Mars is a true Wonderland world that has sung its enticing Sirens’ song for centuries to those who seek to solve its many mysteries. Indeed, the two moons of Mars, named Phobos and Deimos, present some mesmerizing mysteries all their own. Where did the two moons of Mars come from? For a long time their strange irregular shapes have suggested that they were both born asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter–only to be snared by the Red Planet’s gravitational embrace when they wandered too close to what was to become their adopted parent-planet. However, in April 2018, planetary scientists at the Southwest Research Institute (SwRI) in San Antonio, Texas, presented an alternative scenario to explain the origin of these two tiny potato-shaped moons. The new theory proposes that Phobos and Deimos were really born as the result of an ancient impact when a small dwarf protoplanet blasted into the primordial Mars. The paper describing this new model is published in the April 16, 2018 issue of the journal Science Advances.
The primordial Solar System has frequently been likened to a “cosmic shooting gallery” where objects, large and small, were continually crashing into one another–wreaking havoc. The ancient giant collision between the young Mars and an ill-fated protoplanet would have been almost identical to the one that is generally thought to have been responsible for the formation of Earth’s own large Moon. According to this model, Earth’s Moon was born when a doomed Mars-sized protoplanet named Theia crashed into our still-forming planet.
Astronomers have been debating the origin of the mysterious duo of Martian moons for decades. The perplexing puzzle, that has been difficult to solve, is whether the moons are really captured asteroids or were born instead from a debris disk whirling around the primordial Mars. This surrounding debris disk would have originated as a result of the proposed giant impact. This giant impact model explaining the origin of Phobos and Deimos has been considered the most promising explanation. Alas, earlier models of this process were hindered by low numerical resolution, as well as overly simplified modeling technology.
In the case of the giant impact model between the primordial Earth and the tragedy that was Theia, the violent impact hurled debris into the sky above our planet. Eventually, the debris coalesced to create Earth’s lovely lunar companion.
“Ours is the first self-consistent model to identify the type of impact needed to lead to the formation of Mars’ two small moons,” noted study lead author Dr. Robin Canup in an April 16, 2018 SwRI Press Release. Dr. Canup is an associate vice president in the SwRI Space Science and Engineering Division, as well as one of the leading scientists using large-scale hydrodynamical simulations to model planet-scale collisions, including the favored giant impact Earth-Moon formation model.
Quite A Pair
Ever since their discovery in 1877 by the American astronomer Asaph Hall (1829-1907), Phobos and Deimos have bewitched and bewildered astronomers seeking the elusive answer to the question of how Mars managed to acquire its duo of oddly-shaped little moons. Phobos has an orbit that carries it closer to Mars than its sibling moon, with a semi-major axis of 5,827 miles, as opposed to Deimos’ 14,580 miles.
When a moon is in orbit around its parent-planet, all goes well for both the planet and its moon–just so long as the gravity that is keeping the moon in one piece exceeds the relentless and powerful pull of its planet. The trouble begins if the ill-fated moon wanders too close to the gravitational grip of its destructive parental planet. This is because the tidal forces of the planet start to exceed the gravitational bind that is holding the unlucky moon together–this means that the moon will fall apart. Earth’s relatively large Moon is very lucky because the limit–termed the Roche Limit–is a bit under 10,000 kilometers, and it is a safe and secure 385,000 kilometers from our planet.
Alas, other moons may not be as lucky. This fortunate state of affairs for Earth and its lunar companion is not the case for the Martian moons. Phobos is the larger moon of the duo, at about 22 kilometers in diameter, and it is currently slowly wandering inward towards Mars. Phobos is a doomed little moon-world, because it will approach the Martian Roche Limit in about 20 million years. When it does so, Phobos will be pulled apart, forming a mess of debris that will create a spectacular ring around the Red Planet. In contrast, Deimos–the smaller of the duo–will remain without its companion moon. Deimos orbits its parent-planet at a safer, greater distance. This last surviving Martian moon will become a lonely object lingering in the Martian sky.
