Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 – 23 September 2022
Europlanet Science Congress 2022
Palacio de Congresos de Granada, Spain
18 September – 23 September 2022
TP1
Mercury Science and Exploration

TP1

Understanding the formation, evolution, composition, interior structure, and environment of Mercury is of primary interest to better understand Mercury and the role this terrestrial planet plays in the evolution of our solar system.

NASA’s MESSENGER spacecraft provided many insights and surprising results regarding these goals. MESSENGER data are still under analysis and will continue to provide many important contributions to Mercury science.

However, MESSENGER also raised many questions that are still open and will be addressed by the new joint ESA/JAXA mission to Mercury, BepiColombo, which was successfully launched in October 2018. In October 2021, the first of six flybys of Mercury took place (second flyby in June 2022).

This session welcomes contributions addressing the planet’s geology, surface composition, geodesy, interior structure, exosphere, magnetosphere, gravity, and magnetic fields, based on modeling, laboratory experiments, and observations (ground-based, remote-sensing and in situ). The first analyses of BepiColombo's flyby data from Mercury are welcome. Finally, contributions of concepts of future missions to Mercury are encouraged.

Co-organized by MITM
Convener: Jack Wright | Co-conveners: Joe Zender, Johannes Benkhoff, Go Murakami, Lina Hadid, Noah Jäggi, Beatriz Sanchez-Cano, Willi Exner, Joana S. Oliveira, Alice Lucchetti, Anna Milillo, Valeria Mangano
Orals
| Mon, 19 Sep, 10:00–11:30 (CEST), 15:30–18:30 (CEST)|Room Machado
Posters
| Attendance Mon, 19 Sep, 18:45–20:15 (CEST) | Display Mon, 19 Sep, 08:30–Wed, 21 Sep, 11:00|Poster area Level 1

Session assets

Discussion on Slack

Orals: Mon, 19 Sep | Room Machado

Chairpersons: Joana S. Oliveira, Joe Zender
BepiColombo
10:00–10:10
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EPSC2022-78
Johannes Benkhoff and Go Murakami

BepiColombo has finished more than 50% of its about seven year-long cruise-phase. Launched on 20 October 2018 from the European spaceport Kourou in French Guyana it has successfully performed several flybys ( at Earth, twice at Venus and Mercury). BepiColombo with its state of the art and very comprehensive payload will perform measurements to increase our knowledge on the fundamental questions about Mercury’s evolution, composition, interior, magnetosphere, and exosphere. BepiColombo consists of two orbiters, the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (Mio) and is a joint project between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA).

Since the two spacecraft are in a stacked configuration during the cruise only some of the instruments will perform scientific observations. Mio and MPO are connected to each on-top of the Mercury Transfer Module (MTM). The MTM contains a solar electric propulsion engine and will bring the two spacecraft to Mercury. In late 2025, this ‘stack’ configuration is abandoned, the MTM will be jettisoned, and the individual elements spacecraft are brought into their final Mercury orbit: 480x1500km for MPO, and 590x11640km for Mio.  

Despite the reduced instrument availability, scientific and engineering operations has been scheduled during the cruise phase, especially during the swing-bys. A status of the mission and instruments, science operations plan during cruise, and first results of measurements taken in the first four years since launch will be given.

How to cite: Benkhoff, J. and Murakami, G.: BepiColombo on its cruise to Mercury – first results and mission status, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-78, https://doi.org/10.5194/epsc2022-78, 2022.

10:10–10:25
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EPSC2022-1260
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ECP
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solicited
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MI
Umberto De Filippis, Carlo Lefevre, David Lucchesi, Marco Lucente, Carmelo Magnafico, Roberto Peron, and Francesco Santoli

ISA (Italian Spring Accelerometer) is a scientific payload of the Mercury Planetary Orbiter (MPO) module of the ESA/JAXA BepiColombo mission to planet Mercury and it is the first high-sensitivity accelerometer on-board an interplanetary spacecraft. It will be one of the key instruments to perform Radio Science Experiments during the orbital phase. The instrument is sensitive to any acceleration, greater than 10-8 ms-2Hz-1/2, perturbing the free fall of the spacecraft in the overall gravity field. The main goal of ISA is indeed to measure the so-called Non Gravitational Perturbations (NGP) allowing to reconstruct, a posteriori, the motion of the spacecraft on a geodesic of spacetime. During the first Mercury flyby, performed in October 2021, the spacecraft approached the target planet reaching an altitude above its surface of only 200 km. Thanks to this very low altitude and to the ISA on-board position in cruise configuration, far away from the center of mass of the overall composite spacecraft, the accelerometer has been able to clearly detect the gravity gradient accelerations. Indeed, this is the first direct measurement of the gravity gradient acceleration induced on a spacecraft by the gravity field of a celestial object different from the Earth. Near the closest approach to the planet, the spacecraft entered in eclipse, losing the effect of the solar radiation pressure acting on its surfaces exposed to the Sun. As a consequence, a sudden change of the acceleration was clearly detected by the accelerometer; the measured signal has a magnitude aligned with the expectations, computed considering optical coefficients and spacecraft attitude. In June 2022, BepiColombo will carry out a second flyby that will be very similar, in terms of altitude, attitude and B-plane coordinates, to the first one, representing an almost unique opportunity to compare two similar measurements.

