What secrets about Mars is its moon Phobos hiding?

Mars is a desolate planet that is both Sun-blasted and frozen over. Its atmosphere has been all but obliterated by the ravages of solar radiation over billions of years, but does that mean it is forever lost to time?

There might still be pieces of Martian atmosphere trapped in its largest moon. Phobos may have a name that literally translates to “fear”, but what could be hiding just beneath its surface is nothing to be afraid of. It orbits through charged particles, or ions, that were once part of Mars’ atmosphere but escaped into space. Scientists now believe ions from that lost atmosphere slammed into Phobos as it made its way through them, and are probably still trapped in its surface. Dust from Phobos could potentially tell us how the atmosphere of Mars evolved—or devolved.

“Phobos is original in the Solar System because it is also exposed to ions coming from the atmosphere of the Mars,” UC Berkeley researcher Quentin Nénon, who recently led a study published in Nature Geoscience, told SYFY WIRE.

Runaway ions from Mars may prove or disprove the idea that Mars was once much more Earthlike, with a thick atmosphere and liquid water on its surface. There could have even been some sort of life. Whatever is left of the Red Planet’s atmosphere (if you could even call it that) has less than one percent of Earth’s atmospheric density. Phobos is thought to have collected quite a few ions from Mars because it orbits extremely close to the planet. Because it is tidally locked, like our Moon, so one side is always facing Mars. That side has been exposed to anywhere from 20 to 100 times more ions than the far side, so whatever relics are left of Mars’ atmosphere could be embedded in its surface.

This is where NASA’s MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft comes in. MAVEN has been orbiting Mars since late 2014 in an effort to investigate it lost its atmosphere, and also beams back insight about how the Martian climate has morphed into what it is now. To see if Phobos really was showered with Martian ions, as opposed to solar ions or other random particles flying around, Nénon analyzed MAVEN data from when the spacecraft passed through the moon’s orbit. Particle velocity and kinetic energy measurements from MAVEN’s STATIC (Suprathermal and Thermal Ion Composition) instrument gave away the mass of ions it ran into. Martian ions were singled out this way.

“Most of the ions that impact the surface of Phobos have relatively high velocities, so that they actually penetrate inside the surface or rock,” Nénon said. “While traveling inside the surface, ions are decelerated until they stop somewhere inside the material. Once stopped, they are surrounded by many molecules, so that they cannot escape to space.”

Deimos, the smaller of Mars' two Moons. Phobos (top) could have remnants of its planet's atmosphere lurking just beneath the surface. Credit: NASA

More mysteries surround Phobos and its brother moon, Deimos. Where they came from is unknown. The could have broken off from Mars, as Earth’s Moon is thought to have done, or they might have formed in the same cloud of dust and gas Mars emerged from. They might have even been huge hunks of debris from a collision or asteroids that were captured by Martian gravity. Getting evidence for any of these scenarios would need deeper probing, because the surfaces of Phobos and Deimos are nowhere near the same as they were when they were born. Nénon believes the absence of an atmosphere left both moon vulnerable.

“Phobos does not have an atmosphere to protect its surface, so the primitive composition of rocks exposed to outer space is altered by the harsh space environment,” he said. “Basically, anything that impacts Phobos' surface alters its properties, including sunlight, the solar wind, micrometeoroids and large bodies."

Moons can often tell us about the distant past of the planets they orbit. Our own Moon is the best record we have of the early solar system, because it doesn't have an atmosphere or geological processes such as wind or flowing water to wear away at that evidence (this is unfortunately also the reason that Moon dust can be lethal to both humans and science instruments). Lunar craters that have essentially remained the same as they were four billion years ago have told scientists about the Late Heavy Bombardment, a period of turbulence during which both Earth and its satellite were beaten up by asteroids and other bodies.

Apollo astronauts brought back lunar soil that revealed the violent past Earth and the Moon went through. It would be just about impossible to tell what hit our planet at the same time that the moon was getting bombarded, because our atmosphere along with wind, water, the shifting of soil and other phenomena would have worn away at those craters for eons. Some may be buried deep underground while others disappeared completely. Astronauts didn’t even have to traverse the Moon for scientists to find this out, because samples of its regolith were enough.

“Right now, we do not know which specific insights a sample from Phobos could reveal about the ancient Martian atmosphere,” said Nénon. “The hope is to maybe constrain the composition of the ancient Martian atmosphere. Understanding the composition of Phobos' samples that will be brought back to Earth by MMX in 2029 will be a challenge, and our contribution is to highlight that one important piece of the puzzle, which is the transfer of ions from the atmosphere of Mars to the surface of its moon Phobos.”

Even though Phobos has not been sampled yet, what Nénon and his team were able to find out even without physical evidence is still telling. JAXA’s Martian Moons Exploration (MMX) probe will take off for Phobos in 2024, and is expected to bring something back by 2029, and scientists examining these samples will know what to look out for. They may finally reveal secrets that are still lingering in space.