planetary Na+ is substantially enhanced during the FTE shower intervals compared with the Na+ during other intervals. The total number of Na+ on the dayside magnetosphere in the FTE shower intervals is more than twice the number of Na+ during the intervals without FTE showers. Since FTE shower events depend on solar wind variations, which last tens of minutes to an hour, the corresponding enhancements of exospheric Na+ should be on the scale of same order, i.e., short-term variations.
We have shown that the occurrence and physical properties of FTE showers clearly depend on the magnetic shear angle across the magnetopause current sheet and the magnetosheath plasma β. FTE showers play a dominant role in the flux circulation of Mercury’s Dungey cycle. Furthermore, FTE showers are closely associated with substantial enhancements in the Na+ density in the dayside magnetosphere.
How to cite: Sun, W.-J., Slavin, J., Smith, A., Dewey, R., Poh, G., Jia, X., Raines, J., Livi, S., Saito, Y., Gershman, D., DiBraccio, G., Imber, S., Guo, J., Fu, S., Zong, Q.-G., and Zhao, J.: MESSENGER observations of flux transfer event showers at Mercury: Contributions to Dungey cycle and influences on magnetospheric sodium ions, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-62, https://doi.org/10.5194/epsc2020-62, 2020.
MESSENGER observations used to constrain surface composition suggest a global magma ocean formed on early Mercury [1, 2]. Our study models the coupled evolution of the Hermean magma ocean and atmosphere, and determines the extent of exospheric loss from an atmosphere formed by evaporation of a magma ocean. Using our framework to couple the interior and exterior chemical reservoirs, we evaluate a range of possible atmospheric evolution scenarios for early Mercury. These include the possibility that Mercury was fully-accreted before a mantle stripping event caused by a giant impact  led to degassing, or alternatively that the building blocks of Mercury were already relatively volatile-free.
Assuming an initial surface temperature of 2500 K and an oxygen fugacity fixed at 1 log unit below the Iron-Wüstite buffer, we find that the Hermean magma ocean cooled to 1500 K (the rheological transition) in around 400 to 9000 yrs, depending on the efficiency of radiative heat transfer in the atmosphere. We investigate the behaviour of two endmember cases: (1) a present-day sized Mercury that is volatile free and thus cools in a manner similar to that of a blackbody, and (2) a larger Mercury that is sufficiently volatile-rich to provide a greenhouse atmosphere that delays cooling. During the magma ocean stage, evaporation and sublimation of oxide components from the molten silicate magma contribute to the growth of the atmosphere, in addition to volatile outgassing. For the endmember case with initial volatile abundances based on enstatite chondrite-like precursors, the atmosphere is dominated by CO, CO2, H2, and H2O, with minor abundances of SiO, Na, K, Mg, and Fe gas species. For the endmember case without major volatiles (C, H), the atmospheric composition is dominated by metal- and metal oxide gas species only.
We apply a Monte Carlo (MC) atmospheric loss model  to calculate exospheric losses for all pathways and find that photoionization is the dominant loss mechanism for early Mercury. Cases both with and without major volatiles reveal that the atmosphere lasts between 100 and 250 years before the near-surface of Mercury becomes solid. Preliminary MC results show that photo-dissociation considerably alters the atmospheric composition, efficiently breaking SiO2 into SiO+ and O. Solar wind and magnetospheric plasma leads to rapid evacuation of the ionized species from the neutral exosphere whereas oxygen is lost efficiently by Jeans escape.
Using our coupled framework, future modelling efforts will aim to understand if and how the evaporation of the magma ocean of early Mercury has modified its surface composition, with a view to interpreting BepiColombo observations. Hence our work provides an important step to connect Mercury’s formation, earliest evolution, and upcoming observations.
References:  Vander Kaaden, K. E. and McCubbin, F. M. (2016). Cosmochim. Ac., 173, 246–263.  Berthet, S., Malavergne, V., and Righter, K. (2009). Geochim. Cosmochim. Ac., 73(20), 6402–6420.  Benz, W., et al. (2008). Mercury, pp. 7–20. Springer.  Wurz, P., and Lammer, H. (2003). Icarus, 164(1), 1–13
How to cite: Gamborino, D., Jäggi, N., Bower, D. J., Wolf, A., Sossi, P., Vorburger, A., Oza, A. V., Ligterink, N. F. W., and Wurz, P.: A magma ocean origin for Mercury's earliest exosphere, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-571, https://doi.org/10.5194/epsc2020-571, 2020.
