Phobos and Deimos

Phobos and Deimos (designated Mars I Phobos and Mars II Deimos) are the two natural satellites of the planet Mars and among the smallest regular moons in the Solar System. Discovered in August 1877 by the American astronomer Asaph Hall using the 26-inch (66-cm) Clark refracting telescope at the United States Naval Observatory in Washington, D.C., both moons are named after the twin mythological sons of Ares (Mars) and Aphrodite (Venus) who accompanied their father into battle in Homer's Iliad: Phobos (ancient Greek for "fear" or "panic") and Deimos ("dread" or "terror").

Unlike Earth's spherical Moon (R = 1,737.4 km), both Martian moons are small, triaxially ellipsoidal, heavily cratered bodies too low in mass to achieve hydrostatic equilibrium. The larger, inner satellite, Phobos, has triaxial dimensions of 26.8 × 22.4 × 18.4 km (mean volumetric radius R = 11.08 ± 0.04 km, mass M = 1.0659 × 10¹⁶ kg) and orbits just 5,980 km above the Martian surface at a semi-major axis of a = 9,376 km (2.760 Mars radii)—closer to its primary planet than any other known satellite in the Solar System. The smaller, outer satellite, Deimos, measures 15.0 × 12.2 × 10.4 km (mean radius R = 6.27 ± 0.08 km, mass M = 1.4762 × 10¹⁵ kg) and orbits at a = 23,463.2 km (6.917 Mars radii). Both satellites occupy nearly circular, near-equatorial prograde orbits (e = 0.0151 and 0.00033; i = 1.09° and 1.79°) in synchronous tidal locking with Mars, yet their dark, featureless red-sloped reflectance spectra and low bulk densities (1.872 and 1.471 g/cm³) fuel a central debate in planetary science: whether Phobos and Deimos are captured carbonaceous asteroids from the outer Asteroid Belt or second-generation moons that re-accreted in situ from a circum-Martian giant-impact debris disk.

Asaph Hall's 1877 Discovery

Before 1877, astronomers including William Herschel and Heinrich d'Arrest had searched unsuccessfully for Martian moons, while Jonathan Swift's 1726 satire Gulliver's Travels (and Voltaire's 1752 Micromégas) famously imagined Laputan astronomers discovering two tiny Martian moons orbiting at 3 and 5 planetary diameters in 10 and 21.5 hours—a literary coincidence driven by Johannes Kepler's geometric progression between Venus (0 moons), Earth (1 moon), and Jupiter (4 Galilean moons known at the time). Taking advantage of the favorable perihelic opposition of Mars in August 1877, Asaph Hall deliberately masked the blinding glare of the Martian disk in the eyepiece of the USNO 26-inch refractor to search right up to the planet's limb. Encouragingly urged by his wife, the mathematician Angelina Stickney Hall, not to abandon the search after several empty nights, Hall discovered Deimos on 12 August 1877 and Phobos on 17 August 1877.

Super-Synchronous Orbit of Phobos vs. Sub-Synchronous Deimos

In planetary orbital mechanics, the areosynchronous radius—the orbital distance at which a satellite's Keplerian orbital period matches Mars's sidereal rotation period of 24 hours 37 minutes 22.7 seconds (1.02596 days)—lies at a_sync = 20,428 km (6.01 Mars radii):

• Deimos (a = 23,463 km, P = 30.312 hours): Orbiting slightly outside the synchronous radius, Deimos moves eastward across the Martian sky more slowly than the background stars rotate westward, taking ~2.7 Martian sols to pass from rising in the east to setting in the west while evolving very slowly outward due to tidal torque.
• Phobos (a = 9,376 km, P = 7 hours 39 minutes 12 seconds): Orbiting deep inside the synchronous radius, Phobos completes more than three full orbits per Martian sol. To an observer on the Martian surface, Phobos rises in the west, races eastward across the sky in ~4 hours 15 minutes, and sets in the east twice every Martian day.

