Mars Express Research Articles

Prolonged Lifetime of the Transient Ionized Layer in the Martian Atmosphere Caused by Comet Siding Spring

AbstractIn October 2014, the close encounter between Mars and comet Siding Spring produced a transient ionized layer in the upper atmosphere composed primarily of Mg+ and Fe+ ions. The layer was detected by instruments on three spacecraft, including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express. Analyses of the MARSIS data indicated the transient layer persisted up to 19 h after the comet's closest approach. We report MARSIS observations that suggest the transient layer lasted at least 7 days—and potentially as long as 32 days—after closest approach. During this period, the transient layer was mostly confined to a narrow latitude range between 20°N and 60°N and a longitude range spanning 275°E to 95°E. Since this period coincided with a highly active Sun, we discuss how solar flares may have contributed to the layer's prolonged lifetime.

Observation of Disturbed Plasma Structures in the Environment of the Sun and Near-Earth Space with Radio Sounding and Local Measurements

An experiment on radio sounding of the circumsolar plasma by signals from the Mars Express spacecraft in 2013 was carried out in the period from March 4 through May 31. The investigated characteristics were the frequency of signals in the centimeter and decimeter ranges and the differential frequency. A number of events were recorded in which the intensity of the frequency fluctuations of the signals probing the plasma was several times higher than the background values. As shown by the analysis of observations of solar activity, such as an increase in the flux of X-rays and SOHO LASCO coronagraph data, this is explained by the passage of disturbed plasma streams generated in the solar corona through the spacecraft radio communication path with the Earth. The comparison of the radio transmission data with the results of measurements of the parameters of the near-Earth plasma using the Wind spacecraft in adjacent periods of time. As a result of the analysis of the data on the proton concentration, it became clear that sharp increases in both the average values and fluctuations of this characteristic were also observed near the Earth. The time lag between the events observed in the circumsolar and near-Earth plasma shows that the cause of the disturbances is the increased activity of the same coronal region rotating with the Sun.

Mars Express Observations of Cold Plasma Structures in the Martian Magnetotail

AbstractWe present observations from five Mars Express (MEX) orbits in September 2016 while the spacecraft passed through the Martian induced magnetotail at altitudes up to 3,500 km. On these orbits, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument was operated in Active Ionospheric Sounding (AIS) mode at much higher altitude than normal, acting as a local sounder and detecting cold plasma structures in this region. In this paper we combine MARSIS tail measurements with solar wind data from the Solar Wind Ion Analyzer (SWIA) instrument and the Magnetometer (MAG) from Mars Atmosphere and Volatile EvolutioN (MAVEN) in order to investigate possible factors affecting plasma transport from the dayside and through the terminator. MARSIS observed structured cold ionospheric plasma along its trajectory, at all altitudes and solar zenith angles (SZAs). Isolated regions of cold plasma were also observed on each orbit as the spacecraft crossed the terminator, even at high altitudes. We conclude that the variability of plasma seen in the tail results from a multifactorial transport process, the development of which cannot be attributed to a sole parameter influencing it, despite the availability of simultaneous high quality solar wind measurements.

Machine learning for automatic identification of new minor species

One of the main difficulties to analyze modern spectroscopic datasets is due to the large amount of data. For example, in atmospheric transmittance spectroscopy, the solar occultation channel (SO) of the NOMAD instrument onboard the ESA ExoMars2016 satellite called Trace Gas Orbiter (TGO) had produced ~ 10 millions of spectra in ~ 20000 acquisition sequences since the beginning of the mission in April 2018 until 15 January 2020. Other datasets are even larger with ~ billions of spectra for OMEGA onboard Mars Express or CRISM onboard Mars Reconnaissance Orbiter. Usually, new lines are discovered after a long iterative process of model fitting and manual residual analysis. Here we propose a new method based on unsupervised machine learning, to automatically detect new minor species. Although precise quantification is out of scope, this tool can also be used to quickly summarize the dataset, by giving few endmembers (”source”) and their abundances.The methodology is the following: we proposed a way to approximate the dataset non-linearity by a linear mixture of abundance and source spectra (endmembers). We used unsupervised source separation in form of non-negative matrix factorization to estimate those quantities. Several methods are tested on synthetic and simulation data. Our approach is dedicated to detect minor species spectra rather than precisely quantifying them. On synthetic example, this approach is able to detect chemical compounds present in form of 100 hidden spectra out of 104, at 1.5 times the noise level. Results on simulated spectra of NOMAD-SO targeting CH4 show that detection limits goes in the range of 100–500 ppt in favorable conditions. Results on real martian data from NOMAD-SO show that CO2 and H2O are present, as expected, but CH4 is absent. Nevertheless, we confirm a set of new unexpected lines in the database, attributed by ACS instrument Team to the CO2 magnetic dipole.