Mercury's sodium atmosphere has been the subject of many studies since it's discovery in 1985, with its extended corona and the tail-like structure probably forming its most famous attribute [5, 8, 10, 12]. In addition, the Na atmosphere is temporally and spatially variable, concentrated on the dayside, often enhanced close to the north and south poles (with a moderate north-south asymmetry), and correlates with in situ magnetic field observations. [3, 4, 6, 7, 9, 11]. MESSENGER observations, carried out between 2011 and 2015, show little or no year-to-year variation and a dawn-dusk asymmetry in Mercury's sodium atmosphere . In this presentation we analyse different sodium surface release mechanisms and determine their contribution to the Hermean Na exosphere by comparing our model results to published MESSENGER measurements.
1. MESSENGER observations
The MESSENGER measurement we use as a baseline for this study was presented by  in their Figure 7. According to their observations, the sodium column density ranges from >1012 cm–2 at the surface to ~107 cm–2 at ~3500 km altitude, exhibits a steep fall-off close to the surface, and starts to flatten out at ~700 km.
2. Monte-Carlo Model
The Monte-Carlo model used herein was first presented in 2003 . In this model, a large number of particles (typically on the order of 106) are traced individually ab initio. Source mechanisms relevant to Mercury's exosphere are (i) thermal desorption, (ii) photon-stimulated desorption, (iii) sputtering, and (iv) micro-meteorite impact vaporization. Figure 1 shows the energy distribution functions associated with these four release processes. For this specific analysis, the effect due to radiation pressure, and an improved energy distribution for photon-stimulated desorption was added . Particles are removed from the calculation domain when they (i) are ionised, (ii) fall back and stick to the surface, or (iii) reach the Hill-sphere.
Figure 1: Normalized energy distribution functions for (i) thermal desorption, (ii) photon-stimulated desorption, (iii) sputtering, and (iv) micro-meteorite impact vaporization. The vertical line shows the escape energy.
3. Simulation Parameters & Geometry
Limb scan data from the MASCS/UVVS instrument on MESSENGER presented in  Figure 7 was taken on 16 June 2012 at a local time of ~12:00. On that date, Mercury was at a true anomaly angle of TAA = 99°, which means that the radiation pressure was almost at its maximum. The scan itself extends from just above the surface up to an altitude of several thousand km.
4. Results and Discussion
Our analysis shows that the measurement presented by  can be quantitatively reproduced by particles originating from two different source mechanisms. Close to the surface, thermal desorbed Na particles with a surface temperature of 594 K dominate the exosphere. At higher altitudes, Na particles released by micro-meteorite vaporization dominate. Both the shape and the absolute value of the modelled tangent column density agree well with the measurements.
How to cite: Wurz, P., Vorburger, A., and Gamborino, D.: Mercury's Sodium Exosphere: Interpretion of MESSENGER Observations, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-178, https://doi.org/10.5194/epsc2020-178, 2020.
The interaction between the interplanetary medium and planetary environments gives rise to different phenomena according to the spatio-temporal scales. Here we apply for the first time a novel data analysis method, i.e., the Hilbert-Huang Transform, to discriminate both local and global properties of Venus’ and Mercury’s environments as seen during two MESSENGER flybys. Hence, we may infer that the near-Venus environment is similar in terms of local and global features to the ambient solar wind, possibly related to the induced nature of Venus’ magnetosphere. Conversely, the near-Mercury environment presents some different local features with respect to the ambient solar wind, due to both interaction processes and intrinsic structures of the Hermean environment. Our findings support the ion kinetic nature of the Hermean plasma structures, with the foreshock and the magnetosheath regions being characterized by inhomogeneous ion-kinetic intermittent fluctuations, together with MHD and large-scale fluctuations, the latter being representative of the main structure of the magnetosphere. We also show that the HHT analysis allow to capture and reproduce some interesting features of the Hermean environment as flux transfer events, Kelvin-Helmholtz vortex, and ULF wave activity, thus providing a suitable method for characterizing physical processes of different nature. Our approach demonstrate to be very promising for the characterization of the structure and dynamics of planetary magnetic field at different scales, for the identification of different planetary regions, and for the detection of the “effective” planetary magnetic field that can be used for modelling purposes.
