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  • ddc:523  (15)
  • ddc:550.78  (7)
  • English  (22)
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  • 2020-2023  (22)
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  • 1
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    Formation of chondrule analogs aboard the International Space Station (2021)
    Details
    Publication Date: 2022-03-30
    Description: Chondrules are thought to play a crucial role in planet formation, but the mechanisms leading to their formation are still a matter of unresolved discussion. So far, experiments designed to understand chondrule formation conditions have been carried out only under the influence of terrestrial gravity. In order to introduce more realistic conditions, we developed a chondrule formation experiment, which was carried out at long‐term microgravity aboard the International Space Station. In this experiment, freely levitating forsterite (Mg2SiO4) dust particles were exposed to electric arc discharges, thus simulating chondrule formation via nebular lightning. The arc discharges were able to melt single dust particles completely, which then crystallized with very high cooling rates of 〉105 K h−1. The crystals in the spherules show a crystallographic preferred orientation of the [010] axes perpendicular to the spherule surface, similar to the preferred orientation observed in some natural chondrules. This microstructure is probably the result of crystallization under microgravity conditions. Furthermore, the spherules interacted with the surrounding gas during crystallization. We show that this type of experiment is able to form spherules, which show some similarities with the morphology of chondrules despite very short heating pulses and high cooling rates.
    Description: Carl Zeiss Meditec AG http://dx.doi.org/10.13039/501100002806
    Description: BIOVIA Science Ambassador program
    Description: Bundesministerium für Wirtschaft und Energie http://dx.doi.org/10.13039/501100006360
    Description: Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946
    Description: NanoRacks LLC
    Description: DreamUp
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Dr. Rolf M. Schwiete Stiftung
    Keywords: ddc:549 ; ddc:550.78
    Language: English
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  • 2
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    Joint Inversion of Receiver Functions and Apparent Incidence Angles for Sparse Seismic Data (2021)
    Details
    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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  • 3
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    Paleoclimate Evolution on Titan After Episodic Massive Methane Outgassing Simulated by a Global Climate Model (2021)
    Details
    Publication Date: 2022-03-25
    Description: Titan's paleoclimate after the onset of the putative last major methane outgassing event 700 Myr ago is simulated by a global climate model. If the atmosphere was methane‐depleted prior to outgassing, outgassed methane initially causes warming due to increased greenhouse effect. Further outgassing leads to methane snowfall, which in turn cools the troposphere and surface by an ice‐albedo feedback and thereby initiates a lengthy ice age. Formation of ice sheets begins in the polar region, but with increasing methane inventory the entire globe is eventually covered by surface methane frost as thick as 100 m, with local accumulation on elevated terrains. Among various time‐dependent input parameters the methane inventory by far exerts the greatest control over the climate evolution. As Titan's climate transitions from a dry state via a partially ice‐covered state to a globally ice‐covered state, the circulation and precipitation pattern change profoundly and the tropospheric temperature further decreases. Globally ice‐covered snowball Titan is characterized by weak meridional circulation, weak seasonality and widespread snowfall. Frost ablation begins after the end of outgassing due to photochemical destruction of atmospheric methane. It is conceivable that Titan's polar seas resulted from melting of the polar caps within the past 10 Myr and subsequent drainage to the polar basins. Surface methane frost could only melt when the frost retreated to the polar region, which led to global warming by lowering of the surface albedo at low latitudes and increased greenhouse effect.
    Description: Plain Language Summary: Saturn's moon Titan may have experienced long periods of cold climate in the past when the nitrogen atmosphere contained no methane unlike the present atmosphere. We simulated how Titan's climate may have changed when large amounts of methane were outgassed into such a cold atmosphere as indicated by models of Titan's evolution. The atmosphere can hold a certain amount of methane but the vast majority of outgassed methane condenses out as snow and is deposited on the surface. Bright methane snow on the surface keeps the surface cold and thereby prevents efficient greenhouse warming. Initially, surface methane frost is confined to high latitudes, but eventually the entire globe will be ice‐covered under the assumed total amount of outgassed methane. The seasonal and global pattern of atmospheric circulation and snowfall strongly depend on the degree of frost coverage. The surface frost sublimes away long after outgassing has ceased because methane is destroyed in the atmosphere by photochemistry. Eventually, the polar caps melt, leaving behind the observed polar seas.
