Graphic: Christine Daniloff |
Are we all Martians? According to many
planetary scientists, it’s conceivable that all life on Earth is descended from
organisms that originated on Mars and were carried here aboard meteorites. If
that’s the case, an instrument being developed by researchers at MIT and
Harvard could provide the clinching evidence.
In order to detect signs of past or
present life on Mars—if it is in fact true that we’re related—then a promising
strategy would be to search for DNA or RNA, and specifically for particular
sequences of these molecules that are nearly universal in all forms of
terrestrial life. That’s the strategy being pursued by MIT research scientist
Christopher Carr and postdoctoral associate Clarissa Lui, working with Maria
Zuber, head of MIT’s Department of Earth, Atmospheric and Planetary Sciences
(EAPS), and Gary Ruvkun, a molecular biologist at the Massachusetts General
Hospital and Harvard Univ., who came up with the instrument concept and put
together the initial team. Lui presented a summary of their proposed
instrument, called the Search for Extra-Terrestrial Genomes (SETG), at the IEEE
Aerospace Conference.
The idea is based on several facts that
have now been well established. First, in the early days of the solar system,
the climates on Mars and the Earth were much more similar than they are now, so
life that took hold on one planet could presumably have survived on the other.
Second, an estimated one billion tons of rock have traveled from Mars to Earth,
blasted loose by asteroid impacts and then traveling through interplanetary
space before striking Earth’s surface. Third, microbes have been shown to be
capable of surviving the initial shock of such an impact, and there is some
evidence they could also survive the thousands of years of transit through
space before arriving at another planet.
So the various steps needed for life to
have started on one planet and spread to another are all plausible.
Additionally, orbital dynamics show that it’s about 100 times easier for rocks
to travel from Mars to Earth than the other way. So if life got started there
first, microbes could have been carried here and we might all be its
descendants.
So what?
If we are descendants from Mars, there might be important lessons to be learned
about our own biological origins by studying biochemistry on our neighbor
planet, where biological traces erased long ago here on Earth might have been
preserved in the Martian deep freeze.
The MIT researchers’ device would take
samples of Martian soil and isolate any living microbes that might be present,
or microbial remnants (which can be preserved for about up to a million years
and still contain viable DNA), and separate out the genetic material in order
to use standard biochemical techniques to analyze their genetic sequences.
“It’s a long shot,” Carr
concedes, “but if we go to Mars and find life that’s related to us, we
could have originated on Mars. Or if it started here, it could have been
transferred to Mars.” Either way, “we could be related to life on
Mars. So we should at least be looking for life on Mars that’s related to
us.”
Even a few years ago, that might have
seemed like more of a long shot, but recent Mars orbiter and rover missions
have clearly shown that Mars once had abundant water, and many of the
conditions thought to be needed to support life. And although the surface of
Mars today is too cold and dry to support known life forms, there is evidence
that liquid water may exist not far below the surface. “On Mars today, the
best place to look for life is in the subsurface,” Carr says.
So the team has been developing a device
that could take a sample of Martian soil from below the surface—perhaps dredged
up by a rover equipped with a deep drill—and process it to separate out any
possible organisms, amplify their DNA or RNA using the same techniques used for
forensic DNA testing on Earth, and then use biochemical markers to search for
signs of particular, genetic sequences that are nearly universal among all known
life forms.
The researchers estimate that it could
take two more years to complete the design and testing of a prototype SETG
device. Although the proposed device has not yet been selected for any upcoming
Mars mission, a future mission with a lander or rover equipped with a drill
could potentially carry this life-detection instrument.
No instrument has been sent to Mars
specifically to look for evidence of life since NASA’s twin Viking landers in
1976, which produced tantalizing but ambiguous results. An instrument on the
Mars Science Lander to be launched in the fall will investigate chemistry
relevant to life. The instrument from the MIT-Harvard team directly addresses Earth-like
molecular biology.
Christopher McKay, an astrobiologist at NASA-Ames Research
Center in California who specializes in research
related to the possibility of life on Mars, says this work is “very
interesting and important.” He says, “it is not implausible that life
on Mars will be related to life on Earth and therefore share a common genetics.
In any case it would be important to test this hypothesis.” But he adds
that there is another motive for doing this research as well: “From an
astronaut health and safety point of view and from a return-sample point of
view, there is more to worry about” if there are organisms closely related
to us on Mars, since a microbe that is similar is much more likely to be
infectious to terrestrial life forms than would a totally alien microbe—so it
is very important to be able to detect such life forms if they are present on
Mars. In addition, this method could also detect any biological contamination
on Mars that has been brought by spacecraft from Earth.
This kind of test is something we have
the ability to do, he says, and therefore, although such an experiment has not
yet been formally approved, “it seems improbable to me that we will do a
serious search for life on Mars and not do this test.”
