Mars Moons Phobos and Deimos Origin: Are These Remnants of a Lost Moon?

A new study suggests that Mars' two moons - Phobos and Deimos - actually came from a common progenitor, challenging existing knowledge about the twin moons.

For a long time, Phobos and Deimos - which were named for Ares' (Mars) sons Fear and Panic respectively - were thought to be asteroids captured by the Red Planet and have been orbiting Mars ever since. Additionally, these moons look a lot different from Earth's own Moon, with ours being larger and more spherical in terms of physical properties.

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The recent study titled "Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor," appearing in the journal Nature Astronomy presents recent evidence why the captured theory of Phobos and Deimos might not be accurate at all.

Using geophysical data and models created from the Moon's orbital motion, researchers traced back to billions of years ago - finding that Phobos and Deimos were in approximately the same spot. This basically suggests that they both belonged to a single, larger object at the same point in the past.

Remnants of a Lost Moon

The nature of these twin moons has puzzled researchers since they were first discovered back in 1877. Phobos measures about 14 miles (22 kilometers) across, while Deimos is slightly more than half this size. The new study proposes that between 1 and 2.7 billion years ago, a single moon orbited Mars.

"Phobos and Deimos are the remainders of this lost moon," says Amirhossein Bagheri, a research team member and a doctoral student at the Institute of Geophysics at ETH Zurich in Germany, in a statement from the university. While their unusual appearance led the scientific community to suspect that Phobos and Deimos were asteroids captured in the Red Planet's gravity field, Bagheri noted that this is "where the problems started."

Amir Khan, a senior scientist with the Physics Institute of the University of Zurich and ETH Zurich's Institute of Geophysics, explains that the idea was to trace the orbits and how they have changed since then. Finding a point where their locations overlapped, researchers believed that the two moons came from a single progenitor in the past.

Aside from the overlap in their past orbit and motion with respect to Mars, researchers had to verify this theory further. They had to examine the existing theories based on the interaction between Mars and its moons. It involves a form of energy conversion called dissipation, which depends on various factors such as the size of the planet and its moons, their physical makeup, and the distance between them.

Further Investigations on the Red Planet and Its Moons

The ETH Zurich press release noted the exploration efforts by NASA's InSight mission, including the German university. Specifically, the electronics integrated into the InSight seismometer, which records tremors and meteorite impacts, were developed at ETH. Khan explains that the data from this mission are used to "constrain the Mars model," based on their calculations and the dissipation that occurs in Mars and its moons.

Imaging and measurement data from other Martian probes suggests that both Phobos and Deimos are composed of a very porous material. "There are a lot of cavities inside Phobos, which might contain water ice, and that's where the tides are causing a lot of energy to dissipate," explains Khan.