50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

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DARK STREAK FEATURES IN MONGOLIA AS TERRESTRIAL ANALOGUES OF RECURRENTSLOPE LINEAE ON MARS M. Nakamura1,2, Y. Sekine1, K. Fukushi3, N. Hasebe3, D. Davaadorj4, Y. Takahashi2,
H. Hasegawa5, T. Ogura6, K. Morida3, 1Earth-Life Sci. Inst., Tokyo Inst. Tech., 2Dept. Earth Planet. Sci., Univ. Tokyo, 3Inst. Nat. Environ. Tech., Kanazawa Univ., 4Dept. Geograph., Nation. Univ. Mongolia, 5Faculty Sci. Tech.,Kochi Univ., 6Dept. Soc.-Cul. Environ. Sci., Univ. Tokyo
Introduction: Recurrent Slope Lineae (RSL) are
dark, narrow features that appear and lengthen on steep(~30°) slopes in warm seasons and fade in cold seasonsof Mars [1,2,3]. Although the formation mechanism of
RSL is still in debate, several processes have been proposed, including liquid brine flows triggered by melting of subsurface ice (wet processes) [4,5], and granular flows (dry processes) [2,6]. However, neither wet
nor dry processes can explain all of the observables ofRSL. In especially, the wet processes seem to be inconsistent with the absence of absorptions due to liquidwater on RSL and the fact that RSL appear only on
steep slopes [2].
A bottleneck that makes the formation mechanism
remain unclear is that RSL have been investigatedmainly through remote-sensing data of Mars. Here, wereport similar dark streak features on Earth in cold,
arid areas of central Mongolia. We conduct geologicalsurvey for the terrestrial RSL analogue sites to construct the high-resolution three-dimensional structure
and to analyze soil samples. Based on the formation
mechanism of the terrestrial RSL analogues, we discuss the formation of RSL on current Mars, especiallyaiming to explain the absence of absorption of liquidwater and the fact that RSL appear only on steep
slopes in the context of the wet processes of RSL formation.
Dark streak features in Mongolia: We firstsearched for RSL-like dark, narrow features over
Earth’s surface using Google Earth satellite images.
We focused on arid or semi-arid areas because of lessvegetation, which include Namib, Atacama, and Gobideserts. Among these areas, we find RSL-like darkstreak features only in cold and arid, southern part ofKhangai Mountains in central Mongolia (Fig. 1),
where discontinuous permafrost and underground ice
melt seasonally [7]. Comparing the satellite images
taken at different times, we find that dark streak features in this area show seasonal variation similar to
RSL on Mars; namely, the streaks appear in summer
and fade in winter. Although the slope angles of theterrestrial analogues (~10° or greater) are shallow, theobserved similarities in morphology and seasonal activity (Fig. 1) suggests that the streak features in Mongolia can be promising terrestrial analogues of RSL onMars.

Geological survey & analyses: We then conducted geological survey for two sites of the terrestrial RSLanalogues found in Mongolia in the end of August
2018. One (termed, the site-BH) is located at 10-km
north of Bayankhongor at altitude of 2060 m from thesea level, and the other (termed, the site-GB) is locatedat 25-km north of Gurvanbulag at altitude of 2630 mfrom the sea level. Owing to high elevation of the siteGB, a mean surface temperature at the site-GB (~4ºC)
in the end of August was significantly lower than thatat the site-BH (~10ºC). At both of the sites, dark streakfeatures were found on the slopes during the survey
from visual images taken by an unmanned aerial vehicle (i.e., a drone). By analyzing the visual images takenby the drone with a software of PhotoScan, a threedimensional topographic structure was constructed ateach site with a spatial/vertical resolution of 2–9 cm. A
flow accumulation analysis was performed for the obtained topographic structure using a software of QGIS.
In the geological survey, we measured water contents of soils on site on and outside the dark streakfeatures. The mineralogical compositions and grain
size distributions were measured for the collected soil
samples. During the survey, no water flow was observed at the site-BH, whereas water flow occurred on
the slopes at the site-GB possibly due to melting ofunderground ice (see below). Velocity, depth, andwidth of the water flow were also measured on site at
the site-GB. The chemical composition of the collectedwater was measured with ICP-AES.
(a) Earth(b) Mars

30m

30m

Figure 1. (a) Dark streak features at the site-BH in
central Mongolia (Google Earth image). (b) Recurrent
Slope Lineae (RSL) in Newton Crater, Mars. Portionof image ESP_023045_1380.
Formation mechanism of terrestrial RSL analogues: Although no water flowed during the survey,
dark streak features appeared at the site-BH (Fig. 2).
At the source regions of the dark streak features, small

