Fassett, Caleb I.

Fhirabayashi, Masatoshi

Needham, Debra H.

Thomson, Bradley J.

NASA Technical Reports Server

Local Variations in Lunar Regolith Thickness: Testing a New Model of Regolith Formation near the Apollo 15 Site 
Debra H. Needham1, Caleb I. Fassett1, Masatoshi Hirabayashi2, Bradley J. Thomson3
1NASA/Marshall Space Flight Center, Huntsville, AL 35805, debra.m.hurwitz@nasa.gov; 2Auburn University, Auburn, AL; 3University of Tennessee, Knoxville, Knoxville, TN
Introduction
The formation of regolith is a fundamental surface process on airless terrestrial 
bodies, yet the processes and rates of regolith formation have yet to be fully 
resolved. A new analytical model developed in [1] describes the growth of 
regolith by small, simple craters as a function of time, improving on pre-
existing Monte Carlo estimates for the regolith. In addition, this model 
describes the expected variability in regolith thickness on a given geologic 
unit.  An additional factor to consider is regolith transport and its dependence 
on local slopes.
In this study, we analyze the regolith thickness in the mare units exposed in the 
walls of Hadley Rille (~26°N, 3.6°E; Panel 1, left) to identify trends in outcrop 
exposure and local slope characteristics. We compare these observations to 
those of slopes associated with exposed outcrops in other sinuous rille walls 
(Panels. 2–4) to determine what slopes outcrops are observed and whether 
this is consistent between rilles.  In addition, we look at the elevation that 
outcrops occur below the rille margin (defined here as the 5 degree slope 
contour_, and the distance from this margin to the beginning of the outcrops. 
Both of these values might be expected to increase with time as the rille’s 
topography degrades [2,3].  Results can have implications both for regolith 
transport processes as well as for the age of the feature – steeper gradients are 
expected to have had less elapsed regolith transport, consistent with a younger 
feature age.
Preliminary results indicate that regolith drapes over mare basalt substrate on 
the upper part of sinuous rille walls (Fig. 1, below), so direct measurements 
of outcrop depth beneath crests of sinuous rille walls are not necessarily 
indicative of regolith thickness.
Figure 1. The thickness of layers of regolith exposed in sinuous rille walls are not representive of regolith 
thicknesses in the mare plains due to regolith draping over rocky outcrops of mare basalts. This lens 
artificially increases the apparent thickness of the regolith.
Instead, we measure the slope at which outcrops become apparent. We compare 
observed outcrop-exposing slopes within each sinuous rille as well as between 
sinuous rilles to identify trends.
Results
Slope
• Outcrop exposures reside at lower slopes than angle of repose (36o, [4]).
• Outcrops observed in the walls of Hadley and the northern Marius Hills 
rille reside at similar slopes that are less steep than outcrops observed in 
Vallis Schröteri and Beethoven walls.
• Outcrops in sinuous rille walls can be concealed by internal landslides or 
by higher rates of regolith transport resulting from the proximity of topo-
graphic features outside the sinuous rille.
DiStance anD elevation change from outcrop to 5o Slope contour
• This distance is expected to increase with feature age because as regolith 
is transported, the terrain is rounded, resulting in the retreat of the sinuous 
rille wall and an increase in distance between wall exposures and slope 
threshold [2,3].
• The median distance between outcrops and the slope threshold varies 
widely for each analyzed sinuous rille, suggesting the ages of the analyzed 
features also varies.
• Vallis Schröteri has the largest distance and elevation changes between 
outcrops and slope threshold, which could indicate this is among the older 
features analyzed.
• The northern Marius Hills sinuous rille has the smallest distance and el-
evation changes, and could be among the younger features analyzed.
Key Points
• Our method comparing the slopes and elevations at 
which outcrops appear on sinuous rille walls shows 
promise for understanding (1) Regolith Transport and (2) 
Sinuous Rille Profile Modification with Age.
• Future work will include an additional comparison with 