Martian cloud climatology and life cycle extracted from Mars Express OMEGA spectral images

A Martian water-ice cloud climatology has been extracted from OMEGA data covering 7 Martian years (MY 26–32) on the dayside. We derived two products, the Reversed Ice Cloud Index (ICIR) and the Percentage of Cloudy Pixels (PCP), indicating the mean cloud thickness and nebulosity over a regular grid (1° longitude × 1° latitude × 1° Ls × 1 h Local Time). The ICIR has been shown to be a proxy of the water-ice column derived from the Mars Climate Database.The PCP confirms the existence and location of the main cloud structures mapped with the ICIR, but also gives a more accurate image of the cloud cover. We observed a more dense cloud coverage over Hellas Planitia, the Lunae Planum region and over large volcanoes (Tharsis volcanoes, Olympus and Elysium Montes) in the aphelion belt.For the first time, thanks to the fact that Mars Express is not in Sun-synchronous orbit, we can explore the clouds diurnal cycle at a given season by combining the seven years of observations. However, because of the eccentric orbit, the temporal coverage remains limited. Identified limitations of the dataset are its small size, the difficult distinction between ice clouds and frosts, and the impact of surface albedo on data uncertainty. We could nevertheless study the diurnal cloud life cycle by averaging the data over larger regions: from specific topographic features (covering a few degrees in longitude and latitude) up to large climatic bands (covering all longitudes). We found that in the tropics (25°S – 25°N) around northern summer solstice, the diurnal thermal tide modulates the abundance of clouds, which is reduced around noon (Local Time). At northern midlatitudes (35°N – 55°N), clouds corresponding to the edge of the north polar hood are observed mainly in the morning and around noon during northern winter (Ls = 260° – 30°). Over Chryse Planitia, low lying morning fogs dissipate earlier and earlier in the afternoon during northern winter. Over Argyre, clouds are present over all daytime during two periods, around Ls = 30° and 160°.

Mars' Ionopause: A Matter of Pressures

AbstractThis study assesses under what circumstances the Martian ionopause is formed on the dayside, both in regions where there are strong crustal magnetic fields and areas where these fields are small (<30 nT). Multiple data sets from three MAVEN dayside deep dip campaigns are utilized between periapsis and 600–1,000 km, as well as solar wind observations from Mars Express. The ionopause is identified as a sudden decrease of the electron density with increasing altitude and a simultaneous increase of the electron temperature and variability below 400 km. This is a physically robust approach as the electron temperature is a key parameter in determining the structure of the ionospheric profile, and, therefore, also a strong indicator of the ionopause location. We find that 36% (54%) of the electron density profiles over strong (weak) crustal magnetic field regions had an ionopause event. We also evaluate the roles of ionospheric thermal and magnetic pressures on the ionopause formation as well as the presence of solar wind particles, H+, down to the location of the ionopause. We found that the topside ionosphere is typically magnetized at mostly all altitudes. The ionopause, if formed, occurs where the total ionospheric pressure (magnetic + thermal) equals the upstream solar wind dynamic pressure. Moreover, the lower edge of the ionopause coincides with the altitude where the solar wind flow stops: The thermal pressure suffers a significant reduction with increasing altitude and the solar wind proton density has a prominent increase.