Because Phobos orbits faster than Mars rotates, the gravitational tidal force raised by Phobos on Mars's viscoelastic mantle lags behind Phobos's sub-satellite longitude, exerting a retarding gravitational torque that drains orbital angular momentum from Phobos and transfers it to Mars's spin. Astrometric tracking by Viking, Mars Global Surveyor, Mars Express, and Mars Reconnaissance Orbiter confirms that Phobos's mean motion is accelerating at dn/dt = +(1.27 ± 0.03) × 10⁻³ deg/yr², corresponding to a secular inward orbital decay rate of ~1.8 cm per year (1.8 meters per century) (Lainey et al., 2007). Within 30 to 50 million years, Phobos will spiral inward across the fluid/rubble-pile Roche limit (~5,500–7,000 km from Mars's center), where Martian differential tides will strip away its regolith and disrupt its porous interior into a short-lived planetary ring around Mars before the largest fragments impact the Martian equatorial plains (Black & Mittal, 2015).

Stickney Crater and the Global Groove Network of Phobos

High-resolution imaging by Mariner 9, the Viking orbiters, Mars Global Surveyor, ESA's Mars Express High Resolution Stereo Camera (HRSC), and Mars Reconnaissance Orbiter HiRISE shows that Phobos's leading hemisphere is dominated by Stickney crater (named after Angelina Stickney Hall), a 9.4-km-wide impact basin spanning more than 40% of Phobos's mean diameter (Thomas, 1979). The energetic impact that excavated Stickney came close to shattering Phobos altogether and blanketed the surrounding terrain in asymmetric bluing ejecta lobes.

Even more distinctive are the hundreds of parallel linear grooves (100 to 200 m wide, 10 to 30 m deep, and up to 20 km long) that score almost the entire surface of Phobos except the high-latitude trailing anvil zone. Three mechanisms account for different families of grooves:

1. Secondary Ejecta Chains from Primary Martian Impacts: Rolling and bouncing chains of secondary blocks blasted off the Martian surface by large basin-forming impacts on Mars (Murray et al., 2006).
2. Impact Fracturing and Regolith Drainage: Deep tectonic joints opened by the Stickney impact into which loose, 100-m-thick surface regolith subsequently drained.
3. Ongoing Tidal Extensional Stress: Recent viscoelastic modeling shows that as Phobos spirals closer to its Roche limit at 2.76 R_Mars, increasing tidal stretching opens parallel extensional fractures in its cohesive outer regolith layer overlaying a weak, rubble-pile interior (Hurford et al., 2016).

Deimos: Smooth Regolith Mantling and Downslope Creep

Despite having a similar low geometric albedo (0.068) and ancient crater density at diameters >1 km, Deimos lacks any linear grooves and presents a remarkably smooth, muted topography dominated by two large saddle-like depressions (including the ~10-km south polar basin and the 2.3-km craters Swift and Voltaire). Because Deimos orbits at 6.92 Mars radii—well outside the Roche limit—impact ejecta launched at low velocities from craters on Deimos remains gravitationally trapped within Deimos's circum-Martian torus and is re-accreted onto Deimos within decades, burying small craters under a 20-to-100-meter-thick blanket of fine-grained regolith that creeps downslope into regional topographic lows under microgravity (g ≈ 3 mm/s²).

Explaining the simultaneous properties of Phobos and Deimos—their very low albedos (~0.07) and featureless, red-sloped visible-to-near-infrared reflectance spectra resembling outer-belt D-type or T-type carbonaceous chondrite asteroids, combined with their low-inclination (i < 1.8°), low-eccentricity (e < 0.016) equatorial orbits—poses a classic conundrum between spectroscopy and orbital dynamics:

