Context.
Even after the Rosetta mission, some of the mechanical parameters of comet 67P/Churyumov-Gerasimenko’s surface material are still not well constrained. They are needed to improve our ...understanding of cometary activity or for planning sample return procedures.
Aims.
We discuss the physical process dominating the formation of aeolian-like surface features in the form of moats and wind taillike bedforms around obstacles and investigate the mechanical and geometrical parameters involved.
Methods.
By applying the discrete element method (DEM) in a low-gravity environment, we numerically simulated the dynamics of the surface layer particles and the particle stream involved in the formation of aeolian-like morphological features. The material is composed of polydisperse spherical particles that consist of a mixture of dust and water ice, with interparticle forces given by the Hertz contact model, cohesion, friction, and rolling friction. We determined a working set of parameters that enables simulations to be reasonably realistic and investigated morphological changes when modifying these parameters.
Results.
The aeolian-like surface features are reasonably well reproduced using model materials with a tensile strength on the order of 0.1–1 Pa. Stronger materials and obstacles with round shapes impede the formation of a moat and a wind tail. The integrated dust flux required for the formation of moats and wind tails is on the order of 100 kg m
−2
, which, based on the timescale of morphological changes inferred from Rosetta images, translates to a near-surface particle density on the order of 10
−6
–10
−4
kgm
−3
.
Conclusions.
DEM modeling of the aeolian-like surface features reveals complex formation mechanisms that involve both deposition of ejected material and surface erosion. More numerical work and additional in situ measurements or sample return missions are needed to better investigate mechanical parameters of cometary surface material and to understand the mechanics of cometary activity.
The structure of the upper layer of a comet is a product of its surface activity. The Rosetta Lander Imaging System (ROLIS) on board Philae acquired close-range images of the Agilkia site during its ...descent onto comet 67P/Churyumov-Gerasimenko. These images reveal a photometrically uniform surface covered by regolith composed of debris and blocks ranging in size from centimeters to 5 meters. At the highest resolution of 1 centimeter per pixel, the surface appears granular, with no apparent deposits of unresolved sand-sized particles. The thickness of the regolith varies across the imaged field from 0 to 1 to 2 meters. The presence of aeolian-like features resembling wind tails hints at regolith mobilization and erosion processes. Modeling suggests that abrasion driven by airfall-induced particle "splashing" is responsible for the observed formations.
Our research focuses on the mineralogical characteristics of dark sediment deposits in Martian craters (dunes, dune fields, and sand sheets) and their local sediment sources, carried out on the basis ...of 70 selected localities. Morphological analyses reveal dark layers exposed in a number of crater walls, showing indications of material transport from the layers downwall to the intracrater deposits. Additional sites show dark sediment emerging from several craters floors, suggesting a dark material source beneath the crater floor. Spectral analyses of OMEGA and CRISM data showed for the first time that a direct mineralogical connection exists between the dark layers in the crater walls and the intracrater dune and sand sheet material. Based on these findings, it is shown that these layers are the local sources for the dark aeolian sediment inside the craters. Spectral analyses further indicate that all deposits are nearly of the same mafic mineralogical composition, hinting at a similar origin. We conclude that the mineralogical composition points to a volcanic origin of the material. The dark layers which we infer to be the material sources might represent layers of volcanic ash, almost globally deposited in Early Noachian times and subsequently covered by regolith. Groundwater, impact erosion, and crustal movement led to the highly fragmented situation of the dark layers today. However, burial protected the layers of dark sediment from chemical alteration caused by surficial fluvial processes. Impact erosion after the wet periods on Mars led to the exposure and mobilization of the material today.
