Degradation at the InSightLanding Site, Homestead Hollow , Mars: Constraints from Rock
Heights and Shapes
J. A. Grant1, S.
A. Wilson1, M. Golombek2, A.
Trussell2,3, N. H. Warner4, N.
Williams2, C. M. Weitz5, H.
Abarca2, R. Deen2
1Center for Earth and Planetary Studies, National Air
and Space Museum, Smithsonian Institution, 6th at Independence SW,
Washington, DC, 20560 ORCID: https://orcid.org/0000-0001-8276-1281
2Jet Propulsion Laboratory, California Institute of
Technology, Pasadena, CA,
3California Institute of Technology, Pasadena, CA,
4SUNY Geneseo, Department of Geological Sciences, 1
College Circle, Geneseo, NY 14454
5Planetary Science Institute, 1700 East Fort Lowell,
Tucson, AZ, 85719
Submitted to Earth and Space Science 7/29/21
Abstract: Rock heights and three-dimensional shapes around theInSight lander in Homestead hollow , Mars, provide new
constraints on modification of the degraded 27 m in diameter impact
crater and are a tool for characterizing degradation on regolith-covered
lava plains on Mars. Decreasing average rock height and increasing
percentage of fragments where height comprises the short axis from
outside to within the hollow supports significant ejecta deflation
accompanied by infilling of the interior. Rock relief outside the hollow
is compared with expectations of pristine ejecta thickness and indicates
up to ~40 cm of near-rim early deflation (decreasing to
a few cm out to one diameter) can account for the predicted eolian
component of infilling and that other eolian infilling sources are not
required. Scattered rocks in the hollow are ejecta from subsequent
nearby impacts and their mostly buried expression is consistent with
subsequent long-term degradation estimated to be 10-4m/Myr. Basalt rock shapes at InSight are likely similar to basalt
rock shapes on Earth, but appear more platy, bladed, and elongate in a
triangular form factor plot and more discoidal and bladed in an axes
ratio plot. Nevertheless, addition of 10 cm to near rim rock heights to
account for continued partial embedding in ejecta would result in rock
shapes quite similar to terrestrial rocks. Consistency between
degradation estimates based on current rock relief and rock shape after
accounting for partial embedding in ejecta indicates up to
~30-40 cm early (~0.1 Ga) near-rim
deflation was followed by much lesser long-te rm degradation.
Key Points
Rock heights and shapes at Homestead hollow indicate early
transport of deflated ejecta sediments can account for infilling of the
crater.
Near-rim rock heights and shapes relative to the expected original
versus remnant ejecta thickness indicates ~40 cm
deflation occurred.
Continued exposure of later arriving, small ejecta rocks in the hollow
supports very low estimates of degradation over most hollow history.
Introduction
The InSight mission (Interior Exploration using Seismic
Investigations, Geodesy, and Heat Transport) to Mars landed November
2018 in western Elysium Planitia at
4.502°N, 135.623°E on a
regolith‐covered, Early Amazonian basaltic lava plain capped by
~3 m of impact-formed regolith (Banerdt et al. 2020;
Golombek, Williams et al., 2020; Golombek, Warner et al., 2020). The
lander is within a highly degraded, ~400-700 Myr old,
~27 m-diameter impact crater dubbed “Homestead
hollow ” (Golombek, Warner et al,
2020; Grant et al., 2020; Warner, Grant et al. 2020). The present form
of the hollow (Fig. 1) contrasts with its predicted pristine morphology:
the original ~0.7 m high rim and ~3-4 m
depth has been reduced to a subtle depression only 0.3 m deep.
Corresponding estimates of hollow infilling are between 3-4 m (Grant et
al., 2020; Warner, Grant, et al., 2020). The surface of the interior of
the hollow is characterized by relatively fewer/smaller rock fragments
with respect to the margin and near-rim of the exterior (Fig. 2), though
there are ~3X more rocks in an area dubbed “Rocky
Field ” on the western interior floor relative to the eastern interior
floor (Figs. 1 and 2).
Rocks at InSight are basaltic in composition and likely of
volcanic lava origin based on: (1) relative proximity to north‐south
trending wrinkle ridges (Golombek et al., 2018); (2) the presence of
degraded lobate flow margins in the region (Golombek et al., 2018); (3)
occurrence of platy and ridged surface textures and possible lava
inflation plateaus and volcanic vents (Pan et al., 2020); (4) rocks with
a fairly uniformly fine‐grained aphanitic texture (Golombek, Warner et
al., 2020); (6) the absence of any observable sedimentary structures in
rocks at the landing site; and (7) evidence that the regional Hesperian
transition unit around the lander (Tanaka et al., 2014) experienced an
Early Amazonian‐aged resurfacing event linked to regionally occurring
late volcanism (Warner et al., 2017).
