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A new photograph from the Perseverance rover on Mars shows the smaller of the planet’s two moons—named Deimos—glowing in the Martian sky shortly before sunrise. The origin of the moons has been shrouded in mystery since the beginnings of astronomy. Today, scientists are mostly split between the possibility that they once were parts of Mars itself or that they are asteroids that were captured by the planet’s gravity.
“They have an enigmatic origin,” says Christopher Edwards, a planetary scientist at North Arizona University and an expert in the Martian moons. “They’re a curiosity.”
Deimos has only been captured in photographs from the surface of the Red Planet a few times, and seldom at sunrise. The low light and long exposure make the image hazy and dusky. Many of the white specks in the sky are likely noise, according to NASA, though some may be cosmic rays.
The theory that Deimos and its companion moon Phobos are stray asteroids captured by gravity is supported by observations of reflected light, which suggests they may be composed of carbon-rich rocky material, unlike the iron-rich dust and rocks of Mars surface.
On the other hand, some astronomers have speculated that maybe the moons are made of Mars rock that has somehow become darker through an unknown chemical process. The moons’ orbits are nearly circular and travel in roughly the same plane, which makes some think they formed out of rings of ejecta created by an ancient impact, perhaps between Mars and a large asteroid.
The discovery of the moons of Mars has a curious history.
“The giant impact was my favorite scenario,” says planetary scientist Patrick Michel of the Lagrange Laboratory at the University of Côte d’Azur in France, “because it seemed to be the only way to produce two moons in circular orbits.” But more recently scientists have found that asteroid capture could also produce such a configuration, he says.
Phobos is roughly potato-shaped and only about 14 miles across, but it is the larger moon. It orbits just a few thousand miles above the Martian surface, much closer than any other natural satellite. Deimos is less than half its size and orbits the planet more than 14,000 miles away.
Both Michel and Edwards are part of the Martian Moon Exploration mission due to launch next year, which is designed to bring back rock samples from one of the Martian moons in 2031. These samples could help to settle the question of the origin of the moons. Michel is also a member of the Hera mission, which is bound for the asteroid belt but made a gravity “slingshot” around Mars in March. The slingshot trajectory brought the Hera probe to within about 621 miles of Deimos and allowed it to image the moon’s rarely seen far side; Michel says the data from that flyby is still being processed.
The discovery of the moons of Mars has a curious history. They have been a subject of speculation since 1610, when the pioneering Italian astronomer Galileo Galilei first observed the rings of Saturn through a telescope he’d built himself. Galileo then wrote a cryptic message in letters to announce his discovery—an established scientific practice at the time, to hide his disclosure if it later turned out not to be true. The message was “smaismrmilmepoetaleumibunenugttauiras,” by which Galileo meant “I have observed the highest planet tri-form” (because he imagined the two sections of rings he saw on each side of Saturn were part of the planet).
But his message was misinterpreted by the contemporary astronomer Johannes Kepler, then living in the Austrian city of Linz, who announced in subsequent letters that Galileo had seen two moons around Mars. The supposed discovery aligned with Kepler’s own speculations, in which he reasoned Mars must have more moons than Earth because Galileo had already seen five moons around Jupiter.
But this supposition wasn’t proven true until more than 250 years later, when the American astronomer Asaph Hall first observed the Martian moons in 1877 with a refracting telescope at the U.S. Naval Observatory in Washington D.C.—the largest in the world at that time. Hall gave the Martian moons their names, Deimos and Phobos, after Homer’s names for the sons and charioteers of the war god Ares—the Greek equivalent of the Roman god Mars. (Translations vary, but according to NASA, deimos means dread, while phobos means fear.)
Recent flybys suggest that Deimos is made from slightly different rocky material than Phobos, says Edwards, which seems to support the theory of their origins as captured asteroids. Edwards worked on the Hope mission to Mars led by the United Arab Emirates space agency, which conducted the flybys of Deimos in 2023. But the difference could still be explained if samples from the Mars Moon Exploration mission show the moons are composed of material similar to rocks from Mars, Edwards says. The mission will image both Martian moons over several years and finally attempt a landing on Phobos to gather rock samples for return to Earth.
Asteroid families have distinctive geochemical compositions, and the rock samples could be expected to determine the origin of Phobos, and by extension Deimos (the chances that both moons have different origins are very slim). The Mars Moon Exploration mission “provides a really unique opportunity to solve a lot of questions about these enigmatic moons,” Edwards says. Michel, too, hopes the moon rocks will offer some answers: “It will be good to have real confirmation at last.” 
Lead image: NASA/JPL-Caltech