How to cite: Alberti, T., Milillo, A., Laurenza, M., Massetti, S., Ivanovski, S., Ippolito, A., Plainaki, C., De Angelis, E., Mangano, V., Mura, A., Orsini, S., and Rispoli, R.: The Hilbert-Huang Transform (HHT) as a tool for characterizing dynamical features of Mercury’s and Venus’ magnetospheres, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-89, https://doi.org/10.5194/epsc2020-89, 2020.
We examine the effects of the interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure on the solar wind proton precipitation to the surface of Mercury. We use the Amitis model, a three-dimensional GPU-based hybrid model of plasma (particle ions and fluid electrons), and explain a method we found necessary to accurately calculate plasma precipitation to the surface of Mercury through the highly dynamic Hermean magnetosphere. We use our model to explain ground-based telescope observations of Mercury's neutral sodium exosphere, and compare our simulation results with MESSENGER observations. For the typical solar wind dynamic pressure near the orbit of Mercury (i.e., ~7-8 nPa) our model shows a high solar wind proton flux precipitates through the magnetospheric cusps to the high latitudes on both hemispheres on the dayside with a higher precipitation rate to the southern hemisphere compared to the north, which is associated with the northward displacement of Mercury's intrinsic magnetic dipole. We show that this two peak pattern, which is also a common feature observed for neutral sodium exosphere, is controlled by the radial component (Bx) of the IMF and not the Bz component. Our model also suggests that the southward IMF and its associated magnetic reconnection do not play a major role in controlling plasma precipitation to the surface of Mercury through the magnetospheric cusps, in agreement with MESSENGER observations that show that, unlike the Earth, there is almost no dependence between the IMF angle and magnetic reconnection rate at Mercury. For the typical solar wind dynamic pressure, our model suggests that the solar wind proton precipitation through the cusps is longitudinally centered near noon with ~11o latitudinal extent in the north and ~21o latitudinal extent in the south, which is consistent with MESSENGER observations. We found an anti-correlation in the incidence area on the surface and the incidence particle rate between the northern and southern cusp precipitation such that the total area and the total rate through both of the cusps remain constant and independent of the IMF orientation. We also show that the solar wind proton incidence rate to the entire surface of Mercury is higher when the IMF has a northward component and nearly half of the incidence flux impacts the low latitudes on the nightside. During extreme solar events (e.g., Coronal Mass Ejections) a large area on the dayside surface of Mercury is exposed to the solar wind plasma, especially in the southern hemisphere. Our model suggests that over 70 nPa solar wind dynamic pressure is required for the entire surface of Mercury to be exposed to the solar wind plasma.
How to cite: Fatemi, S., Poppe, A. R., and Barabash, S.: Solar wind proton precipitation to the surface of Mercury, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-31, https://doi.org/10.5194/epsc2020-31, 2020.
Planet surfaces exposed to high energy plasma undergo surface alteration by sputtering (e.g., ). Impinging ions thereby eject particles from the uppermost layers of mineral grains. The effect that sputtering has on surface reflectivity in mid infrared (MIR) is relevant for the interpretation of surface-based spectra as well as future Bepi Colombo MIR spectra. Previous studies have shown that the IR spectrum of a given mixture varies with its porosity, grain size, temperature, emergence angle and mineral to glass ratio [2, 3, 4, 5, 6].
This study uses the mineral diopside (CaMgSi2O6), enstatite (Mg2Si2O6), wollastonite (Ca2Si2O6), and bytownite (Na0.2Ca0.8Al2Si2O8) as analogues for the surfaces of Moon and Mercury. The minerals were ground to flocculation resulting in mean grain sizes of 20-34 µm and then and pressed into pellets with porosities of about 20 %. In addition to bulk powder pellets, wollastonite powder fractions of <30, 30-60 and 60-150 µm were produced and analyzed. Mineral powder pellets were irradiated by He2+ with solar wind characteristic energies of 4 keV under an incident angle of 45°.
Before ion irradiation, MIR spectra show an expected, strong dependence on grain size (Figure 1, e.g., ). First results of irradiated wollastonite bulk powder pellets with mean grain sizes of 34 µm are presented in Figure 2. IR reflectivity after irradiation shows changes in relative peak intensities and the apparent loss of peaks. A similar effect was observed when lowering grain size in the non-irradiated samples. These results suggest that solar wind sputtering effects add to the ambiguity for interpreting MIR data of planetary surfaces.
