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  • ddc:523  (15)
  • English  (15)
  • 2020-2023  (15)
  • 1970-1974
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  • English  (15)
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  • 1
    Publication Date: 2022-12-10
    Description: Differently aged impact melt in lunar samples is key to unveiling the early bombardment history of the Moon. Due to the mixing of melt products ejected from distant craters, the interpretations of the origin of lunar samples are difficult. We use numerical modeling for a better quantitative understanding of the production of impact‐induced melt and in particular its distribution in ejecta blankets for lunar craters with sizes ranging from 1.5 to 50 km. We approximate the lunar stratigraphy with a porosity gradient, which represents the gradual transition from upper regolith via megaregolith to the solid crustal material. For this lunar setting, we quantify the melt production relative to crater volume and derive parameters describing its increasing trend with increasing transient crater size. We found that about 30%–40% of the produced melt is ejected from the crater. The melt concentration in the ejecta blanket increases almost linearly with distance from the crater center, while the thickness of the ejecta blanket decreases following a power law. Our study demonstrates that if in lunar samples the concentration of a melt with a certain age is interpreted to be of a nonlocal origin, these melts could be the impact products of a large crater (〉10 km) located hundreds of kilometers away.
    Description: Plain Language Summary: Lunar samples contain abundant impact‐induced melt that crystallized at different ages. The melt ages record the formation time of its source craters and are key for a better understanding of the lunar bombardment history. In samples, there is not only the melt derived from the sampling region but also some that originate far away by being entrained in the ejecta of distant craters. Recognizing the distant‐derived melt is essential for the more credible sample interpretation, which requires knowledge of the melt distribution in the ejecta. We use numerical modeling to quantify the production of impact‐induced melt and in particular its distribution in ejecta blankets for lunar craters. We found that the melt concentration in the ejecta blanket increases with distance from the crater center. If the concentration of distant‐derived melt of a certain age in lunar samples is rather high (〉30%), it could originate from large craters (〉10 km) located hundreds of kilometers away.
    Description: Key Points: The melt concentration in the ejecta blanket increases almost linearly with distance from the crater center. Near‐surface porosity causes an increase in melt production. Due to decreasing porosity with depth, it is more prominent at small craters. The melt concentration in distal ejecta of crater of 10's km is rather high (〉30%).
    Description: Deutsche Forschungsgemeinschaft DFG http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:523 ; lunar craters ; melt production ; numerical modeling
    Language: English
    Type: doc-type:article
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  • 2
    Publication Date: 2022-10-06
    Description: We analyze Hubble Space Telescope observations of Ganymede made with the Space Telescope Imaging Spectrograph between 1998 and 2017 to generate a brightness map of Ganymede's oxygen emission at 1,356 Å. Our Mercator projected map demonstrates that the brightness along Ganymede's northern and southern auroral ovals strongly varies with longitude. To quantify this variation around Ganymede, we investigate the brightness averaged over 36°‐wide longitude corridors centered around the sub‐Jovian (0° W), leading (90° W), anti‐Jovian (180° W), and trailing (270° W) central longitudes. In the northern hemisphere, the brightness of the auroral oval is 3.7 ± 0.4 times lower in the sub‐Jovian and anti‐Jovian corridors compared to the trailing and leading corridors. The southern oval is overall brighter than the northern oval, and only 2.5 ± 0.2 times fainter on the sub‐ and anti‐Jovian corridors compared to the trailing and leading corridors. This demonstrates that Ganymede's auroral ovals are strongly structured in auroral crescents on the leading side (plasma downstream side) and on the trailing side (plasma upstream side). We also find that the brightness is not symmetric with respect to the 270° meridian, but shifted by ∼20° towards the Jovian‐facing hemisphere. Our map will be useful for subsequent studies to understand the processes that generate the aurora in Ganymede's non‐rotationally driven, sub‐Alfvénic magnetosphere.
