Imagine yourself standing at the edge of a hot spring. Wisps of steam emanate from the ground, rolling across the water's edge in waves. A gentle breeze blows across your face -- pleasant, if not for the noxious smell of rotten eggs, mixed with the scent of a struck match. The center of the spring boils, bubbling away slowly; from above, it looks like a deep blue hole in the world.
Within the warm waters that trail away from the spring, microbes jostle for position. Some have mastered the art of photosynthesis, converting sunlight into energy to survive. Others feed on the rich soup of chemicals provided by the spring. These mats of microorganisms produce carotenoids, the same chemicals that give pumpkins and carrots their typical hues, so the edges of the spring burst with vibrant oranges and yellows.
It's a scene visitors to Yellowstone National Park may have witnessed, but this isn't a scene from Earth.
This is Mars, billions of years ago.
In its earliest days, places like Mars' Gusev crater, formed by a gigantic asteroid impact around 4 billion years ago, were likely home to hot springs. On Earth, hot springs are lively places. Not just because of the throngs of tourists fumbling with iPhones to snap photos, but because their waters are brimming with bacteria, fungi and viruses.
It follows that in similar locations on Mars where water was once present, life may have found a way to thrive in the muck.
Gusev crater is, today, a hollowed-out desert carved into the face of the red planet. Over billions of years, Mars' atmosphere slowly disappeared. Its landscape morphed, its waters dried up. Volcanic activity resurfaced areas of the planet, including Gusev, eradicating any life that may have been present (and that's a big may).
But if there were microbial communities present in those temperate waters, they may still linger in the rock left behind. The chemical mix of a hot spring, rich in the mineral silica, is the perfect preservation material. As the spring dried, the silica would have entombed the still-living microscopic beasts within, frozen like Han Solo in carbonite, burying them underneath Mars' hostile surface.
There may be a vast cemetery of tiny organisms buried beneath the red planet's exterior.
Uncovering the gravesites has long been the holy grail for astrobiologists and a chief science goal of NASA's Mars exploration program. The agency has sent five rovers to the surface of the planet since 1997. Today, its most advanced rover, Perseverance, rolls along the base of a dried-out lake bed known as Jezero, drilling into rocks and collecting samples that may be returned to Earth in the 2030s.
But another space agency, about one-tenth the size of NASA, is thinking outside of the planet-sized box in its search for Martian life. With its Martian Moons Exploration mission, the Japan Aerospace Exploration Agency, or JAXA, later this decade will touch down on a world no spacecraft has visited before: Phobos, one of Mars' mystifying moons.
Scientists at JAXA, and other astronomers, hypothesize that on this curious moon they may find signs of ancient microbes that were catapulted off the surface of Mars and flung across the cosmos. The remains of these unwitting spacefaring organisms have been untouched for millions of years and, soon, could be plucked from Phobos' face and returned to Earth.
When an asteroid collides with a planet, the planet unleashes a mighty sneeze of dust and rock. The faster the asteroid smashes into the surface, the bigger the sneeze.
Much of the planetary snot will fall back to the surface, but if the asteroid impact is powerful enough, the sneeze will fling dust and rock into space. Some of it can even arc across the gap between Mars and Earth, traveling tens or hundreds of millions of miles between the two planets. If the debris survives its plunge through Earth's atmosphere, it smacks into the ground as a meteorite. Over 300 meteorites discovered on Earth originated from the planet next door.
Just like a human sneeze contains microbes, the material ejected by a planet may also contain microscopic life -- or the remnants of it. If the asteroid death blast doesn't melt the rock and the microbes to mere atoms, there's a chance they can float into the cosmos.
Unfortunately for would-be Martian hunters, no microbes lurk within the stuff that makes it to Earth. "Martian meteorites don't have any signatures of Martian life," says Tomohiro Usui, a planetary scientist at JAXA. The atmosphere mostly takes care of that and alters the rock as it burns through the air.
But Mars is scarred by impacts from drifter asteroids that slammed into the surface over the planet's life. If these impacts were to hit in just the right spot, at just the right angle and just the right time, there's a chance the ejected material would make it to Phobos, Mars' curious, potato-shaped moon.
Phobos has the closest orbit of any known moon to its parent body, circling the red planet at a distance of just 6,000 kilometers (3,700 miles), about the same as the distance between Tokyo and Honolulu. For comparison, Earth's moon resides at a distance of around 385,000 kilometers (240,000 miles). Phobos is practically hugging Mars, and moves around the planet so quickly that if you were to observe it from the surface, you'd be able to see it rise and set twice every Martian day.
