Comments by Chris McKay, NASA ARC, on the 8 December 2000 paper by Malin and Edgett regarding layered deposits on Mars
Note: These comments on the 8 December paper by Malin and Edgett regarding layered deposits on Mars, were provided to SpaceRef by Chris McKay, NASA ARC
The paper of Malin and Edgett (2000) shows the BEST yet images of the
layered deposits that have been thought for some time to be lacustrine. Its
a great success story for MGS and a very positive note in a time of
diminished confidence and lowered expectations regarding Mars missions.
In some ways Malin and Edget overstate their case a bit.
Previous
work is described as having given “Speculative affirmative answers…” and
they describe their hypothesis as a “previously unanticipated, Martian
history” and in the conclusion they say “Until now, it has not been certain
that such materials [lake deposits] actually exist on Mars”.
In fact these lake deposits have been well studied for quite some time.
Nonetheless it is a real pleasure to see these new high resolution images
confirming the previous work. (But not really making it “certain”..
these are only pictures after all.)
The important point here is that there are lake sediments on Mars. For
example in the Canyons of Valles Marineris. This was first suggested by
McCauley (1978) and worked extensively by Nedell et al. (1987) (both cited
by Malin and Edgett). McKay and Nedell (1988) suggested the deposits were
carbonates (more on that below).
As summarized by Malin and Edget there are two key points.
- The material is strong (forms cliffs) but appears to erode to dust not
boulders. (Malin and Edget infer this to mean that it is fine grained like
clay or silt). - There are no visible source regions. This is a real puzzle and a real
clue. A particularly good example is Hebes Chasma which is a box canyon.
Hebes Chasma is an interesting place. This canyon is a box canyon about 280
km long. The mesa in the center of the canyon shows layered deposits that
are believed to have been deposited in standing bodies of water (Nedell et
al., 1987) and have been suggested to be carbonates (McKay and Nedell,
1988). The lakes that may have existed in these canyons may actually date
to the period after the intial warm epoch that formed the dendritic
channels and thus are of interest exobiologically as a possible habitat for
life after ambient conditions on the surface of Mars had become too cold
and dry to support life.
How could this material have gotten into this box canyon. Malin and Edgett
propose several models all of which depend on transporting fine grain
solids by air or water but none of them really does the job. The easiest
answer is that there was no transport; the deposits are carbonates created
in place. This is not a new idea it was suggested years ago (McKay, C.P.
and S.S. Nedell, Are there carbonate deposits in the Valles Marineris,
Mars? Icarus, 73, 142-148, 1988. ) and it would explain how the material
got there. Calcium (most likely) or iron (possible for Mars) or magnesium
(maybe) in solution in the groundwater and CO2 from the atmosphere cause
in-situ saturation and precipitation and carbonates deposit without any
solid (clastic) material being transported in from the side. I.e. sediments
without an obvious erosional source.
This should not be confused with evaporates (eg. salts) which, as Malin
and Edget correctly point out only form when water is drying out.
Carbonates can form in deep water easily, see for example the recent work
on carbonate formation in a freshwater lake in British Columbia:
- Laval, B., S.L. Cady, J.C. Pollack, C.P. McKay, J.S. Bird, J.P. Grotzinger, D.C. Ford, and H.R. Bohm, Modern freshwater microbialite analogues for ancient dendritic reef structures, Nature, 407, 626-629, 2000.
My favorite example is the Permian reef structure that is today the
Guadelupe Mountains of New Mexico and Texas. This is a huge carbonate
(layered) structure. The exposed part of the Guadelupe Mtns are about the
size as the mesa in Hebes Chasma.
We also proposed that the lakes in which these carbonates formed
were ice-covered by analogy with the Antarctic dry valley lakes:
(McKay, C.P. and W.L. Davis, Duration of liquid water habitats on early
Mars. Icarus, 90, 214-221, 1991.)
If these are carbonates why can’t we see them spectrally? Good question and
I think the answer may lie in two areas: 1) thin layers of dust, and/or
2) carbonates are unstable (due to UV or acid erosion) at the surface as
suggested by Mukhin et al (1996). So the carbonates may be a few mm below
the surface. Close enough to show their sedimentary pattern but too deep to
be seen spectrally. If carbonates are really unstable this could account
for no boulders in the weathering products.
- (Mukhin, A.P. Koscheev, Yu. P. Dikov, J. Huth, and H. Wanke, Experimental simulations of the decomposition of carbonates and sulphates on Mars. Nature 379, 141-143, 1996.)
So the new images fit perfectly into what many folks have been doing for a
while. Its not so NEW but it sure is NICE to see such splendid
confirmation. Let work now to get a sample back (that’s how we become
“certain”).
Related Links
° 4 December 2000: Evidence of Martian Land of Lakes Discovered, NASA PAO
° 4 December 2000: Science report: sedimentary rocks on Mars may suggest an ancient land of lakes, Science Magazine
Related Images
° 4 December 2000: IMAGES from Science report: sedimentary rocks on Mars may suggest an ancient land of lakes, Science Magazine
° 4 December 2000: Layered Material in West Arabia Terra Crater, MSSS/NASA JPL
° 4 December 2000: Layered Outcrops of Far West Candor Chasma, MSSS/NASA JPL
° 4 December 2000: Sediment History Preserved in Gale Crater Central Mound, MSSS/NASA JPL
° 4 December 2000: Alternating Light- and Dark-toned Layers in Holden Crater, MSSS/NASA JPL
° 4 December 2000: Light-toned Layered Outcrops in Valles Marineris Walls, MSSS/NASA JPL
° 4 December 2000: “White Rock” of Pollack Crater, MSSS/NASA JPL
° 4 December 2000: Illustrations Regarding Early Mars Sedimentary Rocks from the NASA Space Science Update, December 4, 2000, MSSS/NASA JPL