    Description: Plain Language Summary: Northern lights often illuminate the night sky in a shimmering green or red tone at high geographic latitudes. This emission, scientifically referred to as aurora, is a result of electrically charged particles that move along Earth's magnetic field lines and interact with its atmosphere to produce auroral emission. Apart from the Earth, multiple other planets in our solar system also exhibit auroral emission. By characterizing the brightness and structure of these lights, we are therefore able to deduce insights about a planet's atmosphere, magnetic field and the physical processes occurring along the field lines from afar. In this work, we used observations from the Hubble Space Telescope to analyze the auroral emission of Jupiter's largest moon Ganymede. We combined multiple images of Ganymede to create the first complete map that displays the auroral brightness. Our map revealed that the emission on Ganymede's auroral ovals varies strongly in brightness with divisions into two distinct bright and faint regions. They resemble two auroral crescents in the north and south respectively, and demonstrate the uniqueness of Ganymede's aurora in comparison with the auroral ovals of other planets in the solar system.
    Description: Key Points: Brightness map of Ganymede's ultraviolet auroral emission has been constructed based on Hubble Space Telescope observations from 1998 to 2017. Auroral ovals are structured in upstream and downstream “crescents”. Brightness on sub‐Jovian and anti‐Jovian side is strongly reduced by a factor of 3–4 compared to upstream and downstream side.
    Description: European Research Council (ERC)
    Description: http://archive.stsci.edu/hst/
    Keywords: ddc:523
    Language: English
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  • 3
    Publication Date: 2022-10-04
    Description: The seasonal deposition and sublimation of CO2 represents a major element in the Martian volatile cycle. Here, co‐registration strategies are applied to Mars Orbiter Laser Altimeter profiles to obtain spatio‐temporal variations in snow/ice level of the Seasonal South Polar Cap (SSPC), in grid elements of 0.5° in latitude from 60° to 87°S and 10° in longitude. The maximum snow/ice level in the range of 2–2.5 m is observed over the Residual South Polar Cap. Peak level at the Residual South Polar Cap in Martian Year 25 (MY25) are found to be typically ∼0.5 m higher than those in MY24. The total volume is estimated to peak at approximately 9.4 × 1012 m3. In addition, a map of average bulk density of the SSPC during its recession is derived. It implies much more snowfall‐like precipitation at the Residual South Polar Cap and its surroundings than elsewhere on Mars.
    Description: Plain Language Summary: Each Martian year, up to one third of the atmosphere's CO2 is transported from pole to pole, being deposited and sublimated depending on the season. Accurate measurements of snow level and volume variations of the resulting seasonal polar caps can serve as crucial constraints on the Martian volatile cycles. In this study, we apply new approaches of analyzing the Mars Orbiter Laser Altimeter profiles, which lead to spatially and temporally resolved measurements of snow/ice level of the Seasonal South Polar Cap (SSPC). Based on that, the maximum snow level, interannual maximum level change from Martian Year 24 (MY24) to MY25, and how the volume of the SSPC changes with time are measured. We also estimate the bulk density of the snow/ice deposition during southern winter. It is inferred that there is much more snowfall at the Residual South Polar Cap and its surroundings than elsewhere on the planet.
    Description: Key Points: Using co‐registration of Mars Orbiter Laser Altimeter profiles, spatio‐temporal level variations of the seasonal snow/ice deposits at the Martian south pole are obtained. Maximum level can be up to 2.5 m; Peak level increased by ∼0.5 m at the Residual South Polar Cap from Martian Year 24 (MY24) to MY25. Obtained bulk density map of the seasonal deposits implies that snowfall concentrates at the Residual South Polar Cap and its surroundings.