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How to cite: Jäggi, N., Szabo, P. S., Biber, H., Mezger, K., Aumayr, F., Wurz, P., and Galli, A.: Solar wind sputtering on Mercury and the Moon: effects on mid infrared spectra of mineral powder pellet analogues, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-179, https://doi.org/10.5194/epsc2020-179, 2020.
Planets and other objects in the solar system are exposed to a stream of ions originating from the sun, the solar wind. It has sufficient energy to alter the surface of airless bodies like Mercury via sputtering processes and may create a tenuous exosphere of ejected particles surrounding these objects. Knowing the material parameters relevant for sputtering processes is therefore crucial to understand planetary surfaces. Based on previous efforts with a quartz crystal micro balance as catcher, a more flexible system for evaluating sputtering yields of realistic mineral samples was developed. This allows for direct comparison between glassy thin films and mineral pellet samples with varying properties like grain size or porosity.
Irradiating thin films of material deposited onto Quartz Crystal Microbalances (QCM) is a common technique for determining ion sputtering yields. It allows precise measurements of the samples mass change during irradiation with ion beams [1, 2, 3]. When Pulsed Laser Deposition (PLD) is used for the transfer of sample material onto a quartz, the original sample stoichiometry can often be reproduced . Due to material constraints of the quartz crystals, the temperature during deposition may not exceed 846 K , resulting in glassy sample films.
It is however desired to irradiate mineral analogue samples, as the number and angular distribution of sputtered particles might be different for crystals and glassy films and depend on the grain size. One way of approaching this is by weighing a bulk sample before and after irradiation . This technique lacks the high precision of QCMs, which allow determining changes of sample mass equal to less than a monolayer of material . An alternative approach is the detection of ejecta, for example with a pressure gauge, or a QCM as catcher [6, 7].
At TU Wien, a technique using a QCM for catching sputtered material (C-QCM) was developed earlier, showing the ability to determine sputtering yields from bulk samples with such a method . However, this previous setup was limited in its geometry, with the C-QCM placed on a linear manipulator. In this setup, determining the angular distribution of sputtered material required changing distance and tilt angle between sample and C-QCM.
New setup for angular resolved measurements
In the new setup, the sample is mounted on a rotatable holder, which allows for an angular resolved investigation of sputtering yields (angle α in Figure 1). The C-QCM is mounted on a second sample holder and the angle αC between both can be varied independently from α. Sample and C-QCM are placed coaxial, which guarantees a fixed distance and a variable tilt angle. This is of advantage, as the distance dependence of the C-QCM signal follows an inverse-square law and can therefore significantly increase the measurement uncertainty.
Figure 1: Principle of the Measurement using a QCM as a catcher. The distance can be kept constant while the angles sample/ions α and sample/C-QCM αC are varied.
For the irradiation experiments, ions with solar wind velocity of 440 km s-1  are generated in an Electron Cyclotron Resonance ion source. They are then guided towards the sample holder using electromagnets and electrostatic lenses .
To avoid charging up of the insulating mineral pellets, the setup is also equipped with an electron flood gun. It delivers electrons with an energy of about 30 eV and is pointed towards the irradiated area, neutralizing the positive charge otherwise accumulating on the sample.
The sample holder is equipped with an ohmic heater and a K-type thermocouple. Combined with a Quadrupole Mass Spectrometer, not only irradiations at different sample temperatures, but also Thermal Desorption Spectroscopy (TDS) measurements after irradiations are possible. These can give important information about the implantation and diffusion of solar wind ions in minerals.
A QCM thin film sample and a pellet sample can be installed into the experimental chamber at the same time. Both mounts are placed in the same plane which is on the axis of rotation. By moving between both samples with the help of a linear feedthrough, C-QCM signals can be compared directly as the geometry stays the same. This has the additional benefit, that sputtering yields of the thin film sample can be directly evaluated and used for calibration.
This new setup allows investigation of sputtering yield and angular distribution for a wide variety of realistic samples. Effects of grain size, surface roughness and porosity on sputtering can be studied and compared with simulations  to get a more complete picture on sputtering processes on Mercury's surface.
