14:15 〜 14:30
[MIS20-03] Origin of slope streaks and its relationship to hydrogen abundances and thermal inertia on Mars: a case of Medusae Fossae Formation
キーワード：火星、Slope streaks、火星地質学
Slope streaks are a geological phenomenon on the slopes. Morphologically, it is a narrow linear feature showing an albedo considerably darker than its surrounding (Kreslavsky and Head, 2009). They are mainly located on steep slopes in dusty regions in the mid to low latitudes (between latitudes ± 40°) of Mars (Sullivan et al., 2001). There are multiple hypotheses related to the formation process of slope streaks, including a water-free mechanism (dry mechanism) related to the descent along steep slopes of fine-grained particles, such as sand avalanches (Sullivan et al., 2001). The avalanche strips away the bright dust layer from the surface, exposing a darker layer from below. On the other hand, one argues the role of water and other volatiles in the formation of streaks and shows that the distribution of slope streaks coincides with areas of high concentration of Water Equivalent Hydrogen (WEH) (Bhardwaj et al., 2017). However, the physical relationship between WEH and slope streaks has not been clear. This study aims to reveal the substantive relationship between slope streaks and WEH to understand the formation mechanism.
We compared the number of slope streaks in and around the Medusae Fossae Formation (MFF). The MFF is located in the equatorial region (135°E - 257°E, 15°N - 15°S), and the region has characteristics of a high slope streak density distribution (Ferris et al., 2002). This region shows a high abundance of WEH (Wilson et al., 2018) by epithermal neutron data from the Gamma-Ray Spectrometer onboard both on Mars Odyssey (Boynton et al., 2004) and Fine Resolution Epithermal Neutron Detector (Mitrofanov et al., 2018) data onboard the ExoMars Trace Gas Orbiter.
We have identified 11,697 slope streaks in 91 areas and measured the thermal inertia of 317 craters in the MFF. We found a median value of thermal inertia of 160 J m-2 k-1 s-0.5, which means that half of the craters with thermal inertia values above 160 J m-2 k-1 s-0.5 do not have slope streaks. This indicates that slope streaks would occur differently at thermal inertia below and above 160 J m-2 k-1 s-0.5. In the areas with thermal inertia of > 160 J m-2 k-1 s-0.5, WEH abundances and slope streak density show a clear negative correlation: the lower the WEH, the higher the slope streak density.
The identity of the substance related to high WEH is unknown, but the non-detection of hydrated salts in the MFF (Carter et al., 2013) suggests that the WEH signal indicates the possible presence of H2O-ice. The negative correlation between slope streak density and WEH abundances suggests that the existence of moisture inhibited the formation of slope streaks by intergranular cohesion. This implication can support the particle flow (dry ) mechanism. The initiation of particle movement occurs when the threshold shear stress exceeds the resisting gravitational stress, and the initiation of movement is often defined as a situation where the shear velocity exceeds the threshold shear velocity (Ellis and Sherman, 2013). The threshold shear velocity calculated with the WEH data negatively correlates with the slope streak density. This negative correlation suggests that moisture on the Martian surface increases the threshold shear velocity and depresses the sand movement, resulting in the low slope streak density. Since shear velocity indicates the likelihood of initiating particle movement, this study corroborates the hypothesis of a dry mechanism, such as particle flow initiated with grain movement.
We compared the number of slope streaks in and around the Medusae Fossae Formation (MFF). The MFF is located in the equatorial region (135°E - 257°E, 15°N - 15°S), and the region has characteristics of a high slope streak density distribution (Ferris et al., 2002). This region shows a high abundance of WEH (Wilson et al., 2018) by epithermal neutron data from the Gamma-Ray Spectrometer onboard both on Mars Odyssey (Boynton et al., 2004) and Fine Resolution Epithermal Neutron Detector (Mitrofanov et al., 2018) data onboard the ExoMars Trace Gas Orbiter.
We have identified 11,697 slope streaks in 91 areas and measured the thermal inertia of 317 craters in the MFF. We found a median value of thermal inertia of 160 J m-2 k-1 s-0.5, which means that half of the craters with thermal inertia values above 160 J m-2 k-1 s-0.5 do not have slope streaks. This indicates that slope streaks would occur differently at thermal inertia below and above 160 J m-2 k-1 s-0.5. In the areas with thermal inertia of > 160 J m-2 k-1 s-0.5, WEH abundances and slope streak density show a clear negative correlation: the lower the WEH, the higher the slope streak density.
The identity of the substance related to high WEH is unknown, but the non-detection of hydrated salts in the MFF (Carter et al., 2013) suggests that the WEH signal indicates the possible presence of H2O-ice. The negative correlation between slope streak density and WEH abundances suggests that the existence of moisture inhibited the formation of slope streaks by intergranular cohesion. This implication can support the particle flow (dry ) mechanism. The initiation of particle movement occurs when the threshold shear stress exceeds the resisting gravitational stress, and the initiation of movement is often defined as a situation where the shear velocity exceeds the threshold shear velocity (Ellis and Sherman, 2013). The threshold shear velocity calculated with the WEH data negatively correlates with the slope streak density. This negative correlation suggests that moisture on the Martian surface increases the threshold shear velocity and depresses the sand movement, resulting in the low slope streak density. Since shear velocity indicates the likelihood of initiating particle movement, this study corroborates the hypothesis of a dry mechanism, such as particle flow initiated with grain movement.