mare basalt unit ages, more detailed assessment of geologic 
factors influencing results, and analysis of more rilles.
a
Mare Basalt 
Layers
Regolith Layer
Rocky Outcrops
Eroded Boulders
Talus Slope
Draped Regolith
b
References
[1] Hirabayashi, M., et al. (2018), accepted to JGR.; [2] Fassett, C. I. and Thomson, B. J. (2014), J. Geophys. 
Res. - Planets, 119(10), 2255-2271; [3] Fassett, C. I. and Thomson, B. J. (2015), 46th LPSC, abstract 1120; [4] 
Wagner, R. V., et al. (2013), 44th LPSC, abstract 2924.
Acknowledgements
We would like to acknowledge the Lunar Reconnaissance Orbiter and Kaguya/SELENE mission teams for 
the excellent data used in this research.
Vallis Schröteri
Aristarchus Plateau
Vallis Schröteri
Herodotus
Oceanus 
Procellarum
A A’
Inner Rima Schröteri
A
A’
Topographic Profile of Vallis Schröteri
0
20
40
60
80
100
120
0 20 40 60
Slope (degrees)
Vallis Schroteri Outcrop Slopes
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Distance (m)
Distance from Outcrop to 5 deg Slope
0
20
40
60
80
100
120
140
160
0 50 100 150
Elevation (degrees)
Elevation between Outcrop & Slope
Fr
eq
ue
nc
y
Vallis Schröteri has a typical clustering of outcrop slopes, but distance and elevation change between 
outcrops and slope threshold vary widely, possibly due to local slumping or substrate inhomogeneity.
Rima Beethoven
Rima Prinz
Rima Beethoven
Ha
rbi
ng
er 
Mo
un
tai
ns
Prinz Crater
Oceanus Procellarum
0
20
40
60
80
100
120
0 20 40 60
Slope (degrees)
Beethoven Outcrop Slopes
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Distance (m)
Distance from Outcrop to 5 deg Slope
0
20
40
60
80
100
120
140
160
0 50 100 150
Elevation (degrees)
Elevation between Outcrop & Slope
Fr
eq
ue
nc
y
Topographic Profile of Rima Beethoven
A
A’
A
A’
Beethoven has a similar high slope of 25o for outcrop exposures as Vallis Schröteri, but a much shorter 
distance and smaller elevation change, potentially consistent with a younger age for Beethoven.
Marius Hills Sinuous Rilles
Marius rilles
A A’
0
20
40
60
80
100
120
0 20 40 60
Slope (degrees)
Marius N Outcrop Slopes
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Distance (m)
Distance from Outcrop to 5 deg Slope
0
20
40
60
80
100
120
140
160
0 50 100 150
Elevation (degrees)
Elevation between Outcrop & Slope
Fr
eq
ue
nc
y
0
20
40
60
80
100
120
0 20 40 60
Slope (degrees)
Marius S Outcrop Slopes
0
50
100
150
200
250
300
350
400
450
500
0 100 200 300 400 500
Distance (m)
Distance from Outcrop to 5 Deg Slope
0
20
40
60
80
100
120
140
160
0 50 100 150
Elevation (degrees)
Elevation between Outcrop & Slope
Fr
eq
ue
nc
y
Marius Hills
-1250
-1200
-1150
-1100
-1050
-1000
-950
-900
-850
-800
0 200 400 600 800
E
le
va
tio
n 
(m
)
Distance (m)
S Marius Rille N Marius Rille
Topographic Profile of Marius Rilles
B
B’
A A’
A
A’
B
B’
The northern Marius Hills rille has lower outcrop slopes than the southern rille but similar distances 
and elevation changes between outcrops and slope threshold. 
The slopes may indicate a younger age for the northern rille. Alternatively, the different outcrop slopes 
may indicate variations in local topography and substrate influences on sinuous rille wall exposures. 
Rima Hadley
Palus Putredinis
Ri
ma
 H
ad
ley
Ap
pe
nin
e M
ou
nta
ins
Highlands
A
A’
A A’
Topographic Profile of Rima Hadley
Fr
eq
ue
nc
y
0
20
40
60
80
100
120
0 20 40 60
Slope (degrees)
Hadley Outcrop Slopes
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
Distance (m)
Distance from Outcrop to 5 deg Slope
0
20
40
60
80
100
120
140
160
0 50 100 150
Elevation (degrees)
Elevation between Outcrop & Slope
Hadley outcrops are fairly consistently located ~76 m from the slope threshold at slopes of ~17o. Outliers 
in distance are mainly due to a slump block artificially lowering outcrops on the rille wall.
In each panel, Lunar Orbiter and Kaguya Terrain Camera images were used for context and 
sketch maps, contoured LROC NAC stereo imagery and DTM data were used to observe out-
crops in sinuous rille walls, and LOLA tracks were used to create rille topographic profiles.
https://ntrs.nasa.gov/search.jsp?R=20180001996 2019-08-29T17:40:45+00:00Z


Local Variations in Lunar Regolith Thickness: Testing a New Model of Regolith Formation near the Apollo 15 Site

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