If an observer stood upon the Martian surface near its equator, full Phobos would appear to be approximately one-third as large as Earth’s full Moon. However, Phobos would look considerably smaller if the observer stood further away from the Martian equator–and it would be completely invisible if the observer gazed up at the Martian sky while standing on one of its polar ice caps. Deimos looks more like an especially bright star or planet when viewed by an observer on Earth. There are no total solar eclipses on Mars. This is because the moons are much too small to completely block the Sun. In dramatic contrast, total lunar eclipses of Phobos occur almost every night.
The motions of the Martian moons would appear very different from those of Earth’s own Moon. The speed-demon Phobos rises in the west, sets in the east, and then rises again only eleven hours later. On the other hand, Deimos–being just outside synchronous orbit–rises as expected in the east. However, Deimos performs this feat very slowly. In spite of its 30-hour orbit around its parent-planet, it takes 2.7 days for Deimos to set in the west as it lazily falls behind the rotation of Mars.
Both Martian moons are tidally locked, always showing the same face towards Mars. Several string craters have been observed pockmarking the Martian surface, and they are inclined further from the equator the older they are. This suggests that there may once have been many small moons that perished in the way currently predicted for the doomed Phobos–and that the Martian crust as a whole shifted between these events. In contrast, Deimos is far enough away from its parent-planet to have its orbit slowly boosted instead–as is also the case for Earth’s own Moon. When Earth’s Moon was born it was much closer to our planet. The primordial Moon was a considerably larger object in Earth’s ancient sky than it is now. As time went by, Earth’s Moon traveled farther and farther away; appearing to be smaller and smaller in the sky as a result.
The birthplace of the Martian moons is a subject of hot debate. Both little moons have much in common with carbonaceous C-type asteroids, with albedo, density, and spectra very similar to those of C- or D-type asteroids. Because of this similarity, one theory suggests that both moons may be captured Main Belt asteroids. However, both Phobos and Deimos have circular orbits that are located almost exactly in Mars’s equatorial plane. For this reason, a capture origin requires a mechanism for circularizing the initially highly eccentric orbits, and adjusting their inclinations into the equatorial plane. This would have probably resulted from a combination of atmospheric drag and tidal forces–although it is not clear that enough time was available for this to happen in the case of Deimos. Circular orbits are an indication that the orbiting body was born where it is, while eccentric orbits indicate the opposite. Another problem with the capture theory is that the capture itself requires dissipation of energy. The atmosphere of Mars today is much too thin to capture a Phobos-sized object by way of atmospheric braking. However, a capture may have possibly occurred if the original body was really a binary asteroid that separated as a result of tidal forces.
A Blast In The Martian Past
The new model proposes a much smaller impacting protoplanet than those considered in previous studies. The catastrophic impact thought to have created Earth’s Moon occurred approximately 4.5 billion years ago–a time when our 4.6 billion year old Solar System was very young. The Earth’s diameter is about 9,000 miles, while the diameter of Mars is just a bit over 4,200 miles. Earth’s Moon is a little over 2,100 miles in diameter, about one-fourth the size of Earth.
Phobos and Deimos formed within the same time frame. Both tiny moons hug their parent-planet in close orbits. The proposed Phobos-Deimos forming impactor would have been approximately the same size as the asteroid Vesta–the second-largest inhabitant of the Main Asteroid Belt after the dwarf planet Ceres. Vesta sports a diameter of 326 miles, while Ceres is about 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. 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,” Dr. Julien Salmon explained in the April 16, 2018 SwRI Press Release. Dr. Salmon is a research scientist at the SwRI.
These new findings are important for the Japan Aerospace Exploration Agency (JAXA) Mars Moons eXploration (MMX) mission, which is scheduled to launch in 2024. MMX will include a NASA-provided instrument. The MMX spacecraft will visit the Red Planet’s two little moons, as well as land on the surface of Phobos in order to obtain a surface sample that will be returned to Earth for study in 2029.
“A primary objective of the MMX mission is to determine the origin of Mars’s moons, and having a model that predicts… the moons’ compositiions would… provide a key constraint for achieving that goal,” Dr. Canup explained in the April 16, 2018 SwRI Press Release.
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