    Description: China Scholarship Council
    Description: Deutsche Forschungsgemeinschaft
    Description: Institut National des Sciences de l’Univers
    Description: Centre National de la Recherche Scientifique
    Description: Centre National d’Etudes Spatiales
    Description: https://pds-geosciences.wustl.edu/missions/mgs/pedr.html
    Description: https://naif.jpl.nasa.gov/pub/naif/pds/data/mgs-m-spice-6-v1.0/mgsp_1000/data/
    Description: https://doi.org/10.17632/z59b9nd6s9.2
    Description: https://doi.org/10.14768/8cba4407-d6a0-4d16-aeaf-d0ebfd2b480a
    Keywords: ddc:523
    Language: English
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  • 4
    Publication Date: 2022-09-30
    Description: On October 7, 2008, the asteroid 2008 TC3 exploded as it entered the Earth’s atmosphere, producing significant dust (in the atmosphere) and delivering thousands of stones in a strewn field in Sudan, collectively known as the Almahata Sitta (AhS) stones. About 600 fragments were officially recovered in 2008 and 2009. Further rocks were collected since the fall event by local people. From these stones, 249 were classified at the Institut für Planetologie in Münster (MS) known as MS‐xxx or MS‐MU‐xxx AhS subsamples. Most of these rocks are ureilitic in origin (168; 67%): 87 coarse‐grained ureilites, 60 fine‐grained ureilites, 15 ureilites with variable texture/mineralogy, four trachyandesites, and two polymict breccias. We identified 81 non‐ureilitic fragments, corresponding to 33% of the recovered samples studied in Münster. These included chondrites, namely 65 enstatite chondrites (43 EL; 22 EH), 11 ordinary chondrites (OC), one carbonaceous chondrite, and one unique R‐like chondrite. Furthermore, three samples represent a unique type of enstatite achondrite. Since all AhS stones must be regarded as individual specimens independent from each other, the number of fresh ureilite and enstatite chondrite falls in our meteorite collections has been increased by several hundred percent. Overall, the samples weigh between 〈1 and 250 g and have a mean mass of ~15 g. If we consider—almost 15 years after the fall—the mass calculations, observations during and after the asteroid entered the atmosphere, the mineralogy of the C1 stones AhS 91A and AhS 671, and the experimental work on fitting the asteroid spectrum (e.g., Goodrich et al., 2019; Jenniskens et al., 2010; Shaddad et al., 2010), the main portion of the meteoroid was likely made of the fine‐grained (carbonaceous) dust and was mostly lost in the atmosphere. In particular, the fact that C1 materials were found has important implications for interpreting asteroid 2008 TC3's early spectroscopic results. Goodrich et al. (2019) correctly suggested that if scientists had not recovered the “water‐free” samples (e.g., ureilites, enstatites, and OC) from the AhS strewn field, 2008 TC3 would have been assumed to be a carbonaceous chondrite meteoroid. Considering that the dominating mass of the exploding meteoroid consisted of carbonaceous materials, asteroid 2008 TC3 cannot be classified as a polymict ureilite; consequently, we state that the asteroid was a polymict carbonaceous chondrite breccia, specifically a polymict C1 object that may have formed by late accretion at least 50–100 Ma after calcium–aluminum‐rich inclusions.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Alexander von Humboldt Foundation http://dx.doi.org/10.13039/100005156
    Keywords: ddc:549.112 ; ddc:523
    Language: English
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  • 5
    Publication Date: 2022-09-22
    Description: Io's movement relative to the plasma in Jupiter's magnetosphere creates Alfvén waves propagating along the magnetic field lines which are partially reflected along their path. These waves are the root cause for auroral emission, which is subdivided into the Io Footprint (IFP), its tail and leading spot. New observations of the Juno spacecraft by Mura et al. (2018, https://doi.org/10.1126/science.aat1450) have shown puzzling substructure of the footprint and its tail. In these observations, the symmetry between the poleward and equatorward part of the footprint tail is broken and the tail spots are alternatingly displaced. We show that the location of these bright spots in the tail are consistent with Alfvén waves reflected at the boundary of the Io torus and Jupiter's ionosphere. Then, we investigate three different mechanisms to explain this phenomenon: (a) The Hall effect in Io's ionosphere, (b) travel time differences of Alfvén waves between Io's Jupiter facing and its opposing side and (c) asymmetries in Io's atmosphere. For that, we use magnetohydrodynamic simulations within an idealized geometry of the system. We use the Poynting flux near the Jovian ionosphere as a proxy for the morphology of the generated footprint and its tail. We find that the Hall effect is the most important mechanism under consideration to break the symmetry causing the “Alternating Alfvén spot street.” The travel time differences contributes to enhance this effect. We find no evidence that the inhomogeneities in Io's atmosphere contribute significantly to the location or shape of the tail spots.
    Description: Key Points: Hall effect in Io's ionosphere produces Poynting flux morphology similar to observed alternating Alfvén spot street in Io footprint tail (IFP). Alfvén wave travel time difference and asymmetries in Io's atmosphere are not sufficient to produce observed structures in IFP. IFP emission inter‐spot distance correlates with reflected Alfvén waves.
