Overview

Phobos is the largest irregular natural satellite of Mars. It is the 85th-largest moon in the Solar system, and it is the largest moon in the Martian system. It got its name from Phobos, the Greek god of fear and panic, who was the son of Ares. It measures 21.84km across in diameter and has an average temperature of -51.424 °C. Phobos makes four and a half orbits in the time that its companion, Deimos, makes only one orbit. The moon has an uneven shape because the gravity of Phobos is too weak to reach hydrostatic equilibrium. In fact, if you jumped on Phobos, it would jump 51.6 meters and you would need to wait 12 minutes to land back on the ground. Depending on how strong the jump is, you might not be able to come back to the ground and you'd orbit Mars. Phobos lacks an atmosphere due to the solar winds stripping away any particles, and it has no circumstances to create a protective magnetic field because of its insufficient mass. With an albedo of about 0.071, it is one of the least reflective bodies in the Solar System.

Past formation

The origin of Phobos as well as Deimos is still contentious up to this day. One hypothesis claims that Phobos is a former asteroid belt that got too close and was grabbed by Mars to sit in orbit. There is some supporting evidence available, like spectra, albedo, and densities that are remarkably comparable to those of carbonaceous C- or D-type asteroids with which it shares characteristics of carbonaceous C- or D-type asteroids with which it shares characteristics. However it has some flaws like both Martian moons orbital eccentricity being almost zero and Phobos as well as Deimos has a close to zero orbital inclination, every natural captured asteroid orbits in a high orbital eccentricity, high orbital inclination, and sometimes going opposite direction to the parent rotational direction, that being the case, a method for circularising the originally very eccentric orbit and correcting its inclination into the equatorial plane is therefore needed for a capture origin; this mechanism will most likely involve a combination of atmospheric drag and tidal forces.^[1] Energy must also be dissipated during capture. For atmospheric braking to successfully trap a Phobos-sized object on Mars, the atmosphere is currently too thin. Thermal infrared observations of Phobos point to a composition dominated by phyllosilicates, which are well-known from the surface of Mars. Once further ruling out a capture origin, the spectra are dissimilar from those of all kinds of chondrite meteorites.^[2]

Another possibility is that Mars was originally surrounded by numerous bodies the size of Phobos and Deimos and was thrown into orbit by a collision with a huge planetesimal. Both sets of data are consistent with the current theory for the formation of Earth's moon, which postulates that Phobos was formed from debris expelled by an impact on Mars that reaccreted in Martian orbit.^[3] The formation of Mars' moons may have begun with a massive impact with a protoplanet that was just one-third the mass of Mars and left a ring around it. A big moon was created by the ring's inner portion. Phobos and Deimos were created by gravitational interactions between this moon and the outer ring. The two little moons, however, continued to orbit Mars after the giant moon later collided with it. The high porosity and fine-grained surface of the moons support this notion. Fine-grained material would be produced by the outer disc.^[4][5]According to simulations, the object that struck Mars had to be between Ceres and Vesta in size since a greater impact would have produced a wider disc and more moons, which would have prohibited the survival of small moons like Phobos and Deimos.

Future destruction

Phobos' orbit goes closer to Mars at a rate of about 2 cm per Earth year or 1.8 meters every 100 Earth years, meaning its orbit is unstable due to the moon's orbit being inside the synchronous orbit radius, an orbit where the orbital period of that object orbiting is identical to the rotation period of that parent, and roughly somewhere between 30 and 50 million years it will inevitably break apart into a ring system due to passing the Roche radius or it will inevitably collide with Mars, creating a impact crater on the surface of Mars. ^[6]

Gallery