1. 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, Houston TX USA

2. Ken Jones’ Study of Martian Crater Morphologies
Global study of craters >15km diam.
General morphology classes: H/M/S/F
Regional statistics
H M S F

3. Jones’ Theory: Obliteration Spike

4. Model of spike’s effect on crater classes

5. Modelling how time-variable erosion affects crater morphologies
Obliteration time history
Total
fresh
slight
moderate
heavily
…degraded
(Chapman, 1974)

6. Signature of “episode” in morphologic statistics
Among intermediate sized craters (tens of km diameter), smaller ones (~10 km) are most heavily degraded, largest ones (>30 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

7. 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.

8. 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.

9. 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

10. 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

11. Degrees of Terrain Softening
None Some A Lot

12. 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!

13. 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