Could life on Earth have originated on Mars? over the past two decades that question has left the pages of science fiction and entered the mainstream of empirical science. Planetary scientists have found that rocks from Mars do make their way to Earth; in fact, we estimate that a ton of Martian material strikes our planet every year. Microorganisms might have come along for the ride. The impacts that launched these rocks into Earth-bound trajectories were violent, high-pressure events, but experiments show that certain species would survive. On passing through Earth’s atmosphere, Martian meteoroids are heated only a few millimeters in from their surfaces, so any microbes deeper inside would not burn up [see “Did Life Come from Another World?” by David Warmflash and Benjamin Weiss; Scientific American, November 2005].
In between takeoff and landing, organisms would need to survive the coast through interplanetary space inside their rocky vessels. Orbital analyses indicate that most Mars meteoroids take thousands or millions of years to get here, but a few (about one in 10 million) arrive within a year or so. Could a bug cling to life for that length of time? The quest for an answer is about to begin.
This month the Russian Federal Space Agency plans to launch the Grunt probe to the Martian moon Phobos. It carries a basketball-size capsule that will collect a scoop of Phobosian soil and return it to Earth in 2014. Within this capsule is a smaller container developed by the Planetary Society, the Living Interplanetary Flight Experiment (LIFE), packed with terrestrial organisms. A soil sample with a mixed population of microorganisms from Israel’s Negev Desert lies at the center. Surrounding it are 30 small tubes with 10 species, representing all three domains of Earth’s biology: bacteria, archaea and eukaryotes. Five of these species flew on the space shuttle Endeavour’s final mission in May as a dress rehearsal.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Our team chose organisms either because they are the terrestrial analogues of putative Martian organisms or because they will let us see just how hardy the hardiest microbes really are.
Bacteria
One of the bugs is Deinococcus radiodurans, famous for being able to survive when its DNA is zapped with enormous doses of radiation. I have been studying the D. radiodurans samples that took the Endeavour trip and feel quite sure their cousins will survive the trip to Phobos and back. Comparing the robustness of genetically different individuals may give new insights into exactly how these organisms tolerate radiation, desiccation and extreme cold.
Whereas D. radiodurans tolerates radiation without changing its cellular form, other bacteria retreat into hardened structures known as endospores. Our experiment includes two of them.
Bacillus subtilis has a long history as a test species in spaceflight experiments. One of my Phobos LIFE colleagues, Gerda Horneck of the German Aerospace Center, has been sending B. subtilis into orbit since the 1960s and demonstrated that its endospores can survive for up to six years in space, coated by only a thin layer of dust, which protects against solar ultraviolet rays. Interplanetary space adds the hazard of charged particle radiation, which is more penetrating.
Our other bacillus, B. safensis, was first discovered 10 years ago in the Spacecraft Assembly Facility at the NASA Jet Propulsion Laboratory. Technicians there were sterilizing the Mars Odyssey orbiter to prevent it from contaminating the Red Planet with terrestrial organisms, which might confound future searches for life or, worse, kill any indigenous organisms. Test swabs revealed a species that managed to survive. (Out of the same concerns about contamination, we designed the canister to comply with planetary protection guidelines set by the Committee on Space Research of the International Council for Science.)
Archaea
Resembling bacteria but sharing more of their biochemistry with eukaryotes, archaea are grouped into their own domain. Methanothermobacter wolfeii was chosen not because it is especially resilient but because it produces methane. The Martian atmosphere contains traces of this gas, and some scientists have suggested it comes from microbes akin to M. wolfeii.
We included Haloarcula marismortui for a similar reason. Native to the Dead Sea, it is a salt lover, as any Martian organisms would probably need to be. To avoid freezing, liquid water on the Red Planet must be briny. In fact, one Mars meteorite, Nakhla, shows evidence it was immersed in an ancient brine.
Thriving in volcanically heated ocean sediment, Pyrococcus furiosus is no model for life on Mars, but we included it as an experimental control. If our organisms die, we need to be able to tell whether it was the stress of the space environment or the heat of atmospheric reentry that killed them. If P. furiosus is the only survivor, we will be able to blame the heat.
Eukaryotes
Eukaryotes are organisms with nucleated cells, like human cells. We doubt they would have ever made the journey from Mars, but we felt we should study their resilience to space, anyway. One species we included is the commonly studied yeast Saccharomyces cerevisiae.
Tiny animals and plants will be flying, too. Tardigrades, known affectionately as water bears, are invertebrates about 1.5 millimeters long with small clawed legs. They are extremely resistant to radiation, temperature extremes and even the space vacuum. Representing plants are seeds of Arabidopsis thaliana. Like B. subtilis, A. thaliana is a veteran space organism, having traveled twice in Apollo capsules.
When the Grunt capsule returns to Earth in 2014, the recovery team will extract the biomodule and send it to ATCC, a biology laboratory in Virginia. Using instruments designed specifically for this purpose, engineers will open the biomodule and distribute samples to participating researchers. Then, at last, we will know whether life can make the leap from planet to planet.
