Stick
a shovel in the ground and scoop. That’s about how deep scientists need
to go in order to find evidence for ancient life on Mars, if there is
any to be found, a new study suggests. That’s within reach of Curiosity,
the Mars Science Laboratory rover expected to land on the Red Planet
next month.
The
new findings, which suggest optimal depths and locations to probe for
organic molecules like those that compose living organisms as we know
them, could help the newest Mars rover scout for evidence of life
beneath the surface and within rocks. The results suggest that, should
Mars harbor simple organic molecules, NASA’s prospects for discovering
them during Curiosity’s explorations are better than previously thought,
said Alexander Pavlov of the NASA Goddard Space Flight Center in
Greenbelt, Maryland, lead author of the study.
While
these simple molecules could provide evidence of ancient Martian life,
they could also stem from other sources like meteorites and volcanoes.
Complex organic molecules could hint more strongly at the possibility of
past life on the planet. These molecules, made up of 10 or more carbon
atoms, could resemble known building blocks of life such as the amino
acids that make up proteins.
Although
complex carbon structures are trickier to find because they’re more
vulnerable to cosmic radiation that continuously bombards and penetrates
the surface of the Red Planet, the new research by Pavlov and his
colleagues provides suggestions for where to start looking. The amounts
of radiation that rock and soil is exposed to over time, and how deep
that radiation penetrates—an indicator of how deep a rover would have to
sample to find intact organic molecules—is a subject of ongoing
research.
The
scientists report that chances of finding these molecules in the first 2
cm (0.8 inches) of Martian soil is close to zero. That top layer, they
calculate, will absorb a total of 500 million grays of cosmic radiation
over the course of one billion years—capable of destroying all organic
material. A mere 50 grays, absorbed immediately or over time, would
cause almost certain death to a human.
However,
within 5 to 10 cm (2 to 4 inches) beneath the surface, the amount of
radiation reduces tenfold, to 50 million grays. Although that’s still
extreme, the team reports that simple organic molecules, such as a
single formaldehyde molecule, could exist at this depth – and in some
places, specifically young craters, the complex building blocks of life
could remain as well.
The study is scheduled to be published 7 July in Geophysical Research Letters, a journal of the American Geophysical Union.
“Right
now the challenge is that past Martian landers haven’t seen any organic
material whatsoever,” Pavlov said. “We know that organic molecules have
to be there but we can’t find any of them in the soil.”
As
Mars revolves around the Sun, it is constantly bombarded by very small
meteors and interplanetary dust particles, which have plenty of organic
compounds in them, Pavlov said. Therefore, over time they would have
accumulated at the Martian surface.
The
Mars Science Laboratory is the newest and largest of NASA’s Martian
landers and is scheduled to touch down August 2012. Curiosity doesn’t
have a shovel but, equipped with drilling technology, it will collect,
store, and analyze samples of Martian material down to 5 cm below the
surface of rock and soil. Past Martian rovers have only collected loose
soil atop the surface that has been directly exposed to cosmic
radiation, making the possibility for detecting organic molecules
exceedingly slim.
When
evaluating how deep organic molecules might persist beneath the
surface, previous studies have mainly focused on the maximum depth,
approximately 1.5 m (5 feet), that cosmic radiation reaches because
beyond that point organic molecules could survive, unharmed, for
billions of years, Pavlov said. However, drilling to 1.5 m or deeper is
currently too expensive to engineer for a Martian rover.
So
the team focused on more attainable depths—the first 20 cm (8 in) below
the surface. They modeled the complex scenario of cosmic ray
accumulation and its effects on organic molecules using a collection of
important variables, including Martian rock and soil composition,
changes in the planet’s atmospheric density over time, and cosmic rays’
various energy levels.
In
addition to the finding that some simple carbon-containing molecules
could exist within 10 cm (4 in) depth, the scientists emphasize that
certain regions on Mars may have radiation levels far lower than 50
million grays near the surface—and so more complex molecules like amino
acids could remain intact.
In
order to find these molecules within the rover’s drilling range (1 to 5
cm), the scientists found the best bet is to look at “fresh” craters
that are no more than 10 million years old, unlike past expeditionary
sites that mainly sampled from landscapes undisturbed for billions of
years.
Compared
to Martian landscapes undisturbed for one billion years or more,
relatively young craters exhibit freshly exposed rock and soil that was
once deeper beneath the surface. . The new research indicates that this
material will have been near the surface for a short enough period of
time that it’s overall exposure to harmful radiation would not have been
enough to wipe out organic molecules.
“When
you have a chance to drill, don’t waste it on perfectly preserved
(landscapes),” Pavlov said. “You want to go to fresh craters because
there’s probably a better chance to detect complex organic molecules.
Let Nature work for you.”
Lewis
Dartnell, a postdoctoral researcher at the University College London in
the U.K., said the paper was a nice study that combined results from
other studies with the latest radiation modeling. Dartnell was not part
of the study, but has published previous work involving effects of
cosmic radiation on the Martian surface.
“The
next logical step,” Dartnell said, “is to actually experiment and have a
radiation source hit amino acids with radiation of similar energies as
cosmic rays and determine how quickly those amino acids are destroyed
because models can only do so much.”
Curiosity
is set to land in Gale crater—the same crater where the Spirit rover
landed in 2004– on August 6. Whether this 3.5-billion-year-old crater
has fresher craters within it is uncertain. However, Pavlov hopes that
his team’s findings will at least help guide NASA on where to drill once
the rover has landed and influence where future generations of rover
landers will touch down.
Source: American Geophysical Union