    Description: Regional Computing Center of the University of Cologne (RRZK)
    Description: European Research Council (ERC)
    Description: http://plutocode.ph.unito.it/download.html
    Keywords: ddc:523
    Language: English
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  • 6
    Publication Date: 2022-06-17
    Description: In preparation for future human habitats on Mars, it is important to understand the Martian radiation environment. Mars does not have an intrinsic magnetic field and Galactic cosmic ray (GCR) particles may directly propagate through and interact with its atmosphere before reaching the surface and subsurface of Mars. However, Mars has many high mountains and low‐altitude craters where the atmospheric thickness can be more than 10 times different from one another. We thus consider the influence of the atmospheric depths on the Martian radiation levels including the absorbed dose, dose equivalent and body effective dose rates induced by GCRs at varying heights above and below the Martian surface. The state‐of‐the‐art Atmospheric Radiation Interaction Simulator based on GEometry And Tracking Monte Carlo method has been employed for simulating particle interactions with the Martian atmosphere and terrain. We find that higher surface pressures can effectively reduce the heavy ion contribution to the radiation, especially the biologically weighted radiation quantity. However, enhanced shielding (both by the atmosphere and the subsurface material) can considerably enhance the production of secondary neutrons which contribute significantly to the effective dose. In fact, both neutron flux and effective dose peak at around 30 cm below the surface. This is a critical concern when using the Martian surface material to mitigate radiation risks. Based on the calculated effective dose, we finally estimate some optimized shielding depths, under different surface pressures (corresponding to different altitudes) and various heliospheric modulation conditions. This may serve for designing future Martian habitats.
    Description: Plain Language Summary: Thanks to Earth's magnetic field and atmosphere, high‐energy cosmic particles can be efficiently shielded from causing radiation risks for humans on Earth. However, for crewed space missions, in particular long‐term missions to Mars, space radiation is a major risk for the health of astronauts. Mars does not have an intrinsic global magnetic field and its atmosphere is too thin to effectively shield against radiation. Here, we model the Martian radiation environment induced by omnipresent cosmic rays in Mars's atmosphere and terrain. Given that Mars has many high mountains and low‐altitude craters where the atmospheric thickness can be more than 10 times different from one another, we also consider different model setups with different atmospheric profiles. We find that with more shielding the heavy ion contribution to the radiation is reduced while the neutron contribution is enhanced. For a given threshold of the annual biologically weighted radiation effective dose, for example, 100 mSv, the required regolith depth ranges between about 1 and 1.6 m. At a deep crater where the surface pressure is higher, the needed extra regolith shielding is slightly smaller. Our study may serve for mitigating radiation risks when designing future Martian habitats using natural surface material as shielding protection.
    Description: Key Points: We calculate dose, dose equivalent, and effective dose rates induced by various components of galactic cosmic rays on and below Mars surface. Surface pressure which is related to geographic altitude influences the surface and subsurface radiation level. Subsurface secondary neutrons contribute significantly to the effective dose and are a critical concern for radiation risks on Mars.
    Description: CAS strategic priority program
    Description: National Natural Science Foundation of China (NSFC) http://dx.doi.org/10.13039/501100001809
    Description: CNSA pre‐research Project on Civil Aerospace Technologies
    Description: The Key Research Program of the Chinese Academy of Sciences
    Description: German Aerospace Center (DLR)
    Description: https://et-wiki.physik.uni-kiel.de/atris/atris
    Keywords: ddc:523
    Language: English
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  • 7
    Publication Date: 2022-03-30
    Description: The estimation of crustal structure and thickness is essential in understanding the formation and evolution of terrestrial planets. Initial planetary missions with seismic instrumentation on board face the additional challenge of dealing with seismic activity levels that are only poorly constrained a priori. For example, the lack of plate tectonics on Mars leads to low seismicity, which could, in turn, hinder the application of many terrestrial data analysis techniques. Here we propose using a joint inversion of receiver functions and apparent incidence angles, which contain information on absolute S‐wave velocities of the subsurface. Since receiver function inversions suffer from a velocity depth trade‐off, we in addition exploit a simple relation that defines apparent S‐wave velocity as a function of observed apparent P‐wave incidence angles to constrain the parameter space. We then use the Neighborhood Algorithm for the inversion of a suitable joint objective function. The resulting ensemble of models is then used to derive uncertainty estimates for each model parameter. In preparation for the analysis of data from the InSight mission, we show the application of our proposed method on Mars synthetics and sparse terrestrial data sets from different geological settings using both single and multiple events. We use information‐theoretic statistical tests as model selection criteria and discuss their relevance and implications in a seismological framework.
