[meteorite-list] Mars Rover's Meteorite Discovery Triggers Questions

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 20 12:48:21 2005
Message-ID: <200501201748.JAA01370_at_zagami.jpl.nasa.gov>

http://www.space.com/missionlaunches/mars_meteor_050120.html

Mars Rover's Meteorite Discovery Triggers Questions
By Leonard David
space.com
20 January 2005

The discovery of an iron meteorite sitting on Mars by NASA's Opportunity
rover has kick-started a wide-ranging discussion as to what the find may
be telling us about the planet itself, past water conditions there, and
just how peppered the red planet might be with the fallen objects.

Roughly the size of a basketball, the object is mostly made of iron and
nickel, and is the first meteorite of any type ever identified on
another planet.

Now labeled Heat Shield Rock, the meteorite was found at the robot's
exploration zone, a huge empty parking lot-like tract of martian real
estate called Meridiani Planum.

Once scientists here on Earth spotted the odd-looking rock, the
Opportunity rover was wheeled into position. The robot was then
commanded to utilize a suite of science instruments to survey the meteorite.

New line of inquiry

How does the finding of the meteorite scale given all the Mars rover
science accumulated to date?

"Finding one meteorite is surprising and interesting, but not by itself
what I'd call one of our most important discoveries," responded Steve
Squyres of Cornell University in Ithaca, New York and principal
investigator for the science instruments on both Opportunity and Spirit,
its sister ship also busy at work but on the other side of Mars.

"The important thing, I think, is that this now opens up a whole new
line of inquiry for us," Squyres said, calling attention to several
questions, such as: How common are meteorites at Meridiani? What does
the concentration of meteorites tell us about erosion rates on the plains?

"We've seen lots of little rocks on the plains, but with this one
exception -- and Bounce Rock -- we've never stopped to look at one,"
Squyres told SPACE.com.

In April of last year, the rover studied "Bounce Rock", an odd,
football-sized object that Opportunity struck while bouncing to a stop
inside protective airbags on landing day over a year ago. Scientists
noted at the time that the rock's elemental composition was unlike
anything seen on Mars before, with similarities to a meteorite tagged
EETA79001 that was found in Antarctica in 1979.

"So what fraction of the rocks on the plains are meteorites? We haven't
really thought much about meteorites until now, but this discovery has
really opened our eyes to the question. As we work to answer it, we may
learn quite a bit about the long-term history of the plains," Squyres said.

Other scientists contacted by SPACE.com offered their thoughts on the
meteorite find.

The Antarctica experience

An iron meteorite being found on Mars would be "a truly remarkable
discovery," said Carl Agee, Director of the Institute of Meteoritics in
the Department of Earth and Planetary Sciences at the University of New
Mexico, Albuquerque, New Mexico.

"Here at the Institute," Agee said, "we're always a little skeptical
about 'dense black rocks' until some careful examination and analyses
have been done. On the other hand, we're finding many meteorites in the
deserts of Earth, so why not on Mars?"

Agee pointed out that running across a meteorite on another planetary
body would be a first. No meteorite was ever found on the Moon - even
with all the survey work done there by both robots and humans, he said.

"The fact that a robot found it is even more astounding, since a human
walking on Mars would probably be much better at spotting meteorites in
a landscape. We know this from the Antarctica experience," Agee
explained, pointing to the on-going treks to that part of our planet and
the concentrations of meteorites found there.

"The meteorite flux and the cratering rate on Mars is also a very
interesting problem. Perhaps the flux is higher than we thought and thus
meteorites are more abundant. This would have an effect on assumed
cratering rates which are used to calibrate the martian geologic time
scale," Agee said.

If this is an iron meteorite found by Opportunity, then it seems to have
survived the oxidizing surface environment of Mars by not rusting away
yet, Agee noted. "This may mean that it has been sitting on the surface
for a short length of time, either because it was buried and is now
being exposed by erosion or it fell to Mars recently."

A fluke..or?

Clark Chapman, a planetary scientist at Southwest Research Institute in
Boulder, Colorado, said he was very skeptical when first hearing about
the meteorite on Mars.

"On the Earth, after all, even in most favored locations, a lot of
people can cover an awful lot of ground before finding a meteorite.
While Mars is a less 'active' world than Earth, it has sand dunes and
dust storms...a lot going on," Chapman said. "And Spirit and
Opportunity, amazing though they are, are surveying very small regions
very slowly."

Chapman said the meteorite finding is a lesson learned.

"As often happens with theoretical expectations, solid experimental and
observational reality has intruded. No doubt it will spur people to
think more carefully about how common meteorites might be on the surface
of Mars," Chapman said. "Maybe this is a fluke. But, more likely, we
will learn something about Martian surface processes as a result of this
unexpected discovery."

Opportunity's luck

Coming across a meteorite on Mars that is apparently iron in composition
"is pretty amazing, but is consistent with Opportunity's luck - lands in a
little crater with fascinating rocks exposed - then finds a meteorite,"
said G. Jeffrey Taylor of the Hawaii Institute of Geophysics and
Planetology at the University of Hawaii in Honolulu.

Taylor said the meteorite is perched like they are when found in
Antarctica. In that case they are revealed by sublimation of the ice,
after being transported a long way.

Unless it fell very recently, the meteorite on Mars was probably
buried "then excavated when the surrounding dirt was removed", Taylor
speculated. "This leads to some interesting possibilities and questions."

First, the meteorite is not weathered much, indicating dry conditions
since it landed. "That is consistent with the presence of minerals that
are easily altered by water, such as olivine," he said, which would rot
away in several thousand to a few tens of thousands of years --
depending on temperature -- if sitting in water.

Might be plenty of them

Another observation, Taylor noted, is how would such a meteorite get
buried in the first place - by brief flood, wind, or by self-burial when
it whacked into Mars? That earns an "I dunno", he said.

Furthermore, assuming it was buried, what processes uncovered the
meteorite? Taylor advised that wind is a likely possibility, but
wondered how long that would that take.

Determining when that meteorite landed is an answer that can't be known
absolutely without someone bringing a piece to Earth, Taylor said.

But the question of how many meteorites litter the surface of Mars is an
intriguing one.

"It seems amazingly lucky for Opportunity to find this, even given that
rover's luck. So there might be plenty of them. This might have to do
with proximity to the asteroid belt, but especially the dry conditions
on Mars. That's where we find lots of meteorites on Earth, in dry
deserts of northern Africa, Australia, and New Mexico, and the cold
desert of Antarctica," Taylor concluded.

Meteorite collector's dream site

"It is way cool!" That's the short and simple view from Laurie Leshin,
Director of the Center for Meteorite Studies in the Department of
Geological Sciences at Arizona State University in Tempe, Arizona.

"I would very much like to see them examine some of the other rocks
strewn along the plains of Meridiani -- maybe this is the meteorite
collector's dream site -- like Antarctica here on Earth. It would be
interesting to see if they can find some other types of meteorites,"
Leshin said.

Leshin placed the meteorite find on Mars into a larger picture.

Dubbed "Heat Shield Rock" the meteorite can be considered an extension
of the story of water at the Meridiani site, Leshin said. "The rocks at
Meridiani indicate water in the past, and this meteorite -- with its
apparently fresh metallic surface -- probably shows how little water has
been involved in the recent history of this place. It reinforces our
notion that Mars in the past may have supported extensive liquid water,
but that Mars today is a dry, cold desert," she said.

Leshin's bottom line: "Turns out that a very harsh place for life is a
great place to preserve meteorites!"
Received on Thu 20 Jan 2005 12:48:06 PM PST