Its proximity to the red planet has led JAXA scientists and engineers to speculate about the potential for finding the remnants of Martian microbes on the moon's surface.
"If Martian life once existed and was widespread elsewhere on Mars, the chance that its dead remains exist also on Phobos is, in my opinion, relatively high," says Ryuki Hyodo, a planetary scientist at JAXA's Institute of Space and Astronautical Science.
When Hyodo speaks of "dead remains," he is referring to a series of biosignatures the JAXA team has dubbed "shigai," for "sterilized and harshly irradiated genes, and ancient imprints." "Dead remains" is the English translation for shigai, and the term was coined in an August 2021 article published in the journal Science.
It's possible, JAXA believes, that Phobos could be a satellite cemetery, unwittingly holding molecular evidence of long-dead microorganisms. Mars' second moon, Deimos, may also contain pieces of Mars on its surface, but it orbits at a much greater distance and is about half the size of Phobos. Why go to the moons instead of Mars itself? Simply put, it's easier.
Haruna Saguhara, an analytical chemist at JAXA, explains that the molecular evidence present on Phobos' surface could be anything from bacteria-like features stamped in rock or the chemical signatures of ancient microbes -- fats, DNA and carbon compounds indicative of life.
Hyodo notes that any impact that kicked up Martian soil wouldn't need to reach high velocities to land on Phobos because the moon is within Mars' gravitational sphere. "This is why potential biosignatures could be delivered to Phobos without being destroyed by an impact process," he says. That makes searching for shigai on Phobos a viable option.
"I buy that argument," says Steven Ruff, a planetary geologist at Arizona State University and creator of the Mars Guy YouTube channel, "but it will be a challenge."
To understand how likely JAXA's Mars-shot is at finding remnants of Martian life on Phobos, it's important to understand why scientists believe Mars itself may have been habitable in its ancient past.
Columbia Hills is the perfect place to start. Located in the center of Gusev Crater, these hills were explored by NASA's rolling Mars laboratory Spirit in 2005. Ruff and other scientists hypothesize that Columbia Hills was once home to ancient hot springs.
Spirit was equipped with a spectrometer, which can determine the chemical composition of rocks, and the rover snapped hundreds of photos of an area known as the Home Plate. It showed that this region contained fields of opaline silica, which, on Earth, form in hydrothermal systems like hot springs. The nodular deposits are associated with preservation of microbial life in places like El Tatio, Chile, a hot spring that provides some of the most Mars-like conditions we know of.
"The story of what Spirit found in the Columbia Hills of Gusev Crater is quite provocative with regard to potential evidence for life billions of years ago," says Ruff, emphasizing what Spirit found isn't proof of life but reason to explore further.
There is still debate over the existence of a hydrothermal system at Columbia Hills and what might have caused the opaline silica deposits. Confirmation, Ruff says, would require a sample-return mission. Without one, scientists are left to pore over the data from Spirit and other Martian robots to try to pull apart the story of Columbia Hills.
There are some small mercies, though. Mars doesn't have plate tectonics, which have reshaped the surface of Earth over eons, deforming the rocks and obscuring fossils and biosignatures of ancient life forms on our planet.
"The preservation potential of those hot spring deposits in very ancient rocks on Mars is much better than what we see on Earth," says Tara Djokic, a geologist at the Australian Museum.
Djokic and her colleagues have studied ancient hot springs in the Pilbara, Western Australia, and found evidence of 3.5-billion-year-old cyanobacteria, a type of photosynthesizing microbe. If similar life arose on Mars in the same timespan as it did on Earth, it stands to reason it could be locked away within the soil of Columbia Hills.
And if any of these rich, potentially habitable areas on Mars were bombarded by asteroids, perhaps the ejected mess could have made it to Phobos. If any agency is capable of finding out, it's JAXA.
Retrieving samples from small cosmic bodies, millions of miles from Earth, is a JAXA specialty.
In 2013, it launched the Hayabusa2 spacecraft to a 4.6-billion-year-old asteroid known as Ryugu. The ancient rock provided a treasure trove of intriguing samples for JAXA to collect, and Hayabusa2 was able to make two swift retrievals. In December 2020, it returned the rocks to Earth by slinging a sample capsule toward a landing spot in outback Australia. It was the second time JAXA had been able to retrieve samples from an asteroid and bring them home.
