[meteorite-list] How Mars Got Its Rust

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu May 6 13:20:36 2004
Message-ID: <200405061720.KAA09481_at_zagami.jpl.nasa.gov>

http://www.nature.com/nsu/040503/040503-6.html

How Mars got its rust

Model explains why the red planet is so red.

MARK PEPLOW
Nature Science Update
6 May 2004

Why is Mars so much rustier than the Earth? The red planet
has more than twice as much iron oxide in its outer layers
as our own, yet most planet scientists reckon the two
bodies were formed from the same materials.

David Rubie and colleagues from the University of
Bayreuth, Germany, say they have an answer: the intense
heat inside the early Earth was enough to convert a lot of
iron oxide into molten metallic iron, which seeped down into
the planet to form a huge liquid core.

Mars never achieved the temperatures needed for this process
simply because it is smaller, they say. This left more iron oxide in the
upper layers of the planet, which led to its distinctive russet hue and
relatively puny iron core.

"Our model shows that the planets could have formed from the same
material and then evolved to their present compositions and internal
structure," says Rubie.

To reach their conclusions, the team used a hydraulic press to
squeeze a sample of iron, nickel and oxygen to more than 175,000
times atmospheric pressure while heating it up to 2,400 ?C. These
experiments, published today in Nature[1], helped them to understand
how oxygen and iron would have behaved in the planets' early
magma oceans.

Crimson tide

About 4 billion years ago, the newly formed Earth and its
neighbouring rocky planets were hammered by intense meteor
bombardment. This would have melted the planets' surfaces into
oceans of magma. Rubie estimates that on Earth, these seas of
molten rock were about 1,800 km deep. Below that lay a solid mantle
and, finally, a core of molten iron.

The Earth is almost twice the diameter of Mars and is ten times more
massive. This means that the bottom of the Earth's magma ocean
would have been under much more pressure than that on Mars,
simply because there was more material pressing down from above.

On Earth, the pressure would have raised the magma's temperature
to more than 3,200 ?C, at which point iron oxide readily converts into
metallic iron and dissolved oxygen. The liquid iron would then have
rained downwards through the magma, creating a relatively large core
and leaving a paltry 8% iron oxide in the outer mantle. The process
was probably complete within the first 30 million years of the planet's
life, says Rubie.

But on Mars, the magma ocean would not have reached more than
about 2,200 ?C. In such an ocean, iron oxide would have been
perfectly stable. Rubie's experiments predict that these conditions
would leave behind a solid outer mantle that contains about 18% iron
oxide, precisely matching observations of martian geology. This also
explains why the martian core makes up a much smaller fraction of
planet mass than the Earth's core does.

"I do not know of any other explanation for Mars's rustiness," says
John Murray, a planetary scientist at the Open University in Milton
Keynes, UK. He adds that as most theories of planet formation
assume that Mars and Earth started off with the same materials, an
explanation of how they became so different is fundamental to
understanding how our solar system evolved.

References

    1. Rubie, D. C., Gessmann, C. K. & Frost, D. J. Nature, 429, 58
       - 61, doi:10.1038/nature02473 (2004). |Article|
Received on Thu 06 May 2004 01:20:11 PM PDT