Aims . This study aims to analyze Phobos’ photometric properties using Mars Express mission observations to support the Martian Moons exploration mission (MMX) devoted to the investigation of the ...Martian system and to the return of Phobos samples. Methods . We analyzed resolved images of Phobos acquired between 2004 and 2022 by the High Resolution Stereo Camera (HRSC) on board the Mars Express spacecraft at a resolution ranging from ~30 m px −1 to 330 m px −1 . We used data acquired with the blue, green, red, and IR filters of HRSC and the panchromatic data of the Super Resolution Channel (SRC). The SRC data are unique because they cover small phase angles (0.2–10°), permitting the investigation of the Phobos opposition effect. We simulated illumination and geometric conditions for the different observations using the Marx Express and the camera spice kernels provided by the HRSC team. We performed photometric analysis using the Hapke model for both integrated and disk-resolved data. Results . The Phobos phase function is characterized by a strong opposition effect due to shadow hiding, with an amplitude and a half-width of the opposition surge of 2.28±0.03 and 0.0573±0.0001, respectively. Overall, the surface of Phobos is dark, with a geometric albedo of 6.8% in the green filter and backscattering. Its single-scattering albedo (SSA) value (7.2% in the green filter) is much higher than what has been found for primitive asteroids and cometary nuclei and is close to the values reported in the literature for Ceres. We also found a surface porosity of 87%, indicating the presence of a thick dust mantle or of fractal aggregates on the top surface. The SSA maps revealed high reflectance variability, with the blue unit area in the northeast Stickney rim being up to 65% brighter than average, while the Stickney floor is among the darkest regions, with reflectance 10 to 20% lower than average. Photometric modeling of the regions of interest selected in the red and blue units indicates that red unit terrains have a stronger opposition effect and a smaller SSA value than the blue ones, but they have similar porosity and backscattering properties. Conclusions . The HRSC data provide a unique investigation of the Phobos phase function and opposition surge, which is valuable information for the MMX observational planning. The Phobos opposition surge, surface porosity, phase integral, and spectral slope are very similar to the values observed for the comet 67P and for Jupiter family comets in general. Based on these similarities, we formulate a hypothesis that the Mars satellites might be the results of a binary or bilobated comet captured by Mars.
Wind‐formed features are abundant in Oxia Planum (Mars), the landing site of the 2022 ExoMars mission, which shows geological evidence for a past wet environment. Studies of aeolian bedforms at the ...landing site were focused on assessing the risk for rover trafficability, however their potential in recording climatic fluctuations has not been explored. Here we show that the landing site experienced multiple climatic changes in the Amazonian, which are recorded by an intriguing set of ridges that we interpret as Periodic Bedrock Ridges (PBRs). Clues for a PBR origin result from ridge regularity, defect terminations, and the presence of preserved megaripples detaching from the PBRs. PBR orientation differs from superimposed transverse aeolian ridges pointing toward a major change in wind regime. Our results provide constrains on PBR formation mechanisms and offer indications on paleo winds that will be crucial for understanding the landing site geology.
Plain Language Summary
Oxia Planum on Mars is the landing site for the ExoMars 2022 mission. The region likely hosted a standing body of water, but the effect of the wind was also important in shaping the landscape. In this study, we first describe a set of linear ridges that, in our interpretation, were sculpted by the wind in a more recent past. We also show that the wind that formed the ridges (Periodic Bedrock Ridges) was blowing from a different direction than the ones that formed younger ripples on top, suggesting a complex geological history of wind erosion and deposition that will be further investigated during the ExoMars mission.
