[meteorite-list] Round Mars Grains Excite NASA

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:20 2004
Message-ID: <200402042126.NAA04658_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/2/hi/science/nature/3460143.stm

Round Mars grains excite Nasa
BBC News
February 4, 2004

Nasa's robot rover Opportunity has found round
grains in the soil of Mars, raising the possibility
they may have been shaped by liquid water.

Photos taken by the rover's microscopic imager
show coarse soil grains of different shapes,
including round ones.

But few of the grains are spherical, which means
other forces could be responsible for their shape.

Scientists say meteorite collisions could have
produced rounded grains by melting the Martian
rock on impact.

Coarse grains

"We saw a bunch of really fine-grained stuff that
we couldn't tell much about and then we saw these
coarser grains on top," said Nasa principal
investigator Steve Squyres, describing the soil's
structure.

"We began to notice that some of these grains looked
awfully darn round."

Mission scientist Hap McSween said there was a
limited number of ways very round grains could have
formed.

One possibility was that grains in moving water had
accumulated layers of material on their outside as
they rolled around. These near-spherical grains are
known as oolites.

But the microscopic imager revealed that very few
of the particles were spherical balls. This meant
that the moderately rounded grains are unlikely to
be oolites.

"Oolites also shouldn't have these these little
bubbles in them."

Molten jet

One explanation, he said, was that large meteors
could have melted Marian rocks, spraying the
resulting molten material out as a fine jet which
cools to form a glass.

If the original Martian rock had water in it at
the time of the impact, gas could have formed in the
molten material, which might explain the small
holes or bubbles in the grains.

"These droplets, as they fly through the air,
develop aerodynamic shapes - intriguing shapes like
dumbells and teardrops, sometimes buttons. And I
think I've seen all three of those shapes while
looking at the [microscopic imager]," Dr McSween
added.

Another possibility is that round grains in the soil
are lapilli: ash particles from a volcanic eruption
that coagulate to form small, rounded pellets.

'Mineral map'

The scientists released a mineral map of part of
Opportunity's crater, which was produced using data
from the Miniature Thermal Emission Spectrometer
(Mini-Tes).

It shows that the grey mineral haematite, which
sometimes forms in the presence of water, is most
concentrated near the rim of the crater.

Scientists also took measurements of the soil using
the rover's Moessbauer spectrometer. It confirms
the presence of a magnetic compound which could be
haematite.

Meanwhile, Opportunity's twin rover Spirit is
recovering gradually from its malfunction on 22
January.

Dr Mark Adler, deputy mission manager for Spirit,
said attempts to provide the rover with its
bearings by getting it to look at the sun had failed.

This could be remedied by re-formatting its flash
memory, thought to be the source of the problem
that crippled it.
Received on Wed 04 Feb 2004 04:26:59 PM PST