Current research attempts to describe morphologic features seen in very high resolution Viking Orbiter images of crater ejecta blankets and interiors, in order to constrain the most likely target properties at the time of crater formation. The Viking Orbiter image data set contains approximately 400 frames at a spatial resolution of better than 10 meters per pixel, and 2,200 frames at a resolution of better than 20 meters per pixel, for which the atmosphere was either clear or only slightly obscured. A search is being conducted of all these images and so far has revealed several examples of both interior and exterior features of impact craters that bear on the nature of the ejecta fluidizing medium. Preliminary observations and speculations on the physical state of the Martian regolith at the time of crater formation are given and briefly discussed

Mouginis-Mark, Peter J.

English

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baitem: BBo&d~)@c ~ U B S  for fie 
Phflcal mte of 1Uhe T q e t  at Time d Hmpct. 
Peter J. Mouginis-Mark Planetary Geosciences, Hawaii Institute of Geophysics, 
University of Hawaii, Honolulu, Hawaii 96822 
From the return of the &st Viking Orbiter images of martian impact craters it was 
evident that the morphology of the ejecta blankets surrounding these craters was 
different from their lunar and mercurian counterparts insofar as much of the ejecta 
appeared to have been emplaced by surface flow (Carr et  al., 1977). Over the last ten 
years, considerable debate has focused on the possible cause(s) for this ejecta fluidization 
and flow. Possible forms that the fluidizing medium may have taken include water ice or 
liquid water within th6 target material (Carr et al., 1977; Boyce, 1979; Johansen. 1979) or 
atmospheric gases interacting with suitable particle sizes within the ejecta curtain (Carr 
et al., 1977; Schultz and Gault, 1979; Schultz, 1986). Indeed, Schultz (1986) draws atten- 
tion to the fact that the fluidized ejecta facies only indicate fluid-like emplacement, and 
laboratory experiments (Schultz and Gault, 1981, 1984) have shown that rampart- 
bordered ejecta facies and ejecta flow lobes can develop without the presence of water. 
A ground water system has been proposed for Mars on a global scale (Carr, 1979; 
Clifford, 1986), and it is believed likely that such a system would influence the occurrence 
and physical state of impact crater ejecta. A major diEculty in correlating these infer- 
ences about ground volatile content with the observed properties of crater ejecta blank- 
ets lies in knowing the extent to which the ground water system (the deep megaregolith) 
was filled. Many of the large outflow channels seen on Mars may have formed through the 
eruption of ground water under pressure from the megaregolith beneath the permafrost, 
which is estimated to have been about 1 km thick (Carr, 1979). Carr (1986) concluded 
that the megaregolith below this permafrost layer, planetwide, might contain no less than 
the equivalent of a water layer 350 meters thick, while Clifford's (1986) calculations of 
outgassed HaO on Mars would suggest that if more than a few percent of the quantity of 
water required to saturate the pore volume of the cryosphere were present, then a sub- 
permafrost water system of substantial proportions would result. After saturation of the 
pore volume of the cryosphere, the equivalent of an additional 100 meters of HzO inven- 
tory would be sufficient to create an acquifer nearly 4.3 km deep. This small volume of 
water is all that is required to produce this acquifer because the predicted pore volume 
at depth is very low (Clifford, 1986). It is further predicted that much of the water origi- 
nally at  depths of less than 1 km would remain in situ for most of martian history (Fanale 
eE al., 1986), thus constituting a possible method for fluidizing impact crater ejecta. 
Because of the importance that the presence of liquid water, liquid brines, or ground 
ice close to the surface would hold in global models of volatile abundance and physical 
state (e.g., Fanale and Jakosky, 1982; Fanale et d., 1986), more precise observational 
information is needed to help constrain these theoretical models of volatile distribution 
and state. One approach to identifying both the former presence (or absence) and physi- 
cal state of subsurface volatiles in the martian past comes from their inferred influence 
in controlling the morphology of rampart craters and crater ejecta blankets. Key 
testable morphologic indicators are thought to be the identification (or lack thereof) of 
melt water channels and small-scale topography on the surface and edges of the ejecta 
blankets, and the degree of stability of the parent crater walls (Mouginis-Mark, 1986). 