 Hayderer G., et al.: A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion–surface collisions, Rev. Sci. Instrum., 70, 3696, 1999.
 Hijazi H., et al.: Kinetic and potential sputtering of an anorthite-like glassy thin film, J. Geophys. Res. Planets, 122, 1597, 2017.
 Szabo P. S., et al.: Solar wind sputtering of wollastonite as a lunar analogue material – Comparisons between experiments and simulations, Icarus, 314, 98, 2018.
 Cook R. K., et al.: Piezoelectric constants of alpha-and beta-quartz at various temperatures, Phys. Rev., 80, 712, 1950.
 Goschnick J., et al.: Sputter yields and erosion rates for low energy ion bombardment of multielemental powders, Nucl. Instrum. Methods Phys. Res. B, 83, 339, 1993.
 Galli A., et al.: Surface charging of thick porous water ice layers relevant for ion sputtering experiments, Planet. Space Sci., 126, 63, 2016.
 Berger B. M., et al.: Sputtering measurements using a quartz crystal microbalance as a catcher, Nucl. Instrum. Methods Phys. Res. B, 406, 533, 2017.
 Wurz P., et al.: Self-consistent modelling of Mercury’s exosphere by sputtering, micro-meteorite impact and photon-stimulated desorption, Planet. Space Sci., 58, 1599, 2010.
 Dobes K., et al.: Transient effects during sputtering of aC: H surfaces by nitrogen ions, Nucl. Instrum. Methods Phys. Res. B, 286, 20, 2012.
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How to cite: Biber, H., Szabo, P. S., Cupak, C., Jäggi, N., Galli, A., Wurz, P., and Aumayr, F.: A quartz crystal microbalance for investigating the angular distribution of particles sputtered from realistic mineral samples, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-264, https://doi.org/10.5194/epsc2020-264, 2020.
Until almost a decade ago Mercury was considered by some scientists to be very similar to the Moon. Both are small rocky bodies in the inner solar system with thin exospheres and no large scale traces of recent geological activity. However Mercury’s surface reflects much less sunlight than the Moon, making it an extremely dark body. To explain the reasons for this difference, significant abundances of iron and titanium (and their oxides) were proposed for the Hermean surface after the first NASA MESSENGER flyby of Mercury. But once in orbit around the planet, the NASA MESSENGER GRS instruments found very small abundances of iron, confirming earlier ground-based spectroscopy observations, and virtually no titanium. Therefore neither of the elements can account for this diversity. Recent analysis of MESSENGER data acquired for the darkest regions of Mercury’s surface suggest that the unknown darkening material could be carbon, in particular as the mineral graphite  whose abundance in the darker regions is predicted to be 1 to 3 wt% higher than the surroundings.
Last year at the Planetary Spectroscopy Laboratory (PSL) of the Institute of Planetary Research (DLR, Berlin) we measured reflectance spectra of graphite from UV to TIR spectral range (0.2 to 20 µm) under several phase angles. Reflectance spectra of samples were measured fresh and after heating (100° to 400° C, step 100° C) in vacuum for 8 hours. Reflectance spectra of komatiite (chosen as Mercury surface simulant, after ) was measured alone and mixed with few % of graphite to reproduce the results from , following the same procedure as for graphite. [JH1]
Emissivity spectra for the same set of samples was measured in vacuum (< 0.8 mbar) for successive steps of surface temperatures from 100°C to 400°C in the TIR spectral range (1 to ~ 18 µm) in preparation for analysis of data collected by the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS), a spectrometer developed by DLR, jointly with the Wilhelms Universität in Münster, on board of the ESA BepiColombo Mercury Planetary Orbiter (MPO) on its way to Mercury since October 2018 .
Emissivity spectra of pure Komatiite, graphite, and their mixture (graphite is 5% in weight) were measured along 2 consecutive days to reproduce day and night insulation conditions on Mercury. Effects of graphite mixed to Komatiite could be seen in the Christiansen Feature (CF) and surrounding regions (Figure 1).
Figure 1. Emissivity spectra for the 3 Mercury analogues measured in vacuum at 100°, 200°, 300°, and 400° C over the MERTIS spectral range. Graphite spectral feature is highlighted in the region around 8 µm.