    Description: Key Points: Massive methane outgassing into Titan's atmosphere should have caused global ice sheets if the atmosphere was previously depleted in methane. Climate of methane snowball Titan is characterized by weak circulation, low temperature, weak seasonality and widespread snowfall. Melting polar caps in geologically recent past may have resulted in polar seas.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:523 ; ddc:551.5
    Language: English
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  • 4
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    The Spectrum of Slip Behaviors of a Granular Fault Gouge Analogue Governed by Rate and State Friction (2021)
    Details
    Publication Date: 2022-03-25
    Description: Currently, it is unknown how seismic and aseismic slip influences the recurrence and magnitude of earthquakes. Modern seismic hazard assessment is therefore based on statistics combined with numerical simulations of fault slip and stress transfer. To improve the underlying statistical models we conduct low velocity shear experiments with glass micro‐beads as fault gouge analogue at confining stresses of 5–20 kPa. As a result, we show that characteristic slip events emerge, ranging from fast and large slip to small scale oscillating creep and stable sliding. In particular, we observe small scale slip events that occur immediately before large scale slip events for a specific set of experiments. Similar to natural faults we find a separation of scales by several orders of magnitude for slow events and fast events. Enhanced creep and transient dilatational events pinpoint that the granular analogue is close to failure. From slide‐hold‐slide tests, we find that the rate‐and‐state properties are in the same range as estimates for natural faults and fault rocks. The fault shows velocity weakening characteristics with a reduction of frictional strength between 0.8% and 1.3% per e‐fold increase in sliding velocity. Furthermore, the slip modes that are observed in the normal shear experiments are in good agreement with analytical solutions. Our findings highlight the influence of micromechanical processes on macroscopic fault behavior. The comprehensive data set associated with this study can act as a benchmark for numerical simulations and improve the understanding of observations of natural faults.
    Description: Plain Language Summary: Earthquakes occur when two continental plates slide past each other. The motion is concentrated at the interface of the two plates which is called a fault. In many cases the fault is filled with granular material, called gouge, that supports the pressure between the plates. Therefore, the properties of this gouge determine how fast and how large an earthquake can be. It also has an influence on the time between earthquakes. In our study, we examine a simplified version of a fault gouge in a simple small‐scale model. Instead of rock material we use glass beads and measure how different conditions affect the motion of the model. We find that our model reproduces features of fault gouge because it shows similar behavior. When there is no motion our model fault becomes stronger with a rate equal to fault gouge. Also, the type of strengthening is analogous to fault gouge. During slip, the glass beads become weaker as the slip velocity increases in a similar manner as in natural faults. These results improve the understanding of computer simulations and natural observations.
    Description: Key Points: Slip modes in granular gouge are akin to natural fault slip. Glass beads are a suitable granular analogue for fault gouge and show rate‐and‐state dependent friction. Enhanced creep and small scale events are signals for imminent failure and indicate fault criticality.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Description: 亥姆霍兹联合会致力, Helmholtz‐Zentrum Potsdam ‐ Deutsches GeoForschungsZentrum GFZ (GFZ) http://dx.doi.org/10.13039/501100010956
    Keywords: ddc:550.78
    Language: English
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  • 5
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    Temperatures Near the Lunar Poles and Their Correlation With Hydrogen Predicted by LEND (2021)
    Details
    Publication Date: 2022-03-24
    Description: The lunar polar regions offer permanently shadowed regions (PSRs) representing the only regions which are cold enough for water ice to accumulate on the surface. The Lunar Exploration Neutron Detector (LEND) aboard the Lunar Reconnaissance Orbiter (LRO) has mapped the polar regions for their hydrogen abundance which possibly resides there in the form of water ice. Neutron suppression regions (NSRs) are regions of excessive hydrogen concentrations and were previously identified using LEND data. At each pole, we applied thermal modeling to three NSRs and one unclassified region to evaluate the correlation between hydrogen concentrations and temperatures. Our thermal model delivers temperature estimates for the surface and for 29 layers in the sub‐surface down to 2 m depth. We compared our temperature maps at each layer to LEND neutron suppression maps to reveal the range of depths at which both maps correlate best. As anticipated, we find the three south polar NSRs which are coincident with PSRs in agreement with respective (near)‐surface temperatures that support the accumulation of water ice. Water ice is suspected to be present in the upper ≈19 cm layer of regolith. The three north polar NSRs however lie in non‐PSR areas and are counter‐intuitive as such that most surfaces reach temperatures that are too high for water ice to exist. However, we find that temperatures are cold enough in the shallow sub‐surface and suggest water ice to be present at depths down to ≈35–65 cm. Additionally we find ideal conditions for ice pumping into the sub‐surface at the north polar NSRs. The reported depths are observable by LEND and can, at least in part, explain the existence and shape of the observed hydrogen signal. Although we can substantiate the anticipated correlation between hydrogen abundance and temperature the converse argument cannot be made.