Prior studies concluded that hollow degradation was dominated by eolian
and mass wasting processes with lesser impact contributions that
stripped and lowered the near-rim and ejecta, resulting in nearly
complete infilling of the crater interior and burial of any rocks lining
the hollow (Golombek, Warner et al., 2020; Grant et al., 2020; Warner,
Grant, et al., 2020). Early degradation of the hollow was relatively
more rapid and dominated by deflation of fines from the ejecta, some of
which were transported downwind and infilled the crater, and was
accompanied by mass-wasting along the crater wall. Early degradation
continued until coarser fragments created a surface lag on the ejecta
and/or sufficient relief around partially exposed rocks created a
boundary layer that precluded further stripping while accumulating
eolian fill stabilized the walls of the hollow (Grant et al., 2020;
Warner, Grant, et al., 2020). Comparisons made using orbital data
covering nearby, similar-sized craters in varying stages of degradation
(Sweeney et al., 2018; Warner, Grant, et al., 2020) revealed thatHomestead hollow assumed something close to its present form
within the first 0.05-0.1 Ga after formation (Grant et al., 2020;
Warner, Grant, et al., 2020a). At least some rocks visible in the hollow
(e.g., Rocky Field on the western side; see Golombek, Warner et
al, 2020) are likely related to ejecta from subsequent, nearby impacts,
and are therefore not associated with hollow formation: their continued
partial exposure highlights evidence for very slow modification that
occurred over most of the history of the hollow (Grant et al., 2020;
Warner, Grant, et al., 2020). The cumulative result of hollow
degradation is characterized primarily by an increase in the number and
relative relief of rocks along the margin and exterior of the hollow
relative to the filled interior (Fig. 2), and there is no significant
topographic transition between the two areas (Warner, Grant, et al.,
2020). Nevertheless, the relative relief and shapes of rocks in and
around the hollow has not been quantified, and thus possible
implications for the amount of exterior stripping and accompanying
infilling relative to mass wasting or direct infilling related to ejecta
from later impacts are not yet been well-defined.
Rock height (vertical measurement of the rock above the adjacent local
surface) and three-dimensional shape (various comparisons between the
short, intermediate and long axes of a rock) are important parameters
used to assess the varying amount and number of geomorphic processes
affecting a landscape over time and help identify where erosion
dominates over burial and/or help to identify locales where new
materials may have been introduced as infilling deposits (Fig. 2).
Rock shape can also be used to help identify the processes and intensity
of degradation processes responsible for the characteristics of a rock
population, thereby informing (or constraining) which processes have
been active and how they may have varied with location over time. A
number of studies have examined the two- and three-dimensional shape of
rocks (i.e., using either two or three of the principal axes of a rock)
on the Earth and Mars. Two-dimensional rock shape characterization can
focus on sphericity (Corey, 1949), angularity (Krumbein, 1941) or other
characteristics of a rock and is often applied to rocks on Mars where
information on all three principal rocks axes is precluded by lack of
accurate height information (e.g., Garvin et al., 1981; Yingst et al.,
2007, 2008, 2010, 2013; Craddock and Golombek, 2016). Three-dimensional
evaluation of rock shapes requires accurate measure of all dimensions
and can include various ratios or relations between the long,
intermediate, and short axes of a rock (e.g., Zingg, 1935; Sneed and
Folk, 1958). These relationships are used to characterize rocks as more
rounded (compact or equant), disc-shaped, bladed, or elongate (rod). As
general examples, Graham and Midgley (2000) showed that
three-dimensional rock form is a discriminator between scree, moraine,
and frost-shattered deposits. Three-dimensional shape characteristics
for terrestrial basalt rocks in a wide range of geologic and weathering
environments, including impact, however, show little variability (e.g.,
Craddock and Golombek, 2016; Ehlmann et al., 2008; Kumar et al., 2014),
likely a reflection of control by cooling fractures on how the rocks
break (Craddock and Golombek, 2016). Three-dimensional information
related to rock axes at InSight is available via processing of
stereo images acquired by the lander, enabling direct comparison of rock
shapes on Mars to those associated with basalt rocks on the Earth.
Our analysis of variations in the height and shape of basalt rocks
around the InSight lander can provide additional constraints on
geomorphic processes active at the landing site over time to further
constrain the responsible processes, their timing, and amount that
resulted in hollow degradation. For example, variations in rock height
together with information of degradation processes can quantifying
differences in relief from outside to within the hollow that relates to
regions of net erosion versus deposition. Moreover, the relief on rock
fragments in ejecta around the hollow, where stripping likely
predominates, can be equated to the amount of surface lowering that has
occurred when coupled with expectations of the pristine thickness and
nature of the deposit. We also explore whether there are significant
differences in three-dimensional rock shapes at InSight relative
to the more uniform shapes observed in terrestrial basalt rocks in a
range of environments (e.g., Ehlmann et al., 2008; Kumar et al., 2014;
Craddock and Golombek, 2016) and examine how differences in shape may
relate to their immediate setting and/or formation and modification of
the hollow. For example, the observed versus expected form of the basalt
rock fragments outside the hollow coupled with rock relief and
expectations of the pristine ejecta thickness can help to predict the
degree to which rocks remain embedded in the regolith versus being fully
exposed on the surface, further informing the amount of stripping that
has occurred. We focus on comparison of shape parameters for the basalt
rocks at Homestead hollow to shape parameters for basalt rocks in
the ejecta deposit at the simple-structure Lonar impact crater, in India
(Kumar et al., 2014) to minimize any unlikely, minor uncertainties in
interpretation related to differences geologic setting. Our approach
helps detail where and how much degradation occurred at theInSight landing site, and whether processes occurring locally on
and within the hollow can account for the current degraded appearance or
whether additional sediments from more distant sources are also
required. The approach provides a template that can be used to constrain
degradation occurring on widespread, broadly similar volcanic surfaces
(e.g., Grant et al., 2004, 2020; Tanaka et al., 2014; Warner, Schuyler,
et al., 2020) emplaced over latter portions of Mars history.
