Phobos

Key facts

Diameter

22.5 km1% of Earth's Moon

Mass

1.07 × 10¹⁶ kg

Surface gravity

0.0057 m/s²0.0006 g

Density

1,876 kg/m³

Distance from Mars

9,376 km≈ 2.8 Mars radii

Orbital period

7.7 hoursSame as its rotation (tidally locked)

Discovered

1877Asaph Hall

Surface

Dark, dusty regolith scarred by grooves and craters

Surface temperature

Roughly −112 °C at night to −4 °C in sunlight

Rotation

Tidally locked, always showing the same face to Mars

Notable feature

Stickney crater, about 9 km wide

Named after

Phobos, Greek personification of fear and panic

Values follow NASA/JPL planetary satellite parameters: mean diameter, mean distance (semi-major axis) and sidereal orbital period.

Surface and geology

Phobos is one of the darkest objects in the Solar System, reflecting only about 7 per cent of incoming sunlight. Its surface is covered in fine regolith that may be a metre or more deep, and its spectrum resembles some primitive, carbon-rich asteroids, although that resemblance is part of an ongoing debate about its origin.

The dominant landmark is Stickney crater, roughly 9 kilometres wide, almost half the moon's shortest dimension. The impact that made it must have come close to shattering Phobos entirely. Stickney is named after Angeline Stickney Hall, who encouraged her husband, the astronomer Asaph Hall, to keep searching for Martian moons.

Phobos is also streaked with long, parallel grooves, many of them radiating from the Stickney region. Scientists have proposed several explanations: fractures from the Stickney impact, chains of craters from debris blasted off Mars, or stretch marks created as Mars's tides slowly pull the moon apart.

Interior and composition

With a density of under 2 grams per cubic centimetre, Phobos is far lighter than solid rock. Scientists think it is highly porous, perhaps a loosely bound rubble pile with a thin, cohesive outer layer, and it may contain some ice. Its exact composition remains uncertain because no sample has yet been studied in a laboratory.

Orbit and rotation

Phobos completes an orbit in about 7 hours 39 minutes, faster than Mars rotates. Seen from the Martian surface it therefore rises in the west and sets in the east, crossing the sky roughly twice a day. Because it sits inside synchronous orbit, Mars's tides rob it of orbital energy and drag it inwards by about 1.8 metres per century.

That shrinkage adds up. In roughly 30 to 50 million years, Phobos is expected either to strike Mars or, more likely according to many models, to be ripped apart by tidal forces and spread into a ring of debris around the planet.

Origin

For decades Phobos was described as a captured asteroid, because its dark colour and lumpy shape resemble small bodies in the asteroid belt. Its nearly circular orbit close to Mars's equator is hard to explain that way, however.

Many researchers now favour the idea that Phobos and Deimos formed from debris thrown into orbit after a giant impact on Mars, rather like a miniature version of Earth's Moon. Some models even suggest Mars has had a cycle of rings and moons. Returning samples from Phobos should help settle the question.

Exploration

Phobos has been photographed by many Mars orbiters and rovers, yet no mission has successfully landed on it. Several dedicated Soviet and Russian attempts ended in failure, and the next serious attempt is a Japanese sample-return mission.

The moons of Mars

Surprising facts about Phobos

• From the Martian surface, Phobos looks only about a third as wide as the full Moon does from Earth, and it never fully covers the Sun; NASA rovers have filmed it crossing the solar disc as a lumpy black silhouette.
• Phobos orbits so low that from Mars's polar regions it never rises above the horizon at all.
• It circles Mars about three times in a single Martian day, so an observer could watch it rise twice between sunrise and sunset.
• Phobos is drifting towards Mars by roughly 1.8 metres every century and may end up as a ring around the planet.
• Its biggest crater is named after Angeline Stickney, whose encouragement is credited with Asaph Hall's discovery of both Martian moons in 1877.

Keep exploring

• Deimos, moon of Mars: mars's small, smooth outer moon
• The Moon, moon of Earth: earth's ancient, ever-present companion
• Mars: the planet guide
• Watch Phobos orbit Mars in the interactive 3D solar system
• All the major moons of the Solar System

Sources

• NASA Science — Phobos
• JPL Solar System Dynamics — Planetary satellite physical parameters
• Wikipedia — Phobos