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A granular debris flow on the wall of Stevinus A (downhill to the bottom and to the right, in the original full-size LROC Featured Image).
A 7 meter boulder impedes the progress of the flow, which bifurcates
and reconnects about 10 meters further downhill. LROC Narrow Angle
Camera (NAC) observation M154893929R, LRO orbit 7960, March 16, 2011. Detail from LROC Featured Image Dry debris or liquid flow? by Lillian Ostrach, June 3, 2011 [NASA/GSFC/Arizona State University].
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Paul D. Spudis
The Once & Future Moon
Smithsonian Air & Space
Although the Moon’s gravity is low, only about 0.165 of the Earth,
rock and soil move down slope over time. In geology, such processes are
called
mass wasting
and is one of the principal sources of erosion on the Moon (the other
being meteorite bombardment). Mass wasting includes both gradual,
infinitesimally slow soil creep on slopes and rapid, catastrophic mass
movements, called
landslides. Long trains of rock debris can form
scree slopes, loose fragments lying precariously at the critical angle beyond which they move, the
angle of repose.
Because impact craters make steep walls and the larger ones bring up
peaks in their centers, most mass wasting on the Moon is found in and
around impact craters of all sizes.
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| Dark and light streaks on crater walls on the Moon. (click HERE to enlarge) [NASA/GSFC/Arizona State University]. |
As the number of
high resolution images
taken from the LRO mission continues to proliferate, several
interesting and under-appreciated lunar surface phenomena are becoming
more apparent. Among the fresh craters of the Moon, we find light and
dark steaks on the walls of the ubiquitous craters of the Moon.
Although it is not surprising that material might move or flow down
steep slopes on the Moon, the appearance of these flows can be
startlingly similar to those seen on other planets, particularly Mars,
where such streaks have been cited as evidence for the presence of
subsurface water.
The new narrow angle LRO camera can see objects on the surface
smaller than one meter (typically, 50 cm per pixel resolution). These
new views have shown us a wide diversity of new features within impact
craters and have given us a new appreciation for mass wasting. Larger
crater walls are slumped, with stair step-like wall terraces,
concentrically arranged around the crater between rim and floor. In
detail, these terraces show ponds of dark material that seem to collect
in low areas. Most of this material looks like it was once molten but
now congealed; it is probably solidified impact melt. Flows of melt may
cascade down and over the walls of fresh craters.
However, many “flows” of both dark and light material on the Moon seem to consist of loose
fragments of rock debris lying on steep slopes.
These debris flows show a variety of morphologies, including simple
flow shapes, cascades, ponding, and fan-like termini. Sometimes the
dark and light flows intermingle within a single crater while others
show only one type. These debris flows can usually be traced back to
outcrops of bedrock in the upper portions of the crater wall. As the
bedrock erodes (usually by meteorite erosion and disaggregation due to
the intense fracturing induced by the original impact that formed the
crater), it sheds small fragments that train down slope, forming
flow-like landforms.
Because crater walls are uneven, undulating surfaces, the rates of
down slope movement can vary widely over small distances. This
sometimes results in multiple, overlapping flows of debris. Factors
that
control the albedo
(reflectivity) of the debris flows are not well understood. It could
be related to composition (for example, dark, iron-rich mare basalt vs.
white, anorthositic highland rocks). Another factor might be particle
size; small pebble-sized rock flows could be bright as new, fresh
surfaces are constantly exposed. Flows that contain mixed soil might be
darker than normal, as this soil could cover the fragments and reduce
its average reflectivity. But while all these factors may be of
significance to one degree or another, the brightness of a streak is not
particularly indicative of origin.
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| Dark streaks on crater walls, Mars. (click HERE to enlarge) |
On Mars, many
dark streaks are evident
on crater walls and, as on the Moon, come in a wide variety of forms
and occurrences. Martian dark streaks have been variously interpreted
as being caused by compositional and particle size differences, but the
most popular idea is that the
dark streaks are wet soil,
i.e., they represent areas where liquid water is seeping out from the
planet’s subsurface and moistening the surface. One observation
supporting this idea is an apparent correlation of some of the dark
streaks with surface temperature, with warmer slopes showing more. As
liquid water is not stable on the martian surface,
salt-rich brines (which would have much lower melting points than pure water) have been invoked as the possible liquid phase.
The
dark streaks on the crater walls of the Moon
call water-related interpretations of similar features on Mars into
question. The nature of down slope movement on Mars is likely to be
controlled by even more diverse factors than the lunar case. For
example,
large landslides partly cover the floor of the Valles Marineris,
the large canyon system on Mars. These landslides can extend tens of
kilometers across the valley floor and the mass flow might have been
lubricated by trapped atmospheric gas; this “cushioning” effect occurs
within some landslides on the Earth. Such a process would not occur on
the Moon. The diversity of geological processes on Mars suggests that
explanations for dark wall streaks could encompass many more
possibilities than simple wetting of the surface.
Although the existence of dark lunar streaks does not negate
water-related interpretations of similar features on Mars, they do call
attention to the need to keep alternative hypotheses in mind. For many
years (and with some success), planetary geologists have extrapolated
landforms and processes (thought to be understood) on Earth, to similar
appearing features on the planets. In the case of the dark streaks,
terrestrial water seepages in the desert can be darker than surrounding
desiccated terrain. A wide variety of evidence indicates that water is
present in the subsurface on Mars but sometimes other effects such as
rock composition or particle size are responsible for the streaks and
alternatives to seepage should always be kept in mind.