Diagnostic features are likely to be physically small in size (probably less than a kilome- 
ter in extent) and so have not previously been considered either in global studies of rarn- 
part craters (e.g. Johansen, 1979; Horner and Greeley, 1986) or cratering models (e.g., 
Schultz and Gault, 198% Schultz, 1986). The curreht research therefore attempts to 
describe morphologic features seen in very high resolution (8 to 17 meters per pixel) Vik- 
ing Orbiter images of crater ejecta blankets and interiors, in order to constrain the most 
likely target properties at the time of crater formation. 
https://ntrs.nasa.gov/search.jsp?R=19870014074 2020-03-20T10:31:02+00:00Z
The Viking Orbiter image data set contains approximately 400 frames at a spatial 
resolution of better than 10 meters per pixel, and 2,200 frames at a resolution of better 
than 20 meters per pixel, for which the atmosphere was either clear or only slightly 
obscured (J. Zimbelman, pers. comm. 1986). A search is being conducted of all these 
images and so far has revealed several examples of both interior and exterior features of 
impact craters that bear on the nature of the ejecta fluidizing medium. Preliminary 
observations and speculations on the physical state of the martian regolith at the time of 
crater formation are as follows: 
1) The ejecta lobes retain much of the small-scale structure that was produced during 
ejecta deposition. Radial striations, pressure ridges and the sharp distal ramparts 
are all well preserved on craters ranging in size from 4.5-26 krn in diameter. The 
general absence of remobilized materials as a consequence of water sapping from the 
emplaced ejecta suggests that the lobes possessed appreciable mechanical strength 
at the time of their emplacement and that the liquid water content of the flows was 
very low. No evidence to support the proposal by Woronow (1981) that the ejecta con- 
tained 16 to 72% water has been found. 
2) For two ejecta deposits within the basin Schiaparelli that have been partially eroded, it 
is evident that large blocks were transported within the ejecta flow, and that some of 
these blocks were most likely ice-rich. Subsequent melting or ablation of this ice has 
led to collapse of the ejecta deposit in isolated places to form features similar to ter- 
res trial kettle holes. 
3) The same eroded ejecta blankets reveal that flow striations extend through the thick- 
ness of the ejecta deposit, rather than simply beirig superficial features. This would 
imply that the ejecta flow was non-turbulent. akin to poorly fluidized terrestrial 
pyroclastic flows. 
4) Most of the crater interiors studied to date reveal few signs of melt water release or 
the abnormal amounts of slumping that would be expected if the rim material were 
water-rich. 
5) Rare examples of surface flow can be found both on the ejecta deposits and crater 
inner walls. In the case of a small crater southwest of Bakhuysen, it is believed that 
this surface flow was made possible due to the formation of this small crater on the 
ejecta blanket of Bakhuysen. For the cfater Cerulli, no obvious explanation for the 
unusually volatile-rich wall material presents itself, but it is clear that extensive sur- 
face flow, expressed by numerous small channels on the wall and continuous ejecta 
blanket, took place following the formation of the crater. 
It should be remembered that only a very small number of craters, all in a narrow 
latitude band (predominately in the southern hemisphere) have been included in this 
investigation, primarily due to the sparse coverage of the high resolution %king Orbiter 
images. Individually, the above observations do not argue convincingly for a single set of 
properties for the target material or the derived ejecta; small volumes of liquid water, 
larger volumes of ice, or purely atmospheric effects could all be used to explain ejecta 
fluidization. Taken together, however, it is believed that the observational data supports 
the notion that ice, rather than water or atmospheric effects, dominated both the 
emplacement process and the mode of ejecta fluidization for the majority of rampart 
craters studied here. By implication, ice must have been present in the target rocks in 
significant amounts (a few to perhaps 10 volume percent?) at the time of impact. The sur- 
face channels (both on the ejecta blankets and within craters) and kettle holes argue 
against atmospheric effects dominating ejecta fluidization, while the stability of small- 
scale topography and only the rare occurrence of surface channels indicates that liquid 
water was not commonly associated with the ejecta. 
