Thickness of proximal ejecta from the Orientale Basin
from Lunar Orbiter Laser Altimeter (LOLA) data:
Implications for multi-ring basin formation
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Fassett, Caleb I., James W. Head, David E. Smith, Maria T.
Zuber, and Gregory A. Neumann. “Thickness of Proximal Ejecta
from the Orientale Basin from Lunar Orbiter Laser Altimeter
(LOLA) Data: Implications for Multi-Ring Basin Formation.”
Geophys. Res. Lett. 38, no. 17 (September 2011): , L17201, p.15.
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http://dx.doi.org/10.1029/2011gl048502
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American Geophysical Union
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Final published version
Accessed
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http://hdl.handle.net/1721.1/85848
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GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L17201, doi:10.1029/2011GL048502, 2011
Thickness of proximal ejecta from the Orientale Basin from Lunar
Orbiter Laser Altimeter (LOLA) data: Implications
for multi‐ring basin formation
Caleb I. Fassett,1 James W. Head,1 David E. Smith,2,3 Maria T. Zuber,2
and Gregory A. Neumann3
Received 13 June 2011; revised 22 July 2011; accepted 27 July 2011; published 1 September 2011.
[1] Quantifying the ejecta distribution around large lunar
basins is important to understanding the origin of basin
rings, the volume of the transient cavity, the depth of sampling, and the nature of the basin formation processes. We
have used newly obtained altimetry data of the Moon from
the Lunar Orbiter Laser Altimeter (LOLA) instrument to
estimate the thickness of ejecta in the region surrounding
the Orientale impact basin, the youngest and best preserved
large basin on the Moon. Our measurements yield ejecta
thicknesses of ∼2900 m near the Cordillera Mountains, the
topographic rim of Orientale, decaying to ∼1 km in thickness at a range of 215 km. These measurements imply a volume of ejecta in the region from the Cordillera ring to a
radial range of one basin diameter of ∼2.9 × 106 km3 and
permit the derivation of an ejecta‐thickness decay model,
which can be compared with estimates for the volume of
excavation and the size of the transient cavity. These data
are consistent with the Outer Rook Mountains as the
approximate location of the transient cavity’s rim crest
and suggest a volume of ∼4.8 × 106 km3 for the total amount
of basin ejecta exterior to this location. Citation: Fassett, C. I.,
J. W. Head, D. E. Smith, M. T. Zuber, and G. A. Neumann (2011),
Thickness of proximal ejecta from the Orientale Basin from Lunar
Orbiter Laser Altimeter (LOLA) data: Implications for multi‐ring
basin formation, Geophys. Res. Lett., 38, L17201, doi:10.1029/
2011GL048502.
1. Introduction
[2] Early in the history of the Moon, basin‐scale impact
events modified millions of square kilometers in a geological instant, excavating deep into the lunar crust and perhaps
mantle, and spreading ejecta radially over areas often
approaching a lunar hemisphere [Moore et al., 1974; Head,
1974; Melosh, 1989; Head et al., 1993; Spudis, 1993].
Despite the significance of basins in planetary history, key
questions remain about the volume of basin ejecta, the origin
of basin rings, the size and geometry of the transient cavity,
and the depth of excavation and sampling.
[3] The Orientale impact basin (D = 930 km) is the most
recent large multi‐ringed lunar impact basin, and for this
1
Department of Geological Sciences, Brown University, Providence,
Rhode Island, USA.
2
Department of Earth, Atmospheric, and Planetary Sciences,
Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.
3
Solar System Exploration Division, NASA Goddard Space Flight
Center, Greenbelt, Maryland, USA.
Copyright 2011 by the American Geophysical Union.
