Phobos and Deimos, two satellites of Mars, originated from disintegration of a much larger moon between 1 and 2.7 billion years ago, according to new computer simulations and an analysis of data from NASA’s InSight mission.
Mars is kept company by two cratered moons — an inner moon named Phobos and an outer moon named Deimos. Image credit: NASA / JPL-Caltech / Malin Space Science Systems / Texas A&M University.
Phobos and Deimos were discovered by the American astronomer Asaph Hall at the Naval Observatory on August 18, 1877.
These potato-shaped satellites orbit Mars in synchronous rotation with inclinations of only 0.01 and 0.92 degrees relative to the equatorial plane of the planet.
Although Phobos and Deimos are small, their peculiar orbits hide important secrets about their past.
“Earth’s Moon is essentially spherical, while the moons of Mars are very irregularly shaped — like potatoes,” said Amirhossein Bagheri, a doctoral student at the Institute of Geophysics at ETH Zurich, adding
“Phobos and Deimos look more like asteroids than natural moons. This led people to suspect that they might in fact be asteroids that were captured in Mars’s gravity field.”
“But that’s where the problems started: captured objects would be expected to follow an eccentric orbit around the planet, and that orbit would be at a random inclination; in contradiction to this hypothesis, the orbits of the Martian moons are almost circular and move in the equatorial plane of Mars. So, what is the explanation for the current orbits of Phobos and Deimos?”
To solve this dynamic problem, Bagheri and colleagues relied on computer simulations and seismic data from NASA’s InSight mission.
“The idea was to trace the orbits and their changes back into the past,” said Dr. Amir Khan, a researcher at the Physics Institute of the University of Zurich and the Institute of Geophysics at ETH Zurich.
“As it turned out, the orbits of Phobos and Deimos appeared to have crossed in the past. This means that the moons were very likely in the same place and therefore have the same origin.”
The researchers concluded that a larger celestial body was orbiting Mars back then.
This ancient moon was probably hit by another body between 1 and 2.7 billion years ago and disintegrated as a result.
“Phobos and Deimos are the remnants of that lost moon,” Bagheri said.
The team’s simulations also show that, while Deimos very slowly continues to ascend, Phobos will impact on Mars in 39 million years or tidally disintegrate into a ring.
“The results stand to be improved with Mars InSight geophysical data, in particular the dissipation in Mars and its frequency dependence that control the orbital history of Phobos,” the scientists said.
“The upcoming Martian Moons Exploration mission will also provide crucial information on the moons’ interiors, which will help to settle the question of their origin.”
The findings appear in the journal Nature Astronomy.
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A. Bagheri et al. Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor. Nat Astron, published online February 22, 2021; doi: 10.1038/s41550-021-01306-2
