A colossal impact between proto-Mars and a Vesta-to-Ceres-sized object likely produced the two Martian moons, Phobos and Deimos, according to new research.
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.
The origin of Phobos and Deimos has been debated for decades. The question is whether the bodies were asteroids captured intact by Mars gravity or whether the small satellites formed from an equatorial disk of debris, as is most consistent with their nearly circular and co-planar orbits.
The production of a disk by an impact with the Red Planet seemed promising, but prior models of this process were limited by low numerical resolution and overly simplified modeling techniques.
“Ours is the first self-consistent model to identify the type of impact needed to lead to the formation of Mars’ small moons,” said study lead author Dr. Robin Canup, of the Southwest Research Institute.
“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 and Ceres — is needed, rather than a giant impactor.”
“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.”
The new model invokes a much smaller impactor than considered previously.
Our Moon may have formed when a Mars-sized object crashed into the proto-Earth 4.5 billion years ago, and the resulting debris coalesced into the Earth-Moon system.
The Earth’s diameter is about 7,918 miles (12,742 km), while Mars’ diameter is 4,217 miles (6,785 km). The Moon is 2,159 miles (3,474 km) in diameter, about one-fourth the size of Earth. While they formed in the same timeframe, Deimos and Phobos are very small, with diameters of only 3.9 miles (6.2 km) and 14 miles (22 km) respectively, and orbit very close to Mars.
The proposed Phobos-Deimos forming impactor would be between the size of the asteroid Vesta, which has a diameter of 326 miles (525 km), and the dwarf planet Ceres, which is 587 miles (945 km) wide.
“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 Dr. Julien Salmon, also from the Southwest Research Institute.
“The outer portions of the disk accumulate into Phobos and Deimos, 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.”
The study was published in the April 18, 2018 issue of the journal Science Advances.
_____
Robin Canup & Julien Salmon. 2018. Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres sized body with Mars. Science Advances 4 (4): eaar6887; doi: 10.1126/sciadv.aar6887
