The impact cratering record documents the history of resurfacing events on Mars. The morphology and distribution of layered (rampart) ejecta blankets provide insights into the presence of volatiles in the upper crust [1-4]. The physical properties of the crust and history of water have been revealed through recent quantitative studies of the geometry of Martian craters [5-91. Here, we present the results from a study focused on impact craters in Utopia Planitia and the Elysium Mons province to infer the history and properties of resurfacing episodes

Black, B. A.

Stewart, S. T.

NASA Technical Reports Server

Workshop on Role of Volatiles and Atmospheres on Martian Impact Craters 25 
IMPACT CRATER GEOMETRIES PROVIDE EVIDENCE FOR ICE-RICH LAYERS AT LOW 
LATITUDES ON MARS. B. A. Black and S. T. Stewart, Harvard University (Department of Earth and Planetary 
Sciences, 20 Oxford St., Cambridge, MA 021 38, bblack@fas.harvard.edu, sstewart@eps.harvard.edu). 
Introduction. The impact cratering record 
documents the history of resurfacing events on Mars. 
The morphology and distribution of layered (rampart) 
ejecta blankets provide insights into the presence of 
volatiles in the upper crust [1-4]. The physical 
properties of the crust and history of water have been 
revealed through recent quantitative studies of the 
geometry of Martian craters [5-91. Here, we present 
the results from a study focused on impact craters in 
Utopia Planitia and the Elysium Mons province to 
infer the history and properties of resurfacing episodes. 
Crater Measurements. Using the HMars program, 
an interactive toolkit for measuring crater geometries 
based on the Mars Orbiter Laser Altimeter (MOLA) 
data [5, 71, we measured 384 craters in Utopia, Isidis, 
Elysium, the Vastitas Borealis Formation W F ) ,  
Acidalia, Solis Planum, and Lunae Planum. Of these, 
208 well-resolved craters (all fresh and intermediately 
degraded craters greater than 4 km diameter) in the 
Utopia region were studied in detail. 
The MOLA PEDR altimetry profiles are gridded 
using Delaunay triangulation at a resolution of 0.6 
Wpixel.  Rim height, cavity depth, and diameter 
measurements are conducted on the PEDR data and 
volume measurements are based on the gridded data. 
The accuracy of measurements with the m a r s  
program are -10% for rim height and <30% for ejecta 
volumes based on measurements of simulated craters 
on realistic background terrains [5,  71. 
Results. Our results (Fig. 1) expand previous studies 
of crater geometries using the m a r s  program and 
demonstrate significant differences in crater 
geometries with region on Mars. For example, VBF 
craters (T) are distinctly softened compared to Utopia, 
with in-filled cavities and degraded rims. In the 
Utopia-Elysium region, we found a set of fresh craters 
with apparent ejecta volumes in excess of expected 
excavation volumes (identified by -k and in Fig. 1). 
Fresh craters are defined by cavity depths, rim heights, 
and visual inspection of THEMIS imagery. These 
“excess-ejecta” (EE) craters have volumes above the 
apparent pre-impact surfaces ( Vubove) several times 
larger than their cavity volumes (VCmQ). We defined 
two classes of these excess-ejecta craters for 
illustrative purposes. Class one (EE-I) contains the 
freshest craters with the highest proportions of excess 
C ! m  t-m (?%-E) contains craters have slightly lower 
criteria for freshness and excess ejecta, defined as 
ejecta: ds/D~>0.08, HR/DR>O.O~, vuh~cm,,,‘l .5. 
Craters with ds/D~>0.06, &/D~>0.023, 
Va~vJVc-$l .25 that do not fall in class EE-I. 
We investigated the geologic setting of EE craters 
and found a spatial correlation with certain flows from 
Elysium (Fig. 2). Seventy percent of the craters were 
located on lava and sediment units associated with 
Amazonian flows extending from the northwest of the 
Elysium rise [lo]. No EE craters were found in Isidis 
Planitia, Acidalia Planitia, Solis Planum, or Lunae 
Planum, although surveys of the last three regions 
were not complete. 
In our survey, we identified eight EE-I craters in 
Utopia-Elysium. The amount of excess ejecta is 
calculated using two independent methods (Table 1). 
First, the volume above the apparent pre-impact 
surface, Vobove, is compared to the crater cavity volume, 
Vcm,w Second, the observed ejecta, Vohs-eiec,a, is 
calculated by subtracting a conservative model for the 
uplifted surface (defined at a radius r by HR(r/RR)-5.5), 
where HR and RR are the rim height and radius of the 
crater), from v&,w The observed ejecta volume, 
vobs-,c,u, is compared to the expected ejecta volume, 
Vepec,e~-eiec,a, for the crater diameter [ 111. The excess 
volume ratios from the independent methods are in 
very good agreement, confirming that the excess 
volume is robust. 
Discussion. Several scenarios were examined to 