How to cite: De Filippis, U., Lefevre, C., Lucchesi, D., Lucente, M., Magnafico, C., Peron, R., and Santoli, F.: First NGP measurements at Mercury, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1260, https://doi.org/10.5194/epsc2022-1260, 2022.

10:25–10:35
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EPSC2022-353
Giovanni Munaretto, Gabriele Cremonese, Emanuele Simioni, Alice Lucchetti, Maurizio Pajola, and Matteo Massironi

Introduction: The physical properties of a particulate surface, like roughness, grain size, shape and transparency affect how it reflects the incoming light. This concept is used in planetary photometry to infer the surface properties of a celestial object from multiple observations taken from different directions and solar illumination (Hapke et al. 2012). Models linking the observed surface brightness with parameters related to physical properties of the surface have been established. The estimation of such parameters is referred as photometric modelling (Domingue et al., 2016). On Mercury, this technique has been employed to construct monochrome and color global mosaics, but it was never applied to investigate local surface features (Domingue et al., 2016). Therefore, the photometric modelling of Mercury’s surface features represents a novel and useful tool to investigate their nature. In addition, the identification of high-performance photometric models of any given surface material over multiple wavelengths enables to accurately predict the amount of reflected sunlight that will be observed through remote cameras and spectrometers.

In this abstract, we first describe our modelling approach, discuss its improvement with respect to current available photometric models of Mercury, and present a few science cases in which it has been applied. Then, we will also show how this methodology is being applied for the calibration of SIMBIO-SYS observations that will be acquired during the Mercury Orbit Insertion (MOI) phase of the mission.

Methodology: we first analyze the Tyagaraja and Canova craters hollows (i.e., tens meters to several km-sized shallow, irregular, flat‐floored depressions characterized by bright interiors and haloes, Blewett et al., 2011), which are covered by multiple overlapping 8 filter MDIS/WAC (Hawkins et al., 2007) images with resolution higher than 665 m/px and phase angles from 30° to > 100°.  Over this region, we construct a latitude-longitude sampling grid with 665 m spacing. For each point we retrieve the surface reflectance and the solar illumination and observation angles using the 3D information of the global USGS DTM and the spacecraft and Sun position information within the observation SPICE kernels. This dataset is fitted with the Hapke and Kaasalainen-Shkuratov photometric models and estimates of their parameter are obtained for each point of the grid (see for example Fig 1C).

Results

Modelling performance: Our results suggest that photometric models derived from the inversion of multiple, overlapping observations are more accurate, especially for bright targets, rather than global photometric models of Mercury (Fig 1A,B). Overall, we estimate a modelling accuracy of better than 10% at 3σ, comparable with the radiometric noise level of the observations.

Hollows results: Our results suggest that hollows are more backscattering than the floor of the crater in which they form. This is consistent with hollows being made of a material rich in holes and/or vescicles, in agreement with a formation by devolatilization. In addition, we find that they are smoother than the crater floor, consistently with the emplacement of a fine particles halo during hollow growth.

Figure 1. Modeled vs Observed reflectance obtained with our KS3 model (gold) and the global one for Mercury for Canova (A) and Tyagaraja (B) craters. C) Example of Hapke single scattering albedo map of Tyagaraja crater. D) MOI SIMBIO-SYS footprints and the Regions of Interests (ROI) that are being analyzed for the cross-calibration of the three channels.

Cross calibration of the SIMBIO-SYS channels: SIMBIO-SYS is a suite of three instruments, a high-resolution imager (HRIC), a stereocamera (STC) and a spectrometer (VIHI). Because of the dual-spacecraft configuration of BepiColombo, standard star observations with SIMBIO-SYS are not feasible until the satellite will be in its nominal orbit around Mercury. It is therefore pivotal to identify regions on the surface of Mercury with well-defined spectrophotometric properties that can be used as ground truth for the initial cross calibration of the three channels, prior to standard stars observations. Such regions will be also used to verify the integration times before the planning of the Global Mapping phase and to characterize the instrument straylight and pixel response non uniformity. The ROI will be observed during the Mercury Orbit Insertion (MOI) orbits where no standard stars pointing are still allowed. Therefore, we applied our methodology to identify ROIs within these orbits that currently have at least 5 MDIS/WAC observations, phase angles > 110° and resolutions higher than 665 m/px (Fig 1D). The retrieved photometric models for these regions will be used to determine accurate spectra (at the MDIS/WAC wavelengths) that will be interpolated within the SIMBIO-SYS photometric system and used to cross-calibrate the three channels of the instrument.