0094‐8276/11/2011GL048502
reason, it has long provided a type‐example for basin formation processes [Moore et al., 1974; Head, 1974; Spudis,
1993, and references therein]. The lack of measurements of
lunar topography at high spatial resolution and precision has
complicated prior attempts [e.g., McGetchin et al., 1973;
Moore et al., 1974; Head et al., 1975] to characterize the
thickness of Orientale ejecta and its radial decay away from the
basin. The high‐precision altimetric data recently acquired
by the Lunar Orbiter Laser Altimeter (LOLA) instrument
onboard the Lunar Reconnaissance Orbiter (LRO) [Smith
et al., 2010] now makes it possible to provide a more refined
and accurate ejecta‐thickness estimate. We use a digital terrain model (DTM) of LOLA data for the Orientale region
gridded to 128 pixels per degree (237 m/pixel; interpolation
of LOLA point observations) for all measurements.
2. Methods
[4] The first of the techniques used to estimate the
thickness of Orientale ejecta relies on LOLA to measure the
amount of material in partially filled, pre‐Orientale craters,
many of which are clearly recognizable in Figure 1a. There
are 154 such pre‐Orientale craters with D = 20 to 100 km,
and we have measured their observed rim crest‐to‐floor
relief. The difference between the observed relief in these
craters and the expected relief in a fresh crater of the same
size represents a firm upper limit on the amount of ejecta
(or, strictly, degradation of relief) contributed by Orientale
to the crater’s morphometry [e.g., Moore et al., 1974]
(Figure 2a; see also Text S1 of the auxiliary material).1 In
general, we would expect this to be an overestimate of the
ejecta thickness, since most craters on the lunar surface have
experienced some amount of mass wasting, weathering, or
infill that would have reduced the relief of the pre‐Orientale
crater population before the basin‐forming impact. We use
the empirical morphometric relationships derived by Pike
[1977] for the expected relief of fresh lunar craters.
[5] A second technique we apply to assess the thickness of
ejecta is to find the smallest crater which survived the Orientale
basin‐forming impact at a given radial range (Figure 2b and
Text S1 of the auxiliary material). Since small craters are far
more common on planetary surfaces than larger craters, we
can be confident that numerous smaller craters must have
existed prior to the Orientale event and been erased, and the
smallest surviving crater provides information about the
scale below which all pre‐existing craters were erased, again
applying Pike’s [1977] relationships. We divide the area
1
Auxiliary materials are available in the HTML. doi:10.1029/
2011GL048502.
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Figure 1. (a) A portrayal of the LOLA DTM of Orientale, with regional trends removed to help emphasize the alteration of
topography surrounding the basin by ejecta. Note that pre‐Orientale craters are visible in its surroundings, but most have
been significantly altered by ejecta. Major rings of the basin are labelled; CR: Cordillera Mountain ring, R = 465 km; OR:
Outer Rook ring, R = 310 km; IR: Inner Rook ring, R = 240 km; ID: Inner Depression, R = 160 km. (b) Locations where
measurements were obtained that constrain the ejecta thickness (dots, fill measurements, Figure 2a; triangles, direct measurements of lobes, see Text S1 of the auxiliary material). Base map is a LOLA DTM superposed on a LOLA hillshade.
around Orientale into four intervals of increasing radial
range at 100 km increments from the Cordillera (Figure 2b).
[6] A third approach to estimating the ejecta thickness is
the direct measurement of ejecta in instances where it is
possible to infer the elevation of a pre‐Orientale surface (see
Text S1 of the auxiliary material). This method is subject to
few assumptions, but suffers from the disadvantages that the
ejecta can be measured only where it is relatively thin and
spatially heterogeneous, and where the deposit is dominated
by surface flow of ejecta and local debris, a distinct physical
process. Because of these uncertainties, we rely on the other
two techniques for the observations and ejecta‐thickness
models described below.