explain the apparent excess ejecta. Based on the 
geological history of the region, the locations of the 
EE craters, and the crater geometries, we interpret the 
excess material in the ejecta blanket as evidence for 
removal of a volatile-rich layer surrounding the 
craters. The two volume ratios (Table 1) imply that the 
apparent excess ejecta is a result of deflation of the 
pre-impact surface level producing perched ejecta. 
Among the eight EE-I craters, the average apparent 
ejecta thickness is -45 m larger than expected. 
We rejected scenarios involving removal of material 
by scouring events (e.g., debris or lahar flows) because 
of the symmetric preservation of ejecta around most of 
the craters. Our preferred model is sublimation of one 
or more water ice-rich surface deposits emplaced 
before the impact events under a different climate. 
After the impact events, the ejecta blanket covers and 
insulates a portion of the layer. As the climate 
changed, the unprotected icy layer surrounding the 
ejecta blanket sublimates more quickly, leaving an 
apparently lower pre-impact surface. The inner ejecta 
blanket of most EE-I craters appear softened (Fig. 3), 
suggesting partial sublimation or relaxation. 
https://ntrs.nasa.gov/search.jsp?R=20050201860 2019-08-29T19:32:19+00:00Z
26 LPI Contribution No. 1273 
Such an ice-rich layer may have been emplaced 
during a glacial period on Mars. Climate simulations 
suggest that the flank of Elysium Mons may 
accumulate ice deposits during high obliquity periods 
[12]. Alternatively, the clustering of EE craters on 
Elysium flow deposits may indicate that many of these 
flows were fluidized with liquid water. 
We estimate the time scale for sublimation of an icy 
layer [following methods of 13, 141. Longevity 
depends strongly on the thickness of the layer, ice 
fraction, burial depth, porosity of the insulating ejecta, 
and temperature. At an average temperature of 210 K, 
a 45-m thick pure ice layer, buried under 20 m of 
ejecta (10% porous, 10-pm pores), would sublimate in 
-55 Ma. A 45-m thick partial ice layer would 
sublimate in a few 10's Ma in the present climate. 
Conclusions. Craters with perched ejecta in the 
region of Utopia Planitia provide evidence for an ice- 
rich layer preserved by overlying ejecta blankets. 
These observations support climate models and 
geomorphologic observations that suggest Martian 
obliquity cycles produced recent glacial periods and 
deposition of low- and mid-latitude ice [8, 12, 15-17]. 
The inference of ice-rich layers, residing below the 
penetration depth of the Neutron Spectrometer [ 181, is 
testable with ground-penetrating radar or drilling. 
Acknowledgments. This work was supported by NASA 
NAGS13474 and the Harvard College Research Program. 
References. [I] Cave, J.A. (1993) JGR 98, 11079. [2] 
Cintala, M.J. and P.J. Mouginis-Mark (1980) GRL 7,329. [3] 
Barlow, N.G. and T.L. Bradley (1990) Icarus 87, 156. [4] 
Stewart, S.T., et al. (2004) Shock Compression ofCondensed 
Matter -- 2003, AIP, 1484. [5] Valiant, G.J. and S.T. Stewart 
(2004) LPSC 34, 1293. [6] Boyce, J.M., et al. (2005) JGR 
110, E03008. [7] Stewart, S.T. and G.J. Valiant (2004) 7th 
Mars Crater Consortium Meeting. [SI Meresse, S. ,  et al. 
(2005) LPSC 36, 1748. [9] Black, B.A. (2005), B.A. thesis, 
Harvard. [IO] Russell, P.S. and J.W. Head (2003) JGR 108, 
5064. [ l l ]  Kring, D.A. (1995) JGR 100, 16979. [12] 
Mischna, M.A., et al. (2003) JGR 108, 5062. [13] Stewart, 
S.T. and F. Nimmo (2002) JGR 107,5069. [I41 Moore, J.M., 
et al. (1996) lcarus 122, 63. [15] Hauber, E., et al. (2005) 
Nature 434, 356. [16] Head, J.W., et al. (2005) Nature 434, 
346. [17] Laskar, J., et al. (2004) Icarus 170, 343. [18] 
Feldman, W.C., et al. (2002) Science 297, 75. 
Table 1. Excess Ejecta Volume. Ratios of observed 
apparent ejecta volume to expected ejecta volume and ratios 
of volume above apparent pre-impact surface to cavity 
volume for EE-I craters. 
2 '  .I 
0.00 0.02 0.04 0.06 
Rim Height I Rim Diameter (HR I DR) 
-1  F 
0.00 0.05 0.10 0.15 0.20 
Depth from Surface I Rim Diameter (ds / DR) 
Fig. 1. Log of ratio of volume above apparent pre-impact 
surface to cavity volume as a function of A. rim height / rim 
diameter and B. depth from surface / rim diameter. Craters 
on the right are considered the freshest. +: class one excess- 
ejecta craters; 0: class two excess-ejecta craters. Log ratio 
of zero indicates Vobove = VCLNifY. 
Fig. 2. Locations of excess ejecta craters, overlaid on 
shaded relief topography and map of Elysium flow units 
[from 101. Symbols identify different crater groups (defined 
in Fig. 1). White f: EE-I craters; white 0: EE-I1 craters. 


Impact Crater Geometries Provide Evidence for Ice-rich Layers at Low Latitudes on Mars

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server