A Landscape Evolution Perspective on
How Young is Young on the Lunar Surface
3. Ina Caldera
A maximum age of 10-32 Myr has been suggested for Ina. Based on
its topography, we find maximum ages of 50 to 400 Myr for profiles
Aa, Bb, Cc and 5 to 40 Myr for Dd, Ee, and Ff. However, more stringent
limits exist on the most recent activity. Both profiles Ee and Ff
have ~15-100 cm deep troughs at the base of scarps with the local
topographic curvature of 0.1-0.2 m-1. With these curvatures, these
troughs should infill at ~0.5-3 m/Myr, or within 1-2 Myr, yet they
remain topographic lows. This implies they formed at the base of
these scarps recently, or are forming currently (in a geologic sense).
2
Dept. of Astronomy, Mount Holyoke College ([email protected]; @marslakes)
2
Center for Remote Sensing, Boston University ([email protected])
1
-260
Elevation (m)
Caleb I. Fassett and Bradley J. Thomson
1
Aa, κt=4975
Bb, κt=1050
Vertical
Ex. = 4x
Cc, κt=775
-300
Ee, κt=675
Ff, κt=70
I. Introduction and Summary
-340
0
Model
Data
Dd, κt=425
100
200
300
400
500
600
Distance (m)
Figure 3. (top) Ina topography from LROC stereo.
High-resolution topography and image data have renewed interest in the possibility that localized volcanism and
tectonism occurred on the Moon in the last tens-to-hundreds of million of years. In a recent study, we used impact
crater degradation to constrain how fast the lunar surface changes. This methodology can also be applied directly to
landforms besides craters to provide a new source of information about their age. An important caveat is that, unlike
for craters where the initial topography is well known, for other landforms we do not generally know their initial form.
Because diffusion cannot be run backwards, we are thus mostly limited to constraining the maximum age of landforms.
4. Small Scarps, Wrinkle ridges, and
Graben: Lee-Lincoln and Virtanen
Many fresh tectonic features on the Moon are potentially young. LeeLincoln scarp, which was examined closely during Apollo 17, has an
extension onto the steep slopes (15-18°) of North Massif. Because of
these steep slopes, the scarp would be efficiently erased if the most
recent tectonic activity was ancient. Our best fit maximum age for
the scarp is 75 Myr. Small graben northeast of Virtanen crater are
consistent with more recent activity, with best fit ages of 1-2 Myr.
1135
A-A’, κt=15
1130
1115
B-B’, κt=39
1110
Vertical
Ex. = 4x
Model
Data
1105
0
50
100
150
Distance (m)
Elevation (m)
We also looked at Hadley Rille and the well-preserved Eratosthenian lava flows in Imbrium. Hadley Rille has infilled
and widened since its formation. Our estimates for the rille’s maximum age are consistent with ages from Apollo 15
samples, although a wide range of scenarios can reasonably match the rille’s current topography. The margins of the
young lava flows in Imbrium are less degraded than expected from either crater statistics or from the degradation of
craters on their surface. The most straightforward explanation is that the margins of these flows are more resistant to
lateral erosion and degradation than expected.
NASA/GSFC/ASU
(left) LROC oblique view of Ina (M1108203502LR).
Elevation (m)
Nonetheless, our results are consistent with the interpretation that lunar features that have been hypothesized to be
young - small lobate scarps, graben, and irregular mare patches - are in fact young (less than 100 Myr, but in some
cases much younger than that). In fact, in the case of Ina Caldera, its small-scale topography suggests it is actively
evolving today.
(bottom) Profiles of Ina with best fit diffusion models (assuming initially
vertical scarps). Exterior-to-interior on smooth unit: A-a; B-b; C-c. From
smooth hillocks to troughs: D-d, E-e, F-f. Note the rounded forms of most
profiles.
-1950
200
A-A’, κt=1225
-2100
-2250
0
Vertical
Ex. = 1.5x
Model
Data
300
600
900
Distance (m)
Figure 4. (top two) LROC stereo topography of Virtanen graben with
resulting profiles and fits.
(bottom row) LROC stereo of Lee-Lincoln scarp, with profiles and fits.
Mosaic from source images
on the Lunar Surface Journal
(left) Composite mosaic of the Lee-Lincoln scarp extension onto North
Massif from Apollo 17 during the traverse from Station 2a to 3.