Measuring Rock Dimensions
The orthogonal dimensions (minimum and maximum horizontal together with
height) of rocks around the InSight lander were measured from an
orthomosaic digital elevation model (DEM) covering much of the nearfield
surrounding the lander. Image mosaics of the surface surrounding the
landing site were acquired using the arm-mounted Instrument Deployment
Camera (IDC, angular resolution of 0.82 mrad/pixel at the center of the
image, see Maki et al., 2018). A nearly complete panorama was taken of
the landscape around the lander and consists of 283 IDC stereo images
(Fig. 2). These images were used to create the orthoimage and DEM of the
local environment (Fig. 3). As summarized from Golombek et al. (2021),
IDC image resolution varied from 0.12 cm/pixel to 2.8 cm/pixel with
increasing range and the DEM has elevation postings every 5 mm. The
panorama orthomosaic was bundle adjusted (Abarca et al., 2019), except
for the west region, which is separate from the rest of the panorama
(Figs. 2 and 3). Error in stereo coordinates is the result of
uncertainty in the robotic arm position and stereo processing errors.
Nevertheless, pre-launch tests of the surface in front of the lander had
a mean horizontal accuracy of 11 mm, a mean absolute vertical accuracy
of 6.5 mm, and mean relative vertical accuracy of 5 mm. After landing,
56 images taken in front of the lander on sol 12 and had a spatial
accuracy between adjacent stereo frames of 1.9 mm overall with a maximum
error between frames of 4 mm. Images further from the lander, including
horizon images, were bundle adjusted to those close to the lander. The
error within each stereo pair is characterized by the stereo range error
(Maki et al., 2018) of the IDC camera, and range error in the DEM
increases from <1 cm to ~13 cm at a distance
of 10 m from the lander. Spatial uncertainties in the orthomosaic of
<4 mm in the workspace to <1 cm at 10 m distance do
not have an appreciable effect on derived rock sizes because they are
based on relative measurements of rocks over small distance within the
orthomosaic and the spatial uncertainties are smaller than that
associated with the overall orthomosaic. Golombek et al. (2021) further
estimated that the IDC pixel-scale is 0.5 cm at 5 m distance and that
80-95% of the rocks in the DEM down to 0.01 m diameter could be counted
out to 10 m. Finally, it is unlikely that a significant number of small
rock fragments are undercounted along the hollow margin and exterior as
a result of shadowing by larger blocks because of the height of the IDC
during imaging (above the lander deck) and the limited range of the
measurements. With these points in mind, rock fragments >1
cm were measured within 10 m of the lander that covers a significant
portion of the hollow interior, the hollow margin to the west-north and
nearest few meters of the hollow exterior to the west-north (Fig. 3).
For the horizontal dimensions of the rock fragments in and around the
hollow, we utilized a population of 2,034 rock fragments examined by
Golombek et al. (2021) that characterized the rock size-frequency
distribution. As summarized from Golombek et al. (2021), horizontal rock
dimensions were made by digitizing polygonal outlines of visible rocks
in the orthomosaic in ArcGIS. A convex hull was calculated providing
minimum and maximum (non-vertical) axes enclosing a rock. The minimum
axis is calculated as the shortest distance between any 2 vertices of
the minimum bounding polygon while the maximum axis is calculated as the
longest distance between any 2 vertices of the minimum bounding polygon.