    Description: Plain Language Summary: The lunar poles have quite unique illumination conditions. For instance, the Sun never shines on some crater floors. As a consequence, the floors of those craters are very cold and dark. Here, water ice can accumulate on the surface and can be preserved for long periods of time. One of the instruments mounted on the Moon‐orbiting satellite Lunar Reconnaissance Orbiter is capable of detecting areas where hydrogen is located, which is assumed to be present in the form of water ice. For instance, the instrument detected several areas at the lunar poles where a lot more water ice is found than at other locations. For these special locations, we calculated the temperatures at the surface and near sub‐surface to see whether they are indeed cold enough for water to freeze. At some of these locations, surface temperatures turn out to be too warm. However, we found that at these warm surfaces where no water ice can exist it can be transported into the sub‐surface and survive there. This mechanism is referred to as ice pumping. In summary, we could show that temperatures at all these special locations are usually cold enough for water ice, either right at the surface or within the first meter of soil.
    Description: Key Points: Some neutron suppression regions (NSRs) form from surface ice deposits while others may form through ice pumping in the sub‐surface. NSRs identified by Lunar Exploration Neutron Detector correlate well with low surface temperatures in permanently shadowed regions (PSRs) and are in agreement with sub‐surface temperatures in non‐PSR.
    Description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659
    Keywords: ddc:523 ; ddc:551.5
    Language: English
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  • 6
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    Directionality of the Martian Surface Radiation and Derivation of the Upward Albedo Radiation (2021)
    Details
    Publication Date: 2022-03-24
    Description: Since 2012 August, the Radiation Assessment Detector (RAD) on the Curiosity rover has been characterizing the Martian surface radiation field which is essential in preparation for future crewed Mars missions. RAD observed radiation dose is influenced by variable topographical features as the rover traverses through the terrain. In particular, while Curiosity was parked near a butte in the Murray Buttes area, we find a decrease of the dose rate by (5 ± 1)% as 19% of the sky was obstructed, versus 10% in an average reference period. Combining a zenith‐angle‐dependent radiation model and the rover panoramic visibility map leads to a predicted reduction of the downward dose by ∼12% due to the obstruction, larger than the observed decrease. With the hypothesis that this difference is attributable to albedo radiation coming from the butte, we estimate the (flat‐terrain) albedo radiation to be about 19% of the total surface dose.
    Description: Plain Language Summary: Interplanetary space is filled with energetic particles that can affect the health of astronauts, for example, by causing late‐arising cancer and possibly hereditary diseases. Mars lacks a global magnetic field and its atmosphere is very thin compared to Earth's. Thus its surface is exposed to such space radiation which presents risks to future humans on Mars. Mitigation strategies could include using natural geological structures on Mars, for example, cave skylights and lava tubes and even simple buttes, for protection. The Radiation Assessment Detector (RAD) on the Curiosity rover has observed a decrease of the radiation absorbed dose rate by (5 ± 1)% while Curiosity was parked near a butte. This provides the first direct illustration that Mars's surface features may serve as potential radiation shelters for future missions. However, when exploiting such shielding possibilities, the secondary radiation generated in the terrain of Mars that is, emitted backwards must also be considered. Combining the RAD observation with a radiation transport model, we derive such “reflected” radiation dose on a flat terrain to be about 19% of the total surface dose.
    Description: Key Points: The Martian surface radiation is influenced by topographical features. The surface downward radiation dose of particles traversing through the atmosphere depends on the zenith angle. The surface upward radiation dose is about 19% of the total dose.
