Mars with its two moons, Phobos and Deimos. Phobos orbits a mere 3,700 mile3s (6,000 km) above the surface, while Deimos is almost 15,000 miles (24,000 kilometers) away from the planet. In comparison, there is an almost 384,000 kilometers mean distance between the surface of the Earth and our elliptically orbiting moon. With the moons so close to Mars, debris from meteorite impacts on the planet can easily land on the moons. (NASA/JPL-Caltech/University of Arizona)

Sometime in the early to mid-2020s, the capsule of the Japanese Martian Moons eXploration (MMX) mission is scheduled to arrive at the moons of Mars – Phobos and Deimos.

These are small and desolate places, but one goal of the mission is large: to collect samples from the moons and bring them back to Earth.

If it succeeds, the return would likely be the first ever from Mars or its moons — since planned sample return efforts from the planet itself will be considerably more challenging and so will take longer to plan and carry out.

The Mars moon mission has the potential to bring back significant information about their host planet, the early days of our solar system, and the origins and make-up of the moons themselves.

It also has the potential, theoretically at least, to bring back Martian life, or signatures of past Martian microbial life. And similarly, it has the potential to bring Earth life to one of the moons.

Hidenori Genda, an ELSI planetary scientist with a long-lasting interest in the effects of giant planetary impacts, such as the one that formed our moon. His work has also focused on atmospheres, oceans, and life beyond Earth. (Nerissa Escanlar)

Under the general protocols of what is called “planetary protection,” this is a paramount issue and is why the Japan Aerospace Exploration Agency (JAXA) was obliged to assess the likelihood of any such biological transfers with MMX.

To make that assessment, the agency turned to a panel of experts that included planetary scientist, principal investigator, and associate professor Hidenori Genda of Tokyo’s Earth-Life Science Institute.

The panel’s report to JAXA and the journal Life Sciences in Space Research concluded that microbial biology (if it ever existed) on early Mars could have been kicked up by incoming meteorites, and subsequently traveled the relatively short distance through space to land on Phobos and Deimos.

However, the panel’s conclusions were unambiguous: the severe radiation these microbes would encounter on the way would make sure anything once living was now dead.

“In our analysis, we calculate the possibility of biology on Mars and it spreading to the moons,” Genda said. “But we did not find conditions under which more than an insignificant amount of that possible life could survive on the moons. And so there is no danger of contamination when returning a sample from the moons.”

However, he said, there is a decent chance that microbial “dead bodies” from Mars are on the surface of the moons and could – if the mission gets lucky – be drilled or scooped up for the voyage back to Earth.

The science board recommended that the mission be designated “unrestricted Earth-return” by the Planetary Protection Panel of the international Committee on Space Research (COSPAR). And last year that panel agreed with the JAXA assessment – which means there is less than a one-in-a-million chance that any living Mars microbe will exist in the sampling area on Phobos and Deimos.

In practical terms, the designation allows JAXA, and its international partners, to undertake their mission without extraordinary measures to protect the Earth and their sample on return.

In making their assessment, the JAXA-appointed panel hypothesized the potential concentration of microbes present on Mars by looking to analog environments on Earth – the Atacama Desert in Chile, as well as permafrost and dry sections of Antarctica.

Then they assessed if and how microbes in rocks in the Mars crater Zunil – where potential life would be relatively protected from deadly radiation – could be dispersed by an incoming meteorite, and then to what extent the ejecta would spread to the moons. Because they are both so close to their host planet – Phobos is the moon closest to its host planet in our solar system – they would no doubt have samples of Martian material, and perhaps of ancient Martian material when it was warmer and wetter on the planet, and had a much greater chance of being home to biology.

NASA’s Mars Reconnaissance Orbiter approached to within 6,800 kilometers of Phobos to capture this enhanced-color view of the moon. Phobos is a small, irregularly shaped object with a mean radius of 7.1 miles (11) km and is seven times as massive as the tiny outer moon, Deimos (NASA / JPL -U. Arizona)

The JAXA team concluded that, while the spread of Mars materials to the moons could and probably would be substantial, 70 to 80 percent of the possible microbes were likely to be dispersed all over the moons’ surfaces and then rapidly sterilized by solar and galactic radiation.

The rest could be protected within larger ejected rocks and by thick layers of regolith in new craters on the moons, created by the in-coming Mars rocks. But all told, the chance of microbes being present would be 1,000 to 10,000 times less than in the Mars crater – where none may exist at all anyway.

The COSPAR Category V designation (unrestricted Earth-return because the moons are considered to have “no indigenous life forms”) will permit JAXA to use a range of regolith-collection techniques if they meet certain conditions. They can, for instance, use a coring device that can drill down 3.14 inches (8 centimeters) to collect up to 3.5 ounces (100 gram) samples from Phobos.

For its sample collection, JAXA opted to drill about .8 inches (2 cms) ) into the surface to collect at least 0.35 ounces (10 grams) of soil into the sample container. The spacecraft will then take off from Phobos and make several flybys of the smaller moon Deimos before sending the Sample Return Capsule back to Earth, arriving in July 2029.

Just last month, JAXA approved the mission transition from the pre-project phase to the project phase; an important milestone towards launch. The mission focus will now switch from predominantly research and analysis to developing software and hardware development for the mission.


