17:15 〜 18:45
[MIS19-P01] The role of inter-regolith cohesion in the formation of Martian slope streaks in the Medusae Fossae Formation and its surroundings
キーワード：火星、Slope streaks、Medusae Fossae Formation、Water Equivalent Hydrogen、レゴリス
Martian slope streaks are narrow linear features with a lower albedo than their surroundings [1]. They form continuously even today and therefore have the potential to provide a state of the Martian environment. However, the formation mechanism of the slope streaks is not clear due to the lack of spatial resolution of the Remote Sensing data. One of the hypotheses suggests that the slope streaks are the manifestations on the surface linked to dust avalanches, evidenced by their morphological resemblance and occurrence in areas with low thermal inertia [1]. Additionally, the role of water and other volatiles (e.g. CO2) in the formation of slope streaks has also been discussed [2-4].
To complement this hypothesis, Bhardwaj et al. (2017) [4] found that the global distribution of slope streaks broadly coincides with areas of high-water Equivalent Hydrogen (WEH) abundance observed by the Mars Odessey Neutron Spectrometer (MONS) at 300 km/pix [5]. The involvement of water in the formation of slope streaks on Mars implies the presence of liquid processes, raising implications for hydrological dynamics, potential subsurface water sources, and the overall understanding of Martian geomorphology and environmental conditions. In our study, to clarify the role of WEH on the origin of slope streaks we compared the density of slope streaks in each pixel of the WEH map and verified the involvement of WEH in the formation of slope streaks. The Medusae Fossae Formation and its surroundings were selected as the study area as it covers the highest and lowest hydrogen abundance regions in low latitudes on Mars [6]. Understanding how WEH contributes to the development of slope streaks, we will provide insights into the existing aqueous environment on Mars.
Thermal inertia indicates the size of surface grains on Mars [7]. We found that the craters with thermal inertia values below 160 J m-2 k-1 s-0.5 had slope streaks. Contrastingly, about half of the craters with Thermal inertia values above 160 J m-2 k-1 s-0.5 did not have slope streaks. Additionally, in areas with Thermal inertia below 160 J m-2 k-1 s-0.5, there was a clear negative correlation between slope streak density and WEH abundance.
By utilizing the model proposed by Presley and Christensen in 1997 [7] and presuming a constant Martian atmospheric pressure of 6 hPa, thermal inertia of 160 J m-2 K-1 s-0.5 is indicative of a grain size approximating 40 µm. Conventionally, surface materials are stable at their inherent angle of repose, a proclivity accentuated with diminutive grains. Notably, the angle of repose undergoes a marked ascent for particles below 50 µm, attributed to an augmented intergranular cohesion between grains [8]. The prevalence of slope streaks in regions with particles measuring 40 µm or less, as discerned in this investigation, strongly implies an increased likelihood of their formation in locales where the force of intergranular cohesion is more robust. Furthermore, moisture in soils functions to augment intergranular cohesion, assuming a pivotal role in exerting influence on the stability of surface materials. For a constant evaporation amount of surface moisture, the rate of change in intergranular cohesion would be greater in low WEH abundance areas compared to high WEH abundance areas. The change in intergranular cohesion associated with the phase transition of moisture decreases the angle of repose and causes the descent of excess deposited material, which could have led to the formation of slope streaks.
References: [1] Sullivan R. et al. (2001) JGR Planets., 106 (E10), 23607-23633. [2] Kreslavsky M. A. and Head J. W. (2009) Icarus, 201 (2), 517-527. [3] Schorghofer N. et al. (2002) GRL, 29 (23), 41-1. [4] Bhardwaj A. et al. (2017) Sci reports, 7(1), 1-14. [5] Feldman W. C. et al., (2004), JGR, 109, E09006. [6] Wilson J. T. et al. (2018) Icarus, 299 148-160. [7] Presley M. A. and Christensen P. R. (1997) JGR planets, 102, 6551–6566. [8] Lumay G. et al. (2012) Powder Technol. 224, 19-27
To complement this hypothesis, Bhardwaj et al. (2017) [4] found that the global distribution of slope streaks broadly coincides with areas of high-water Equivalent Hydrogen (WEH) abundance observed by the Mars Odessey Neutron Spectrometer (MONS) at 300 km/pix [5]. The involvement of water in the formation of slope streaks on Mars implies the presence of liquid processes, raising implications for hydrological dynamics, potential subsurface water sources, and the overall understanding of Martian geomorphology and environmental conditions. In our study, to clarify the role of WEH on the origin of slope streaks we compared the density of slope streaks in each pixel of the WEH map and verified the involvement of WEH in the formation of slope streaks. The Medusae Fossae Formation and its surroundings were selected as the study area as it covers the highest and lowest hydrogen abundance regions in low latitudes on Mars [6]. Understanding how WEH contributes to the development of slope streaks, we will provide insights into the existing aqueous environment on Mars.
Thermal inertia indicates the size of surface grains on Mars [7]. We found that the craters with thermal inertia values below 160 J m-2 k-1 s-0.5 had slope streaks. Contrastingly, about half of the craters with Thermal inertia values above 160 J m-2 k-1 s-0.5 did not have slope streaks. Additionally, in areas with Thermal inertia below 160 J m-2 k-1 s-0.5, there was a clear negative correlation between slope streak density and WEH abundance.
By utilizing the model proposed by Presley and Christensen in 1997 [7] and presuming a constant Martian atmospheric pressure of 6 hPa, thermal inertia of 160 J m-2 K-1 s-0.5 is indicative of a grain size approximating 40 µm. Conventionally, surface materials are stable at their inherent angle of repose, a proclivity accentuated with diminutive grains. Notably, the angle of repose undergoes a marked ascent for particles below 50 µm, attributed to an augmented intergranular cohesion between grains [8]. The prevalence of slope streaks in regions with particles measuring 40 µm or less, as discerned in this investigation, strongly implies an increased likelihood of their formation in locales where the force of intergranular cohesion is more robust. Furthermore, moisture in soils functions to augment intergranular cohesion, assuming a pivotal role in exerting influence on the stability of surface materials. For a constant evaporation amount of surface moisture, the rate of change in intergranular cohesion would be greater in low WEH abundance areas compared to high WEH abundance areas. The change in intergranular cohesion associated with the phase transition of moisture decreases the angle of repose and causes the descent of excess deposited material, which could have led to the formation of slope streaks.
References: [1] Sullivan R. et al. (2001) JGR Planets., 106 (E10), 23607-23633. [2] Kreslavsky M. A. and Head J. W. (2009) Icarus, 201 (2), 517-527. [3] Schorghofer N. et al. (2002) GRL, 29 (23), 41-1. [4] Bhardwaj A. et al. (2017) Sci reports, 7(1), 1-14. [5] Feldman W. C. et al., (2004), JGR, 109, E09006. [6] Wilson J. T. et al. (2018) Icarus, 299 148-160. [7] Presley M. A. and Christensen P. R. (1997) JGR planets, 102, 6551–6566. [8] Lumay G. et al. (2012) Powder Technol. 224, 19-27