Phobos
Phobos is the larger and innermost of the two natural satellites of Mars. It has an irregular shape and orbits only about 6,000 km above the planet's surface. One revolution takes less than eight hours, substantially shorter than a Martian day. Mars's other moon, Deimos, follows a more distant orbit and moves much more slowly.[5][6]
Size and physical characteristics
Phobos measures approximately 27 × 22 × 18 km. It lacks the rounded shape of the Moon and the large satellites of the giant planets because its own gravity is too weak to smooth major irregularities. Its longest dimension is comparable to the distance between neighbouring districts of a large city.[2]
The volumetric mean radius is about 11 km, the mass approximately 1.06 × 1016 kg, and the mean density about 1.87 g/cm³. This is substantially lower than the density of most solid rocks.[1]
Surface gravity is of the order of 0.006 m/s², more than one thousand times weaker than on Earth. It varies from place to place because of the moon's irregular shape and rotation. Even a small movement can significantly change the trajectory of a spacecraft operating near the surface.[1][8]
Orbit and rotation
Phobos orbits approximately 9,400 km from the centre of Mars. Its orbit is nearly circular and lies close to the planet's equatorial plane. One revolution takes about 7 hours 39 minutes.[5][6]
The moon rotates on its axis in the same amount of time and therefore keeps one side facing Mars. Phobos travels around Mars faster than the planet rotates; from the Martian surface, it rises in the west and sets in the east.[4][6]
Because the orbit is so low, Phobos cannot be seen from the entire surface of Mars. Its path across the sky and viewing conditions vary strongly with the observer's latitude and position.[2]
Surface and Stickney crater
Phobos has a dark surface covered with impact craters, boulders and loose regolith. Repeated impacts have fragmented its upper layers. Images show debris moving downslope and darker streaks of disturbed material.[9]
The largest and most conspicuous feature is Stickney crater, approximately 9 km across—about one third the moon's longest dimension. It is named after mathematician Chloe Angeline Stickney Hall, the wife of Asaph Hall.[2]
An impact of this scale substantially altered the appearance of such a small moon. Smaller craters, dust and large fragments cover Stickney's floor and slopes.[2][4]
Grooves and internal structure
Numerous grooves cross the surface, forming long lines and groups of parallel depressions. Their origin remains unresolved. Proposed explanations include the movement of impact debris and deformation caused by tidal forces from Mars.[4]
The low mean density indicates that the interior may contain substantial voids. Mars Express flyby data support a model of a porous rubble pile, in which blocks of different sizes are held together by gravity rather than forming a completely monolithic body.[10]
In such models, void space may account for roughly 25–35% of the volume. The estimate depends on the assumed composition, and the exact distribution of material inside Phobos remains unknown.[10]
Temperature and lack of atmosphere
Phobos has no atmosphere to retain or transport heat. Adjacent illuminated and shaded regions can therefore have very different temperatures.[2][3]
During Mars Global Surveyor observations in 1998, well-lit slopes reached about −4 °C, while shaded areas were approximately −112 °C. These values describe the specific viewing conditions. The large difference supports the presence of a loose, fine-grained surface layer that heats and cools relatively quickly.[3]
Origin
The origin of Phobos and Deimos remains unresolved. One leading hypothesis proposes that Mars captured small asteroid-like bodies. Another links the moons to debris placed in orbit by a large impact on Mars.[8]
The dark surface and some spectral properties resemble primitive asteroids. However, the nearly circular orbits close to Mars's equator require explanation; simple capture alone does not reproduce every observed feature. Laboratory analysis of surface samples is expected to distinguish between possible scenarios.[11]
Orbital evolution
Tidal interaction causes Phobos to spiral gradually inward. Its orbit is currently shrinking by about 1.8 metres per century.[2]
Over tens of millions of years, tidal forces may break the moon apart to form a ring, or Phobos may collide with Mars. NASA gives an approximate timescale of 50 million years; the precise outcome and timing depend on the moon's internal structure and strength.[2]
Discovery and exploration
Asaph Hall discovered Phobos during the night of 17–18 August 1877 at the United States Naval Observatory, shortly after finding Deimos. Their names come from Greek mythology: Phobos personifies fear and Deimos terror.[7][6]
Mariner 9 photographed both moons in 1971–1972. Later Mars orbiters continued the investigation. Mars Global Surveyor measured surface-temperature differences, while the HiRISE camera on Mars Reconnaissance Orbiter returned detailed images of the craters and grooves.[12][3][13]
Europe's Mars Express performed close flybys, viewing Phobos from different directions and measuring its gravitational influence. The data help constrain the moon's shape, terrain and internal structure.[4][10]
Mars rovers observe Phobos from the planet's surface. Perseverance, for example, has recorded transits of its irregular silhouette across the solar disk. These observations refine the moon's orbital motion.[14]
MMX mission
Japan's Martian Moons eXploration—MMX mission will investigate Phobos in detail, observe Deimos and return samples to Earth. An H3 rocket successfully launched the spacecraft on 20 October 2026 Japan Standard Time, or 19 October UTC. MMX separated from the upper stage about 71 minutes after liftoff and entered its planned interplanetary trajectory.[15]
The following day, JAXA announced completion of the initial critical operations phase: the spacecraft was communicating, its solar arrays had deployed and its attitude was stabilised.[16] The mission plan includes deployment of the small Idefix rover, surface investigations and collection of at least 10 g of material. Sample return is expected in 2031. Its composition should help establish the origin of the Martian moons and the history of material in the Mars system.[8][11]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Planetary Satellite Physical Parameters // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Phobos // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 MOC Image of Phobos with TES Temperature Overlay // NASA Jet Propulsion Laboratory. 1998.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Mars Express tracks the phases of Phobos // European Space Agency. 2019.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Planetary Satellite Mean Elements // NASA Jet Propulsion Laboratory, Solar System Dynamics.
- ↑ 6.0 6.1 6.2 6.3 6.4 NASA's Hubble Sees Martian Moon Orbiting the Red Planet // NASA Science. 2017.
- ↑ 7.0 7.1 Fantastic Phobos // European Space Agency. 2012.
- ↑ 8.0 8.1 8.2 Mars moon lander set to launch on 19 October // European Space Agency. 13 October 2026.
- ↑ Martian Moon Phobos Hip-Deep in Powder // NASA Science. 1998.
- ↑ 10.0 10.1 10.2 Phobos flyby success // European Space Agency. 4 March 2010.
- ↑ 11.0 11.1 Mission Overview / Mission Flow // JAXA, Martian Moons eXploration.
- ↑ Mariner 9 // NASA Science.
- ↑ Phobos colour 2008 — NASA/JPL-Caltech/University of Arizona image description // Wikimedia Commons.
- ↑ NASA's Perseverance Captures 'Googly Eye' During Solar Eclipse // NASA Jet Propulsion Laboratory. 2024.
- ↑ Launch result of the Martian Moons eXploration spacecraft aboard H3 Launch Vehicle flight No.10 // JAXA. 20 October 2026.
- ↑ Completion of the Initial Critical Operations Phase of the Martian Moons eXploration Spacecraft // JAXA. 21 October 2026.