These measurements are exactly horizontal with no elevation information,
yielding the small and large axes for each rock fragment. To measure the
height of each fragment, we used the same dataset of 2,034 rocks noted
above (Golombek et al. 2021), but removed rocks that were not entirely
covered by the 5 mm DEM. The height (Z) of each rock was derived from
the 5mm DEM (with 0.5-1 mm vertical precision) using the Add Surface
Information plug-in in ArcGIS which calculated the difference between
the maximum (Z_MAX) and minimum (Z_MIN) values for each fragment (as
mapped by the bounding polygon). Measurements are rounded to the nearest
centimeter to maximize confidence in the estimated dimensions. Our final
database of rock dimensions includes 2,004 rocks (Fig. 3), distributed
in three areas: A) the hollow interior (n=1,850, including Rocky
Field and more rock-free portions of the interior); B) the hollow
margin (n=68) defined as rocks within ~1 m of the zone
of increased rock density relative to the hollow interior (Fig. 2); and
C) the hollow exterior (n=85) within a few meters beyond the hollow
margin to the west-northwest of the lander. Fragment dimensions (minimum
horizontal, maximum horizontal, and height) were used to evaluate any
systematic changes in rock height or shape within the hollow interior,
along the hollow margin, or exterior to the hollow. Our initial
assumption is that the rock sizes lining the interior and along the wall
(margin) and near-rim should all be generally similar given they are the
last materials being excavated during crater formation (average rock
size should decrease with increasing distance from the rim). It is
likely, however, that increasing thickness of fill from the margin
towards the hollow interior results in at least partial burial of many
rocks emplaced during the impact-formation of the crater.
Measuring Rock Shape
Rock shapes for fragments in and around the Homestead hollow were
derived using commonly used calculations
(Sneed and Folk, 1958) for maximum
projection sphericity (\(\varphi\)), deviation from compactness
(D ), and form factor
(F ):
\(\varphi=\sqrt[3]{\frac{S}{\text{LI}}^{2}}\)(1)
\(D=\frac{S}{L}\) (2)
\(F=\ \frac{L-I}{L-S}\) (3)
Where L , I , and S correspond to the long ,intermediate , and short axes of each rock fragment,
respectively. Ratios of the length of each axis are then plotted to
obtain the form factor using the equations (1)-(3) in the TRI-PLOT excel
program published by Graham and Midgley (2000) to constrain the
distribution of compact, platy, bladed, and elongate fragments. In
addition, the ratios of the rock fragment intermediate to long axis
versus fragment short to intermediate axis was plotted separately (after
Zingg, 1935) to characterize whether fragments are more equant,
discoidal, bladed, or rod-shaped relative to expectations from studies
of terrestrial basalt rocks (e.g., Ehlmann et al., 2008; Kumar et al.,
2014; Craddock and Golombek, 2016).
Results
Rock heights on the exterior, margin, and interior of the hollow (Fig.
3) are often the shortest measured axis (height is the short axis 78%
of exterior rocks, 91% of margin rocks, and 97% of interior rocks)
(Table 1, see also Grant et al. (2021) data repository for additional
details). For the exterior of the hollow, the 10 tallest rocks stand
between 23-30 cm in exposed relief (five largest rocks are
>28 cm in height), and 22% of the rocks are
>10 cm tall. The 10 tallest rocks along the hollow margin
stand between 4-33 cm in exposed relief (five largest rocks are 25-34 cm
in height), with 10% of the rocks > 10 cm tall. In the
hollow interior, the 10 tallest rocks stand between 14-26 cm in exposed
relief, but less than 1% of the rocks are > 10 cm tall
(Table 1). The decrease in rock height from the exterior to the interior
of the hollow is also apparent in the overall average height of rocks
within each zone, with rocks outside the hollow, around the hollow
margin, and inside the hollow measuring 6 cm, 4 cm, and 1 cm tall,
respectively (Table 1, Fig. 4). The standard error associated with the
height in each zone indicates that the differences are significant (Fig.
4), especially between the exterior and the interior of the hollow. To
evaluate whether the difference in the height of rocks outside versus
within the hollow is related to larger versus smaller rocks within the
hollow, height values were normalized by dividing the rock height by the
average of the width and length. Resultant values for the exterior rocks
are 0.42 (0.65 standard deviation) versus 0.27 (0.34 standard deviation)
and 0.25 (0.42 standard deviation) for the margin and interior rocks,
respectively. These values are consistent with a wide range in rock
sizes in all three areas, but that are generally higher and more exposed
around the exterior of the hollow.
With the above in mind, study of rock shape focused on rock fragments on
the exterior and margin of the hollow (see also Grant et al. (2021) data
repository for additional details) because they are more exposed,
consistent with predictions (Golombek, Warner, et al., 2020; Grant et
al., 2020; Warner, Grant, et al., 2020). For exterior fragments, the
maximum projection sphericity (equation 1) ranges from 0.11 to 0.87 with
a mean of 0.35 (0.21 standard deviation) and from 0.10 to 0.81 with a
mean of 0.33 (0.15 standard deviation) for the margin (Table 1). The
deviation from compactness (equation 2) is 0.02 to 0.74 with a mean of
0.18 (0.18 standard deviation) for the exterior and from 0.02 to 0.73
with a mean of 0.16 (0.13 standard deviation) for the margin (Table 1).
The form factor (equation 3) for the exterior ranges from 0.18 to 0.97
with a mean of 0.57 (0.18 standard deviation) for the exterior and from
0.05 to 0.90 with a mean of 0.52 (0.17 standard deviation) for the
margin (Table 1). By comparison,
basalt rock fragments in the ejecta deposit at Lonar crater have a
maximum projection sphericity 0.11 to 1.0 with a mean of 0.72, the
deviation from compactness is 0.18 to 1.0 with a mean of 0.53, and the
form factor ranges from 0 to 1 with a mean of 0.56 (Table 1). As such,
the most similar shape parameter between the rock fragments atInSight and in the ejecta at Lonar crater is the form factor.
