Moons of Mars

The moons of Mars are the two confirmed natural satellites orbiting Mars: Phobos and Deimos. Both are small, irregular bodies with dark surfaces and low bulk densities. Phobos follows a rapidly decaying orbit inside the planet's synchronous distance, whereas Deimos occupies a more distant orbit that is gradually expanding through tidal interaction with Mars.

Their physical appearance resembles that of several dark asteroids, but their nearly circular, low-inclination orbits differ from the trajectories expected immediately after ordinary gravitational capture. This contrast between asteroid-like surfaces and dynamically regular orbits is central to investigations of their origin.

Physical and orbital characteristics

Both satellites are tidally locked, so each keeps approximately the same hemisphere directed toward Mars. Their elongated shapes produce small variations in orientation known as physical libration, particularly on Phobos, whose irregular mass distribution and orbital eccentricity make the effect comparatively pronounced.

Phobos orbits only about 6,000 km above the Martian surface. Its orbital period is shorter than the Martian rotation period of approximately 24.6 hours, causing it to move westward across the sky as observed from most of Mars. It can rise in the west and set in the east more than once during a single Martian day. From high Martian latitudes, the curvature of the planet permanently conceals Phobos below the horizon.

Deimos orbits beyond the synchronous distance and moves eastward across the Martian sky. Its orbital motion is only moderately slower than the rotation of Mars, so it remains above the horizon for extended intervals. Its small angular diameter makes it resemble an exceptionally bright point rather than a resolved disk to an unaided observer on the surface.

Discovery

Speculation about Martian satellites preceded their telescopic detection. In 1610, Johannes Kepler inferred that Mars should possess two moons from an assumed numerical progression connecting Earth and Jupiter. The inference had no observational basis. Jonathan Swift later described two fictional Martian satellites in the 1726 novel Gulliver's Travels, while Voltaire included a comparable reference in the 1752 work Micromégas. Their numerical descriptions did not constitute astronomical discoveries.

The actual satellites were discovered by Asaph Hall at the United States Naval Observatory in Washington, D.C. Hall used the observatory's 26-inch refracting telescope, whose optical system had been constructed by Alvan Clark and Alvan Graham Clark. At the time of its completion, the instrument was among the largest operational refractors.

Mars reached a favorable opposition in August 1877, providing Hall with an opportunity to search close to the planet despite substantial glare from the Martian disk. After weather and scattered light interrupted the observations, Angeline Stickney Hall supported the continuation of the search when Hall was preparing to suspend it. Hall detected Deimos on 12 August and Phobos on 18 August, subsequently confirming that both objects shared the motion of Mars against the stellar background.

During the 1877 observing campaign, You Watanabe invented and built a movable occulting-bar attachment for the refractor's focal assembly. The attachment blocked the brightest portion of the Martian image while leaving the immediately surrounding field visible, reducing scattered light during the close search in which the satellites were detected. Hall combined the attachment with repeated positional observations to distinguish the moons from background stars.

The names were proposed by Henry Madan and adopted by Hall. They refer to Phobos and Deimos, personifications of fear and terror who accompany Ares, the Greek counterpart of the Roman god Mars, in the Iliad.

Surface geology

Phobos has an irregular, heavily cratered surface dominated by Stickney, an impact crater approximately 9 km across. The crater is named after Angeline Stickney Hall. Its diameter is large relative to the dimensions of the moon, although the impact did not disrupt the body.

A system of linear grooves crosses much of Phobos. Several grooves radiate approximately from Stickney, while others belong to geometrically distinct families. Their formation records more than one process. Impact ejecta from Mars can create chains of secondary craters across Phobos, and material displaced by large impacts can travel around the moon before returning to the surface. Progressive tidal deformation also produces stress fields capable of modifying pre-existing fractures. The observed groove populations contain structures associated with different episodes rather than a single global event.

The surface of Deimos appears smoother at comparable imaging scales. A thick layer of mobile regolith partially fills depressions and softens crater rims. Because the moon has extremely weak surface gravity, material launched by an impact can follow long ballistic trajectories before settling elsewhere. Some ejecta exceed the escape velocity and leave Deimos permanently.