    Description: Strategic Priority Program of CAS
    Description: NSFC
    Description: CNSA pre‐research project on civil aerospace technologies
    Description: NASA, Jet Propulsion Laboratory (JPL) http://dx.doi.org/10.13039/100006196
    Description: Deutsches Zentrum für Luft‐und Raumfahrt (DLR) http://dx.doi.org/10.13039/501100002946
    Keywords: ddc:523
    Language: English
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  • 7
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    Induced Magnetic Fields and Plasma Motions in the Inner Part of the Martian Magnetosphere (2021)
    Details
    Publication Date: 2022-03-24
    Description: Analysis of Mars Atmosphere and Volatile Evolution (MAVEN)/Supra‐Thermal And Thermal Ion Composition observations in the Martian upper atmosphere, bounded at higher altitudes by the shocked solar wind, shows that the draping of interplanetary magnetic field penetrates down to low altitudes (∼200−250 km) and governs dynamics of the ionosphere. The upper ionospheric plasma is driven into motion flowing around Mars similar to the shocked solar wind in the adjacent magnetosheath. Such a fluid‐like motion is accompanied by ion acceleration caused by the bending of the magnetic field, leading to ion extraction and finally to ion pickup. Extraction of ions and their acceleration produces a recoil effect of the bulk ionosphere in the opposite direction. This provides a strong asymmetry in ion dynamics in two different hemispheres, accompanied by wrapping of the magnetic field lines around Mars and respective reconnection.
    Description: Plain Language Summary: Although the Martian magnetosphere is hybrid and contains components of the induced and intrinsic magnetosphere, is possible to display these components by using the specific coordinate systems. Here we study the properties of the induced magnetosphere using the data obtained by MAVEN spacecraft. The interplanetary magnetic field penetrates deep into the Martian ionosphere draping around Mars and drive to the motion dense ionospheric plasma. Draping features and the induced plasma motions occur different in two hemispheres determined by the direction of the motional electric field in the solar wind. Ion acceleration and extraction is accompanied by a recoil effect that leads to a shift and asymmetry of the ionosphere.
    Description: Key Points: Draping of the interplanetary magnetic field around Mars penetrates deep to the ionosphere enveloping the planet and driving the ionosphere to the bulk motion. Draping and motion of the ionospheric plasma is characterized by asymmetry by the direction of the motional electric field in solar wind. Ion acceleration and extraction from the ionosphere is accompanied by a shift of the bulk ionosphere in the opposite direction.
    Description: National Aeronautics and Space Administration http://dx.doi.org/10.13039/100000104
    Description: DFG http://dx.doi.org/10.13039/501100001659
    Description: Russian Science Foundation http://dx.doi.org/10.13039/501100006769
    Keywords: ddc:523 ; ddc:551.5
    Language: English
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  • 8
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    Rheological and Mechanical Layering of the Crust Underneath Thumbprint Terrains in Arcadia Planitia, Mars (2021)
    Details
    Publication Date: 2022-03-28
    Description: In the area of Arcadia Planitia in the Northern hemisphere of Mars, mounds indicating fluid and sediment emissions have been already recognized. Here, we show that through fractal and fracture‐spacing analyses of a large vent population it is possible to infer the mechanical layering of the underlying subsurface. Our work includes the mapping of an entire population of 9,028 vents over an area of 122,000 km2. The analysis of mound distribution at the surface led to the formulation of inferences about the subsurface feeding conduits, and to the identification of three mechanical discontinuities at c. 4–5, c. 14–23, and c. 50–55 km. This evidence matches the mechanical stratigraphy recorded by the InSight NASA mission, and is in agreement with independent previous subsurface global modeling, supporting our conclusions.
    Description: Plain Language Summary: The Martian northern hemisphere displays mounds interpreted to be the result of sediment and water erupting onto the surface. We analyzed the mounds spatial distribution and found patterns that reflects the extent at depth of the subsurface conduits that fed those mounds (array of fractures, i.e., high permeability pathways) allowing the sediment and water upwelling. These conduits thus connect the surface to the source of the erupted materials at depth. These source levels are located at the base of layers characterized by mechanical properties different from the adjacent ones (e.g., loose sediments vs. crystalline bedrock). Such layers are hence referred as mechanical discontinuities. We identified three discontinuities: at c. 4–5, c. 14–23, and c. 50–55 km. Our outcomes match the mechanical stratigraphy recorded by the InSight NASA mission, and is in agreement with independent previous subsurface global modeling, supporting our conclusions.
