Evidence for Extensive Degradation in the Noachian from the Impact Cratering Record

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Evidence for Extensive Degradation in the
Noachian from the Impact Cratering Record
Clark R. Chapman
Southwest Research Inst.
Boulder, CO USA
Vernadsky-Brown Microsymposium 35 The Noachian
Period in the History of Mars March 9/10, 2002
Houston TX USA
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Ken Jones Study of Martian Crater Morphologies

• Global study of craters gt15km diam.
• General morphology classes H/M/S/F
• Regional
  statistics

H M S
F
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Jones Theory Obliteration Spike
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Model of spikes effect on crater classes
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Modelling how time-variable erosion affects
crater morphologies
Obliteration time history
Total fresh slight moderate heavily degraded
(Chapman, 1974)
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Signature of episode in morphologic statistics
Among intermediate sized craters (tens of km
diameter), smaller ones (10 km) are most heavily
degraded, largest ones (gt30 km) only modestly
degraded or nearly fresh.

• (a) sequence f,s,m,h indicates incomplete-ness
  due to resolution
• (b) Mars data (Jones)
• (c) Sequence h,m,s,f indicates obliteration
  episode smaller craters are most affected,
  largest ones least affected

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Regional variations in intensity or duration of
spike

• Case 1 obliteration episode same duration,
  varies in intensity regionally
• Case 2 obliteration episode same intensity,
  varies in duration regionally
• Data suggest Case 1 but need refined studies at
  high res.

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Absolute age of obliteration

• Early Mariner 9 interpretations had obliteration
  tied to the declining early cratering flux.
• Depending on calibration of absolute ages, the
  obliteration could have happened toward the end
  of the decline (a), or considerably later (b).
• But the important conclusion is that it was
  decoupled from the end of the early bombardment.

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LHB on Mars?
Mars ?
LHB

• One Mars meteorite (and only one ALH84001) is
  very old and has an Ar-Ar age of 3.9 Ga
  statistics of ONE (Ash et al., 1996)
• Meteorite degassing ages are very spread out
  compared with lunar LHB and somewhat spread out
  compared with lunar rocks
• Evidence is dissimilar!
• Different impact histories or
• Different selection biases

Lunar rock de-gassing ages
Kring Cohen 2002
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Valley Networks How do they Affect Crater
Morphologies?

• Valleys follow low-lands in MOLA topography, but
  originate within few km of divides (Irwin
  Howard, 2001)
• Crater rims can be eroded, but valleys rarely cut
  through craters
• But associated processes could flatten floors

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Degrees of Terrain Softening
None
Some A
Lot
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A wide variety of surface modication processes...

• Many kinds of processes, many different
  signatures
• Lacustrine, oceanic
• Volcanic
• Aeolian (dunes, storms)
• Tectonic
• Glacial
• Rivers and streams
• Subterranean flow
• Creep
• Isostatic adjustment
• Superimposed cratering
• Etc., etc.

Wind Volcanism No modifica-tion at all!
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Conclusions about crater obliteration in the
Noachian

• Toward the end of the early bombardment (LHB?)
  there was an epoch of intense modification of the
  southern uplands
• Since it was not tied to the declining cratering
  rate, it was due to some other cause, or causes
• The obliteration was global varied somewhat in
  strength regionally
• Detailed morphologic studies should reveal the
  cause, or causes

MOC image and hi-res footprint