According to the April 18, 2018 issue of Science Advances, in the article entitled the “Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres-sized body with Mars”… oh wait… I kind of just gave away the whole thing right there.
Before we get lost in space, for those who are interested, go to the bottom of this article and click on the link to the music video by M|A|R|R|S - Pump Up The Volume, a 1987 video with some cool music - helping birth British acid house music - and some old-school video footage of early manned space flight.
Anyhow, according to the article, scientists from the Southwest Research Institute theorize that the small, misshapen moons of Mars—Phobos and Deimos—were formed after a single impact of the young proto-Mars and a dwarf-planet-sized object similar in size to the largest asteroids Vesta and Ceres.
Now, an object smashing into a planet to create a moon is not new news—Earth and Luna (our moon) was formed in a similar fashion, though the Earth impact is suspected to have been much, much larger than the Martian one.
Luna may have formed when a Mars-sized object crashed into the nascent Earth 4.5 billion years ago, and the resulting debris coalesced into the Earth-Moon system.
Scientists have been discussing the origins of Mars' two moon for a while, wondering if they were simply asteroids captured by the planet's gravity or if they were formed from the common form of an equatorial disk of debris.
While others had thought of an impact as the cause, test models 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’ two small moons,” says lead author Dr. Robin Canup, an associate vice-president in the Southwest Research Institute Space Science and Engineering Division. Canup is one of the leading scientists using large-scale hydrodynamical simulations to model planet-scale collisions, including the prevailing Earth-Moon formation model.
“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,” Canup explains. “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 Mars model invokes a much smaller impactor than considered previously. Earth’s diameter is about 8,000 miles, while Mars’ diameter is just over 4,200 miles. The Moon is just over 2,100 miles 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 7.5 miles and 14 miles 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, and the dwarf planet Ceres, which is 587 miles 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,” says the paper’s second author, Dr. Julien Salmon, an Southwest Research Instituteresearch scientist. “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.”
That's cool... but what the heck does any of this have to do with Japan?
Well, the Japan Aerospace Exploration Agency (JAXA) has an upcoming Mars Moons eXploration (MMX) mission where it will try and determine the origin of the two moons of Mars.
The MMX mission is scheduled to launch in 2024, and will visit both moons with a planned landing on the surface of Phobos to take a surface sample before it returns to Earth in 2029.
“A primary objective of the MMX mission is to determine the origin of Mars’ moons, 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,” Canup acknowledges.
The mission will also take aboard a special NASA (National Aeronautics and Space Administration) tool... of which I know little about, suffice to say that the JAXA mission is one that NASA is interested in.
As for the research done in the “Origin of Phobos and Deimos by the impact of a Vesta-to-Ceres-sized body with Mars,” it was funded by NASA’s Solar System Exploration Research Virtual Institute (SSERVI) in Silicon Valley, and by NASA’s Emerging Worlds program. The research was conducted as part of the Institute for the Science of Exploration Targets (ISET), a SSERVI team from SwRI’s office in Boulder, Colorado.
PS: For your listening pleasure:
PPS: The image at the top of this article is from http://www.seasky.org/solar-system/mars-menu.html, and as it correctly states it is not to scale, but it does show how misshapen the two moons of Mars are.