    Description: Key Points: We present a complete mapping of a large vent population in the Arcadia Planitia region of the northern plains of Mars. We reconstructed the subsurface mechanical layering underlying the vent field using spatial distribution analysis. These analyses proved to be efficient and open the possibility of collecting subsurface rheological data from areas beyond InSight reach.
    Description: H2020 Excellent Science (H2020 Priority Excellent Science) http://dx.doi.org/10.13039/100010662
    Description: DLR Management Board Young Research Group Leader Program
    Description: Executive Board Member for Space Research and Technology
    Keywords: ddc:523
    Language: English
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  • 9
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    Effects of CO2 and Ocean Mixing on Miocene and Pliocene Temperature Gradients (2022)
    Details
    Publication Date: 2022-03-28
    Description: Cenozoic climate changes have been linked to tectonic activity and variations in atmospheric CO2 concentrations. Here, we present Miocene and Pliocene sensitivity experiments performed with the climate model COSMOS. The experiments contain changes with respect to paleogeography, ocean gateway configuration, and atmospheric CO2 concentrations, as well as a range of vertical mixing coefficients in the ocean. For the mid‐Miocene, we show that the impact of ocean mixing on surface temperature is comparable to the effect of the possible range in reconstructed CO2 concentrations. In combination with stronger vertical mixing, relatively moderate CO2 concentrations of 450 ppmv enable global‐mean surface, deep‐water, and meridional temperature characteristics representative of mid‐Miocene Climatic Optimum (MMCO) reconstructions. The Miocene climate shows a reduced meridional temperature gradient and reduced seasonality. In the case of enhanced mixing, surface and deep ocean temperatures show significant warming of up to 5–10°C and an Arctic temperature anomaly of 〉12°C. In the Pliocene simulations, the impact of vertical mixing and CO2 is less important for the deep ocean, which we interpret as a different sensitivity dependence on the background state and mixed layer dynamics. We find a significant reduction in surface albedo and effective emissivity for either a high level of atmospheric CO2 or increased vertical mixing. Our mixing sensitivity experiments provide a warm deep ocean via ocean heat uptake. We propose that the mixing hypothesis can be tested by reconstructions of the thermocline and seasonal paleoclimate data indicating a lower seasonality relative to today.
    Description: Plain Language Summary: Cenozoic climate changes have been associated with tectonic changes and altered atmospheric CO2 concentrations. Here, we present Miocene and Pliocene computer simulations where we changed paleogeography, ocean gateways, and atmospheric CO2 concentrations as well as vertical mixing in the ocean. We show that the effect of ocean mixing on temperature is comparable to the respective effect of a possible range of CO2 concentrations. In combination with stronger vertical mixing, relatively moderate CO2 concentrations of 450 ppmv allow surface and deep‐water temperatures representative for reconstructions of the climate optimum of the mid‐Miocene. In the Pliocene simulations, the influence of vertical mixing and CO2 is less important than in the Miocene. We provide a possible mechanism of ocean heat absorption, albedo, and emissivity changes including a deeper oceanic mixing layer and a lower seasonality in the Miocene compared to today.
    Description: Key Points: Miocene experiment with standard mixing and atmospheric CO2 of 600 ppm captures large‐scale temperature characteristics of the mid‐Miocene. With enhanced ocean mixing the temperature characteristics and meridional temperature gradient can be reproduced with a CO2 level of 450 ppm. Miocene shows a strong warming at polar latitudes and reduced seasonality, vertical mixing, and CO2 are less important for the Pliocene.
    Description: Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) http://dx.doi.org/10.13039/501100003207
    Description: Helmholtz Association (亥姆霍兹联合会致力) http://dx.doi.org/10.13039/501100009318
    Description: Helmholtz Climate Initiative RE‐KLIM
    Keywords: ddc:550.78
    Language: English
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  • 10
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    Formation of the Thebe Extension in the Ring System of Jupiter (2021)
    Details
    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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  • 11
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    Global Drivers and Transport Mechanisms of Lunar Rockfalls (2021)
    Details
    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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  • 12
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    Geological History of Southeastern Gorgonum Chaos, Mars: A Story of Water and Wind (2021)
    Details
    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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    Pore Space Topology Controls Ultrasonic Waveforms in Dry Volcanic Rocks (2022)
    Details
    Publication Date: 2022-12-06
    Description: Pore space controls the mechanical and transport properties of rocks. At the laboratory scale, seismic modeling is usually performed in relatively homogeneous settings, and the influence of the pore space on the recorded wavefields is determined by rock‐fluid interactions. Understanding this influence in dry rocks is instrumental for assessing the impact of pore topology on waves propagating in heterogeneous environments, such as volcanoes. Here, we simulated the propagation of shear waves as a function of pore space parameters in computational models built as proxies for volcanic rocks. The spectral‐element simulations provide results comparable with ultrasonic experiments, and the outcome shows that the size, shape, volume, and location of pores impact amplitudes and phases. These variations intensify in waveform coda after multiple scattering. Our results confirm that pore topology is one of the primary regulators of the propagation of elastic waves in dry rocks regardless of porosity.
