Mars is a favorite of many planetary scientists, who continue to visit it through increasingly advanced robotic explorers. But don’t forget that our planetary neighbor is adorned with two moons: Little Phobos, a lumpy mass 17 miles wide; and tiny Deimos, only 9 miles long. Their ancient Greek names can mean “fear” and “fear,” but the aesthetics of these Lilliputian space potatoes inspire just the opposite.
They don’t look as interesting as Jupiter and Saturn’s volcanic or icy moons, nor is their desolation as extreme or diverse as Earth’s moon. But that hasn’t stopped generations of planetary scientists from being eager to get a closer look at the ramshackle duo.
The Soviet Union and, later, Russia have tried three times to reach Phobos, but software bugs and launch disasters have condemned each attempt. Scientists in the US have tried and, so far, failed to convince NASA’s powers that it will be worth a mission to both moons. The next big hope is Japan, whose goal is to launch a Phobos heist mission in 2024 that will try to steal some of its rocks.
What is all this fuss about? For many, the desire to visit Phobos and Deimos was galvanized by their deeply mysterious nature. “They are super rare, confusing and interesting,” said Abigail Fraeman, a planetary scientist studying Mars, Phobos and Deimos at NASA’s Jet Propulsion Laboratory.
We don’t know where the moons come from because they look like strange asteroids to the red planet, but they behave as by-products of Mars’s early, impact-laden history. And if that Japanese mission manages to take some samples and decode the chemistry of the shattered moons, we could discover its origins. By doing so, we will not only gain a better understanding of Mars’ ancient past. We will also be able to look back in time to the chaos that shaped the early solar system.
Much of what we know about Phobos and Deimos comes from fortuitous observations. The rovers on the surface, as well as the mechanical eyes that orbit around Mars, are sometimes in the right place to tilt their cameras and take photos of the two moons. Phobos, being larger and closer to Mars, can be seen in greater detail: a deformed disaster marked by a large crater and multiple grooves that seem to be made by the claws of a cosmic cat.
Remote observations of its surfaces have revealed no outstanding mineral characteristics or textures that can definitively detail the moons’ overall compositions and ultimately their origins, said Laura Kerber, associate scientist for the Mars Odyssey spacecraft project at the Laboratory. Jet Propulsion System.
“They check all the boxes that are consistent with them like these captured asteroids,” said Dr. Fraeman, debris mosaics that drifted too close to Mars long ago and were trapped in the planet’s orbit.
But both moons orbit the equator in a neat and orderly circular fashion, suggesting that they joined together from a debris disk that danced around a young Mars. It’s hard to capture an asteroid and make it “end up in this beautiful symmetrical, circular orbit,” said Jeffrey Plaut, the project scientist for the Mars Odyssey mission.
Mars, which has a tenth of Earth’s mass, has a relatively weak gravitational pull, making it seem unlikely that it can capture rapidly passing asteroids, said Tomohiro Usui, a robotic planetary exploration expert at the Exploration Agency Japan Aerospace. But if they were formed from a debris disk launched from Mars after a colossal impact, Deimos should be orbiting closer to Mars than it is today.
Reconciling their appearances with their orbits is difficult.
“They just shouldn’t exist,” said Dr. Fraeman. “They don’t make any sense.”
Chosen as brothers, both moons may not even have the same origin story.
In less than 100 million years, said Matija Ćuk, a research scientist at the SETI Institute in Mountain View, California, Phobos will get so close to Mars that its gravity will rip apart the moon, transforming into a beautiful ring system.
It won’t be the first time, some scientists say. Recent calculations suggest that Phobos was once 20 times more massive. But, according to one hypothesis, it headed towards Mars and shattered ring material, much of which rained down on Mars. The remaining ring material was grouped into a new smaller Phobos. This cycle has been repeated several times over billions of years, with Phobos decreasing with each cycle completed.
Although made of ancient matter, the Phobos we see today may have been assembled just 200 million years ago. If Phobos were confirmed to be a randomly grouped mass, it would be a revelation, suggesting that ring planets are the norm for our solar system.
Tiny Deimos, on the other hand, may have remained intact for 3.5 billion years. It is on its way to completely escape Mars, says Dr. Usui, to meander through the void.
NASA’s Perseverance rover, which will launch on July 22, is the first stage in an extended series of missions to bring pristine samples from Mars to Earth for study. The atmosphere of Mars, ancient volcanism and the flow of water have eroded many of the first rocks on the planet. But if Phobos is made from Martian remains of a massive impact shortly after the planet was born, then the moon has preserved the earliest memories of Mars.
“Many theories suggest that terrestrial planets, including Earth, formed dry, and the water was delivered by icy meteorites that dispersed inland,” said Dr. Usui. “If the moons are captured asteroids, they are evidence of this process in action, and their composition shows what materials were delivered to early Earth.”
Meteorites colliding with Mars could have covered Phobos in a thin layer of Martian dust from all over the planet. This matter can be very young and extremely old, and taking advantage of it will help scientists discover “how Mars could have progressed from a livable world to an uninhabitable one,” said Dr. Usui.
With a successful (albeit troublesome) asteroid sample return mission under its belt and another speeding up Earth with more asteroid matter, the Japanese space agency has a “pretty good track record” in snatching space rocks, Dr. Fraeman said . Hopes are understandably high for exploration of Japan’s Martian Moons, or MMX mission, which will arrive on Mars in 2025.
If successful, these huge planetary science questions may finally have clear answers. We will get to see not only Mars, but also Earth, in a new light.
“For me, that’s really cool,” said Dr. Fraeman.
