The 2020s are set to be a monumental decade for the search for life on Mars.
But it’s possible the best place to look for life on Mars isn’t on Mars at all.
In a new perspective published Thursday in the journal Science, Japan Aerospace Exploration Agency (JAXA) scientists Ryuki Hyodo and Tomohiro Usui explain why JAXA believes the Martian moon Phobos could provide a unique opportunity to assess whether Mars ever contained life, and how the space agency’s upcoming Martian Moons eXploration (MMX) mission plans to test that hypothesis in 2024.
Why look for Martian life on its moon? — Over billions of years, meteor strikes on Mars have ejected pieces of the planet outward fast enough to fling that debris into orbit.
Some pieces have even fallen to Earth, such as the Allan Hills 84001 meteorite, which was discovered in Antarctica and was — and still is — believed by some scientists to contain evidence of ancient microbial life on Mars by way of organic molecules.
It’s possible similar organic molecules will be found on Phobos, along with chemical biosignatures, fragments of DNA, and fossilized microorganisms. At least, that’s what scientists are hoping.
Hyodo and Usui point out in their perspective article that both of Mars’s moons, Phobos and Deimos, are believed to have collected more Martian meteor strike debris than ever landed on Earth, with the amount of Martian material having collected on the larger and closer orbiting Phobos mixed with the moon’s native regolith at levels estimated at more than 1,000 part per million.
The equivalent of many more Allan Hills 84001 meteorites may well exist on Phobos, providing many more opportunities to find signs of past life on Mars.
Similarly, a mission to Phobos has advantages over current and planned missions to Mars in terms of breadth of samples.
Hunting for alien life on Phobos and Mars
NASA’s Perseverance rover is currently collecting Martian soil samples for later return to Earth, and the upcoming ESA ExoMars Rosalind Franklin rover mission will dig deeper into Mars than ever before in search of signs of life.
But despite being mobile, rover missions only sample relatively limited geological areas on Mars such as the Jezero Crater in the case of Perseverance.
“Is a rock in the U.S. desert always the same as a rock in a hot spring in Japan? The answer is obviously no,” Hyodo tells Inverse.
Sampling Mars material from Phobos can provide more diversity, he and Usui write in the paper, because:
“The random nature of the crater-forming impacts on Mars statistically delivers all possible martian materials, from sedimentary to igneous rocks that cover all of its geological eras.”
There could be more parts of Mars smeared over Phobos, in other words, allowing for a broader survey in one sample.
Why is Phobos important to Mars exploration?
Phobos is a unique opportunity when it comes to hunting for life: Its relationship with Mars means it’s unlikely that, should evidence be found, it came from anywhere beyond the Red Planet.
Hyodo says he’s considered the possibility of early Earth material being blasted to the moon by meteor impacts but notes that given Earths’ heavier gravity and atmosphere, it would require a much larger impact than what it takes to blast material free from Mars. Our Moon is also much further away than Phobos is to Mars.
“If a big impact occurs, it is possible to reach the Moon. However, such a big impact is statistically rare,” Hyodo says. “Even if it occurs, its significant impact energy would melt — or even vaporize — the ejecta.” This would destroy signs of life recorded in the material.
Any actual lifeforms on bits of Mars ejected into space would likely have perished by now, if not during the initial impact, Hyodo and Usui write. But the weaker Martian gravity and thinner atmosphere suggest their possible remnants might still persist in the Phobos regolith.
So rather than searching for actual microbes, the Martian Moons eXploration will be seeking Sterilized and Harshly Irradiated Genes, and Ancient Imprints, or SHIGAI, the acronym spelling a Japanese word meaning “dead remains.”
These could include chemical biosignatures, fragments of DNA, or even fossilized microorganisms sterilized by radiation, but otherwise preserved in the vacuum on the airless Phobos.
The case for the Martian Moons eXploration mission — The Martian Moons eXploration spacecraft is currently expected to launch in 2024 and spend time observing both Phobos and Deimos.
The probe will then touch down to take multiple pneumatic samples of Phobos’s regolith, around 10 grams of material, and return them to Earth in 2029.
It’s a familiar type of mission for the Japanese space agency, if a bit longer distance, considering JAXA’s Hayabusa and Hayabusa 2 missions successfully returned samples from near-Earth asteroids in 2010 and 2020.
“JAXA is the only one, at this moment, that successfully brought samples from asteroids back to Earth,” Hyodo says. The Martian Moons eXploration will build upon those experiences.
Before now, numerous missions to Mars have photographed Phobos, beginning with NASA’s Mariner 7 in 1969, but missions targeting the Martian moon themselves have been failures.
The Soviet Union launched two probes to Phobos in July 1988, Phobos 1 and 2, but Phobos1 was lost en route in September, while Phobos 2 died shortly after reaching Phobos in January 1989.
The Russian Space Agency attempted a sample return mission to Phobos in 2011 named Fobos-Grunt, but after safely reaching Earth orbit, the spacecraft failed to initiate the burn necessary to fly on to Mars and was eventually lost in Earth’s atmosphere.
The Martian Moons eXploration will not be the first mission to Phobos, but if all goes well, it will be the first successful one.