Key Points
We present the first evidence for a periodic bedrock ridge (PBRs) pattern from the ExoMars 2022 landing site
Formative paleowind directions are extrapolated from PBRs and transverse aeolian ridges
Evidence for an Amazonian change in the wind regime are provided
This review paper summarizes the observations and results of the Mars Express Mission and its application in the analysis of geological processes and landforms on Mars during the last 20 years. The ...Mars Express observations provided an extended data base allowing a comparative evaluation of different geological surface landforms and their time-based delimitation. High-resolution imagery and digital elevations models on a local to regional scale and spectral measurements are the basis for geological analyses of water-related surface processes on Mars. This includes the nature and discharges of valley networks, formation timescale of deltas, volumina of sedimentary deposits as well as estimating the age of geological units by crater size–frequency distribution measurements. Both the quantifying of geological processes and the determination of absolute model ages allows to constraint the evolution of Martian water-related activity in space and time. Comparative age estimation of fluvial, glacial, and lacustrine deposits, as well as their timing and episodicity, has revealed the nature and evolution of the Martian surface hydrological cycle. Fluvial and lacustrine activity phases are spread over a time span from Noachian until Amazonian periods, but detailed studies show that they have been interrupted by multiple and long-lasting phases of cessation and quiescent. In addition, evidence of glacial activity shows discrete phases of enhanced intensity correlating with increased spin-axis obliquity amplitude. The episodicity of geological processes, erosion, deposition, and glaciation on Mars demonstrate a close correlation between individual surface processes and endogenic activity as well as spin-axis/orbital variations and changing climate condition.
This review describes the dynamic phenomena in the atmosphere of Mars that are visible in images taken in the visual range through cloud formation and dust lifting. We describe the properties of ...atmospheric features traced by aerosols covering a large range of spatial and temporal scales, including dynamical interpretations and modelling when available. We present the areographic distribution and the daily and seasonal cycles of those atmospheric phenomena. We rely primarily on images taken by cameras on Mars Express.
The High Resolution Stereo Camera (HRSC) of ESA׳s Mars Express is designed to map and investigate the topography of Mars. The camera, in particular its Super Resolution Channel (SRC), also obtains ...images of Phobos and Deimos on a regular basis. As HRSC is a push broom scanning instrument with nine CCD line detectors mounted in parallel, its unique feature is the ability to obtain along-track stereo images and four colors during a single orbital pass. The sub-pixel accuracy of 3D points derived from stereo analysis allows producing DTMs with grid size of up to 50m and height accuracy on the order of one image ground pixel and better, as well as corresponding orthoimages. Such data products have been produced systematically for approximately 40% of the surface of Mars so far, while global shape models and a near-global orthoimage mosaic could be produced for Phobos. HRSC is also unique because it bridges between laser altimetry and topography data derived from other stereo imaging instruments, and provides geodetic reference data and geological context to a variety of non-stereo datasets. This paper, in addition to an overview of the status and evolution of the experiment, provides a review of relevant methods applied for 3D reconstruction and mapping, and respective achievements. We will also review the methodology of specific approaches to science analysis based on joint analysis of DTM and orthoimage information, or benefitting from high accuracy of co-registration between multiple datasets, such as studies using multi-temporal or multi-angular observations, from the fields of geomorphology, structural geology, compositional mapping, and atmospheric science. Related exemplary results from analysis of HRSC data will be discussed. After 10 years of operation, HRSC covered about 70% of the surface by panchromatic images at 10–20m/pixel, and about 97% at better than 100m/pixel. As the areas with contiguous coverage by stereo data are increasingly abundant, we also present original data related to the analysis of image blocks and address methodology aspects of newly established procedures for the generation of multi-orbit DTMs and image mosaics. The current results suggest that multi-orbit DTMs with grid spacing of 50m can be feasible for large parts of the surface, as well as brightness-adjusted image mosaics with co-registration accuracy of adjacent strips on the order of one pixel, and at the highest image resolution available. These characteristics are demonstrated by regional multi-orbit data products covering the MC-11 (East) quadrangle of Mars, representing the first prototype of a new HRSC data product level.
•We present an overview of the main characteristics and operational achievements of HRSC after ten years in orbit.•Demonstrate significant improvements in HRSC data product accuracy based on in-flight data.•Present a new HRSC data product level based on integration of data from multiple orbits.•Review methods and exemplary results for mapping and science analysis, typically based on precise co-registration and joint analysis of DTMs and orthoimages.