AUTOMATIC EVALUATION OF THE INITIAL GEOPOSITIONING ACCURACY FOR LARGE AREA PLANETARY REMOTE SENSING IMAGES

Abstract. The photogrammetric processing of large area planetary remote sensing images is still a very challenging work. In addition to the lack of ground control data and poor tie points extraction, the insufficient knowledge of the initial geopositioning accuracy of the planetary images also increases the difficulty of processing. This paper presents an automatic evaluation method of the initial geopositioning accuracy for large area planetary remote sensing images. The accuracy evaluation method was conducted through image matching on approximate orthophotos derived using coarse resolution digital elevation model (DEM). To improve the orthophotos generation efficiency of linear pushbroom images, a fast ground-to-image transformation algorithm and multi-threaded parallel computing are adopted. The classical normalized cross correlation (NCC) and pyramid matching schemes are used to perform image matching between overlapping orthophotos. Because the conjugate points on orthophotos contain geographic coordinates, we can derive the statistics information (e.g., maximum errors, mean errors and standard deviation) about the geopositioning accuracy of the planetary images. Although it’s actually an evaluation result of relative accuracy, the quantitative geopositioning accuracy information of stereopairs can be used to (1) specify the search window size and the starting position of conjugate points for tie points extraction; (2) set the weight value of bundle adjustment; and (3) identify images with abnormal geopositioning accuracy. Tens of Mars Express (MEX) High Resolution Stereo Camera (HRSC) images were used to conduct the test. The experimental results demonstrate that the proposed method shows high computational efficiency and automation degree. The automatic evaluation of the initial geopositioning accuracy of the planetary images is helpful to produce large area planetary mapping products.

EVALUATING STEREO DTM QUALITY AT JEZERO CRATER, MARS WITH HRSC, CTX, AND HIRISE IMAGES

Abstract. We have used a high-precision, high-resolution digital terrain model (DTM) of the NASA Mars 2020 rover Perseverance landing site in Jezero crater based on mosaicked images from the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (MRO HiRISE) camera as a reference dataset to evaluate DTMs based on Mars Express High Resolution Stereo Camera (MEX HRSC) and MRO Context camera (CTX) images. Results are consistent with our earlier HRSC-HiRISE comparisons at the Mars Science Laboratory (MSL) Curiosity landing site in Gale crater, confirming that those results were not compromised by the small area compared and potential problems with spatial registration. Specifically, height errors are on the order of half a pixel and correspond to an image matching error of 0.2–0.3 pixel but estimates of horizontal resolution are 10–20 pixels. Products from the HRSC team pipeline at DLR are smoother but more precise vertically than those produced by using the commercial stereo package SOCET SET®. The DLR products are also homogenous in quality, whereas the SOCET products are less smoothed and have higher errors in rougher terrain. Despite this weak variation, our results are consistent with a rule of thumb of 0.2–0.3 pixel matching precision based on many prior studies. Horizontal resolution is significantly coarser than the DTM ground sample distance (GSD), which is typically 3–5 pixels.

Martian Topographic Roughness Spectra and Its Influence on Bistatic Radar Scattering

There are few studies on predicting fully bistatic scattering from the rough surface of Mars, though some bistatic radar observations have been made, such as in the MARS EXPRESS mission. To better understand the interaction of radar signals with a planetary surface in bistatic radar observations, the topographic-scale roughness of Mars, characterized by a two-dimensional power spectrum density (2D-PSD), is examined in view of its global roughness variations and scale dependence on geological units. The analysis shows that the Martian 2D-PSD is strongly dependent on the geological units and that it lies between Gaussian and exponential functions, with a power index equal to 1.9. The bistatic scattering coefficients are calculated by an advanced integral equation model (AIEM) with the 2D-PSD as the input. It shows that the specific surface roughness spectrum and the dielectric inhomogeneity should be taken into account in interpreting the bistatic radar scattering response.

The Global Search for Liquid Water on Mars from Orbit: Current and Future Perspectives.