When plotted (Sneed and Folk, 1958; Zingg, 1935) and compared to the
shape of basaltic fragments in ejecta at Lonar crater (Kumar et al.,
2014) large differences can be seen (Figs. 5 and 6): the rock fragments
in all three zones in and around the hollow at InSight appear
significantly more platy, bladed, and elongate in the triangular form
factor plot (Fig. 5) and the bulk of fragments inside the hollow are
discoidal and bladed and rocks along the hollow margin and exterior are
more bladed in the axes ratio plot (Fig. 6). By contrast, the rock
fragments at Lonar are mostly compact, compact platy, compact bladed,
compact elongated, platy, bladed, and elongate in the triangular form
factor plot (Fig. 5), whereas ejecta fragments at Lonar are mostly
equant with lesser, but significant disc- and blade-shaped fragments in
the axes ratio plot (Fig. 6). Although Lonar is a relatively fresh
impact crater where rocks have been subjected to more rapidly weathering
versus the very degraded appearance of Homestead hollow ,
evaluation of basalt rock shapes from diverse geologic and weathering
environments on Earth indicates there should be little effect on
expected rocks shapes (Craddock and Golombek, 2016).
Discussion
The greater rock height outside the hollow is confirmed after
normalizing for possible variations in overall rock size that indicates
differences are not solely the result of larger versus smaller rocks
around and within the hollow (Fig. 4). This observation coupled with an
increasing percentage of rocks where height corresponds to the measured
short axis from outside to around to within the hollow supports the
greater exposure of rocks on the near rim relative to the interior and
is consistent with inferred degradation of the hollow dominated by
stripping fines from the exposed ejecta deposit accompanied by downwind
deposition and infilling of the hollow (Golombek, Warner et al., 2020;
Grant et al., 2020; Warner, Grant, et al., 2020). Reasonable
expectations of relative rocks sizes around and within the pristine
crater, however, suggest that the difference in rock height between the
exterior and interior of the hollow is not a simple reflection of the
difference between the total of exterior stripping versus interior
infilling that has occurred.
Examination of the ejecta remnants around Homestead hollow and
other fresh, nearby small craters reveals only a few meter-scale rocks
(Golombek, Williams et al., 2020; Warner et al., 2017) and implies those
lining the original floor and walls of the hollow are of a similar scale
and are of a lesser size than the depth of fill. This contention is
supported by the paucity of rocks observed in HiRISE images of craters
<30-40 m in the vicinity of the landing site (Warner et al.,
2017) and the expectation that the original 3-4 m depth of the hollow
largely limited impact excavation to the pre-existing impact regolith
(Banerdt et al., 2020; Golombek, Williams et al., 2020; Golombek, Warner
et al., 2020) rather than excavating larger rocks from more competent
material below that could yield larger rocks (that result in Rocky
Ejecta Craters or RECs, see Warner, Grant, et al., 2020). In addition,
the local regolith has locally experienced approximately three impact
events that would be responsible for breaking any large blocks into
smaller fragments (Charalambous et al., 2020; Golombek et al., 2018,
2021). However, even if the hollow-forming impact occurred into bedrock,
the maximum expected fragment size of 0.9-2.8 m (based on the relation
between crater size and largest associated rock described in Moore
(1971) makes it very likely that rocks lining the original hollow floor
are buried beneath the fill (Grant et al., 2020). Hence, the difference
in rock height between the exterior and interior of the hollow is only a
partial reflection of the deflation versus infilling that has occurred.
More likely, the generally smaller size and distribution of exposed
interior rocks (e.g., forming Rocky Field ) is consistent with
later arriving ejecta fragments from nearby impacts as described in
Grant et al. (2020) that have since been partially buried.
Nevertheless, consideration of the rock height/exposure on the near-rim
relative to expected characteristics of the pristine ejecta can be used
to understand the amount of deflation that has occurred there. To start,
the approximate pristine ejecta thickness around the hollow can be
estimated using:
Et =
0.14Rt 0.74(r /Rt )−3.0(4)
Where the ejecta deposit (Et , measured in meters)
can be related to the transient crater radius Rt(where the transient crater diameter Dt is 0.84D
(D is the final diameter)) and radial distance r from the crater
rim, where r >Rt (McGetchin et
al., 1973). Predicted ejecta thickness around the pristineHomestead hollow ranges from ~50 cm at the rim to
around 30 cm three meters outside the rim, 20 cm five meters beyond the
rim, 10 cm at 1 radius, and only a couple of cm at 1 D range.
Based on analogy with ejecta deposits around Meteor Crater, AZ, (Grant
and Schultz, 1993) and Lonar crater (Kumar et al., 2014), the pristine
ejecta deposit around Homestead hollow likely graded from more
clast rich, perhaps clast supported, to matrix rich, perhaps
matrix-supported deposit with increasing radial distance from the rim.