    Description: Key Points: We propose the joint inversion of receiver functions and apparent S‐wave velocity curves to estimate crustal thickness. Using the Neighborhood Algorithm, we show how a full uncertainty estimate can be computed from an ensemble solution. The method is applied to Martian synthetics and terrestrial data sets comprising single and multiple events.
    Description: IMPRS
    Description: Emeritus group
    Description: DLR German Space Agency
    Description: http://www.orfeus-eu.org/data/eida/
    Description: http://instaseis.ethz.ch/marssynthetics/
    Keywords: ddc:622.1592 ; ddc:523
    Language: English
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  • 8
    Publication Date: 2022-03-29
    Description: Jupiter's tenuous dust ring system is embedded in the planet's inner magnetosphere, and—among other structures—contains a very tenuous protrusion called the Thebe extension. In an attempt to explain the existence of this swath of particles beyond Thebe's orbit, Hamilton and Krüger (2008), https://doi.org/10.1038/nature06886 proposed that the dust particle motion is driven by a shadow resonance caused by variable dust charging on the day and night side of Jupiter. However, the model by Divine and Garrett (1983), https://doi.org/10.1029/ja088ia09p06889 together with recent observations by the Juno spacecraft indicates a warm and rather dense inner magnetosphere of Jupiter which implies that the mechanism of the shadow resonance does not work. Instead, we argue that dust grains ejected from Thebe due to micrometeoroid bombardment become the source of dust in the Thebe extension. We show that large (grain radii of a few micrometers up to multi‐micrometers) charged dust grains having significant initial velocities oscillate in the Thebe extension. Smaller charged grains (with sub‐micrometer radii) ejected from Thebe do not spend much time in the Thebe extension and migrate into the Thebe ring. At the same time, if such grains are ejected from larger dust grains in the Thebe extension due to fragmentation, they continue to oscillate within the Thebe extension for years. We argue that fragmentation of large dust grains in the Thebe extensions could be the main source of sub‐micrometer grains detected in the Thebe extension.
    Description: Key Points: In Jupiter's warm and dense inner magnetosphere dust grains acquire high negative electric charges. Dust particles ejected from Thebe with sufficient speeds contribute to the formation of the Thebe extension. Instead of shadow resonances as suggested earlier an alternative mechanism is suggested for the formation of the Thebe extension.
    Description: Max Planck Institute for Solar System Research
    Keywords: ddc:523
    Language: English
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  • 9
    Publication Date: 2022-03-29
    Description: The long‐ and short‐term drivers and transport mechanisms of lunar rockfalls are currently not well understood, but could provide valuable information about the geologic processes that still shape the surface of the Moon today. Here, we compare the global distribution of rockfalls with relevant geophysical data, such as seismic, topographic, thermal, gravity anomaly, and tidal displacement data sets. Rockfalls appear to predominantly occur (a) on equator‐facing slopes and thus in regions with large thermal amplitudes, (b) on slope angles well above‐average (Δ ∼ 10°), and (c) in regions with above‐average rock abundance. We do not observe a qualitatively or statistically relevant relation between rockfall abundance, monitored Apollo‐era shallow seismic activity, and the distribution of visible tectogenetic features. Informed by our global analysis, we conduct a targeted, in‐depth study of 687 rockfall boulders and trajectories in 13 sites across the Moon, including 7 craters, 2 volcanic vents, 2 tectonic structures, and 2 unclassified geomorphic regions. We identify four different source region types, where the type appears to control the occurrence of rockfalls. The source region type in turn is controlled by surface age rather than geomorphic context. We find that rockfall trajectories are mainly controlled by the trigger energy and the geometry of the slope. Our results suggest that erratic small‐scale impacts (mainly in old, Imbrian‐Nectarian, shallow terranes), aided by solar‐induced thermal fatigue of fractured bedrock (mainly in young, Copernican‐Eratosthenian steep terranes), were the dominant, global‐scale long‐ and short‐term drivers of rockfalls in the Moon's recent geologic past.
