Something’s not quite right about the moons of Mars. They are too small — Phobos is 17 miles across, and Deimos is a mere nine miles in length. And they aren’t round, but lumpy, misshaped objects. Frankly, they don’t resemble moons at all.
“They look like asteroids, they smell like asteroids, as well as looking like potatoes,” said James O’Donoghue, a planetary astronomer at the University of Reading in England. Perhaps, then, astronomers have suggested, they are asteroids — two space rocks captured long ago by Mars’s gravity.
A study published Wednesday in the journal Icarus makes a case that the moons did indeed start out in asteroid form. But it’s not the genesis everyone was expecting. Using supercomputer-powered simulations, scientists describe a situation in which a large-enough asteroid was captured by Mars long ago and torn to shreds by the planet’s gravity, briefly forming a debris cloud — and possibly a ring system — around Mars that ultimately clumped together to form two moons.
“What they’ve got here is really compelling,” said Dr. O’Donoghue, who wasn’t involved with the study. “I’m sold.”
The notion that Phobos and Deimos may be captured asteroids has long come up against one major problem: Their orbits are too circular, and too neatly aligned around Mars’s equator. Asteroids approach planets at all sorts of angles, and if these moons were once asteroids, their orbits would be expected to be tilted, and perhaps be somewhat oval-shape.
That they aren’t seems suspicious, and supports the theory that they were forged another way. This is akin to the favored origin story of Earth’s own satellite, wherein a Mars-size object slammed into the nascent world, creating a spray of debris, which glued itself together to form our moon.
If something similar had happened to Mars, scientists would expect this debris to form a disk aligned to the Martian equator, leading to the construction of moons with the same orbital alignment — something observed today.
That’s where the new simulation and its proposed crumbling asteroid come into play. Such an object may have had a mass comparable to that of Vesta, the second-most massive object in the asteroid belt between Mars and Jupiter. And when it plunged toward the Red Planet, it would have gotten extremely close: anywhere from about 3,200 miles above the rust-hued desert to grazing the surface.
Consequently, the gravity of Mars would have ripped the asteroid into fragments. “Some of them can escape from the system completely,” said Jacob Kegerreis, who is a research scientist at NASA’s Ames Research Center in Mountain View, Calif., and one of the authors of the study. But “some of them can hit Mars,” he said, scarring the surface with smoldering craters.
Many shards would remain in orbit around the planet. Over time, these shards would be affected by the gravitational influence of both Mars and the sun. And eventually, several of these fragments would careen into one another, sending out sprays of debris that would then collide into other fragments.
This destruction derby would create a cloud of asteroidal rubble that eventually aligns itself with the Martian equator. The detritus that stayed far enough from the planet would eventually clump together to form Phobos and Deimos.
These potato moons may look tranquil now. But clearly, “the solar system used to be a much messier place,” Dr. Kegerreis said.
The final word on the origins of Mars’s moons will be sought by the Martian Moons eXploration mission, or MMX, a Japanese space agency spacecraft. Set to launch in the next couple of years, MMX aims to retrieve a geological sample from Phobos and return it to Earth, which will reveal whether its composition is Martian or asteroidal.
Whatever that mission finds, the story of at least one of these moons is far from over.
When the proposed asteroid was destroyed, any of its debris lingering closer to Mars would have transformed into Saturnlike rings. These rings were only temporary features — much like Phobos itself, which is slowly tumbling toward Mars. In the next 50 million years or so, it will either spectacularly crash into the planet or will splinter to form a new Martian ring.
Everything in the solar system may seem permanent and unchanging to us Earthlings. But what is true down here is equally true out there: “Nothing lasts forever,” Dr. O’Donoghue said.
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