Although thinner, ejecta further from the rim was likely characterized
by fewer large fragments and more abundant fines, and may have supplied
relatively more sediment (i.e., per unit volume) for transport due to a
lesser abundance of lag or surface armoring fragments. Moreover, the
multiple impact gardening of the regolith into which Homestead hollow
formed would have produced abundant sand-sized material at the expense
of fewer larger rocks in accordance with expectations from fragmentation
during three impact events (Golombek, Charalambous et al., 2020;
Golombek et al., 2018, Golombek et al., 2021). Finally, the numerous
rocks that are exposed around the margin and exterior of Homestead
hollow implies the hollow-forming impact did not occur into
pre-existing fines filling an older crater, in which case even more
fines and fewer clasts would be expected (Grant et al., 2020). In any of
these situations, variable numbers of relatively large (10s of cm up to
~1 m) fragments would likely be present, but more
common/numerous in the vicinity of the rim-crest.
Hence, the pristine ejecta deposit around the hollow was likely
characterized by a wide range of fragment sizes, but with a paucity of
large fragments standing in significant relief. The pristine surface
would have been in disequilibrium with regional geomorphic thresholds
(Grant et al., 2020; Warner, Grant, et al., 2020) and susceptible to
deflation that would have continued until fragments too large to be
transported accumulated as an armoring lag and/or increasing rock relief
created boundary layer that precluded further eolian transport (Grant et
al., 2020). With these points in mind, differing amounts of amount of
eolian deflation from the ejecta around the hollow can be approximated
and compared to the volume associated with fill inside the hollow.
The volume of fill within the 27 m-diameter hollow can be constrained to
first order using an original depth of ~3-4 m that is
now 0.3 m post-infilling (Warner, Grant, et al., 2020) and yields a
total fill volume of ~500-700 m3. If
on order of 60% of the infill is due to diffusional infilling from
around the hollow and 40% is due to eolian infilling (based on
evaluation of ~100 m-diameter craters in the region,
Sweeney et al., 2018) then the eolian contribution to infilling is
~200-280 m3. The small size and
limited pristine depth of Homestead hollow suggests steep slopes
facilitating diffusional infilling were of limited extent and would have
been stabilized by increasing accumulation of eolian infill. Hence, the
actual eolian component to infilling may have been somewhat greater than
40%.
It is unlikely that the 6 cm greater average relief of fragments on the
near-rim of the hollow reflects a uniform amount of ejecta stripping
from the entire deposit given the greater exposed relief on many of the
near-rim rocks (as compared to the likely appearance of the pristine
deposit) and the limited thickness of ejecta beyond about 1R from the
rim. (Golombek, Warner et al, 2020; Grant et al., 2020; Warner, Grant,
et al., 2020). Despite that, an average of 6 cm deflation to a distance
of 1D from the rim would supply ~300
m3 of sediment, close to what is required if all the
all of the fill was eolian. Prevailing, reversing northwest‐southeast
winds (Spiga et al., 2018), however, would cause a significant fraction
of sediment to be transported downrange and bypass the hollow. If half
of the deflated sediment was blown back into the hollow, the volume
contributing to infilling could be ~150
m3 and somewhat less than the lower end estimate of a
40% eolian contribution to the total fill volume. A more realistic
estimate of the eolian erosion from the ejecta might be obtained from
the relief on exposed near-rim rocks, given most were likely buried or
expressed limited relief in the pristine deposit, coupled with use of a
decreasing amount of deflation with increasing range that better
reflects the decreasing thickness of ejecta with increasing distance
from the rim. The maximum relief of rocks around the hollow is
~0.3 m as measured in the DEM and there are a few
instances of rock relief as large as 0.4 m (First Rock,
~20 m distance) and 0.5 m (Hanging Rock,
~21 m distance) beyond the limit of the DEM (Golombek,
Williams et al, 2020; 2021). Based on those rock heights and estimates
of pristine ejecta thickness that decrease outside the rim, deflation of
0.4 m to a range of 3 m beyond the rim, 0.25 m between 3-5 m beyond the
rim, 0.13 m 5-15 m beyond the rim, and 0.03 m from 15-27 m beyond the
rim yields a sediment volume of ~440
m3. While this exceeds the volume of the expected
eolian contribution to hollow infilling, it is a reasonable match if
~50% is blown back into the crater by the prevailing
winds and the remaining deflated sediments bypassed the hollow. If the
volume of fill in the hollow is on the lower end of estimates and/or a
slightly larger percentage of deflated sediments are blown back into the
crater, then eolian contributions could approach 50% or slightly more,
but likely would not comprise an overwhelming fraction of the infilling
sediments.
