Martian moons likely formed after impact with Ceres-sized object

By | Published: April 18, 2018 | Last updated on May 18, 2023
Based on new research, scientists think that the martian moons Phobos and Deimos likely formed after a catastrophic collision between Mars and a dwarf-planet-sized object.


According to a new study published today in Science Advances, Mars’ two moons — Phobos and Deimos — likely formed following an impact between proto-Mars and a minor planet roughly the size of Ceres, which is 587 miles wide (950 kilometers).

To carry out the study, the researchers created a new model that took into account interactions between martian moons, then simulated how a disk of material would form and evolve around Mars following a collision between the Red Planet and a large object. The novel approach allowed the team to track ejecta from the impact with resolutions a full order of magnitude better than previous studies.

“We used state-of-the-art models to show that a Vesta-to-Ceres-sized impactor can produce a disk consistent with the formation of Mars‘ small moons,” said co-author Julien Salmon, a research scientist at the Southwest Research Institute (SwRI), in a press release.

“The outer portions of the disk accumulate into Phobos and Deimos,” said Salmon, “while the inner portions of the disk accumulate into larger moons that eventually spiral inward and are assimilated into Mars. Larger impacts advocated in prior works produce massive disks and more massive inner moons that prevent the survival of tiny moons like Phobos and Deimos.”

This composite image compares the sizes of Mars’ two moons, Phobos (middle) and Deimos (far left), with the size of Earth’s Moon (right), which is about 100 times bigger than Phobos.
NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M University
Astronomers have long debated the origin of Mars’ tiny satellites. Some posit that Phobos and Deimos, which have diameters of 14 miles and 7.5 miles respectively, are actually intact asteroids that were captured by the gravity of Mars. But due to the fact that both moons orbit in nearly the same plane and trace almost circular paths, many astronomers lean toward the theory that Phobos and Deimos were produced inside a disk of debris that encircled Mars following a powerful collision with another object.

Though prior researchers have attempted to model such a collision, the simulations typically fall short due to low resolution and oversimplified models. “Ours is the first self-consistent model to identify the type of impact needed to lead to the formation of Mars’ two small moons,” said lead author Robin Canup, an associate vice president in the SwRI Space Science and Engineering Division.

“A key result of the new work is the size of the impactor; we find that a large impactor — similar in size to the largest asteroids Vesta [326 miles wide] and Ceres [587 miles wide] — is needed, rather than a giant impactor,” said Canup. “The model also predicts that the two moons are derived primarily from material originating in Mars, so their bulk compositions should be similar to that of Mars for most elements. However, heating of the ejecta and the low escape velocity from Mars suggests that water vapor would have been lost, implying that the moons will be dry if they formed by impact.”

As part of the new study, researchers modeled a collision between a Ceres-sized object and Mars. The results of the 3-D simulation show that the impact produces a disk of debris around the Red Planet that is primarily made of material from Mars. The outer rim of the disk eventually coalesces into two small martian moons — Phobos and Deimos.
Southwest Research Institute
The new research is particularly timely because the Japan Aerospace Exploration Agency (JAXA) is currently preparing for its Mars Moons eXploration (MMX) mission, which will launch a spacecraft in the early 2020s that will visit both martian moons, as well as land on Phobos to collect a surface sample that will be returned to Earth for further analysis.

“A primary objective of the MMX mission is to determine the origin of Mars’ moons,” said Canup. “And having a model that predicts what the moons’ compositions would be if they formed by impact provides a key constraint for achieving that goal.”

To learn much more about Mars, check out our FREE Guide to Settling Mars, jointly produced by Astronomy and Discover magazines.