Detecting Our Martian Cousins

Feb 15, 2010 by Michael Schirber
Comet 1975 V1 (also known as "Comet West"). Comets and asteroids may help spread life from one planet to another. Image credit: ESO


The same thing may have happened across the ocean of space. Some researchers speculate that life on Mars - if there is any - may be composed of "island species" that were carried away from Earth on interplanetary meteorites.



The idea isn't too crazy. We already have evidence that some biologically important molecules, such as the ingredients for , are delivered by comets. And we know that around 3.5 billion years ago, impacts often kicked up rocks from the Earth's surface and launched them into space.



A SETG prototype will have its first field test this year with funding from NASA's Science and Technology Instrument Development program.

The Viking landers carried four instruments designed to search for signs of martian life: a gas chromatograph/mass spectrometer, as well as experiments for gas exchange, labeled release, and pyrolytic release. Credit: NASA

DNA detection

This is the first time DNA analyzing equipment has been proposed for a planetary mission. Other "life-detectors" - like those that were onboard the Viking landers - search for or .



But Ruvkun argues that his team's technique can detect a single DNA molecule in a sample, whereas other tests for biology - such as identifying chemical constituents with mass spectrometry - are not nearly as sensitive.


Of course, if DNA were found on Mars, the first reaction would be that we brought it there on our spacecraft. To rule out this sort of contamination, the SETG instrument would sequence part of the DNA and compare it to the genetic codes of Earth species.

Owing to their cosmic isolation, any Martians should have evolved very differently from Earthlings. However, if we do share a common ancestor as Ruvkun and others presume, then we'll have to make room for the Martians on our tree of life.

The SETG project is putting together an instrument that could go to Mars and search soil or ice samples for the presence of DNA. Image credit: UC Berkeley

Immutable DNA

Analyzing alien DNA will require a common reference point - that is, a stretch of DNA that would likely be conserved in both Martians and Earthlings.






A primer for life

Ruvkun's group plans to use highly conserved segments of the 16S ribosomal RNA gene as "primers" for amplifying DNA.

Primers like these are commonly used by biologists to locate and replicate specific pieces of DNA. Multiple single-strand copies of the primer segment are added to a sample, where they will latch onto any DNA that has the corresponding code. It's like very selective Velcro.

The binding by the primer induces replication of the matching DNA strand by enzymes that are also added to the mix.

In the case of SETG, the proposed strategy is to add primers from the 16S ribosomal RNA to soil or ice collected from the Martian surface. If there is DNA in the sample and that DNA happens to match the primer sequence somewhere in its , a copy will be made of that portion of the sample's DNA.

This replication step can be repeated many times, leading to a million-fold amplification in Martian DNA containing the primer sequence. The amplified DNA can then be detected with fluorescent probes for abundant DNA, a simple first test of DNA amplification.

[Note: if Martians are DNA-based or RNA-based but lack the primer sequence, then the SETG device may still detect them with other tests. The problem is that without the primer as a reference point the researchers won't be able to say very much about what's been detected.]

The amplification process - called polymerase chain reaction, or PCR for short - is so commonplace now that junior high school students are using it, Ruvkun says. It should therefore be sufficiently robust for a space instrument.

If DNA is detected, the SETG device will then sequence 100 or more base pairs around the primer sequence. This adjacent code is not normally conserved - it will therefore have tiny mutations in it that can identify what sort of life-form is the owner of this DNA.

If the sample were contaminated by Earthling DNA, then the SETG researchers should be able to recognize signatures in the adjacent code that will pinpoint whether the contamination comes from a human or a bacteria or something else familiar to us.

But if nothing on Earth matches the observed sequence, Ruvkun and his colleagues will claim to have found our long-lost Martian cousins. And since DNA from fossilized life probably can't survive very long on Mars (just as it has a limited lifetime on Earth), such a detection would presumably mean that these cousins are still alive and well.

On the flip side, it's highly unlikely that extinct Martian life will have left us any DNA fossils. "If Mars was sterilized a billion years ago, then we're toast," Ruvkun says.

But he believes that if Mars was once inhabited, it still is.

"Never bet against life," he says. "We can't imagine all the environments that organisms can survive in."

Field test

Ruvkun and his colleagues have built a prototype of their DNA analyzer that is about the size of a shoe box.

To help calibrate this instrument, the team will travel to Argentina to the Copahue Volcano, which is considered to be one of the most Mars-like environments on Earth. The prototype will be used to sequence the DNA of some of the hearty microbes that live in the acidic runoff from the volcano.

If it were to get the green light to go to Mars, the SETG instrument would need to shrink down considerably. Full developmental costs might run about $30 million, according to co-investigator Maria Zuber of the Massachusetts Institute of Technology.

But long before that, Ruvkun says they'll need to go through a kind of "high school dance," in which different payload projects come together to try and pair up for a future mission that might be a decade out.


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3 / 5 (2) Feb 16, 2010

Somehow, even if they found a ton of "biomass" they would still argue that it might have been created by some nonliving process. THEN they would want to send another half billion dollar mission taking 10 years to put together to see if it contains any DNA.

Apparently Mr. Pace is much younger than me. I would like to see this question answered in MY lifetime.


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