3:30 PM - 5:00 PM
[PPS05-P20] Effects of hydrogen content on surface flow on inner wall of the Martian craters
Keywords:Mars, crater, surface, flow, hydrogen
Observations of epithermal neutrons by Mars Odyssey Neutron Spectrometer have revealed that there is spatial heterogeneity in the subsurface hydrogen distribution on Mars (Feldman et al., 2002, Wilson et al., 2018). These subsurface hydrogens may affect topographic degradation processes on Mars. On the inner slopes of craters on Mars, there are linear topographic features that indicate the flow of particles toward the crater center. The purpose of this study is to evaluate the effect of the hydrogen content on the fluidity of these slopes. For this purpose, we focus on small craters with diameters ranging from a few to several tens of meters on the inner slopes of large craters with diameters of about 10 km on Mars, and investigated how the number density of these small craters changes with hydrogen content.
Based on the subsurface hydrogen distribution map (Wilson et al., 2018), we selected large craters on the regions with high and low hydrogen contents and counted small craters on their inner slopes. We also counted small craters on the ejecta blankets of the large craters to estimate the formation age of the large craters. The slope angles of counting areas were calculated from MOLA elevation data.
In almost all of the large craters analyzed in this study, the number densities of small craters on the slopes is lower than those on the ejecta blankets. This result indicates that the small craters have been erased by surface flows on the slopes. To understand the flow processes on the slopes, we investigated the relationships between the number densities of small craters and the slope angles and between the number densities and the formation age of large craters.
If the formation and erasure rates of small craters on the slopes are in equilibrium, the number density of small craters is expected to depend on the slope angle because on steeper slope the erasure rate of small craters becomes faster. However, there is no clear dependence of the number density of small craters on steepness of slopes. On the other hand, if the formation rate of small craters exceeds the erasure rate, the older the formation age of the large crater is, the higher the number density of small craters on the slope should be. However, such a trend was not observed.
Considering the possibility that the topographic degradation processes may differ from region to region, we evaluated only the effects of surface flow on the slopes by calculating the ratio of the number density of small craters on the slope to that on the ejecta blanket. The results show that the slope/ejecta ratio of small crater density tends to be higher in hydrogen-rich regions than hydrogen-poor regions, suggesting that small craters may be less likely to be erased on slopes with hydrogen-rich regions. An experimental study have indicated that the development of water ice tendril structures between regolith particles can bind them together (Siegler et al., 2022). Our results may indicate that the adsorption of water ice among regolith particles in the hydrogen-rich regions caused them to lose their fluidity.
Based on the subsurface hydrogen distribution map (Wilson et al., 2018), we selected large craters on the regions with high and low hydrogen contents and counted small craters on their inner slopes. We also counted small craters on the ejecta blankets of the large craters to estimate the formation age of the large craters. The slope angles of counting areas were calculated from MOLA elevation data.
In almost all of the large craters analyzed in this study, the number densities of small craters on the slopes is lower than those on the ejecta blankets. This result indicates that the small craters have been erased by surface flows on the slopes. To understand the flow processes on the slopes, we investigated the relationships between the number densities of small craters and the slope angles and between the number densities and the formation age of large craters.
If the formation and erasure rates of small craters on the slopes are in equilibrium, the number density of small craters is expected to depend on the slope angle because on steeper slope the erasure rate of small craters becomes faster. However, there is no clear dependence of the number density of small craters on steepness of slopes. On the other hand, if the formation rate of small craters exceeds the erasure rate, the older the formation age of the large crater is, the higher the number density of small craters on the slope should be. However, such a trend was not observed.
Considering the possibility that the topographic degradation processes may differ from region to region, we evaluated only the effects of surface flow on the slopes by calculating the ratio of the number density of small craters on the slope to that on the ejecta blanket. The results show that the slope/ejecta ratio of small crater density tends to be higher in hydrogen-rich regions than hydrogen-poor regions, suggesting that small craters may be less likely to be erased on slopes with hydrogen-rich regions. An experimental study have indicated that the development of water ice tendril structures between regolith particles can bind them together (Siegler et al., 2022). Our results may indicate that the adsorption of water ice among regolith particles in the hydrogen-rich regions caused them to lose their fluidity.