3. Observations
[7] The distribution of locations where we have measured
Orientale ejecta thicknesses is shown in Figure 1b, and
thicknesses derived are shown in Figure 2. Following
McGetchin et al. [1973], we assume that the radial variation
in ejecta thickness, t, away from the Cordillera ring (which
has thickness TCR) can be expressed by a power function of
the range r (measured from the center of Orientale), scaled
by the radius RCR (= 465 km):
t ¼ TCR ðr=RCR ÞB
ð1Þ
[8] From this equation, we treat both TCR and B as independent unknowns and compute a nonlinear least squares fit
to the lower envelope of the data in Figure 2a and thickness
estimates from crater survival in Figure 2b. The resulting
thickness at the Cordillera ring is TCR = 2900 ± 300 m,
decaying with a power law exponent of B = 2.8 ± 0.5 (the
quoted uncertainties are 95% confidence intervals from the
non‐linear least square fit). This ejecta‐thickness profile
should be understood as an average and local variations are
common. Since this estimate is derived from burial and
relief degradation, it excludes structural uplift that is known
to occur close to crater rim crests, but includes an unknown
component of locally derived material incorporated in the
ejecta deposit that increases as a function of increasing
radial range [e.g., Oberbeck, 1975].
[9] It has been argued that Orientale formed by an oblique
impact on the basis of other morphological and morphometric characteristics, such as asymmetry in the far‐field
ejecta (secondary crater chains) [Scott et al., 1977; Wilhelms,
1987; Schultz, 1996; Schultz and Papamarcos, 2010]. Although
we do not see a major asymmetry in near‐field ejecta
thickness, the scale of variation in near‐field ejecta thickness
may simply be unresolvable using our techniques.
[10] To assess how directly this ejecta decay profile
affected nearby topography and how it might contribute to
the elevation of the basin rim, we can measure the topography at the Cordillera ring and compare it to topography at
some distance away. From the ejecta profiles, we expect
∼2 km more ejecta deposited at the Cordillera ring than at
r/RCR = 1.5 (half of a basin radius from the Cordillera).
Comparing the average elevation of the Cordillera rim (m =
3.7 km from the LOLA DTM; s = 2.6 km) and the average
elevation of terrain at r/RCR = 1.5 (m = 1.3 km; s = 3 km), we
find a ∼2400 m difference (note that the 1 − s values given
here reflect azimuthal variability in elevations, not errors in
altimetric measurements, which are negligible). The similarity of the ∼2000 m expected change (from the ejecta
profile alone) and 2400 m difference (in observed, average
elevation difference) suggest that much of the elevation
change between r/RCR = 1 and r/RCR = 1.5 is a result of the
decay in ejecta thickness; some of the additional ∼400 m may
be the result of structural uplift.
4. Discussion
[11] In this section, we compare our measurements to
earlier estimates for the ejecta thickness and its decay. We
also use the derived ejecta‐thickness function to calculate
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Figure 2. (a) Distribution of fill material within craters
near Orientale based on their reduced relief when compared
to fresh craters of the same size. Since this reduction in relief
reflects the sum of pre‐Orientale infill and Orientale ejecta,
it is generally an overestimate of ejecta thickness. For this
reason, the lower envelope on the data is relied upon for fitting. (b) Estimates for the thickness of ejecta required to
erase the small crater population below the size of the smallest surviving craters at various ranges. The dashed thickness line reflects uncertainty in our assumptions about the
degradation state of the fresh craters that were erased and
the size below which all craters were erased (see Text S1
of the auxiliary material); the thin solid line in range is
the 100 km range over which we binned our measurements.
In Figures 2a and 2b the primary x‐axis is normalized to the
Cordillera radius (RCR = 465 km), and the black line is the
ejecta‐thickness model, T = 2900(r/RCR)−2.8.
the ejecta volume outside the Cordillera ring and extrapolate
this function inward to assess the volumes of ejecta implied
as a function of the location of the transient cavity rim crest.
4.1. Comparisons of Past Estimates of Ejecta
Thickness and Decay
[12] A wide range of estimates have been made for the
thickness of ejecta at the rim crest of the transient cavity,
and for the ejecta‐thickness decay function. McGetchin et al.