50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

ter (Ca2+: 1.4 mM) strongly suggests that the water isderived from subsurface at the site-GB (e.g., melting ofunderground ice). If melting of underground ice at thesite-BH induces water flow with a similar friction velocity of ~30 mm/sec, the water flow is sufficient tocause the selective traction, leaving to the dark streakfeatures after dryness of the surface water.
Friction velocity V∗ (mm/sec)cannels with width of ~5–10 cm and depth of ~1–2 cm
are found. The upstream of the dark streaks are dented
with depth < 1 cm, which becomes shallower toward
the downstream. These observations suggest that erosion of surface materials would have occurred alongwith the dark streak features in the past. This view is
supported by our results of the flow accumulation
analysis for the constructed three-dimensional structure.
The flow directions expected from the analysis agreewith the directions of the dark streaks at the site-BH.
Given that the slope angle (10–12°) at the site-BH isshallower than the angle of repose (~30°), erosion
would have been caused by surface runoff, rather thangranular flows.

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200On Earth180160140120100806040200On Mars

10100

1000

10000

particle size of grain (µm)

Figure 3. Relationship between threshold friction velocity of water flow and particle size of grain based onref. [8].

Figure 2. A close-up picture of a source region of adark streak feature at the site-BH. The source rocks ofthe soils (the boulders in the upstream) are granite.
Although the dark streak features would have been
formed by water erosion, darkening is not caused by
the presence of liquid water. Water contents (27±9%;1s) in surface soils on the dark streaks at the site-BH
show no significant difference from those outside thestreaks (22±5%; 1s) within errors. On the other hand,the surface of the dark streaks contains less brightsands (grain ~300–2000 µm, composed mainly ofquartz and plagioclase) and more dark silts (grain ~10–200 µm, composed mainly of biotite and hornblende),
compared with those outside the dark streaks. These
results suggest that selective traction of the brightsand-sized grains by surface runoff results in remnantof the dark silt-sized grains on the slopes, which darkens the streak features at the site-BH.
Such selective traction of the bright sands can occur if friction velocity of water flow ranges ~10–30
mm/sec (Fig. 3), where only sand grains can move but
silt-sized grains cannot [8]. At the other site of RSL
analogue (i.e., the site-GB), water flow was observed
along with the dark streak features during the survey.
At the site-GB, water flows with ~3 mm in depth and~10 cm in width at the velocity of ~0.1 m/sec, whichcorresponds to ~30 mm/sec of friction velocity. A Cacarbonate-containing composition of the collected wa-Implications for RSL on Mars: Based on theformation mechanism of the terrestrial RSL analogues,
we can explain the two issues in the wet processes for
RSL formation on Mars; 1) no remote-sensing detection of liquid water, and 2) the fact that RSL appear
only on steep slopes. At RSL sites on Mars, a thin layer of fine sands is considered to cover basaltic bedrocks on the slopes [9]. If liquid water/brine flows due
to melting of underground ice, this can remove finesands on the slopes. Consequently, dark streak features
would appear on the slopes due to exposure of underlying basaltic bedrocks even after dryness of water.
Figure 3 suggests that water flow at the site-GB
with ~30 mm/sec of friction velocity can also transport
fine sands on Mars. To achieve this friction velocity onMars, steeper slopes would be required due to the lower gravity than Earth. Using the Manning equation, wesuggest that a slope with angle > ~25–30° would berequired to achieve surface flow with this friction velocity, if the depth and width of water flow are same as
those of the terrestrial analogues. Our results suggestthat RSL on Mars would appear only on steep slopes
due to its low gravity.
References: [1] McEwen et al. (2011) Science, 333,740; [2] McEwen et al. (2014) Nat. Geosci., 7, 53; [3]Dundas et al. (2017) Nat. Geosci., 10, 903; [4] Stillman et al. (2016) Icarus, 265, 125; [5] Ojha et al.
(2015) Nat. Geosci., 8, 829; [6] Schmidt et al. (2017)Nat. Geosci., 10, 270; [7] Szumińska (2016) Sediment.
Geol. 340, 62; [8] Melosh (2011) Planetary Surface
Processes, Cambridge Univ. Press, pp. 500; [9] Ruffand Christensen (2002) J. Geophys. Res., 107, 5127