    Description: Plain Language Summary: Pores control the non‐elastic behavior and, in general, the petrophysical and mechanical properties of rocks. Such properties are essential to assess potential resources such as aquifers and reservoirs or hazards posed by earthquakes, volcanoes, and constructions. The factors controlling the elasticity of rocks are texture, pore space and the fluids filling the pores. While volcanoes represent a key target for rock characterization, measuring and modeling these factors in volcanic rocks remains challenging due to their intrinsic heterogeneities. In this study, we analyzed how pore space parameters influence the overall elastic properties of rocks by changing one parameter at a time. We created synthetic samples and performed computational simulations that show the individual contribution of the amount, size, location, and shape on waveform phases and amplitudes. The findings demonstrate that we can constrain the pore space in heterogeneous rocks in simple but realistic scenarios. Our results are the first step to provide computationaly‐driven forward models of seismic signals in heterogeneous volcanic media, necessary to predict the responses of volcanic rocks to stress.
    Description: Key Points: Computational modeling quantifies the influence of pore space topology on S‐wave propagation in volcanic rocks. Amount, size and location of pores impact ultrasonic wave propagation in dry rocks independently of porosity. Path effects dominate the waveforms and depend on the location of the pores.
    Description: https://doi.org/10.17632/b5p54xtvv9.3
    Keywords: ddc:550.78 ; volcanic rocks ; pore space topology ; S-wave propagation ; computational modeling
    Language: English
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  • 14
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    Melt Production and Ejection From Lunar Intermediate‐Sized Impact Craters: Where Is the Molten Material Deposited? (2022)
    Details
    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
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  • 15
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    Formation of fused aggregates under long‐term microgravity conditions aboard the ISS with implications for early solar system particle aggregation (2022)
    Koch, Tamara E. ; Spahr, Dominik ; Tkalcec, Beverley J. ; [et al.]
    Details
    Publication Date: 2022-10-13
    Description: In order to gain further insights into early solar system aggregation processes, we carried out an experiment on board the International Space Station, which allowed us to study the behavior of dust particles exposed to electric arc discharges under long‐term microgravity. The experiment led to the formation of robust, elongated, fluffy aggregates, which were studied by scanning electron microscopy, electron backscatter diffraction, and synchrotron micro‐computed tomography. The morphologies of these aggregates strongly resemble the typical shapes of fractal fluffy‐type calcium‐aluminum‐rich inclusions (CAIs). We conclude that a small amount of melting could have supplied the required stability for such fractal structures to have survived transportation and aggregation to and compaction within planetesimals. Other aggregates produced in our experiment have a massy morphology and contain relict grains, likely resulting from the collision of grains with different degrees of melting, also observed in some natural CAIs. Some particles are surrounded by igneous rims, which remind in thickness and crystal orientation of Wark–Lovering rims; another aggregate shows similarities to disk‐shaped CAIs. These results imply that a (flash‐)heating event with subsequent aggregation could have been involved in the formation of different morphological CAI characteristics.