    Description: Plain Language Summary: The processes that drive rockfall occurrence are largely unknown, but could provide valuable information about the past and current evolution of the Moon's surface and interior. We compare the global distribution of rockfalls with a series of maps, such as seismic, topographic, thermal, and gravity anomaly maps and observe that rockfalls mainly occur (a) on equator‐facing slopes and thus in regions with large temperature differences, (b) on slope angles above‐average, and (c) in regions with rocky surfaces. We do not observe a relation between rockfall abundance, Apollo‐era seismic activity, and the distribution of visible tectonic features. Informed by our global‐scale analysis we study 687 rockfalls in 13 sites of interest in greater detail, including volcanic‐, tectonic‐, and impact‐related geomorphic regions. We observe that the source region type appears to control rockfall occurrence, which in turn is controlled by the surface age. We find that the lunar rockfall transport process appears to be mainly controlled by the driver energy and the steepness of the slope. Our results suggest that small‐scale impacts (mainly in old, shallow regions) and solar‐driven thermal breakdown of fractured bedrock (mainly in young, steep regions) were the main, global‐scale drivers of rockfalls in the Moon's recent geologic past.
    Description: Key Points: We study the drivers and transport mechanisms of lunar rockfalls on a local and global scale. The two dominant, global‐scale rockfall drivers appear to be: (a) impacts and (b) solar‐driven thermal fatigue. The rockfall driver depends on the source region age and type rather than the geomorphic context.
    Description: Max Planck Institute for Solar System Research
    Description: ETH Zurich
    Description: Engineering Geology group, Department of Earth Sciences, International Max Planck Research School
    Description: http://wms.lroc.asu.edu/lroc/search
    Keywords: ddc:523
    Language: English
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  • 10
    Publication Date: 2022-03-29
    Description: Gorgonum Chaos is part of the Eridania paleolake in Terra Sirenum and displays a number of prominent light‐toned morphological features that bear a record of the regional climatic conditions throughout most of Martian history. Based on an intergrated analysis of orbital data, we mapped a 1,500 km2 area in the southeast of Gorgonum Chaos. Morphologic, spectroscopic, and stratigraphic analyses were used to determine age and composition of the main geological units in the area. We identified four major geological units with decreasing content of hydrated minerals from the oldest to the youngest units, which were completely free of hydrated minerals. In the study area, phyllosilicate‐rich Noachian units compose the majority of the basin floor. Deposits enriched with evaporites were formed around the Noachian/Hesperian transition and erosion created prominent inverted morphologies. Loess‐like material without significant amounts of hydrated minerals was deposited until the late Hesperian. The youngest unit is an Amazonian layer free of hydrated minerals that originated from volcanic activities. This succession of minerals reflects the transition from more humid climatic conditions with the ability to sustain liquid water on the planet's surface during the Noachian to the hyper‐arid Amazonian environment we observe currently on Mars.
    Description: Plain Language Summary: Gorgonum Chaos is part of the Eridania basin, which is a former lake system located at the southern hemisphere of Mars. The landforms observed in this area and their variable brightness suggest dramatically changing climatic conditions during the history of Mars. In an area of 1,500 km2 in size, we analyzed different landforms, their spectral characteristics, and their temporal sequences to determine the age and composition of the geological units. We found four major geologic units whose content of hydrated minerals decreased dramatically from oldest to youngest. The oldest unit with a high content of clay minerals is about 3.7 billion years old and formed the former lake bottom. This is followed by a younger unit with a mineral composition that indicates desiccation of the lake and erosion by wind. These, in turn, were largely covered by materials deposited by wind, which show very little evidence of liquid water. The last and youngest unit is volcanic in origin and completely devoid of minerals indicative of water. This succession of minerals reflects the evolution of the Martian surface, which was capable to sustain liquid water in the early stages of the planet and lost this ability during a drastic climate change.
    Description: Key Points: We produced a geological map of southeastern Gorgonum Chaos. We find a succession from phyllosilicates to olivine in aeolian deposits. The presence of water decreases over time.
    Keywords: ddc:523
    Language: English
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