5. Hadley Rille
q=-κ∇h
Combining this with a statement of material conservation:
∂h/∂t=-∇⋅q
results in topographic evolution following the classic diffusion
equation:
20000
15000
|∇2h|=7.26×10-5 m-1
∂h/∂t=0.4 mm/Myr
90th-percentile abs. curvature:| ∇2h|=5.5×10-4 m-1
(from gridded LOLA 128 ppd; 236m/px)
∂h/∂t=~3 mm/Myr
These values illustrate why small-scale, sharp features, which
are, in general, rare on the Moon, are likely young, because
otherwise they would be degraded beyond recognition.
Mosaic by David Harland
(Lunar Surface Journal)
Elevation (m)
(offset for clarity)
-2100
B-B’ (y-100)
κt=18100
-2200
-2300
Initial
Model
Data
-2400
3000
0
Vertical
Ex. = 5x
1000
2000
Distance (m)
3000
Figure 5. (top) Topography of Hadley rille from the Kaguya Terrain Camera.
(below) Fits to profiles, assuming initially vertical walls (left panel) or a
triangular profile (right panel) with walls slopes of ~32-35°.
(left) Mosaic of Hadley rille from the Apollo 15 Station 1 pan.
6. Imbrium Lava Flows
5000
0
By measuring how fast craters evolve (Fig. 1, Fig. 2), we have
estimated κ (Fig. 2). Our crater degradation measurements
suggest an average κ of 5.5 m2/Myr over the last ~3 Gyr; the
effective diffusivity for the steepest / youngest slopes could be
enhanced by a factor of ~3×.
Median absolute curvature: (from gridded LOLA 128 ppd; 236m/px) B-B’ (y-200)
κt=25300
-2000
10000
∂h/∂t=κ∇2h
20
0
0.5
1
Young Craters:
Higher κ
1.5
2
2.5
Age (Ga)
3
Middle Lunar History:
~Constant κ
3.5
>~3.1 Ga
Increasing κ
15
κ
From this κ and measurements of topographic curvature, we can
infer the local erosion (or gradation) rates:
-2300
Initial
Model
Data
10
5
0
0
0.5
1
1.5
2
2.5
Age (Ga)
3
Some of the most prominent lava flows on the Moon are observed
in west Imbrium. These flows are Eratosthenian in age, 2.1±0.3
Gyr in the Neukum chronology. Crater degradation measurements
give a consistent age, ~1.7±0.5 Gyr (median κt =8050, κtage=12400).
However, topographic profiles of the flow’s margins have best-fit
diffusion profiles of κt~3500-6000, a factor of 2-4× less degraded than
expected given the degradation of craters on the flow’s surface and
the flow’s inferred age. We hypothesize that the margin of these lava
flows are more resistant to erosion than expected.
3.5
50
40
A-A’, κt=3490
B-B’, κt=4550
Vertical
Ex. = 10x
30
20
10
0
0
Figure 2. (top) Crater degradation state as a function of age (assuming
the Neukum chronology function).
(bottom) The effective diffusivity experienced by a crater of a given age.
The upturn in recent times may be a result of non-linear diffusion of
steep initial crater slopes, rather than a secular upturn in the background
degradation rate as a function of time.
60 D-D’, κt=6090
Relative Elevation (m)
(offset for clarity)
If lunar surface topography evolves diffusively, as expected from
both theory and observations, the downslope material flux q is a
function of the diffusivity, κ, and gradient of the topography h:
-2100
-2200
A-A’ κt=9600
-1900
-2000
Vertical
-2400 Ex. = 5x
0
1000
2000
Distance (m)
25000
Crater Degradation State, κt
2. Lunar Crater Degradation, Diffusivity,
and Erosion Rates
A-A’ κt=22500
-1900
Elevation (m)
(offset for clarity)
Figure 1. Illustration of how six craters that have a measured diameter of 1 km appear at different degradation states. The depth-diameter ratio decreases from ~0.2 to ~0.1
over >3 Ga, and crater topography becomes more irregular as degradation continues.
Apollo 15 samples suggest Hadley rille is ~3.3 Gyr old (κt~18600).
The rille’s rim and inner slopes are smoothed and appear to have
undergone topographic diffusion. Maximum best fit κt values of
~22000-25000 (t~3.5-3.7 Gyr) are found assuming initially vertical
walls. If the walls were initially 32-35°, the best fit κt values are
decreased, and can match, or fall below, the expected κt. Results are
thus non-unique and a wide range of models can approximate the
rille’s current topography. However, such models generally imply that
the rille has widened and infilled by tens-to-hundreds of meters.
C-C’, κt=4210
Model
Data
100 200 300 400 500 600 700
Distance (m)
NASA/JSC/ASU
Figure 6. (top) LROC stereo topography of Imbrium flows.
(bottom) Profiles of flows with best fit diffusion models (assuming
initially vertical scarp).
(left) Composite of Apollo 15 view of young lava flows.