LUNAR SINUOUS RILLES: ANALYSIS OF MORPHOLOGY, TOPOGRAPHY, AND
MINERALOGY, AND IMPLICATIONS FOR A THERMAL EROSION ORIGIN. D. M.
Hurwitz1, J. W. Head1, and L. Wilson2 1Department of Geological Sciences, Brown University,
Providence RI 02912, debra_hurwitz@brown.edu 2Environmental Science Division, Lancaster
University, Lancaster, UK, l.wilson@lancaster.ac.uk.
Introduction: Sinuous rilles, meandering
channel-like features ranging from tens of meters
to several kilometers in width and from a few to
several hundreds of kilometers in length, have
long been considered enigmas on the surfaces of
terrestrial planets.
Sinuous rilles are most
commonly observed within the maria regions of
the Moon, but some observed features have
sources within adjacent highlands materials and
rilles have been observed to continue through
highlands materials for considerable lengths.
Sources of sinuous rilles range in morphology
from elongate or irregularly shaped depressions,
such as in the case of Rima Hadley, to large
crater-like depressions, such as in the case of
Rima Prinz. Channels of sinuous rilles range
from u-shaped to v-shaped morphologies and
either merge with other sinuous rilles or tectonic
rilles or terminate abruptly or gradually into the
maria.
Post-Apollo studies link sinuous rille
formation with the emplacement of lava flows,
and a growing appreciation for the ascent and
eruption of lunar magma has led to hypotheses
consistent with very high effusion rates of lowviscosity lava.
The characteristics of the
proposed lavas likely yield lavas with high
Reynolds numbers, suggesting that turbulent flow
and thus thermal erosion of the substrate is likely
[1, 2]. More recent numerical/ analytical models
have therefore modeled the formation of sinuous
rilles as thermally erosive channels that generate
a crust at the flow surface [3-5].
To apply updated models for thermally
erosive lava flows [4, 5], we have been
examining properties of sinuous rilles, including
1) source region topography and morphology to
assess implications of source geometry for
thermal erosion theories [6, 7]; 2) lengths to
assess possible flow durations; 3) slopes in
order to assess important parameters associated
with thermal erosion; 4) marginal deposits in
order to distinguish between leveed lava flows
and thermally erosive flows; and 5) spectral
properties of units in which sinuous rilles occur
in order to assess the units that they may be
eroding and to detect the presence of possible
distal deposits. These properties will be used to
assess and update theories of thermal erosion as
an origin for the formation of sinuous rilles and
to distinguish between those features associated
with thermal erosion and those formed by other
processes.
Preliminary
Results:
Apollo
topophotographic images 41B4S1(50), 41B4S2(50),
41B4S3(50), and 39A3S1(50), with maximum
vertical (horizontal) resolutions of 20 m (61 m),
35 m (81 m), 27 m (102 m), and 38 m (65 m),
respectively, were used to manually collect
detailed topographic profiles both across and
along Rima Hadley (image 41) and Rima Prinz
(image 39). These profiles were then plotted
and analyzed in order to better constrain the
origin of these sinuous rilles (Figures 1 and 2).
Our initial studies have focused on Rima
Hadley and Rima Prinz and also include sinuous
rilles observed on the Aristarchus Plateau and in
other lunar regions. Topographic profiles were
collected for Rima Hadley and Rima Prinz using
high resolution Apollo orthotopographic maps
(1:50,000). Rima Hadley is characterized by a
v-shaped channel <1.5 km wide and ~300 m
deep, and the channel has smooth, rimless edges
and an uphill slope of 2.7 m/km. Rima Prinz is
characterized by a u-shaped channel <2 km wide
and 150 m deep, and the channel also has
smooth, rimless edges and a downward slope of
5.7 m/km. The rimless margins of these sinuous
rilles support a thermally erosive origin because
a thermally erosive channel would cut into the
substrate rather than deposit material at the
surface. The different channel morphologies
might be a result of different substrate
compositions, different lava temperatures or
viscosities (thus Reynolds numbers), or varied
amounts of subsequent modification via
slumping. Additional profiles and analysis are
needed to distinguish between these causes and
to better understand the effects of Reynolds
number on channel morphology.
Future Work: Future analysis of the various
morpohologic, topographic, and compositional
Figure 1: Rime Hadley and Rima Prinz topographic profile
tracks mapped on Lunar Topographic Orthophotomap
image 41B4 (1:250,000).
properties will be used to further enhance our
understanding of the origin of sinuous rilles.
Major outstanding questions include: 1) what
source topographies and morphologies are
observed, and how do these properties relate to
the thermal erosive capabilities of the lava, 2)
what does sinuous rille slope imply about the
thermal erosivity of the lava, 3) what are the
spectral properties of sinuous rille substrates
and is there evidence for distal deposition, and
5) is the origin of lunar sinuous rilles similar to
the origin of sinuous rilles found on Venus,
Mars, and beyond.
Figure 2: Topographic profiles from high resolution Lunar
topophotomap images (1:50,000) of Rima Prinz and Rima
Hadley. Profiles display changes in topography at the base
of each rille, along the entire length visible in the given
images (fig. 1) from source to terminus. In addition, a
profile is also shown across each rille, showing a typical
cross section of the two features. Rima Prinz is a u-shaped
channel that has a commonly observed downward slope of
~5.7 m/km, while Rima Hadley is a v-shaped channel that
has an unusual upward slope of ~2.7 m/km.
References: [1] Hulme, G. (1973) Mod. Geol., 4, 107117. [2] Carr, M. H. (1974) Icarus, 22, 1-23. [3] G. Hulme
(1982) Surv. Geophys., 5(3), 245-279. [4] D. A. Williams
et al. (1998) JGR, 103(B11), 27,533-27,549. [5] D. A.
Williams et al. (2000) JGR, 105(E8), 20,189-20,205. [6]
Head, J. W. and Wilson, L. (1980) 11th LPSC, 426-427. [7]
Wilson, L. and Head, J. W. (1980) 11th LPSC, 1260-1262.
[8] McEwen et al. (1994) Science, 266, 1858-1862. [9] D.
T. Blewett and B. R. Hawke (2001) Meteor. Plan. Sci., 36,
701-730.