1. The Captured Asteroid Hypothesis: Spectrally, Phobos and Deimos closely match dark carbonaceous asteroids (such as the Tagish Lake meteorite parent body) or Centaur minor planets scattered inward from beyond the snow line during giant-planet migration. However, capturing two independent heliocentric asteroids into circular, near-equatorial orbits around a low-mass planet like Mars via aerodynamic drag through a primordial circum-Martian nebula or tidal dissipation requires fine-tuned timing just before nebula dispersal, and captured orbits ordinarily remain eccentric and highly inclined (Burns, 1992).
2. The Giant-Impact Circum-Martian Debris Disk Hypothesis: Dynamically, two small moons on circular, coplanar equatorial orbits are the natural outcome of in-situ accretion from an equatorial debris disk generated when a protoplanet (0.01 to 0.03 M_⊕) struck early Mars—the same impact proposed to have excavated the Borealis northern lowlands basin (Craddock, 2011; Rosenblatt et al., 2016). In numerical simulations by Rosenblatt et al. (2016), a massive inner moon initially accreted inside the synchronous radius from the dense inner impact disk, gravitationally shepherding outer-disk debris into two resonances that formed Phobos and Deimos before the large inner moon tidally decayed into Mars within ~5 Myr. Under this impact model, Phobos and Deimos consist of space-weathered, volatile-depleted Martian silicate mantle and impactor material mixed with 25%–35% macroporosity, while mid-infrared thermal emission spectra obtained by Mars Global Surveyor TES and Mars Express PFS show phyllosilicate/feldspar signatures consistent with altered Martian silicates.

To resolve this origin debate definitively, the Japan Aerospace Exploration Agency (JAXA), in partnership with CNES, DLR, and NASA, developed the Martian Moons eXploration (MMX) mission to orbit Phobos, deploy the IDEFIX rover onto its surface, collect >10 grams of Phobos regolith from >2 cm depth, and return the pristine samples to Earth for laboratory isotopic and mineralogical analysis.

Why does Phobos rise in the west and set in the east on Mars?

Phobos orbits just 5,980 km above the Martian surface and completes one full orbit around Mars in 7 hours 39 minutes—more than three times faster than Mars's 24-hour 37-minute rotation period. Because Phobos moves eastward in its orbit faster than the Martian surface rotates beneath it, an observer on Mars sees Phobos rise in the west, cross the sky in about 4.25 hours, and set in the east twice each Martian day.

Will Phobos eventually crash into Mars or form a planetary ring?

Because Phobos orbits inside Mars's synchronous orbit, Martian tidal forces are pulling Phobos inward by 1.8 cm per year (1.8 meters per century). In approximately 30 to 50 million years, Phobos will cross the Roche limit, where tidal stretching will tear its porous rubble-pile interior apart into a temporary planetary ring around Mars for 1 to 100 million years while larger intact boulders spiral down to impact the Martian equator.

Are Phobos and Deimos captured asteroids or debris from a giant impact on Mars?

This remains an active scientific question tested by JAXA's MMX sample-return mission. Their dark, red-sloped visible spectra and low densities (1.87 and 1.47 g/cm³) resemble porous D-type carbonaceous asteroids from the outer Asteroid Belt, whereas their nearly circular, coplanar equatorial orbits (i < 1.8°, e < 0.016) strongly favor in-situ formation from an equatorial debris disk blasted into orbit by a giant impact on early Mars.

1. Thomas, P. (1979). "Surface features of Phobos and Deimos." Icarus, 40(2), 223–243. https://doi.org/10.1016/0019-1035(79)90069-190069-1)
2. Lainey, V., Dehant, V., & Pätzold, M. (2007). "First numerical ephemerides of the Martian moons." Astronomy & Astrophysics, 465(3), 1075–1084. https://doi.org/10.1051/0004-6361:20065466
3. Craddock, R. A. (2011). "Are Phobos and Deimos the result of a giant impact?" Icarus, 211(2), 1150–1161. https://doi.org/10.1016/j.icarus.2010.10.023
4. Black, B. A., & Mittal, T. (2015). "The demise of Phobos and development of a Martian ring system." Nature Geoscience, 8(12), 913–917. https://doi.org/10.1038/ngeo2583
5. Rosenblatt, R., Charnoz, S., Dunseath, K. M., Terao-Dunseath, M., Trinh, A., Hyodo, R., Genda, H., & Toupin, S. (2016). "Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons." Nature Geoscience, 9(8), 581–583. https://doi.org/10.1038/ngeo2742