Due to its significance in astrobiology, assessing the amount and state of liquid water present on Mars today has become one of the drivers of its exploration. Subglacial water was identified by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) aboard the European Space Agency spacecraft Mars Express through the analysis of echoes, coming from a depth of about 1.5 km, which were stronger than surface echoes. The cause of this anomalous characteristic is the high relative permittivity of water-bearing materials, resulting in a high reflection coefficient. A determining factor in the occurrence of such strong echoes is the low attenuation of the MARSIS radar pulse in cold water ice, the main constituent of the Martian polar caps. The present analysis clarifies that the conditions causing exceptionally strong subsurface echoes occur solely in the Martian polar caps, and that the detection of subsurface water under a predominantly rocky surface layer using radar sounding will require thorough electromagnetic modeling, complicated by the lack of knowledge of many subsurface physical parameters. Higher-frequency radar sounders such as SHARAD cannot penetrate deep enough to detect basal echoes over the thickest part of the polar caps. Alternative methods such as rover-borne Ground Penetrating Radar and time-domain electromagnetic sounding are not capable of providing global coverage. MARSIS observations over the Martian polar caps have been limited by the need to downlink data before on-board processing, but their number will increase in coming years. The Chinese mission to Mars that is to be launched in 2020, Tianwen-1, will carry a subsurface sounding radar operating at frequencies that are close to those of MARSIS, and the expected signal-to-noise ratio of subsurface detection will likely be sufficient for identifying anomalously bright subsurface reflectors. The search for subsurface water through radar sounding is thus far from being concluded.

Mars's Red (575–625 nm) Seasonal Approximate Reflectivity Averaged Over Mars Years 24–28 From Mars Orbiter Camera

AbstractThe Mars Global Surveyor spacecraft successfully orbited Mars for over nine Earth years, its instruments returning a plethora of data that still today form important, key data sets in Mars research. A relatively underutilized data set was returned from the Mars Orbiter Camera (MOC), specifically the wide‐angle component (MOC‐WA), despite returning over 150,000 images in “blue” and “red” wavelengths. The median imaging time of day was 14:00, resulting in few shadows, making it difficult to use for geology, while better cameras arrived within a few years. However, the limb‐to‐limb coverage of the MOC‐WA survey images provides an important resource, where the imaging time that was poor for geology is good for measuring reflectivity. To‐date, the only published albedo maps of Mars are from a companion MGS instrument, TES, with wide wavelength coverage and low spatial resolution, and the OMEGA instrument on Mars Express. Modern processing of the MOC‐WA images, presented here, allow for the creation of new reflectivity maps in a narrow wavelength range that can be used for photometric control of other data sets and understanding broad‐scale surface changes on Mars. Four reflectivity maps were made, at cardinal Ls points, averaging images taken within just a week of each Ls but across multiple Mars years. From these maps, several trends are revealed though most have been previously described, including significant differences in seasonal frost cover surrounding each residual polar ice cap, and significant changes across Syrtis Major, Solis Lacus, and the Tharsis volcanoes.

Oxygen Atom Escape from the Martian Atmosphere during Proton Auroral Events

We present the model calculation results of the atomic oxygen loss rate from the Martian atmosphere induced by precipitation of high-energy protons and hydrogen atoms (H/H+) from the solar wind plasma. Penetration of energetic protons and hydrogen atoms from the solar wind plasma to the upper atmosphere of Mars at altitudes of 100−250 km is accompanied by the momentum and energy transfer in collisions with the main component, atomic oxygen. This process is considered as atmospheric gas sputtering during proton auroral events, which is accompanied by formation of the suprathermal hydrogen and oxygen atom fluxes escaping from the atmosphere. When calculating the formation rate of suprathermal atoms, the modified Monto Carlo kinetic model was used. This model was earlier developed to analyze the data of the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) and the Solar Wind Ion Analyzer (SWIA) onboard the Mars Express (MEX) and the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, respectively. We study the processes of kinetics and transport of hot oxygen atoms in the transition zone (from the thermosphere to the exosphere) of Mars’ upper atmosphere. The kinetic energy distribution functions for suprathermal oxygen atoms were calculated. It has been shown that, during proton auroral events on Mars, the exosphere is populated with a significant number of suprathermal oxygen atoms, the kinetic energy of which reaches the escape energy, 2 eV. In addition to photochemical sources, a hot fraction is formed in the oxygen corona; and a nonthermal flux of atomic oxygen escaping from the Martian atmosphere is produced during proton aurora events. Proton aurorae are sporadic auroral events. Consequently, according to the estimates obtained from the recent MAVEN observations the magnitude of the precipitation-induced escaping flux of hot oxygen atoms may become prevailing over the photochemical sources under conditions of the extreme solar events such as solar flares and coronal mass ejections.

