Mohawk Valley Astronomical Society

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Mystery Moons

by Perry Pezzolanella

Phobos is an interesting and mysterious moon that orbits Mars along with its smaller companion, Deimos. Both moons are oddities and share a common question: Did they form from a huge impact that struck Mars or are they captured asteroids?

Phobos is potato-shaped, measuring 16x14x11 miles and orbits only 3700 miles above the Martian surface. It orbits Mars in only 7.65 hours, much faster than Mars’ rotation of 24.6 hours. This means that Phobos rises in the west and sets in the east three times every Martian day. Phobos orbits so close to Mars that it can never be seen from latitudes greater than 67º. Deimos is truly tiny, measuring 9x7x6 miles and orbits 12,500 miles above the Martian surface. It orbits Mars in 30 hours so it rises in the east and sets in the west 60 hours later. Deimos cannot be seen from the surface of Mars at latitudes greater than 82º.

Both moons are rocky and riddled with craters with Phobos marred by a series of linear fractures. These could be caused by the giant 6-mile crater known as Stickney, but the fractures do not line up and may be caused by the gravitational tug of Mars slowly tearing Phobos apart. This is a good clue that Phobos may not be solid, but more like a rubble pile. Deimos is more stable with only craters and a thicker coating of regolith. These moons are probably not captured asteroids as they should be in highly elliptical orbits and randomly oriented around Mars’ axis. Instead, Phobos and Deimos are in nearly circular orbits right above the equator. This makes the impact theory more likely considering the Earth’s Moon was created from a giant impact and orbits within 5º of the equator in nearly a circular orbit. Material from the impact would condense over the equatorial plane and form a moon or multiple moons. Mars may have had one larger moon that formed at just the right spot for it to quickly fracture. The pieces scattered just enough where one moon formed just far enough from Mars to not be drawn back down closer to Mars and fracture again. That moon may be Deimos. Another cluster of debris formed closer to Mars to become Phobos and was subjected to tides that drew it closer to Mars. It is possible that Phobos may have been a larger moon that was repeatedly shattered and reassemble each time. Phobos would end up smaller each time as some of the debris was drawn down to the Martian surface by atmospheric drag and gravity. Deimos orbits just beyond Mars’ geosynchronous orbit, the distance at which a satellite’s orbital period equals the planet’s rotation rate. Inside the geosynchronous orbit where Phobos resides, tides will force massive satellites to spiral in towards Mars; outside it, away from Mars. Phobos resides within Mars’ Roche limit where tidal influences will tear the satellite apart. The Roche limit is where the tidal limits of a planet prevent the formation of moons.

The best way to determine the origin of Phobos and Deimos is to land and analyze their soil. The Soviet Union tried in 1988 with Phobos 1&2 to land on Phobos and explore, but both missions failed before reaching it. Russia tried again in 2013 with Phobos Grunt, but the spacecraft failed to leave Earth orbit. Now at long last the Japan Aerospace Exploration Agency (JAXA) is planning to launch the Martian Moons Exploration (MMX) mission during September 2024 with arrival at Mars during August 2025. It will orbit Mars for three years making several close flybys of Phobos and Deimos. A microwave-sized rover will explore Phobos from the surface and the spacecraft itself will land on Phobos gathering surface samples, hopefully no less than several ounces, but anything is better than nothing. MMX will depart Phobos during August 2028 and land back on Earth during July 2029 with its precious samples. With the samples safely in Earth’s labs, scientists will study the origin and evolution of Phobos and Deimos. If the impact scenario is correct, the moons may contain minerals from Mars from a time when it might have been habitable.

Japan has proven it can fly missions to other worlds and successfully return samples as it did with Hyabusa 1 at asteroid Itokawa and Hyabusa 2 at asteroid Ryugu. There is hope that the mystery of the origin of Phobos and Deimos will finally be solved after decades of debate.