    Description: BIOVIA
    Description: Nordlicht GmbH
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: Bundesministerium für Wirtschaft und Energie http://dx.doi.org/10.13039/501100006360
    Description: NanoRacks LLC
    Description: Dr. Rolf M. Schwiete Stiftung http://dx.doi.org/10.13039/501100020027
    Description: Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946
    Description: DreamUp
    Description: Carl Zeiss Meditec AG http://dx.doi.org/10.13039/501100002806
    Keywords: ddc:550.78
    Language: English
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  • 16
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    Asteroid 2008 TC3, not a polymict ureilitic but a polymict C1 chondrite parent body? Survey of 249 Almahata Sitta fragments (2022)
    Details
    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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    Spatio‐Temporal Level Variations of the Martian Seasonal South Polar Cap From Co‐Registration of MOLA Profiles (2022)
    Details
    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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    Mapping the Brightness of Ganymede's Ultraviolet Aurora Using Hubble Space Telescope Observations (2022)
    Details
    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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    Strain Signals Governed by Frictional‐Elastoplastic Interaction of the Upper Plate and Shallow Subduction Megathrust Interface Over Seismic Cycles (2022)
    Details
    Publication Date: 2022-10-06
    Description: The behavior of the shallow portion of the subduction zone, which generates the largest earthquakes and devastating tsunamis, is still insufficiently constrained. Monitoring only a fraction of a single megathrust earthquake cycle and the offshore location of the source of these earthquakes are the foremost reasons for the insufficient understanding. The frictional‐elastoplastic interaction between the megathrust interface and its overlying wedge causes variable surface strain signals such that the wedge strain patterns may reveal the mechanical state of the interface. To contribute to this understanding, we employ Seismotectonic Scale Modeling and simplify elastoplastic megathrust subduction to generate hundreds of analog seismic cycles at a laboratory scale and monitor the surface strain signals over the model's forearc across high to low temporal resolutions. We establish two compressional and critical wedge configurations to explore the mechanical and kinematic interaction between the shallow wedge and the interface. Our results demonstrate that this interaction can partition the wedge into different segments such that the anelastic extensional segment overlays the seismogenic zone at depth. Moreover, the different segments of the wedge may switch their state from compression/extension to extension/compression domains. We highlight that a more segmented upper plate represents megathrust subduction that generates more characteristic and periodic events. Additionally, the strain time series reveals that the strain state may remain quasi‐stable over a few seismic cycles in the coastal zone and then switch to the opposite mode. These observations are crucial for evaluating earthquake‐related morphotectonic markers and short‐term interseismic time series of the coastal regions.
    Description: Key Points: Analog earthquake cycle experiments provide observations to evaluate the surface strain signals from the shallow megathrust. The extensional segment of the forearc overlays the seismogenic zone at depth. The strain state may remain quasi‐stable over a few seismic cycles in the coastal zone.
    Description: SUBITOP Marie Sklodowska‐Curie Action project from the European Union's EU Framework Programme
    Description: Deutsche Forschungsgemeinschaft (CRC 1114) “Scaling Cascades in Complex Systems”
    Description: https://doi.org/10.5880/fidgeo.2022.015
    Keywords: ddc:551.8 ; ddc:550.78
    Language: English
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    Alternating Emission Features in Io's Footprint Tail: Magnetohydrodynamical Simulations of Possible Causes (2022)
    Details
    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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    From the Top of Martian Olympus to Deep Craters and Beneath: Mars Radiation Environment Under Different Atmospheric and Regolith Depths (2022)
    Details
    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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    Scaling Laws in Aeolian Sand Transport Under Low Sand Availability (2022)
    Details
    Publication Date: 2022-11-01
    Description: The transport of sand by wind shapes the Earth's surface and constitutes one major factor for the emission of dust aerosols. The accurate modeling of wind‐blown sand transport is thus important to achieve reliable climate simulations and to make predictions about the propagation of desertification. Previous models of wind‐blown sand were designed to compute sand transport rates over a thick sand layer, such as the surface of large, active sand dunes. However, natural soils encompass a broad range of low sand availability conditions, such as crusted or bare soils. It has been a long‐standing open question how wind‐blown sand transport rates respond to wind velocity when the bare ground is covered by a thin layer of sand. Here we calculate the trajectories of wind‐blown sand grains and find that sand transport rates increase faster with wind speed under low sand availability conditions than over sand dunes. The reason for this behavior is elucidated in our simulations: The hopping sand grains fly higher the less sand is covering the hard surface. We obtain mathematical expressions for the sand transport rates as a function of the thickness of sand covering the bare soil, which will be important to improve climate models.
    Description: Key Points: We introduce a particle‐based model in investigating Aeolian (wind‐blown) sand transport when the sand cover on the soil is sparse. The scaling of the Aeolian transport rate with the wind shear velocity has a dependency on the sand cover thickness. There is an anomaly in the functional dependence of the transport rate on the sand cover thickness, depending on the rigid ground roughness.
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://doi.org/10.6084/m9.figshare.19469501
    Keywords: ddc:550.78
    Language: English
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