A rock that fits in the palm of a scientist's
hand and weighs less than a pound is helping paint a picture of the history of
volcanism on Mars. That's because the little igneous rock broke off the Red
Planet when something slammed into the Martian surface about 1.1 million years
ago. It hurtled through space and then fell to Earth, landing in the desert in
Algeria. Yes, it's a Martian meteorite.
Northwest Africa (NWA) 7635, as the meteorite
has been dubbed, wasn't the only igneous rock ejected from Mars at the time.
Researchers have discovered at least 10 others with the same cosmic ray
exposure, evidence that they, too, flew through space after breaking off Mars
around the same time.
But NWA 7635 doesn't fall neatly into the
same category as those other Martian meteorites, called shergottites. While the
shergottites solidified from lava no more than 600 million years ago, NWA 7635
is about 2.4 billion years old, according to new research published Wednesday
in the journal Science Advances. And this could mean that one Martian volcano
was erupting for at least 2 billion years.
This confirms previous ideas that volcanic
activity in a particular site on Mars can last a long time, says study lead
author Thomas Lapen, a geologist at the University of Houston. Researchers have
two main lines of evidence for the geological processes that shape the surface
of the Red Planet, he explains in a phone interview with The Christian Science
Monitor. Researchers can study these meteorites, or they can study data from
the surface of the planet, as recorded by orbiters, landers, and rovers.
The latter vein of study has long suggested
that big volcanic centers on Mars, such as Tharsis and Elysium, could have
formed as long ago as 3 or 4 billion years ago, says Harry "Hap"
McSween, a geoscientist at the University of Tennessee Knoxville who was not
involved in the research. "It's nice to have confirmation," he tells
the Monitor.
Not everyone is convinced, though. These
Martian meteorites may actually be much older than Dr. Lapen and his colleagues
report, says Audrey Bouvier, Canada Research Chair in Planetary Materials and
curator of the Western Meteorite Collection at the University of Western
Ontario.
Dr. Bouvier, whose own research suggests some
depleted shergottites are actually 4.3 billion years old, explains in an email
to the Monitor that Lapen's dates could actually be shock ages – a sort of
resetting of the rock's minerals as a result of meteor impact. But other
meteorite scientists, including Dr. McSween, don't doubt Lapen's dating.
"I would say I definitely agree with the
age dating," Chris Herd, a geoscientist and curator of meteorites at the
University of Alberta who was not involved in the research, tells the Monitor
in a phone interview. "The group is, especially the first author, very
well-known for doing excellent work with these radiogenic isotopes the age
dating and so I have little doubt about the age that they obtain."
Carl Agee, director of the Institute of
Meteoritics at the University of New Mexico who also wasn't involved in the
research, agrees. "I think that they've made a really good argument for
this case that there's long-lived volcanism in a particular location on
Mars," he says in a phone interview with the Monitor. "And the way
that they do it is hard to argue with."
That's not to say that NWA 7635 doesn't
present other puzzles, the scientists say. "It has a different
mineralogy" from the other shergottites, Dr. Agee says. "It has some
characteristics that are reminiscent of shergottites, but it really is an
outlier."This suggests that perhaps the older rock crystallized under
different conditions from the younger ones, Dr. Herd says. And that introduces
a number of questions.
One way to explain this would be if NWA 7635
was actually ejected from a different site on Mars than the other meteorites
and therefore wasn't actually evidence that one volcanic center was active for
at least 2 billion years. But, Herd says, it would be an unlikely coincidence
if two impactors struck Mars around the same time 1.1 million years ago,
knocking off these meteorites.
Instead, this difference in composition could
say something about the change in the magma bubbling up from the Martian mantle
over time, and therefore the geologic history of Mars more broadly. On Mars,
plate tectonics don't drive geological processes like they do on Earth.
Instead, volcanoes are the result of hotspots arising from deep within the
planet, like the volcanoes that formed Hawaii and Iceland.
Without plate tectonics to move a plate over
the fountain of magma, Martian hot spots can produce much larger volcanoes than
Earth's. This is how researchers explain the size of Mars's Olympus Mons, the
largest volcano known in the solar system. It could also explain why volcanic
centers could be active for as long as this new study suggests.
Although "the wealth of information that
we can get from a single, small sample is really quite amazing," Herd
says, the meteorites would be even more useful if researchers knew where
exactly on the Martian surface they came from.
As such, McSween points out, one of the
objectives of NASA's Mars 2020 mission is to have the rover collect and cache
rocks to be returned to Earth at a future date. But, he adds, returning samples
to Earth will be a tricky, costly process. So, for now, he says, "these
meteorites are the cheapest spacecraft mission."
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