Following up on the study performed last year on the spectral effect of graphite mixed with a mercury simulant soil, we concentrated this year on the effect of graphite on reflectance and emissivity spectra of pure minerals (chosen among the list of mercury analogues) when mixed with 5% graphite. Minerals were selected following the list we already published in : among them, we selected Inosilicates (enstatite and diopside), Plagioclase feldspars (albite, labradorite and bytownite), Alkali feldspars (microcline, orthoclase), and a forsterite Fo90.
The MESSENGER mission discovered that Mercury is richer in volatiles than previously expected. This is especially true for sulfur (S), with an average abundance of 4 wt%. It has been proposed that sulfur in the interior of Mercury is brought to the surface through volcanic activity in the form of sulfides as slag deposits in Mercury hollows and pyroclastic deposits. In the case of Mercury, the effect of thermal weathering affecting the spectral behavior of these sulfides must be studied carefully for their effective detection. To continue with our study, we selected some synthetic sulfides including MgS, FeS, CaS, CrS, TiS, NaS, and MnS . For each sample pure and with 5% graphite mixed, we performed emissivity measurements in the thermal infrared range for sample temperatures from 100 °C to 400 °C (step 100° C), covering the daytime temperature cycle on Mercury's surface. In addition, we measured the spectral reflectance of fresh and thermally processed sulfides over a wide spectral range (0.2–20 μm) and under several phase angles .
This spectral library facilitates the detection of sulfides in the presence of graphite by past and future missions to Mercury by any optical. Emissivity measurements in this study will support the MERTIS instrument on the ESA/JAXA BepiColombo mission, while the reflectance of these samples in 0.2–20 μm at various phase angles will support the interpretation of measurements from past (MDIS, MASCS on MESSENGER) and future missions (SIMBIO-SYS on BepiColombo).
We plan to conclude this systematic study with a follow-up experiment, where it is planned to continue this study using different graphite samples and a finer graphite sample (g.s. < 10 µm) to better simulate the predicted very fine particles to be found on Mercury surface. We will monitor the influence of heating temperature on the spectral features of our samples to understand the effect of impact heating on the graphite layer beneath the Martian crust, as explained in .
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How to cite: Maturilli, A., Helbert, J., Alemanno, G., D'Amore, M., Varatharajan, I., and Hiesinger, H.: Effect of grafite on emissivity and reflectance spectra for Mercury surface simulants, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-275, https://doi.org/10.5194/epsc2020-275, 2020.
Mercury's tectonic history has been dominated by global contraction as the planet cools and shrinks. Previously, fault dips have been estimated by measuring offsets to the rims of craters displaced by fault movement. Here we present the first observations of a fault surface that has been exposed in three dimensions by a volcanic vent.
Contractional tectonism is manifested through features such as lobate scarps, wrinkle ridges, and high relief ridges 1,2. The stresses in Mercury's crust that generated this contractional tectonism have multiple possible origins. These include global contraction 3, tidal despinning 4, true polar wander 5, and mantle convection 6. A combination of factors probably generates many faults, but the dominant driver of contractional tectonics on Mercury is the global contraction of the planet 7. This shrinking may be up to 10 km radially. The estimates of the actual amount of reduction in global radius recorded by Mercury’s crust is 7.1 km if the average fault dip is 25o but as little as 4.7 km if the average fault dip is 35o11. An understanding of the dip of the faults on Mercury provides better input into these estimates and hence offers constraints on Mercury's thermal history.
In the past, only indirect means were available to calculate dips of thrust faults, by using the shortening and vertical offset of craters that straddle faults 10,12. These studies derived an average dip of 25o. Until now, no-fault dip on Mercury has been measured by direct means.
A crater located at 147.6oE, -65.6o N shows a small fault scarp that is visible on the side of a volcanic vent, and the trace is expressed also on the vent wall and floor. This is the first known fault surface on Mercury that is exposed in three dimensions.
While a dip determination on a single fault is not conclusive, this result suggests that the shallower dip estimates are likely more representative estimates for average fault dips on Mercury and so adds weight to the higher end of the estimates of global contraction. Unfortunately, the resolution is insufficient for us to measure displacement by means of off-set features. BepiColombo will provide further opportunities to identify and measure similar features elsewhere on Mercury, and so help constrain the planet’s tectonic processes.
How to cite: Pegg, D. L., Rothery, D. A., Conway, S. J., and Balme, M. R.: A Fault on Mercury Revealed in 3-Dimensions, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-81, https://doi.org/10.5194/epsc2020-81, 2020.
