[meteorite-list] Rusting Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Dec 14 16:51:52 2004
Message-ID: <200412142151.NAA16282_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/asy/default.aspx?c=a&id=2645

Astronomy Magazine
December 2004 astro bytes
December 9, 2004

[snip]

Rusting Mars

Exactly what processes made martian soil the way we find it today - rich
in colorful, and sometimes strongly magnetic, iron oxides and hydroxides
- eludes planetary scientists. To shed light on martian soil chemistry,
a team led by Vincent Chevrier of the University of Aix-en-Provence in
France watched powders of natural and synthetic iron weather for a year
under a simulated martian atmosphere.

Previous studies have looked at the effects of surface weathering on
Mars in the presence of water, but they neglected the fact that, unlike
Earth, Mars has an atmosphere primarily made of carbon dioxide.
Moreover, not all martian rock originated on Mars. According to some
studies, meteorites and interplanetary dust particles may contribute as
much as 30 percent by weight to a clump of martian soil. And these
sources can hold as much as 10 times the quantity of metal, sulfide, or
magnetite found in martian rocks.

Chevrier and colleagues exposed powders of synthetic iron and magnetite
(an iron oxide, Fe3O4) and natural hexagonal pyrrhotite (an iron
sulfide, Fe9S10) for a full year in two atmospheres - water plus carbon
dioxide for one set, water plus hydrogen peroxide for the other. To
speed up chemical reactions and to mimic the warmer climate many suspect
prevailed in the martian past, both temperature and atmospheric pressure
were kept higher than those on the Red Planet today.

Magnetite exhibited no change in structure or composition in either
atmosphere, which suggests that on Mars, as on Earth, the mineral simply
weathers out of bedrock.

Metallic iron remained unchanged even after one year in the peroxide
atmosphere, but in the water atmosphere, it underwent rapid changes. The
sample turned an olive color after 19 days, was a "deep, dusky red" by
day 75, and a "dark, yellowish brown" by the experiment's end. It became
increasingly crystalline, too, eventually forming the minerals siderite
and goethite. Goethite can change to hematite in a waterless
environment, so any goethite that formed when the martian environment
was warmer and wetter may exist under layers of hematite today.

The pyrrhotite turned yellow-red after just 5 days in the peroxide
atmosphere, but it took about 100 days to turn a dark, brownish red in
the water atmosphere. In both cases, though, the same minerals formed:
goethite, sulfur, and small amounts of iron sulfates. One of the
sulfates was jarosite, which was identified at Meridiani Planum by the
Opportunity rover last spring.

The scientists reported their results in the December issue of Geology.
They say the experiment reveals a possible mechanism for creating these
minerals on Mars without invoking features not seen there today, such as
oxygen, acid vapor, or extensive bodies of surface water. -- Francis Reddy
Received on Tue 14 Dec 2004 04:51:45 PM PST