PHOBOS

PHOBOS
Name	Phobos
Caption	Color image of Phobos taken by the Mars Reconnaissance Orbiter.
Discovery date	August 18, 1877
Discoverer	Asaph Hall
Named after	Phobos, Greek god of fear
Adjectives	Phobian
Dimensions	27 × 22 × 18 km
Mean radius	11.2667 km
Mass	1.0659×1016 kg
Surface grav	0.0057 m/s²
Escape velocity	41 km/h
Rotation period	Synchronous
Axial tilt	0°
Albedo	0.071
Temperature	≈ 233 K
Atmosphere	None

PHOBOS. It is the larger and innermost of the two natural satellites of Mars, the other being Deimos. Discovered by American astronomer Asaph Hall in 1877, it orbits remarkably close to the Red Planet, completing a revolution in just 7 hours and 39 minutes. Its rapid motion and irregular, potato-like shape have made it a subject of intense scientific study and a target for multiple spacecraft missions.

Overview

Phobos is a dark, heavily cratered body that circles Mars at a distance of only about 6,000 kilometers from the planet's surface, closer than any other known moon to its primary in the Solar System. This proximity means it rises in the west and sets in the east, appearing to cross the Martian sky twice each Martian day. The moon is named for the Greek deity Phobos, a personification of fear and panic who accompanied his father Ares into battle, with the Roman equivalent being Mars. Observations from missions like Viking 1 and the Mars Express have revealed a world dominated by a massive impact crater and a mysterious system of grooves.

Physical characteristics

Phobos has an irregular shape, measuring approximately 27 by 22 by 18 kilometers, and is not massive enough to be pulled into a sphere by its own gravity. Its surface is dark, resembling that of carbonaceous chondrite meteorites, and is one of the least reflective bodies in the Solar System. The most prominent feature is Stickney, a vast impact crater nearly 9 kilometers in diameter, whose formation likely came close to shattering the moon. Radiating from Stickney are extensive series of linear grooves and chains of small craters, which may be stress fractures or ejecta from impacts on Mars. The surface is also covered with a fine regolith, as seen in images from the European Space Agency's Mars Express.

Orbit and rotation

Phobos orbits Mars at an average distance of 9,378 kilometers, well within the planet's synchronous orbit radius, completing three orbits in a single Martian day. It is in a decaying orbit, moving closer to Mars by about 1.8 centimeters per year due to tidal forces, and is predicted to either break apart into a ring or crash onto the surface in roughly 30 to 50 million years. The moon is in synchronous rotation, meaning it always shows the same face to Mars, much like Earth's Moon does to our planet. Its orbital plane is nearly equatorial and has a very low inclination relative to the Martian equator.

Exploration

Phobos has been visited and studied by several spacecraft, beginning with NASA's Mariner 9 in 1971, which provided the first close-up images. The Viking program orbiters in the 1970s returned higher-resolution data, mapping the moon in detail. The Soviet Phobos program in the late 1980s, including Phobos 2, conducted flybys and remote sensing before failing just prior to a planned landing. More recently, the European Space Agency's Mars Express has performed numerous close flybys, using instruments like the High Resolution Stereo Camera and MARSIS radar to probe its interior. Future missions, such as the Japan Aerospace Exploration Agency's Martian Moons eXploration (MMX), aim to return samples from its surface.

Origin and composition

The origin of Phobos remains debated, with two leading hypotheses. One theory posits it is a captured D-type asteroid from the outer asteroid belt, similar to objects found near Jupiter's Trojan asteroids, based on its spectral similarity to carbonaceous chondrites. The alternative theory suggests it formed in orbit from debris ejected by a giant impact on Mars, analogous to how Earth's Moon formed from a collision with Theia. Data from spectrometers on Mars Express and the now-defunct Phobos 2 indicate a surface composition poor in metals but possibly containing phyllosilicates, suggesting some interaction with water. Its low density implies a porous interior, possibly a "rubble pile" structure held together by gravity.

In popular culture

Phobos has been a fixture in science fiction since its discovery, often depicted as an alien base or a key strategic outpost. It features prominently in novels by authors like Arthur C. Clarke in The Sands of Mars and Greg Bear in Moving Mars. In video games, it is famously the location of the military base where the experiment that opens the portal to Hell occurs in id Software's classic *Doom*. The moon also appears in films such as Mission to Mars and television series like The Expanse, where its orbit and the looming Stickney crater are frequently noted.

Category:Moons of Mars Category:Phobos Category:Astronomical objects discovered in 1877