Rock shapes at Homestead hollow provide an independent check on
whether our deflation estimates are realistic. In contrast to the
expectation of mostly compact, compact platy, compact bladed, compact
elongated, platy, bladed, and elongate rocks (Fig. 5) or mostly equant
(~1/3 of the total) with lesser, disc- and blade-shaped
fragments (Fig. 6) at InSight based on comparison to rocks at
Lonar crater (and elsewhere, Ehlmann et al., 2008; Kumar et al., 2014;
Craddock and Golombek, 2016) there is a preponderance of platy, bladed,
and elongate rocks. Further, at Homestead hollow , the average
maximum projection sphericity (equation 1) of 0.35 and average deviation
from compactness (equation 2) of just under 0.2 are well below the
average of 0.72 and 0.53, respectively, of fragments measured at Lonar
crater. However, the form factor (equation 3) for fragments atInSight averages 0.5-0.6 and is comparable to the average of 0.56
observed at Lonar and may hold a clue to understanding the cause of
other differences.
Possible reasons for apparent differences in rock shape parameters atInSight include: A) error in measurements made in the direction
viewed from the lander if the fragment profile precludes accurate
measure of dimensions; and/or B) the partially buried nature of theInSight fragments (especially inside the hollow) leads to
underestimate of the axis associated with rock height. The former can be
ruled out because there is no systematic change in the shape parameters
with increasing distance from the lander when viewed more obliquely.
For the second possibility, the partially embedded appearance of many of
the near-rim rocks, and more buried appearance of rocks within the
hollow, suggests that the actual rock heights are being underestimated.
The predominance of rock height as the smallest fragment axis, provides
support for this statement. For example, height acts as the short axis
for a large majority (78%) of the fragments on the near-rim and
suggests that underestimation of rock height is skewing the shape
analyses. Equations (1) and (2) emphasize the smallest rock axis in the
numerator that can lead to lesser estimated values of maximum projection
sphericity and deviation from compactness. By contrast, the form factor
includes the smallest axis of each fragment as a difference in the
denominator and could have a lesser impact on the calculated values that
appear to better match what is seen in the InSight versus
terrestrial rocks.
In order to consider the effects of underestimating rock height on
derived rock shapes, we recalculated shape parameters and plots after
addition of relief to rocks on the near-rim to account for their
partially embedded appearance. We focused on fragments around the
exterior of the hollow because they are the best exposed (based on
greater rock relief and somewhat lesser percentage of measured height as
the short fragment axis) and made comparisons to the maximum estimated
thickness of the original ejecta deposit.
The estimated 30-50 cm original thickness of pristine ejecta within a
few meters of the hollow rim (covered by the DEM) coupled with the
~30 cm relief on some partially embedded fragments in
the same zone makes it reasonable that an average ~10 cm
ejecta (plus/minus) buries the base of many fragments. Accordingly, 10
cm was added to the height of each rock on the exterior of the hollow as
an estimate for the rock fraction that remains embedded/buried and
resultant revised heights were used in recalculation of shape
parameters. The revised average values for maximum projection sphericity
are 0.65 (0.09 standard deviation, range between 0.29 and 0.85),
compactness is 0.45 (0.11 standard deviation, range between 0.11 and
0.68), and form factor is 0.49 (0.26 standard deviation, range 0.01 and
0.95), all much closer to the corresponding values of 0.72, 0.53, and
0.56 for ejecta fragments at Lonar crater, respectively. The resultant
updated data cloud for exterior rocks including the revised heights are
shown in the TRI-PLOT (Sneed and Folk, 1958) and mostly superposes the
data cloud for rocks in the ejecta at Lonar crater. Results suggest that
continued ~10 cm embedding of the near-rim exterior
rocks can account for most of the observed differences in rock shape
between Homestead hollow and what is expected from basalt rocks
at Lonar crater (Kumar et al., 2014) and in other terrestrial locations
(e.g., Ehlmann et al., 2008; Craddock and Golombek, 2016). Hence,
~30 cm or slightly more deflation from the near-rim
(decreasing outwards) is supported by greater near-rim rock heights,
deflation estimated from the near-rim based on comparison of rock height
and original ejecta thickness, and consistency between revised rock
relief, estimated original ejecta thickness, and expected rocks shapes.
Interior rocks provide additional clues to the degradation history of
the hollow. Rocks inside the hollow average ~1 cm in
height and more than >99% are less than 10 cm tall and it
is likely that most or all are distal ejecta delivered during nearby
impacts (Grant et al., 2020). Those associated with Rocky Fieldwere likely emplaced within the first 0.1 Ga of hollow history (Sweeney
et al., 2018; Grant et al. 2020; Warner, Grant, et al., 2020). Although
the actual size of the interior fragments is uncertain due to their
partial burial, the ratio of their height to average width and
comparison to average size of more exposed rocks on the rim coupled with
likely emplacement as distal ejecta (Grant et al. 2020) suggests most
are small and likely less than 10 cm tall. Warner, Grant, et al. (2020)
and Grant et al. (2020) estimated initially high degradation rates
during the first ~0.1 Ga after hollow formation quickly
slowed as surrounding ejecta became armored by a coarse lag and/or
increasing surface relief due to around larger rocks created a boundary
layer that precluded eolian transport. Degradation rates following this
early period were estimated to be 10-4 m/Myr for most
of the 0.4-0.7 Ga history of the hollow (Sweeney et al., 2018; Warner,
Grant, et al. 2020) equating to ~3-6 cm of infilling
since emplacement of the fragments forming Rocky Field . The
likely small size, but continued, limited exposure of the fragments inRocky Field and other interior rocks is consistent with these
estimates of very slow degradation over the bulk of hollow history.