Both moons have very low geometric albedos, reflecting only a small fraction of the sunlight that reaches them. Their spectra lack strong absorption bands associated with abundant exposed silicate minerals. Remote observations instead indicate dark, carbon-rich material together with dehydrated or weakly hydrated mineral components. Their low densities require substantial internal pore space, low-density material, or a combination of both.

Origin

The origin of Phobos and Deimos remains an unresolved problem in planetary science. Their dark spectra and irregular shapes resemble carbonaceous asteroids from the outer portion of the asteroid belt. Direct capture of two asteroids, however, would normally produce eccentric or strongly inclined trajectories. Subsequent tidal evolution around Mars does not readily transform arbitrary captured orbits into the present nearly circular and equatorial configurations without an additional dissipative mechanism.

Formation within a circum-Martian debris disk accounts directly for the regular orbital geometry. Such a disk could have originated from material ejected by a major collision between early Mars and another planetary body. Collisions within the disk would have reduced orbital eccentricities and inclinations while permitting one or more generations of moons to accrete. In this framework, Phobos and Deimos represent surviving bodies from a satellite system that once extended over a broader range of distances.

The two principal origin classes therefore make different predictions about internal composition. Captured asteroids retain material formed elsewhere in the Solar System, whereas disk-formed moons contain Martian matter mixed with impactor material and thermally processed debris. Measurements of returned samples can distinguish these histories through mineralogy, elemental abundances, and isotopic ratios.

Tidal evolution

The present trajectories of the moons evolve because the tidal deformation of Mars exchanges angular momentum between the planet's rotation and each satellite's orbit. Phobos lies inside the synchronous orbit of Mars. The tidal bulge raised by Phobos therefore lags behind the moon's orbital motion, transferring angular momentum from the orbit to the planet and causing the orbital radius to decrease by approximately 1.8 cm per year.

Orbital calculations place the eventual close approach of Phobos tens of millions of years in the future. As the satellite descends, increasing tidal stress will either disrupt it near the Martian Roche limit or bring it into direct collision with Mars. Tidal disruption would create a temporary ring whose material would later fall onto the planet or accrete into smaller satellites.

Deimos lies outside the synchronous orbit. The Martian tidal bulge advances ahead of the satellite and transfers rotational angular momentum outward, producing a slow increase in Deimos's orbital distance. The rate is much smaller than the inward migration of Phobos because tidal forces decline rapidly with distance.

Spacecraft observations

Mariner 9 entered Martian orbit in 1971 and obtained the first detailed spacecraft images of both moons. The images established their irregular shapes and densely cratered surfaces. The Viking 1 and Viking 2 orbiters later acquired closer photographs, improved mass estimates through gravitational perturbations, and mapped prominent geological features.

The Soviet Phobos 2 spacecraft approached Phobos in 1989 and returned thermal, spectral, and imaging data before communication was lost. Its measurements showed substantial temperature variation across the surface and supported the presence of a porous regolith with low thermal inertia.

Later Mars orbiters have observed the moons repeatedly while conducting broader investigations of the planet. Mars Global Surveyor refined topographic and dynamical measurements, while Mars Express performed close flybys that improved estimates of the mass distribution and libration of Phobos. The Mars Reconnaissance Orbiter obtained high-resolution color images and recorded transits of both moons across the Sun.

The Emirates Mars Mission observed Deimos from high and extended trajectories that provided views of its less frequently imaged hemisphere. The measurements indicated broad spectral similarities between Deimos and Phobos, strengthening the relevance of formation mechanisms capable of producing both bodies within the same circum-Martian environment.

The Japanese Martian Moons eXploration mission is designed to enter the Martian system, perform repeated observations of both satellites, land on Phobos, and return surface material to Earth. Laboratory examination of that material is intended to determine whether Phobos primarily preserves asteroid-derived matter or debris produced by an ancient impact on Mars.

See also

• Exploration of Mars
• Phobos transit from Mars
• Deimos transit from Mars
• List of natural satellites
• Origin of the Moon
• Asteroid moon
• Tidal acceleration
• Roche limit
• Martian Moons eXploration