Meteorite Found On Mars Yields Clues About Planet's Past

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PASADENA, Calif. -- NASA's Mars Rover Opportunity is investigating a
metallic meteorite the size of a large watermelon that is providing
researchers more details about the Red Planet's environmental
history.

The rock, dubbed "Block Island," is larger than any other known
meteorite on Mars. Scientists calculate it is too massive to have hit
the ground without disintegrating unless Mars had a much thicker
atmosphere than it has now when the rock fell. Atmosphere slows the
descent of meteorites. Additional studies also may provide clues
about how weathering has affected the rock since it fell.

Two weeks ago, Opportunity had driven approximately 600 feet past the
rock in a Mars region called Meridiani Planum. An image the rover had
taken a few days earlier and stored was then transmitted back to
Earth. The image showed the rock is approximately 2 feet in length,
half that in height, and has a bluish tint that distinguishes it from
other rocks in the area. The rover team decided to have Opportunity
backtrack for a closer look, eventually touching Block Island with
its robotic arm.

"There's no question that it is an iron-nickel meteorite," said Ralf
Gellert of the University of Guelph in Ontario, Canada. Gellert is
the lead scientist for the rover's alpha particle X-ray spectrometer,
an instrument on the arm used for identifying key elements in an
object. "We already investigated several spots that showed elemental
variations on the surface. This might tell us if and how the metal
was altered since it landed on Mars."

The microscopic imager on the arm revealed a distinctive triangular
pattern in Block Island's surface texture, matching a pattern common
in iron-nickel meteorites found on Earth.

"Normally this pattern is exposed when the meteorite is cut, polished
and etched with acid," said Tim McCoy, a rover team member from the
Smithsonian Institution in Washington. "Sometimes it shows up on the
surface of meteorites that have been eroded by windblown sand in
deserts, and that appears to be what we see with Block Island."

Opportunity found a smaller iron-nickel meteorite, called "Heat Shield
Rock," in late 2004. At about a half ton or more, Block Island is
roughly 10 times as massive as Heat Shield Rock and several times too
big to have landed intact without more braking than today's Martian
atmosphere could provide.

"Consideration of existing model results indicates a meteorite this
size requires a thicker atmosphere," said rover team member Matt
Golombek of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
"Either Mars has hidden reserves of carbon-dioxide ice that can
supply large amounts of carbon-dioxide gas into the atmosphere during
warm periods of more recent climate cycles, or Block Island fell
billions of years ago."

Spectrometer observations have already identified variations in the
composition of Block Island at different points on the rock's
surface. The differences could result from interaction of the rock
with the Martian environment, where the metal becomes more rusted
from weathering with longer exposures to water vapor or liquid.

"We have lots of iron-nickel meteorites on Earth. We're using this
meteorite as a way to study Mars," said Albert Yen, a rover team
member at JPL. "Before we drive away from Block Island, we intend to
examine more targets on this rock where the images show variations in
color and texture. We're looking to see how extensively the rock
surface has been altered, which helps us understand the history of
the Martian climate since it fell."

When the investigation of Block Island concludes, the team plans to
resume driving Opportunity on a route from Victoria Crater, which the
rover explored for two years, toward the much larger Endeavour
Crater. Opportunity has covered about one-fifth of the 12-mile route
plotted for safe travel to Endeavour since the rover left Victoria
nearly a year ago.

Opportunity and its twin rover, Spirit, landed on Mars in January 2004
for missions originally planned to last for three months. Both rovers
show signs of aging but are still very able to continue to explore
and study Mars.

NASA'S JPL manages the Mars Exploration Rovers Opportunity and Spirit
for NASA's Science Mission Directorate in Washington. To see the
image and obtain more information about the rovers, visit:

http://www.nasa.gov/rovers