A valley network in the Western Libya Montes region extending from 80°E to 83.4°E and 1.2°S to 3.9°N exhibits significant detail about water release processes and duration of erosional activities. ...The valley system originates at the southeastern offshoot of Syrtis Major and drains down to Isidis Planitia over a distance of about 300
km. Midstream, the valley network splits into a shorter eastern and a longer western part. For most of its length, the valley exhibits an interior channel which allows constraining discharges ranging from 15,000
m
3/s to 430,000
m
3/s and yielding sediment volumes up to 250
tons/s. Based on stratigraphic relations, the valley system evolved during a period of about 2.8
billion years with major wet episodes in the Noachian (<
4.1
Ga), the Hesperian (3.6 to 3.0
Ga) and the Amazonian (2.8 to 1.4
Ga). While precipitation dominated the fluvial activity during the Noachian era, as indicated by dendritic drainage pattern, the close correlation of lava deposits and valley source regions suggests that volcanic processes, such as ground ice melting and/or hydrothermal water release, might have played a major role during the Hesperian and Amazonian fluvial activities. In addition, multiple volcanic events in the Hesperian and Amazonian ages show that Syrtis Major was active, at least locally, until 1.4
Ga ago. Discharge estimates demonstrate a significant increase from precipitation-induced to volcanic-triggered water release. Fluvial erosion rates, discharges, sediment transport rates, and the lack of any widespread chemical alteration products, such as sulphates and phyllosilicates that are indicative for large long-lasting standing bodies of water, suggest relatively short valley formation times of only a few thousands of years. Compared with the total age of the valley system, short episodic water release events separated by long dry and inactive periods of some hundreds of millions of years, at least in the Hesperian and Amazonian, seem to be more realistic rather than a continuous flow. The youngest channel segment dates back to less than 1.4
Ga. This indicates the presence of surface runoff at that time, which required atmospheric conditions that support water to be stable on the surface even late in Martian history. In summary, the Western Libya Montes Valley System demonstrates episodic and multi-genetic erosion events over most of the Martian history, with a clear change in the erosion style from precipitation to volcanic-triggered water release during the early Hesperian.
We compare estimates of atmospheric precipitation during the Martian Noachian–Hesperian boundary 3.8Gyr ago as calculated in a radiative-convective column model of the atmosphere with runoff values ...estimated from a geomorphological analysis of dendritic valley network discharge rates. In the atmospheric model, we assume CO2–H2O–N2 atmospheres with surface pressures varying from 20mb to 3bar with input solar luminosity reduced to 75% the modern value.
Results from the valley network analysis are of the order of a few mmd−1 liquid water precipitation (1.5–10.6mmd−1, with a median of 3.1mmd−1). Atmospheric model results are much lower, from about 0.001–1mmd−1 of snowfall (depending on CO2 partial pressure). Hence, the atmospheric model predicts a significantly lower amount of precipitated water than estimated from the geomorphological analysis. Furthermore, global mean surface temperatures are below freezing, i.e. runoff is most likely not directly linked to precipitation. Therefore, our results strongly favor a cold early Mars with episodic snowmelt as a source for runoff.
Our approach is challenged by mostly unconstrained parameters, e.g. greenhouse gas abundance, global meteorology (for example, clouds) and planetary parameters such as obliquity – which affect the atmospheric result – as well as by inherent problems in estimating discharge and runoff on ancient Mars, such as a lack of knowledge on infiltration and evaporation rates and on flooding timescales, which affect the geomorphological data. Nevertheless, our work represents a first step in combining and interpreting quantitative tools applied in early Mars atmospheric and geomorphological studies.
•We present a combination of geological and atmospheric models to constrain early Mars precipitation.•We determine runoff rates for 18 ancient valley networks on Mars.•We estimate mean precipitation for early Mars with an atmospheric model.•Comparison between both approaches suggests a cold, dry early Mars.•Episodic melting events or high-temperature periods may have been responsible for valley network formation.
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