Chemicals found in
Martian rocks by NASA's Curiosity Mars rover suggest the Red Planet once had
more oxygen in its atmosphere than it does now.
Researchers found
high levels of manganese oxides by using a laser-firing instrument on the
rover. This hint of more oxygen in Mars' early atmosphere adds to other
Curiosity findings -- such as evidence about ancient lakes -- revealing how
Earth-like our neighboring planet once was.
This research also
adds important context to other clues about atmospheric oxygen in Mars' past.
The manganese oxides were found in mineral veins within a geological setting
the Curiosity mission has placed in a timeline of ancient environmental
conditions. From that context, the higher oxygen level can be linked to a time
when groundwater was present in the rover's Gale Crater study area.
"The only ways
on Earth that we know how to make these manganese materials involve atmospheric
oxygen or microbes," said Nina Lanza, a planetary scientist at Los Alamos
National Laboratory in New Mexico. "Now we're seeing manganese oxides on
Mars, and we're wondering how the heck these could have formed?"
Microbes seem
far-fetched at this point, but the other alternative -- that the Martian
atmosphere contained more oxygen in the past than it does now -- seems
possible, Lanza said. "These high manganese materials can't form without
lots of liquid water and strongly oxidizing conditions. Here on Earth, we had
lots of water but no widespread deposits of manganese oxides until after the
oxygen levels in our atmosphere rose."
Lanza
is the lead author of a new report about the Martian manganese oxides in
the American Geophysical Union's Geophysical Research Letters. She uses
Curiosity's Chemistry and Camera (ChemCam) instrument, which fires laser pulses
from atop the rover's mast and observes the spectrum of resulting flashes of
plasma to assess targets' chemical makeup.
In Earth's geological
record, the appearance of high concentrations of manganese oxide minerals is an
important marker of a major shift in our atmosphere's composition, from
relatively low oxygen abundances to the oxygen-rich atmosphere we see today.
The presence of the same types of materials on Mars suggests that oxygen levels
rose there, too, before declining to their present values. If that's the case,
how was that oxygen-rich environment formed?
"One potential
way that oxygen could have gotten into the Martian atmosphere is from the
breakdown of water when Mars was losing its magnetic field," said Lanza.
"It's thought that at this time in Mars' history, water was much more
abundant." Yet without a protective magnetic field to shield the surface,
ionizing radiation started splitting water molecules into hydrogen and oxygen.
Because of Mars' relatively low gravity, the planet wasn't able to hold onto
the very light hydrogen atoms, but the heavier oxygen atoms remained behind.
Much of this oxygen went into rocks, leading to the rusty red dust that covers
the surface today. While Mars' famous red iron oxides require only a mildly
oxidizing environment to form, manganese oxides require a strongly oxidizing environment,
more so than previously known for Mars.
Lanza added,
"It's hard to confirm whether this scenario for Martian atmospheric oxygen
actually occurred. But it's important to note that this idea represents a
departure in our understanding for how planetary atmospheres might become
oxygenated." Abundant atmospheric oxygen has been treated as a so-called
biosignature, or a sign of extant life, but this process does not require life.
Curiosity has been
investigating sites in Gale Crater since 2012. The high-manganese materials it
found are in mineral-filled cracks in sandstones in the "Kimberley"
region of the crater. But that's not the only place on Mars where high
manganese abundances have been found. NASA's Opportunity rover, exploring Mars
since 2004, also recently discovered high manganese deposits thousands of miles
from Curiosity. This supports the idea that the conditions needed to form these
materials were present well beyond Gale Crater.
Los
Alamos National Laboratory leads the U.S. and French team that jointly
developed and operates ChemCam. NASA's Jet Propulsion Laboratory, a division
of Caltech in Pasadena, California, built the rover and manages the Curiosity mission for NASA's Science Mission
Directorate, Washington.
Representative
Image
Source: NASA
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