Bs a topic for future investigation, numerical models need to be developed to deter- 
mine the effects of different volumetric amounts of ice that would have to be entrained 
within the ejecta to produce the observed morphologies. At  the present time the high 
mechanical strength of ejecta lobes, as demonstrated by up to several tens to hundreds 
of meters of relief on the lobes and ramparts, would suggest that the volumetric amount 
of ice that was incorporated within the ejecta was quite low. Quantitative estimates for the 
volume of ground ice are needed, but currently estimates appear consistent with the 
regolith/atmosphere models proposed for Mars and can explain the observed landform 
morphology. 
Clearly, however, the observations presented here only pertain to a very small 
number of fresh cratdrs that by chance were imaged at very high resolution by the 
Orbiters. While these craters provide an insight into the physical state of the target 
material for these random areas, image resolution is inadequate to draw firm conclusions 
on volatile state for most of Mars. As a result, it is concluded that the identification of 
small-scale features diagnostic of water sapping and ejecta remobilization on crater 
ejecta blankets constitutes one of the many interesting experiments that could be con- 
ducted with the sub-meter resolution camera planned as part of NASA's Mars Observer 
Mission. 
CES Boyce, J.M. (1979). A method for measuring heat flow in the martian crust 
using impact crater morphology (Abstract). @is. Plan. Geol. Rog . 19781 979, NASA W- 
8[)339, p. 114-118. Carr, M.H. (1979). Formation of martian flood features by release of 
water from confhed aquifers. J. Geophys. Res., vd. 84, p. 2995-3007. Carr, M.H. (1986). 
Mars: A water-rich planet (Abstract). Symposium on Mars: Euolution of  its climate and 
atmosphere LPII Contr. #599. p. 9-1 1. Carr, M.H., L.S. Crumpler, J.A. Cutts, R. Greeley, J.E. 
Guest and H. Masursky (1977). Martian impact craters and emplacement of ejecta by sur- 
face flow. J .  Geophys. Res., vol. 82, p. 4055-4065. Clifford, S.M. (1986). Mars: Crustal pore 
volume, cryosphere depth, and the global occurrence of groundwater (Abs.). Symposium 
on Mars: E)uoLution of its climate and atmosphere, LPI Conk. 599, p. 18-20. Fanale, F.P. 
and B.M. Jakosky (1982). Regolith-atmosphere exchange of water and carbon dioxide on 
Mars: Effects on atmospheric history and climate change. Planet. Space Sci. 801. 30, p. 
819-831. Fanale. F.P., J.R. Salvail, AP. Zent and S.E. Postawko (1986). Global distribution 
and migration of subsurface ice on Mars. Icarus, vol. 67, p. 1-18. Horner, V.M. and R 
Greeley (1986). Effects of elevation and plains thicknesses on martian crater ejecta mor- 
phologies on the ridged plains (Abstract). @ts. Plan. Geol. and Geophys. Bog. 1985, 
lF6BSP1.W8m3 p. 446-448. Howard, K.A and H.G. Wilshire (1975) Flows of impact melt at 
lunar craters. Jour. Research U. S. Geol. Sumey, 801. 3, p. 237-251. Johansen (1979). The 
latitude dependence of martian splosh cratering and its relationship to water (Abstract). 
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(Abstract). Symposium o n  Ma~s: Evolution, of i ts  climate and atmosphere. LPI c@aneP. 
#599. p. 67-69. Schultz, P.H. (1986). Crater ejecta morphology and the presence of water 
on Mars (Abstract). Symposium on Mars: fioLuii.on of i ts climate and atmosphere. LPI 
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Martian rampart craters: Morphologic clues for the physical state of the target at time of impact

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