CO2 retrievals in the Mars daylight thermosphere from its 4.3 μm limb emission measured by OMEGA/MEx

We present a non-local thermodynamic equilibrium retrieval scheme for atmospheric composition and its application to Mars CO2 infrared limb emissions as measured by the OMEGA instrument on board Mars Express (MEx). These emissions are caused by CO2 fluorescence of solar radiation, and thus the retrieval scheme accounts for non-LTE processes. We analyzed the dayside limb observations from a selection of three OMEGA orbits or data qubes. Before the retrieval was applied, we performed a radiometric calibration, cleaned the spectra (including clustering techniques) and generated radiance vertical profiles for each dataset. We also present information on the inversion set up, results on the retrieved CO2 density profiles, as well as the temperature profiles derived from the CO2 densities by assuming hydrostatic equilibrium. An extensive sensitivity study of the retrieval scheme was carried out, including its application to the OMEGA spectra taken at different MEx orbital configurations, to conclude on its performance and to offer recommendations for its systematic use with MEx datasets. The uncertainty due to the instrumental Gain calibration and that caused by the retrieval noise error itself are of large importance for the inversion, but a comparable component of the total error comes from the uncertainties of the temperature provided by the GCM. We demonstrated that, between 120 and 160 km, CO2 profiles can be derived with a precision around 30% and a vertical resolution of about 15 km.

Estimated Minimum Life Span of the Jezero Fluvial Delta (Mars).

The paleo-lake floor at the edge of the Jezero delta has been selected as the NASA 2020 rover landing site. In this article, we demonstrate the sequences of lake filling and delta formation and constrain the minimum life span of the Jezero paleo-lake from sedimentological and hydrological analyses. Two main phases of delta evolution can be recognized by utilizing imagery provided by the High Resolution Imaging Science Experiment (NASA Mars Reconnaissance Orbiter) and High Resolution Stereo Camera (ESA Mars Express): (1) basin infilling before the breaching of the Jezero rim and (2) the delta formation itself. Our results suggest that delta formation occurred over a minimum period of 90-550 years of hydrological activity. Breaching of the Jezero rim occurred in at least three distinct episodes, which spanned a far longer time-period than overall delta formation. This evolutionary history implies that the Jezero-lake floor would have been a haven for fine-grained sediment accumulation and hosted an active environment of significant astrobiological importance.

High methionyl–tRNA synthetase expression predicts poor prognosis in patients with breast cancer

AimsMethionyl–tRNA synthetase (MARS) is known to play a critical role in initiating translation and protection against cellular damages in vivo. The aim of this study was to clarify the role...

Martian Ionopause Boundary: Coincidence With Photoelectron Boundary and Response to Internal and External Drivers

AbstractThe Martian ionopause boundary detected as steep gradients in the local electron density profiles from the Mars Advanced Radar for Subsurface and Ionospheric Sounder on Mars Express is studied individually and statistically and compared to the photoelectron boundary identified by the drop of photoelectron signature due to CO2 and O molecules. In ~90% of the cases where we have electron energy flux data, the ionopause coincides with the photoelectron boundary. The steep density gradients form at the border of the photoelectron region and above. The ionopause is observed in a wide range of latitude, longitude, and altitude. According to remote sounding investigations, the average thickness of the ionopause is ~30 km. The average altitude is between 500 and 700 km on the dayside. The altitude of the ionopause is inversely related to the solar wind dynamic pressure. Strong crustal magnetic fields increase the altitude of the boundary and they have a slight, negative effect on the occurrence only for high values. The ionopause occurs more frequently, and its altitude is higher during southern summer. The average altitude of the ionopause and solar irradiance are correlated with each other. The effect of the extreme ultraviolet flux on the occurrence rate is less noticeable.