The MErcury Surface, Space ENvironnement, GEochemistry and Ranging (MESSENGER) mission highlighted the explosive volatile-driven volcanism on the surface of Mercury (Head et al. 2008). Witnesses of this volcanic activity are irregularly shaped depressions surrounded by deposits that are spectrally bright and red relative to the average surface of Mercury (Head et al. 2008, Kerber et al. 2009). The characteristics of these deposits (association with volcanic vents, spectral anomaly and diffuse margins) and the lack of Hermean atmosphere suggest an emplacement by ejection of pyroclasts along ballistic trajectories. A catalogue of around 200 candidates for pyroclastic deposits was established (Kerber et al. 2011, Goudge et al. 2014, Thomas et al. 2014). Besse et al. (2020) studied 14 of these deposits using the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) on board MESSENGER. In some cases, the size of the deposit estimated in the previous catalogues (Kerber et al. 2011, Goudge et al. 2014, Thomas et al. 2014) using multispectral images is found to be underestimated. The spatial extent of the pyroclastic deposits is crucial to estimate the total amount of lava and the quantity of volatile species needed to drive the volcanic eruption on Mercury. Here we present the preliminary results of a global survey of pyroclastic deposits on the surface of Mercury using a spectral database containing MASCS observations.
Goals and preliminary results
We base our analysis on the MASCS data sets available on the Planetary Data System (PDS), grouped in a spectral database containing observations, along with and metadata provided by the MESSENGER team. The Mercury Surface Spectroscopy (MeSS) project provides MASCS spectra corrected according to the method proposed by Besse et al. (2015). From these corrected spectra we have computed several spectral parameters available in the database such as the UV-Downturn (Goudge et al. 2014) describing the drop of the reflectance in the ultraviolet (UV), spectral slope in the visible domain and the reflectance at 750 nm. The two first spectral parameters were used in the study of Besse et al. (2020) to determine the extent of some pyroclastic deposits. Our main objective is to extend this analysis to all the pyroclastic deposits of Mercury’s surface.
Thanks to the database we confirm that 40% of pyroclastic deposit candidates show a spectral anomaly with MASCS. In fact, the value of the spectral parameters is higher in the deposit than in the surrounding terrain (example in Figure 1). Therefore, we normalized the spectral parameters by a local mean for each cases. These spectral parameters decrease from the vent up to a certain distance before stabilizing around the local mean which we interpret as the limit of the deposit. We have implemented a method that allows us to determine the extent of pyroclastic deposits in a systematic way and with a level of confidence. The first results are in agreement with the previous results obtained by Besse et al. (2020) and show the robustness of the method.
Figure 1: Spectral analysis of Agwo facula using The MeSS (Mercury Surface Spectroscopy) database. Top: Maps of the Agwo facula region, each dot corresponds to MASCS spectrum. The color code shows the value of the spectral parameters (visible slope, UV-Downturn, near-infrared slope). The grey circle represents the limit of the volcanic vent. The squares indicate the regions where the local mean is calculated. Bottom: Normalized spectral parameters versus the distance to the volcanic vent. The horizontal black lines indicate the value of the local mean. The color code represents the geographical azimuth of the observations.
The authors acknowledge the Centre National d'Etudes Spatiales (CNES) for support.
The European Space Agency supported the research of S. Besse and O. Barraud through the European Space Astronomy Centre (ESAC) Faculty council.
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How to cite: Barraud, O., Doressoundiram, A., Besse, S., Cornet, T., and Munoz, C.: New approach for systemic analysis of Hermean pyroclastic deposits, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-301, https://doi.org/10.5194/epsc2020-301, 2020.
Nathair Facula (Figure 1) is the largest and most spectrally distinct of nearly 200 ‘bright red’ spots on Mercury’s surface. Most are accepted to be deposits from explosive volcanic eruptions. Nathair Facula hosts a central pit, but the mid-point of this facula is displaced about 20 km northwards from the middle of the pit. This poses unresolved questions about the eruption mechanism. The vent area is almost certainly a ‘compound vent’ where the eruption site migrated between eruptive episodes. It is unclear how a consistent asymmetry could be repeatedly engendered and reinforced by a series of eruptions rather than averaging out to a symmetrical distribution.