The various steps of Martian material transferred to Phobos (and inferred for Deimos). The SterLim (Sterilisation Limits for Sample Return Planetary Protection Measures) and JAXA (Japan Aerospace Exploration Agency) teams undertook experimental studies or numerical modeling to study each distinct step in the chain from the surface of Mars to that of its moons. The JAXA team presented their findings to the committee and it organized its report around the various steps outlined here. (A. Yamagishi and K. Fujita, JAXA)

The two tiny moons – Phobos (fear) and Deimos (panic) – are named after the horses that pulled the chariot of the Greek war god Ares, the counterpart to the Roman war god Mars.

A heated debate regarding the moons – and a central question for MMX to hopefully resolve – involves how the moons formed. Two schools argue very different scenarios: that the two moons are actually asteroids that once orbited the sun between Mars and Jupiter, or that they were formed by a giant impact as with our moon. Last week’s Many Worlds column about this part of the MMX story is here.)

The martian moons appear to have regolith (surface materials) that resembles that of many asteroids in the outer belt. This has led some scientists to argue that Phobos and Deimos are “captured asteroids.” After a collision in the asteroid belt, the two could have scattered inward toward the sun and were ultimately captured and put into orbit by the gravity of Mars.

But in the late stages of the formation of our solar system, giant impacts such as the moon-forming one that struck the Earth were not unusual. Genda’s work suggests that Mars shows evidence for one such collision as well.

His view is, in part, a result of the fact that the surface of the northern hemisphere of Mars lies an average of 3.5 miles (5.5 km) lower than the southern hemisphere. This suggests that it was hit long ago by a relatively large body which would have ejected enough Martian material to lead to the formation of the moons.

And since Mars and Earth were by all accounts far more similar in their very early days than they are now, new insights into early Mars would help us understand the formation of the Earth and potentially how it became habitable.

The habitability question explains why planetary protection is so important: if spacecraft from Earth were to bring microbes to any object in space, there is always the possibility – however remote – that the microbe could survive and spread. From a scientific perspective, any Earth contamination could forever change Mars and Mars moon exploration and science.

An artist’s concept of Japan’s Mars Moons eXploration (MMX) spacecraft, which is scheduled go launch in 2023.  The mission to designed to, among other things, scoop up material from the surface of the moon Phobos and return it to Earth for analysis.  The mission has been joined by the French and German space agencies and it will be carrying a NASA instrument. (JAXA)

The MMX mission has a “forward” planetary protection rating of Category III, which requires more protective steps than the Category II designation for Hayabusa2, — which is now returning to Earth from the asteroid Ryugu, where it collected surface material. This is primarily because of concerns over what could happen if the MMX spacecraft malfunctioned and crashed into Mars, as well as the need to protect the unlikely presence of biology on the moons. Genda said that his group has not addressed the issue of “forward” contamination and that it is an engineering rather than a scientific issue.

As for “back” contamination of Earth from possible martian or other microbes in a returned sample, the consequences could be significant. That’s why Genda and JAXA were so pleased to report that the possibility of that kind of contamination from the Mars moon mission is vanishingly small.

But planetary protection issues do not always have such happy results, and that has caused some controversy.

Concerns over planetary protection have often seen NASA make great efforts to prevent microbes from going to space. Its martian robots are assembled in clean rooms, with many components baked in ovens or doused in chemicals. Famously, its Viking landers for Mars in the 1970s were baked in purpose-built ovens. But these protections have often been costly and, in the view of some scientists, overly burdensome.

As described in a report released last fall by an independent NASA advisory panel, current NASA and international rules that govern the potential spread of earthly microbes to other planets—and the potential return of alien life back to Earth—are often anachronistic and require broad rethinking,


The primary strategy for preventing contamination of Mars with Earth organisms is to be sure that the hardware intended to reach the planet is clean. Each Mars Exploration Rover complied with requirements to carry a total of no more than 300,000 bacterial spores on any surface from which the spores could get into the martian environment. Technicians assembling the spacecraft and preparing them for launch frequently cleaned surfaces by wiping them with an alcohol solution. Components tolerant of high temperature, such as the parachute and thermal blanketing, were heated to 110 degrees Celsius (230 Fahrenheit) or hotter to kill microbes. (NASA/JPL-Caltech)

And given the level current scientific knowledge about important planetary surfaces, they can be safely loosened, said Alan Stern, a planetary scientist from the Southwest Research Institute in Boulder, Colorado, who led the 12-member panel reviewing NASA’s efforts.

“We want to move from this 1960s–70s point of view that all of Mars was treated one way.” Stern said, because planetary surfaces are more nuanced than the rules now in place to protect them..

The new report seems to echo this view. For example, NASA should move beyond a rigid use of spore counts, which can only tally microbes that can be grown in a laboratory (many can’t), to determine the life inhabiting its spacecraft, the report says. Modern techniques that use genomic sequencing to monitor cleanrooms are now available, and these can be combined with probabilistic risk analyses of whether harmful contamination of another world would be likely.

NASA should also rethink how it classifies the surfaces of the Moon and Mars, the report says. All of the moon is now classified as potentially of interest to research on the origins of life, meaning NASA doesn’t want to contaminate it with imports from Earth.

But few scientists now view the moon as an important site for studying such questions—except for its poles, where ice that might have helped sustain life exists. Reclassifying much of the moon’s surface as nonessential for biological studies would simplify exploration for NASA and other space agencies—along with commercial actors.

Similarly, the report says, much of Mars has been treated as if microbes that landed on its surface could survive and be transported to regions thought to host water and allow the replication of life. But many scientists think that outcome is unlikely and definitely worth rethinking.


A version of this column was first published on the website of the Earth-Life Science Institute (ELSI.)