Variations in the Ionospheric Peak Altitude at Mars in Response to Dust Storms: 13 Years of Observations From the Mars Express Radar Sounder

AbstractPrevious observations have shown that, during Martian dust storms, the peak of the ionosphere rises in altitude. Observational studies of this type, however, have been extremely limited. Using 13 years of ionospheric peak altitude data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on Mars Express, we study how the peak altitude responded to dust storms during six different Mars years (MY). The peak altitude increased ∼10–15 km during all six events, which include a local dust storm (MY 33), three regional dust storms (MYs 27, 29, and 32), and two global dust storms (MYs 28 and 34). The peak altitude's orbit‐to‐orbit variability was exceptionally large at the apexes of the MY 29 and MY 32 dust seasons and dramatically increased during the MY 28 and MY 34 global dust storms. We conclude that dust storms significantly increase upper atmospheric variability, which suggests that they enhance dynamical processes that couple the lower and upper atmospheres, such as upward propagating gravity waves or atmospheric tides.

Quantitative assessment of water content and mineral abundances at Gale crater on Mars with orbital observations

Context. The information of water content can help to improve atmospheric and climate models, and thus provide a better understanding of the past and present role of water and aqueous alteration on Mars. Mineral abundances can provide unique constraints on their formation environments and thus also on the geological and climate evolution of Mars. Aims. In this study, we used a state-of-the-art approach to derive the hydration state and mineral abundances over Gale crater on Mars, analysing hyperspectral visible/near-infrared data from the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) instrument onboard Mars Express and from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument onboard the Mars Reconnaissance Orbiter. Methods. The Discrete Ordinate Transfer model was used to perform atmospheric and thermal correction of the OMEGA and CRISM hyperspectral data in order to derive the surface single scattering albedos (SSAs) at Gale crater, Mars. Water content was estimated using a linear relationship between the derived effective single-particle absorption thickness at 2.9 μm from SSAs and the water weight percentage. Mineral abundances were retrieved by performing the linear spectral unmixing of SSAs from CRISM data. The results were compared with the ground-truth results returned from Curiosity rover. Results. The water content for most areas at Gale crater derived using the OMEGA data is around 2–3 water weight percent (water wt % hereafter), which is in agreement with that derived from the in situ measurements by Curiosity’s Sample Analysis at Mars instrument. However, the sensitivity tests show that uncertainties exist due to the combination of several factors including modelling bias, instrumental issue, and different sensing techniques. The derived mineral abundances using the orbital data are not fully consistent with that derived by Curiosity, and the discrepancy may be due to a combination of dust cover, texture, and particle size effects, as well as the effectiveness of the quantitative model. Conclusions. The ground-truth data from Curiosity provide a critical calibration point for the quantitative method used in the orbital remote-sensing observations. Our analysis indicates that the method presented here has great potential for mapping the water content and mineral abundances on Mars, but caution must be taken when using these abundance results for geological interpretations.