Discussion

Our analysis shows that overlapping multiangular MDIS/WAC observations can be used to invert local photometric models of the surface. This approach allowed us to initially study and characterize the scattering properties of hollows, finding that they are smoother and more back-scattering than the crater floors. This is consistent with hollows being formed by a volatile-release mechanism and emplacement of a halo of fine particles. We also found that this modelling approach is more accurate, in particular for high reflectance, than current global photometric models of Mercury. This led us to apply our methodology to selected locations that will be observed by SIMBIO-SYS during the MOI phase, and that will be used for absolute calibration of its three channels. More details will be presented at the conference.

Acknowledgments and Data

We gratefully acknowledge funding from the Italian Space Agency (ASI) under ASI-INAF agreement 2017- 47-H.0.

 

How to cite: Munaretto, G., Cremonese, G., Simioni, E., Lucchetti, A., Pajola, M., and Massironi, M.: Photometric modelling of MESSENGER/MDIS observations: science results and implications for the calibration of SIMBIO-SYS on BepiColombo, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-353, https://doi.org/10.5194/epsc2022-353, 2022.

10:35–10:45
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EPSC2022-123
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ECP
Rozenn Robidel, Eric Quemerais, Dimitra Koutroumpa, Jean-Yves Chaufray, and François Leblanc

The first flyby of Mercury with BepiColombo (ESA/JAXA joint mission) took place on October 1, 2021. PHEBUS (Probing of Hermean Exosphere By UltraViolet Spectroscopy) was able to observe during this flyby with its EUV detector (Extreme UltraViolet, spectral range 55-155 nm) and its visible channels, one centered on 404 nm (K emission line) and the other centered on 422 nm (Ca emission line). We will refer to the channel dedicated to potassium as c404 and the channel dedicated to calcium as c422. The observation started 30 minutes before the Closest Approach (CA) and lasted one hour, with the line of sight directed to the North and slightly anti-sunwards (Fig1). The slit was removed and the acquisitions were made every 10 seconds with an exposure time of 8 seconds.

Fig1: BepiColombo first Mercury's flyby as seen from the Sun (left panel) and in a Z-ρz plane in the MSO frame (right panel), where . The black circle represents the planet, the red line represents BepiColombo’s trajectory (the shadow transit part is the dotted part) and the grey arrows represent PHEBUS pointing direction.

 

The count rate as a function of time from the visible detectors clearly indicate the observation geometry of the flyby, the transit in the shadow of Mercury in particular, and the maximum of the emission on the dayside after the CA (Fig2). We also notice peaks on top of the general emission profile that take place on the morning side. These peaks occur on both detectors over the same period of 8 seconds.  They may be related to dust particles or the crossing of magnetospheric structures. We can also note a different level of dark at the beginning and the end of the observation on c404. We cannot affirm the same for c422, since the observation did not last long enough and we still observe exospheric calcium at the end. However, the two signals seem similar: c404 signal could be due to Ca contamination or could be the detection of Mn (emission line at 403.1 nm). As a reminder, potassium has not been observed by Messenger at this wavelength (Vervack et al., 2016).

Fig. 2 c422 (blue) and c404 (orange) count rate as a function of time. The gray dashed area corresponds to BepiColombo being in the shadow of Mercury, the solid grey line represents the CA time and the red dotted line corresponds to the level of dark on c404 at the beginning of the observation.

 

We correct the data for different contributions: dark current, zodiacal light and stars present in PHEBUS Field of View (FoV). We then model the c422 signal by removing the peaks and applying a median filter. The corrected signal is then converted to radiance, units of Rayleigh (Fig3). One Rayleigh corresponds to 106/4π photons.cm-2.s-1.sr-1. We consider the solid angle (units of sr) related to the pre-slit computed on ground and the effective area (units of cm2), calibrated in flight.

Fig. 3 Ca radiance as a function of time, the blue curve represents the data while the green curve represents the filtered data.

 

We then consider the data on the dawn side and apply an exponential fit I = I0 e-z/h, where r is the tangent altitude of PHEBUS line of sight above the surface, I0 the radiance at the surface and h is the e-folding scale height (Fig4). The best fit found indicates two different populations with two scale heights: 2 180 km when close to the surface and 8 300 km when farther from the surface of Mercury. Our results differ slightly from those of Burger et al. (2012) (1 840 ±140 km and 1 700 ± 200 km at the north and south poles, respectively) but the observation geometries were different.