Masaki Fujimoto, the deputy director of the Institute of Space and Astronautical Science, says people might be expecting JAXA to do something similar to the Hayabusa2 mission -- and to a degree, it is. Though a tantalizing prospect, the chief goal of the Martian Moons Exploration, or MMX, program isn't to track down signs of past Martian life. Rather, the spacecraft is built to snatch samples from the moon's surface in an attempt to tease apart the competing hypotheses of Phobos' origin.
Scientists have proposed two different theories for the moon's formation. The first posits that Phobos was an asteroid belt object that got knocked a little too close to Mars and was captured by the red planet's gravity. Its orbit, which aligns a little too neatly with Mars' equator, seems to rule this out. The second suggests that Phobos was created after a dwarf planet, about 14 times smaller than Pluto, careened into Mars, launching rock into orbit that eventually coalesced to form the moon.
However, what awaits MMX when it arrives in orbit around Phobos remains unknown. Usui notes that scientists are still unsure "about the conditions at the surface" and what kind of rocks might litter Phobos' face. That's a problem JAXA encountered -- and overcame -- at Ryugu. For MMX, the team has had to adapt its sample retrieval methods a little.
MMX will touch down on Phobos multiple times to collect samples, like Hayabusa2 did. But because of the unknown conditions at the surface, it will be equipped with two separate collection systems. One, the coring system, will drill into the moon and extract a sliver of rock. If Phobos' surface is harder than expected, the system could run into trouble penetrating the ground. "It's pretty risky if we only have the coring system," says Usui.
A second system will blow nitrogen gas onto the surface, kicking up loose soil into the spacecraft's sample grab bag.
Provided one of these systems can pull off the interplanetary heist (and maybe both will), JAXA's Martian moon samples will complement the work already underway on the red planet.
It's not just hot springs that may have been home to Martian microbes.
In a 27-kilometer-wide (16-mile-wide) scar on Mars' face, NASA's Perseverance rover has been rolling along the basin of an ancient crater known as Jezero, sampling rocks with a drill attached to a robotic arm, since February 2021. Jezero was once a huge lake. and scientists believe the ingredients for life to kick off may have been present there, too.
The Perseverance mission is taking a more traditional approach than JAXA to finding life on Mars -- by searching for it at the surface. It forms part one of the most daring mail runs in history.
In September, Perseverance bore a hole into a briefcase-sized rock known as "Rochette" and stored the stolen rubble in titanium tubes in its stomach. At a later date, the rover will deposit the tubes on the surface of Mars, a snaking trail of glinting metal in the red planet's sand. Then, sometime in the next decade, a robotic mail carrier will come and collect the tubes from the surface of Mars and send them back to Earth.
Right now, the mail carrier doesn't exist.
NASA and the European Space Agency are developing a Mars lander and rover specifically designed to pick up the samples, place them in a rocket and blast them from Mars back to Earth. Such a mission would not return to Earth until the early 2030s.
Along with NASA and ESA, the China National Space Administration plans to launch its own Mars sample return mission in 2030 -- a mission buoyed by CNSA's recent touchdown on the red planet in May.
Those missions are important because Kawakatsu, the mission lead for MMX, frames JAXA's mission in the context of its compatriot agencies, with one small tweak. While others look to Mars as the star of the show, JAXA has recast Phobos and Deimos as not just extras, but supporting actors in Martian exploration. "Our approach is very unique in the world," he notes, referring to the cosmic trio as a whole: the "Martian sphere."
The reframing also helps with achieving a prestigious first in JAXA's eyes. With a planned sample return date of 2029, MMX would be the first time samples have been returned from the Martian sphere. While the space agency estimates just 0.1% of Phobos' soil likely originated on Mars, there's a chance MMX could bring back the first samples of the red planet to Earth.
Will it contain shigai? Planetary scientists outside of JAXA say it's plausible, but they're not quite as confident as JAXA scientist Hyodo. It will be like searching for "a needle in a haystack," says Ruff.
However tiny the probability, JAXA analytical chemist Saguhara sees looking for life on Phobos as worthwhile. "If there is a chance, we have to do it," she says.
"If we found shigai on Phobos, that's the biggest deal I can imagine," adds Fujimoto.
Three years from the scheduled liftoff date, with a host of technical work and hardware development still to be undertaken in preparation, JAXA is full steam ahead. Fujimoto notes that the launch window could still slip -- not unusual for a space mission -- but if things continue as planned, by the end of the decade, a fireball containing ancient material could blaze a trail across the Australian night sky, just like it did with Ryugu in 2020.
This time, it might carry the remnants of long-dead microbes that once settled in a distant, ancient hot spring.