[1973] suggested a function for the thickness of ejecta at the
transient cavity rim crest TTR = 0.14 R0.74
TR , with RTR and TTR in
meters, on the basis of nuclear craters, Meteor Crater, and
observations of lunar craters. The McGetchin et al. function,
assuming the Cordillera ring as the location of the transient
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crater rim crest, implies ejecta thicknesses at this location of
2190 m, a value smaller than our measured ejecta thicknesses. This function implies even smaller thickness estimates if the transient cavity is significantly smaller than the
Cordillera Ring, as seems likely. Pike [1974] suggested a
wide range of models for TTR; his equation (11), TTR =
0.033RTR, would imply a thickness of ejecta at the Cordillera
ring of ∼15 km (assuming that the Cordillera ring was the
transient cavity rim crest), at least factor of 5 larger than our
measurements. If, however, the transient crater was well
within the Cordillera ring, as discussed below, the thickness
at the rim of the transient cavity implied by this equation is
potentially consistent with our results. Petro and Pieters
[2006] applied the Housen et al. [1983] scaling [see also
Haskin et al., 2003] to suggest TTR = 0.0078RTR, which
would imply a thickness of 3600 m if the Cordillera rim is
the appropriate radius for the transient crater.
[13] Our ejecta‐thickness measurements also allow derivation of a new power law for the decay of ejecta, with an
exponent of B = 2.8 (±0.5); this is shallower than that
estimated by McGetchin et al. [1973] (B = 3) and steeper
than that determined by Petro and Pieters [2006] (B = 2.61),
who applied the model of Housen et al. [1983], although it
is formally consistent with both. Our measurements and
inferred decay law differ significantly from the monotonic
linear decay suggested by Short and Forman [1972] and the
concave down profile of Cordell [1978]. Both of these
profiles have thicknesses of ejecta of >2 km at r/RCR ∼ 1.5,
which is inconsistent with our observations and the preservation of several 7 to 10 km pre‐Orientale craters at this
range. When fresh, such craters would be expected to have
relief of only 1.5 to 2 km, which would have been destroyed
by deposition of >2 km of ejecta.
4.2. Ejecta Volumes and Comparisons to Estimated
Transient Rim Crest Positions
[14] Using our power law description of the radial decay
of ejecta, we can now integrate this function to calculate the
volume of ejecta deposited in various regions, and evaluate
the results in light of proposed locations of the transient
cavity’s rim crest for Orientale. Some ejecta was undoubtedly deposited at larger radial ranges than we measure [e.g.,
Spudis, 1993; Ghent et al., 2008], but it is likely to be
a small percentage of the total ejecta volume because it
commonly occurs in radial chains and is discontinuous.
[15] The Orientale basin consists of the Cordillera ring,
which defines the topographic basin rim, the Outer Rook, a
ring of continuous inward facing massifs, the Inner Rook, a
ring of peaks, and an inner depression that contains Mare
Orientale (Figure 1a). A variety of these rings have been
suggested to approximate the location of the transient cavity’s rim crest (see discussion in the work of Spudis [1993]),
and our new thickness, volume, and decay law estimates
permit us to assess the plausibility of these assignments
(Table 1). From the Cordillera ring to one basin diameter
from this topographic rim crest (r/RCR = 3), we calculate an
ejecta volume of 2.9 × 106 km3 (+1.2, −0.8).
[16] Assuming a paraboloidal shape for the excavation
cavity, its volume is VEx = 0.5pd(RTR)2 for radius of the
transient and excavation cavity, RTR, and for depth of excavation d, which we assume to be 50 km. This assumed depth
is supported by spectroscopic observations that suggest that
the ejecta and ring massifs of Orientale are predominantly
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Table 1. Parameters of Possible Transient Crater Radiia
Mapped Ring
Inner Dep.
