Sky & Telescope - September 2020 - 14

COSMIC RELIEF by David Grinspoon

Not From
Around
Here
NASA's rovers have found
meteorites on Mars that came
from elsewhere. What secrets
do they hold about Martian
history and climate?
WHEN THE OPPORTUNITY ROVER

SEPTEMBER 2 02 0 * SK Y & TELESCOPE

A golfball-size ironnickel meteorite on
Mars imaged by
the Curiosity rover
in 2016

Mars, and we've discovered an average of about one meteorite every 5 km.
That would be ridiculous luck on some
random drive on Earth, even in deserts
akin to the surface of Mars. Has some
process concentrated meteorites in the
regions the rovers have explored?
Our own planet has places that
naturally concentrate meteorites. In
Antarctica's Alan Hills, for instance,
the ﬂow of ice acts like a conveyer belt,
depositing old rocks in a relatively small
area. Could something similar have
happened where our Mars rovers have
made their meteorite sightings?
I asked my colleague Barbara Cohen
(NASA Goddard Space Flight Center),
who has led the effort to catalog the
meteorites seen on the Red Planet.
(They're now listed in the ofﬁcial
database of meteorites at lpi.usra.edu/
meteor.) The surfaces where both Spirit
and Opportunity roamed are deﬂationary, Cohen told me. "It's similar to the
Sahara Desert, where meteorites fall
onto sand dunes, then as the dunes
blow away you get these lag deposits of
meteorites." The dryness preserves them

for hundreds of millions of years. To
get similar concentrations on Mars, she
said, "you have to infer that both things
are in effect - there's a way to concentrate them and a way to preserve them."
Could the relative lack of stony
meteorites we've found on Mars, at least
so far, simply be because irons resist
erosion better? Perhaps, but Cohen
notes that chondritic meteorites might
not stand out as well from Martian
rocks and thus might have escaped
notice as the rovers passed by. Or maybe
Mars, being farther from the Sun and
closer to the asteroid belt, receives a different mix of space rocks.
Further examination of these
fortuitous ﬁnds on Mars has revealed
that multiple erosional processes
have affected their surface chemistries, including some modiﬁcation by
water. These and other meteorites may
therefore become important probes of
Martian history, even climate change.
■ Contributing Editor DAVID GRINSPOON is a senior scientist at the Planetary Science Institute in Tucson, AZ.

N ASA / JPL- CA LTECH / MSSS

discovered the ﬁrst extramartian rock in
May 2004, it seemed like such a delightful novelty. To travel all that way from
our home planet to explore the Martian
hills, dunes, and craters only to stumble
upon a rock that is not from that world!
What are the chances that among all
the places we could have roved we'd roll
right up to this interloper on Mars?
The chances are not all that poor, as
it turns out. Between Spirit, Opportunity, and Curiosity, we've so far encountered more than 20 of these aliens. How
did Opportunity recognize that ﬁrst
outsider? It has the look of many iron
meteorites on Earth - shiny, metallic,
and eroded with vesicles, or pits.
Astute observers noticed that,
strangely, these rocks seem to have the
same infrared spectrum as the Martian
sky. Their shiny, metallic surfaces act
as mirrors, reﬂecting sky light back
to rover instruments. Once scientists
correctly interpreted this, the spectra
became a tool to ﬁnd more meteorites.
Most meteorites we've spotted on
Mars are iron or stony iron. On Earth
these make up about 80% of ﬁnds but
only 6% of witnessed falls, most of
which are stony meteorites called chondrites. So far we've found no chondrites
on Mars. Therein lies a mystery that
we're still trying to work out.
Our rovers have collectively logged
only about 95 kilometers (60 miles) on

http://lpi.usra.edu/meteor http://lpi.usra.edu/meteor

Sky & Telescope - September 2020

Table of Contents for the Digital Edition of Sky & Telescope - September 2020