An elastic model of Phobos’ libration

Context. Study the rotation of a celestial body is an efficient way to infer its interior structure, and then may give information of its origin and evolution. In this study, based on the latest shape model of Phobos from Mars Express (MEX) mission, the polyhedron approximation approach was used to simulate the gravity field of Phobos. Then, the gravity information was combined with the newest geophysical parameters such as GM and k2 to construct the numerical model of Phobos’ rotation. And with an appropriate angles transformation, we got the librational series respect to Martian mean equator of date. Aims. The purpose of this paper is to develop a numerical model of Phobos’ rotational motion that includes the elastic properties of Phobos. The frequencies analysis of the librational angles calculated from the numerical integration results emphasize the relationship between geophysical properties and dynamics of Phobos. This work will also be useful for a future space mission dedicated to Phobos. Methods. Based on the latest shape model of Phobos from MEX mission, we firstly modeled the gravity field of Phobos, then the gravity coefficients were combined with some of the newest geophysical parameters to simulate the rotational motion of Phobos. To investigate how the elastic properties of Phobos affect its librational motion, we adopted various k2 into our numerical integration. Then the analysis was performed by iterating a frequency analysis and linear least-squares fit of Phobos’ physical librations. From this analysis, we identified the influence of k2 on the largest librational amplitude and its phase. Results. We showed the first ten periods of the librational angles and found that they agree well with the previous numerical results which Phobos was treated as a perfectly rigid body. We also found that the maximum amplitudes of the three parameters of libration are also close to the results from a rigid model, which is mainly due to the inclination of Phobos and moments of inertia. The other amplitudes are slightly different, since the physics contained in our model is different to that of a previous study, specifically, the different low-degree gravity coefficients and ephemeris. The libration in longitude τ has the same quadratic term with previous numerical study, which is consistent with the secular acceleration of Phobos falling onto Mars. We investigated the influence of the tidal Love number k2 on Phobos’ rotation and found a detectable amplitude changes (0.0005°) expected in the future space mission on τ, which provided a potential possibility to constrain the k2 of Phobos by observing its rotation. We also studied the influence of Phobos’ orbit accuracy on its libration and suggested a simultaneous integration of orbit and rotation in future work.

First year of coordinated science observations by Mars Express and ExoMars 2016 Trace Gas Orbiter

Two spacecraft launched and operated by the European Space Agency are currently performing observations in Mars orbit. For >15 years Mars Express has been conducting global surveys of the surface, the atmosphere and the plasma environment of the Red Planet. The Trace Gas Orbiter, the first element of the ExoMars programme, began its science phase in 2018 focusing on investigations of the atmospheric composition with unprecedented sensitivity as well as surface and subsurface studies. The coordination of observation programmes of both spacecraft aims at cross calibration of the instruments and exploitation of new opportunities provided by the presence of two spacecraft whose science operations are performed by two closely collaborating teams at the European Space Astronomy Centre (ESAC).In this paper we describe the first combined observations executed by the Mars Express and Trace Gas Orbiter missions since the start of the TGO operational phase in April 2018 until June 2019. Also included is the science opportunity analysis that has been performed by the Science Operation Centres and instrument teams to identify the observation opportunities until the end of 2020. These results provide a valuable contribution to the scientific community by enabling collaborations within the instrument teams and enhance the scientific outcome of both missions. This information is also valuable to other Mars missions (MAVEN, Mars Reconnaissance Orbiter, Curiosity, …), which may be interested in observing these locations for wider scientific collaboration.In this work we have analysed the simultaneous and quasi-simultaneous opportunities for cross-calibrations and combined observations by both missions, in particular for the vertical atmospheric profiles with solar occultation and the global atmospheric monitoring with nadir observations. As a result of this work we have identified simultaneous solar occultations that can be combined to compare vertical atmospheric profiles of the same region observed by different instruments within <2 minute difference, and many other quasi-simultaneous occultation observations within 15 minute difference at various latitudes and local times. We have also analysed and identified the simultaneous nadir observations that have been planned regularly at different distances and illumination conditions, and studied the seasonal evolution of the orbital crossing points and the alignment of the ground tracks driven by seasonal orbital variations.Lastly we provide an analysis of future opportunities to improve the global coverage of the atmosphere until the end of 2020. This study includes combined opportunities for nadir, solar occultations and a preliminary study of the coverage for limbs and radio occultations between both spacecraft. The resulting observations strongly increase the robustness of both Mars Express and Trace Gas Orbiter investigations due to cross-calibration of the instruments. They significantly increase spatial and temporal coverage, open new opportunities for scientific collaboration and synergy thus enhancing overall science return of both missions.