[meteorite-list] Martian Crater Records Aftermath of Amazon-like Flood

[Ron Baalke]

http://space.newscientist.com/article/dn13333-martian-crater-records-aftermath-of-amazonlike-flood.html

Martian crater records aftermath of Amazon-like flood
David Shiga
New Scientist
February 20, 2008

Billions of yeas ago on Mars, a river suddenly burst to the surface from
underground and flooded a large crater, only to disappear again within a
few decades, according to a new study. Although the water was
short-lived on the surface, it may have been present for longer
underground, potentially creating conditions favourable to life.

Many ancient river channels are among the evidence that liquid water was
once present on Mars, but in many cases it is difficult to know just how
long that water was around.

Now, a new study says the water in at least one location on Mars flowed
for just a few decades before disappearing again. The study, led by Erin
Kraal of Virginia Polytechnic Institute and State University in
Blacksburg, US, is based on the pattern of sediment left behind when an
ancient river emerged from underground, flowed for 20 kilometres, then
drained into a crater.

It was an abrupt and catastrophic event, Kraal says. "It would be like
the Mississippi River suddenly bursting out of the ground and flowing
for 10 years and then stopping," she told New Scientist.

The researchers created a mock crater on Earth about 2 metres across.
Water flowed into it, carrying along sand and depositing a fan of
sediment on the crater floor. Fluctuations in how much sand was eroded
by the flow at any given time led to sediment being deposited in
distinct steps, just like in the Martian crater analysed by the team,
which is 128 km wide.

Water flowing into the Martian crater after the 'stepped' sediment was
first laid down would have either buried the stepped pattern in new
sediment, or cut a channel into it, the researchers say. This allowed
them to deduce that the sediment was all deposited in a single,
uninterrupted episode where water was flowing.

Magma heating

The researchers then calculated the rate and duration of the water flow
using estimates of how much sediment was deposited in the crater and the
size of the 20-km-long channel. They arrived at a flow rate of between
2200 and 800,000 cubic metres per second, with a maximum flow time of
about 90 years. 800,000 cubic metres per second is about five times the
flow rate of the Amazon River.

What could have triggered such a flood? Kraal thinks the water was
probably trapped beneath the surface and was mostly or entirely frozen.
It may have melted suddenly when it was heated by magma.

Even though the water did not last long on the surface at this location,
it could have been present for much longer underground. It may have been
kept liquid in places by the heat from magma, and potentially fostered
life, Kraal says. Because of this possibility, the stepped sediment
deposits would be good places to look for signs of past life on Mars,
she says.

Boulder search

Similar stepped sediment deposits appear in only a few places on Mars,
Kraal says. And it is much less clear how long water was present in
other locations with evidence of past water.

Bethany Ehlmann of Brown University in Providence, Rhode Island, US,
says that at other places on Mars, water appears to have been much
longer-lived on the surface.

Two ancient river deltas called Eberswalde and Jezero appear to have
been "built up over hundreds to thousands of years, in more quiescent
lakes fed by large valley networks, in systems reminiscent of rivers on
Earth", she told New Scientist.

She says it should be possible to test the catastrophic flooding
scenario for the crater analysed by Kraal's team by using NASA's Mars
Reconnaissance Orbiter's camera to look for large boulders in the
sediment, which could only be transported by rapidly flowing water.

Journal reference: Nature <http://www.nature.com/nature/index.html>
(DOI: 10.1038/nature06615)