Inner Rook
Outer Rook
Cordillera
Transient Crater
Radius, RTR
160
240
310
465
km
km
km
km
Volume of Excavation
Crater, VEx
RTC Ejecta Thickness
at Transient Rim
Volume of Ejecta
From RTR to RCR
2.0 × 106 km3
4.5 × 106 km3
7.5 × 106 km3
17.0 × 106 km3
57.5 km (+50.7, −27.2)
18.5 km (+9.9, −6.6)
9.0 km (+3.2, −2.4)
2.9 km (±0.3)
6.6 × 106 km3 (+3.4, −2.2)
3.4 × 106 km3 (+1.1, −0.9)
1.9 × 106 km3 (+0.4, −0.4)
0 km3
Volume of Ejecta
From RCR to 3 RCR
2.9
2.9
2.9
2.9
×
×
×
×
106
106
106
106
km3
km3
km3
km3
(+1.2,
(+1.2,
(+1.2,
(+1.2,
−0.8)
−0.8)
−0.8)
−0.8)
a
This table gives the volume of excavation assuming a paraboloidal cavity with depth of 50 km, the ejecta thickness at the transient crater rim found by
extrapolating the derived ejecta‐thickness model to that location, the volume of ejecta between the transient crater rim and Cordillera, and the volume
measured between the Cordillera rim and one basin diameter away.
feldspathic and lack obvious signatures of lunar mantle
material [Pieters et al., 2009; Yamamoto et al., 2010] as well
as by geophysical modelling [Wieczorek and Phillips, 1999;
Hikida and Wieczorek, 2007]. If the Cordillera ring radius
approximates the radius of the excavation, this would imply
an excavation volume of 17.0 × 106 km3, substantially greater
than the ∼2.9 × 106 km3 of ejecta observed within one basin
diameter of the Cordillera ring.
[17] Extrapolating the power law description of ejecta
inward to the current position of rings inside the Cordillera
by varying radius RTR, we can estimate the thickness of
ejecta at the transient cavity’s rim crest and the additional
ejecta volume expected between RTR and RCR. If the transient radius is expressed as a fraction of the Cordillera
radius, r = RTR/RCR, then the thickness expected at the transient crater rim is:
TTR ¼ TCR B
ð2Þ
[18] If the next innermost ring, the Outer Rook Mountains
approximates the size of the transient cavity [Head, 1974],
then r ≈ 2/3. Thus, given B = 2.8, we would expect 3.1 times
as thick an ejecta deposit at the Outer Rook (∼9000 m) than
at the Cordillera ring, and ∼1.9 × 106 km3 of additional
ejecta would have been emplaced between the Cordillera
ring and Outer Rook ring. This ejecta would have ended up
within the final topographic depression of the basin defined
by the Cordillera ring, with an average thickness of ejecta of
∼5 km in this region, known as the Montes Rook Formation
[Scott et al., 1977]. The deposition of this volume of ejecta
would have provided a significant load that may have
influenced the modification stage of basin formation by
facilitating the collapse of the transient cavity’s rim [e.g.,
Head, 2010]. If we were to include this inferred volume of
ejecta between the Cordillera ring and Outer Rook, a total
ejected volume of ∼4.8 × 106 km3 would be implied (Table 1).
[19] The Inner Rook Mountain ring has also been proposed to represent the approximate position of the transient
cavity rim crest [Floran and Dence, 1976]. This smaller
transient cavity would have an ejecta thickness of ∼18.5 km
at the rim crest, a volume inside the Cordillera ring of ∼3.4 ×
106 km3, and a total volume of ∼6.3 × 106 km3. The inner
depression could also represent the transient cavity rim
crest. Extrapolating our ejecta decay function to this much
smaller transient cavity would imply an ejecta thickness at
the rim crest of >50 km, a volume inside the Cordillera ring
of ∼6.6 × 106 km3, and a total volume of ∼9.5 × 106 km3.
The extremely large volume of ejecta for such a small
transient cavity rules out this ring as a realistic candidate for
the transient cavity’s rim crest. Similarly, the extremely
small volume of ejecta, relative to the excavation cavity of
volume for a Cordillera ring–sized excavation cavity also
suggests that it is unlikely to have been the location of the
transient crater rim.
[20] These ejecta volumes can also be compared to the
volume estimates for the Orientale transient cavity derived
on the basis of alternative approaches. On the basis of gravity
data, Wieczorek and Phillips [1999] determined a volume of
3.1 ± 0.4 × 106 km3 for the transient cavity, with a maximum
excavation depth of ∼50 km and radius of excavation of RTR ∼
200 km (midway between the Inner Rook ring and the inner
depression). More recently, Hikida and Wieczorek [2007]
derived similar values for the radius of the excavation cavity and slightly smaller depths using different gravity‐inversion
techniques.