Nathair Facula’s radius was conservatively estimated at 71 km using MESSENGER fly-by images. However, orbital imagery reveals the radius to be double this. The vent was blasted through intercrater plains, but younger age constraints can be inferred because the facula overlies both smooth plains and ejecta from the c4 crater Rachmaninoff (Wright et al 2019), making it Mansurian or younger in age (younger than 1.7 Ga and possibly younger than 0.85 Ga).
When a facula results from explosive volcanism, the dispersal range can be used to estimate gas content at the seat of the eruption. Having been given their ejection velocity by gas expansion, the explosive ejecta particles would travel on ballistic trajectories in near-vacuum conditions. The large size of Mercury’s faculae supports the consensus that Mercury (or at least its crust) is rich in volatiles. These arguments remain broadly valid even for faculae that may be the cumulative product of a series of eruptions, which is particularly likely in the case of faculae whose central vent is a compound structure comprising multiple overlapping vents. Pegg et al. (2020) have shown that about 70% of vent sites on Mercury fall into this category.
Nathair Facula’s vent structure is a pit measuring 39 km on its long axis and 30 km on its short axis. As shown by Thomas et al. (2014) the brink of this pit is scarcely raised above the level of the surrounding terrain, but inside it plummets steeply (at about 30°) to an uneven floor more than 2 km below, with a greatest depth of about 3 km. In map view the brink is non-elliptical, having some sectors whose curvature is convex inwards, alternating with more strongly curved convex-outward sectors. Unlike many compound vents, this example lacks clear septa forming internal divisions between individual vents, although the floor has changes in texture and level that may reflect migration of the locus of explosive activity over time (Pegg et al., 2020).
The size and shape and asymmetry of Nathair Facula
A facula is a thin surficial deposit. It is visible because its spectral properties contrast with the underlying unit, possibly because of loss of carbon or sulfur from the ejected material. The original visible outer edge of the facula would have been radially gradational, where the cover by volcanic ejecta particles fell from 100% to zero. The spectral distinctiveness of a facula, particularly its thinner distal region, is likely to further blur over time because of impact gardening and general space weathering.
Thomas et al. (2014) and Wright et al. (2019) identified the radius of Nathair Facula visually, on enhanced MESSENGER MDIS images, as 130 km and 120 km respectively. Rather than fitting a notional circle to it, they attempted to show its true shape by drawing a slightly irregular boundary defining a roughly equidimensional deposit. The exact position of this boundary is subjective and depends on the whether it is drawn on the basis of a monochrome or colour image, and the contrast stretching used for display.
Besse et al. (2020) took a more objective approach to defining the edge of this facula (and several others) using spectroscopic data from MESSENGER’s MASCS (Mercury Atmospheric and Surface Composition Spectrometer) instrument, a non-imaging instrument that recorded UV-visible-near IR spectral along the spacecraft ground-tracks. They defined spectral parameters that are extreme in the inner part of faculae and decline to the planetary background value at the edge of the facula: the UV downturn, the VIS slope and the NIR slope. Along a nearly north-south line across Nathair Facula they used these data from one track to determine a radius of 140 km.
Careful inspection of enhanced MESSENGER colour images, such as Figure 1, and the map of Wright et al. (2019; Figure 2) shows that the facula is not actually centred on the vent, but is displaced northwards by about 20 km. Spectral analysis of MDIS data suggests the midpoint of the facula is in the NE of the vent, and reveals a slightly elliptical shape to the faculae edge (Figure 3). We interrogated data from five north-south MASCS crossings of Nathair Facula, distinguishing between data points north and south of the vent. One of these is illustrated in Figure 4, showing that each spectral parameter falls to the background level at a greater distance north than south of the notional vent centre. Table 1 gathers data from a total of five tracks. While each spectral parameter records a slightly different radius, in all cases where a radius can be determined it is greater in the north than in the south. In the case of the UV downturn, the mean and median agree on an approximately 21 km discrepancy between the north and south radii. This would be eliminated if the notional centre were shifted north by 10 km, but that would lie about 4 km north of the midpoint of even the northern part of the vent structure.
Future work will consider other spectral parameters.
Acknowledgements: Mapping and MDIS spectral studies have received funding from the EU Horizon 2020 research and innovation programme under grant agreement No. 776276, Planmap.
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