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

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

Jones’ Theory: Obliteration Spike

Model of spike’s effect on crater classes

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

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

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.

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.

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

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

Degrees of Terrain Softening None Some A Lot

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!

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