Our results provide a new means for constraining hollow degradation that
is consistent with prior predictions of modification history. The
exposed rock relief and revised rock shapes on the near-rim support
deflation of up to 40 cm at the near-rim, decreasing with range, and is
a good match with expected eolian contributions of 40% or slightly more
to hollow infilling: the remainder of infilling is likely the result of
diffusional processes (Sweeney et al. 2018; Warner, Grant, et al., 2020)
with only minimal additional infilling contributions associated with
eolian transport from ejecta around later forming craters. Instead, the
inventory of deflated ejecta sediment bypassing the hollow likely
contributed to eolian bedforms in the vicinity of the landing site
(e.g., Golombek, Warner, et al., 2020).
Further, limited hollow infilling via deflation off ejecta around later
impacts is consistent with early, more rapid infilling followed by very
slow degradation over the bulk of hollow history. Continued exposure ofRocky Field and other interior rocks supports this model. Small
rocks forming Rocky Field were likely emplaced during the first
~0.1 Ga after the hollow formed (Grant et al., 2020;
Warner et al, 2020) on a surface close to what is observed in the hollow
today. And their continued exposure is inconsistent with more
significant infilling associated with eolian transport from ejecta
around later occurring impacts.
Summary
The rocks heights and shapes around and within Homestead hollowprovide independent evidence regarding degradation over time that are
generally consistent with processes inferred from prior studies
(Golombek, Warner, et al. 2020; Grant et al., 2020; Warner, Grant, et
al., 2020). Decreasing average rock height, decreasing largest (exposed)
fragment size, and increasing percentage of fragments where height is
the short axis characterize rocks from outside to within the hollow.
These observations coupled with evaluation of likely pristine ejecta
thickness and properties support net stripping of the ejecta of up to
~40 cm in the near-rim and eolian contribution of 40%
or slightly more to the inventory of sediments filling of the hollow.
Moreover, differences between observed versus expected rock shapes on
the exterior of the hollow are found to relate mostly to underestimating
rock height as a result of continued ~10 cm embedding of
near-rim fragments in the remnant ejecta deposit. Recalculation of rock
shapes to accommodate an average additional height of
~10 cm leads to a close match to expected terrestrial
basalt rock shapes Earth (e.g., Ehlmann et al., 2008; Kumar et al.,
2014; Craddock and Golombek, 2016) and further constrains near-rim
deflation to around ~30 cm or slightly more (with an
additional ~10 cm remaining). Continued exposure of
rocks within the hollow (e.g., Rocky Field ) support estimates of
only 10-4 m/Myr degradation for most of the 0.4-0.7 Ga
history of the hollow (Sweeney et al., 2018; Warner, Grant, et al.
2020). Results suggest that a significant fraction of sediments deflated
from the ejecta bypass the hollow via prevailing winds and are likely
trapped by local topography, with some contributing to numerous nearby
bedforms rather than appreciable infilling of downwind impact
structures.
The estimated amount of degradation inferred from rock relief and shape
around and within Homestead hollow helps to quantify, and is
consistent with, both the current and predicted initial appearance of
the crater as well as the estimated amount of modification and
geomorphic processes it has experienced since formation (Golombek,
Warner, et al., 2020; Grant et al., 2020; Warner, Grant, et al., 2020).
Because volcanic surfaces of intermediate to young age are widespread on
Mars (Tanaka et al., 2014), evaluation of rock heights and shape around
small impact craters can be an important tool for understanding local
degradation history
Acknowledgements, Samples, Data
We thank the Jet Propulsion Laboratory, Lockheed Martin Space Systems,
CNES, and other partner institutions that built and operate theInSight lander. A portion of the work was supported by the
InSight Project at the Jet Propulsion Laboratory, California Institute
of Technology, and under grants 80NSSC18K1625 to J. Grant (includes S.
Wilson and C. Weitz) and 80NSSC18K1624 to N. Warner from the National
Aeronautics and Space Administration. All data used in the paper is
freely accessible at the link provided below in the Data Availability
Statement that follows. This is InSight Contribution Number 229.
Data Availability Statement
All data used in analyses of rocks at the InSight lander and described
in the paper are included in two Microsoft 365 Excel files (and
associated readme explanation Word file) and ArcGIS shape files (and
associated readme explanation Word file, and orthophoto and DEM) that
can be freely accessed and downloaded for viewing at
Smithsonian.figshare:
https://doi.org/10.25573/data.15121554.
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Table 1 and Figures/Captions