[21] The ejecta volume we infer outside the Orientale
topographic basin (outside the Cordillera ring), but within
one basin diameter, is ∼2.9 × 106 km3. This rises to ∼4.8 ×
106 km3 if the ejecta profile is extrapolated back inside the
basin to the Outer Rook ring (R = 310 km). Estimates for the
transient cavity volume derived from lunar gravity analyses
(∼3.1 ± 0.4 × 106 km3) [Wieczorek and Phillips, 1999;
Hikida and Wieczorek, 2007] are more comparable to the
value we observe outside the Cordillera ring (R = 465 km),
but the Cordillera ring radius is considerably greater than
their estimated RTR of ∼200 km. One possibility is that the
radius of the transient cavity is simply larger than inferred in
these gravity models. For example, if the current Outer
Rook approximates the size of the transient cavity, then a
50 km depth of excavation would imply a volume of ∼7.5 ×
106 km3, and a 40 km depth of excavation would yield a
volume of ∼6 × 106 km3, consistent with predicted ejecta
volumes.
[22] Four other factors may contribute to this difference:
(1) the excavation cavity volume of a large basin is greater
than the total ejecta volume deposited outside the crater
[e.g., Schultz et al., 1981]; (2) the shape of the excavation
cavity may be more realistically estimated by a nested cavity
configuration than by assuming a paraboloidal geometry
[Cintala and Grieve, 1998; Head, 2010; Baker et al., 2011];
(3) the ejecta‐thickness model assumes no ‘bulking’ or net
change in density or porosity of the ejecta, and does not
include incorporation of local material into the ejecta; and
(4) extrapolating the ejecta profile from outside the basin into
its interior may not adequately reflect what would be
deposited within the rim of the basin as it formed.
[23] In summary, our new ejecta measurements and inferred
ejecta decay profile are consistent with a transient cavity
radius approximated by the current location of the Outer Rook
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ring and are inconsistent with a much larger (RTR > 400 km)
or smaller (RTR < 200 km) transient cavity.
5. Conclusions
[24] We have used new LOLA altimetry data and a variety
of techniques to estimate the ejecta thickness deposited
within one basin diameter of Orientale. We use this estimate
to find its volume, and calculate a new ejecta‐thickness
decay law. We find that the thickness of ejecta is 2900 ± 300 m
at the Cordillera ring, the main topographic rim of Orientale.
The thickness of ejecta decays with a best‐fit power law
exponent of B = 2.8 ± 0.5. The total volume of ejecta outside
the basin (Cordillera ring) and within one basin diameter
of its rim is ∼2.9 × 106 km3. Extrapolation of the ejecta
thickness profile inward to the next ring, the Outer Rook
Mountains, would imply that an additional ∼1.8 × 106 km3
of ejecta was excavated and deposited between the Outer
Rook and Cordillera ring with average thickness of 5 km.
Candidate locations of the transient cavity’s rim crest either
inside the Outer Rook Ring (Inner Rook Mountains and the
inner depression) or beyond it (the Cordillera ring) are less
consistent with our ejecta‐thickness profiles than the Outer
Rook ring. The total volume of Orientale ejecta we infer is
comparable to, but somewhat larger than, geophysical estimates for the volume of the Orientale excavation cavity
volume. Upcoming higher‐resolution measurements of the
lunar gravity field [Zuber et al., 2008] and improved numerical
modeling of the basin formation process [e.g., Stewart, 2011]
may further address this discrepancy.
[25] Acknowledgment. The Editor thanks Noah Petro and Mark
Cintala for their assistance in evaluating this paper.
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G. A. Neumann, Solar System Exploration Division, NASA Goddard
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D. E. Smith and M. T. Zuber, Department of Earth, Atmospheric, and
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