Transcript No Slide Title

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
Total
fresh
slight
moderate
heavily
…degraded
(Chapman, 1974)
Obliteration time history
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 incompleteness 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.
Mars
LHB
LHB on Mars?
 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)
Lunar rock degassing ages
 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
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



Wind

Volcanism

No modification at all!






Lacustrine, oceanic
Volcanic
Aeolian (dunes, storms)
Tectonic
Glacial
Rivers and streams
Subterranean flow
Creep
Isostatic adjustment
Superimposed cratering
Etc., etc.
Conclusions about crater
obliteration in the Noachian
 Toward the end of the early
MOC image
and hi-res
footprint
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