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Martian Moon Phobos

July 27, 2020

The planets visible with the unaided eye are still known today by the names of mythological deities given them in antiquity. Astronomers have kept this tradition in a similar naming of planets discovered telescopically after that time, as shown in the table. Pluto was demoted from full planet status in 2006, after many similar dwarf planets were discovered. I've included it in the table, since it was a planet from 1930 until that year.

Name Namesake
Mercury Mercury was the messenger of the Roman gods, and also the guide of souls to the underworld.
Venus Venus is the Roman goddess of beauty, love, and desire.
Mars Mars was the Roman god of war. The adjective, martial, derives from his name.
Jupiter Jupiter is the king of the Roman gods, and also the god of the sky and thunder. He is often depicted clutching lightning bolts.
Saturn The Roman god, Saturn, was the father of Jupiter, Neptune, and Pluto.
Uranus The Greek god, Uranus, was the father of Saturn and was at one time the king of the gods.
Neptune Neptune was the the Roman god of the sea.
Pluto Pluto was the Roman god of the underworld.

While the Earth and our Moon are known today by their common names, the Greeks associated the Earth with the goddess, Gaia (Roman goddess, Terra, from which so many Earthy adjectives derive). The Moon was associated with the Greek goddess, Selene, and the Roman goddess, Luna, from which we get the adjective, lunar. Selene was romantically involved with Zeus, as were so many others, as can be read below.

It's estimated that there are more than a million asteroids in the main asteroid belt between Mars and Jupiter. A few of the larger of these have been given traditional names, as listed in the table.

Name Namesake
Ceres Ceres was the Roman goddess of agriculture and grain crops, whence we get the word cereal.
Vesta Vesta was the Roman virgin goddess of the hearth, home, and family, whose temple was maintained by the Vestal Virgins.
Pallas Pallas Athena was an alternate name for the Greek goddess Athena, the goddess of wisdom and handicraft, but also warfare.
Hygiea Hygieia was the Greek goddess of health, from whom we get the word hygiene.

Many planets have moons, and many of these have traditional names. Planetary exploration with spacecraft has revealed nearly eighty moons of Jupiter, but quite a few of the earlier discovered moons have names based on Greek mythology, as listed in the table.

Name Namesake
Io Io was one of the mortal lovers of Zeus and an ancestor of Perseus.
Europa Europa was a mortal woman who was abducted by Zeus, who took the form of a bull on that occasion.
Ganymede Ganymede, who was described by Homer as the most beautiful of mortals, was abducted by Zeus in the form of an eagle, so he could become cup-bearer for the Olympic gods.
Callisto Callisto was a nymph who was raped by Zeus. I enjoyed the Callisto character, portrayed by Hudson Leick, in Xena: Warrior Princess.
Metis Metis was a Titaness and the first wife of Zeus.
Adrastea Adrastea was a nymph who was the secret caregiver for the infant Zeus, who was in danger from his father, Cronus.
Amalthea Amalthea was a goat herding nymph who likewise nurtured the infant Zeus.
Thebe Thebe was another consort of Zeus. As you can see, Zeus had quite a sex drive.

Composite image of the Galilean moons, Io, Europa, Ganymede, and Callisto.

Our tradition of naming Solar System bodies after mythological beings was nearly defeated by Galileo (1564-1642), who discovered the first four moons of Jupiter using the recently invented telescope. In order to secure funding from the de' Medici family, he named these moons the "Medician Stars" in his book, Sidereus Nuncius ("Starry Messenger"), published March, 1610, less than two months after his discovery.

Fortunately, Simon Marius, who discovered the moons independently at about the same time as Galileo, gave them our now common names in 1614 at the suggestion of Johannes Kepler (1571-1630).

(A composite image of the Galilean moons, Io, Europa, Ganymede, and Callisto. Io and Ganymede were imaged by NASA's Galileo spacecraft in June, 1996, while Europa and Callisto were imaged somewhat later. NASA/JPL/DLR image via Wikimedia Commons.)

Exploratory spacecraft have revealed that Saturn has many more moons than Jupiter, seven of which have names derived from Greek and Roman mythology.

Name Namesake
Mimas Mimas was one of the Giants in Greek mythology.
Enceladus Enceladus was another Giant.
Tethys Tethys was the Titaness daughter of Uranus and Gaia, sister and wife of the Titan, Oceanus, and mother of the river gods and the Oceanids.
Dione Dione is usually identified as the mother of Aphrodite (Venus).
Rhea Rhea was the Titaness daughter of Gaia and Uranus. She is also the older sister and wife of Cronus, and is often called the mother of gods.
Titan The Titans were descendants of Gaia and Uranus.
Iapetus Iapetus was the Titan father of Atlas and Prometheus.

The naming of the The moons of Uranus departed from tradition, since they are named for literary characters. Although there is a Cupid, the name comes from a character in Shakespeare's play, Timon of Athens, and not the Cupid who was the son of Venus and Mars. The planet, Neptune, however, has many of its satellites named after mythological creatures of the sea, as seen in the following table.

A sculpture of the Greek god of the sea,Poseidon (Neptune in Roman mythology), at the Port of Copenhagen

A sculpture of the Greek god of the sea, Poseidon (Neptune in Roman mythology), at the Port of Copenhagen.

The word "trident" comes from the combination of Latin words for "three" and "teeth," as in the weapon in this sculpture. Homer's Iliad, however, uses the word τρíαινα for this, a word that just means "threefold." However, in the context, this type of fishing spear must have been implied.

(Photo by Hans Andersen, via Wikimedia Commons.)

Name Namesake
Naiad The Naiads were nymphs who presiding over springs, brooks and other fresh water bodies.
Thalassa Thalassa was a sea goddess
Despina The nymph, Despoina, was a daughter of the ocean god, Poseidon
Galatea Galatea was a Nereid. Her name translates as she who is milk-white
Larissa Larissa was a nymph lover of the sea god, Poseidon, who fathered three sons by her
Hippocamp The Hippocamp is a chimera that's half horse and half fish
Proteus Proteus was a prophetic sea god who changed his shape to avoid capture. His name gives rise to the adjective, protean
Triton Triton was a son of Poseidon, and he was represented as a merman who blew a conch shell like a trumpet
Nereid The Nereids were sea-nymph attendants to Poseidon
Halimede Halimede was one of the many Nereids
Sao Sao was a Nereid associated with sailing
Laomedeia Laomedeia was another Nereid
Psamathe The Nereid, Psamathe, was the goddess of sand beaches and wife of Proteus
Neso Neso was another Nereid

In the past, Pluto was thought to be much larger than it actually is, since telescopic images combined the planet with its large and close satellite, Charon. Planetary exploration revealed Charon and four other moons of Pluto, all named for creatures associated with the god of the underworld.

Name Namesake
Charon Charon was the ferryman who brought the dead across the river, Styx
Styx Styx was the goddess associated with the river, Styx
Nix Nyx was the goddess of darkness and night and the mother of Charon
Kerberos Kerberos is the Greek name for Cerberus, the three-headed dog that guards the gates of the Underworld
Hydra The Lernaean Hydra was the nine-headed serpent guard of Lerna, an entrance to the Underworld

While the gas giant planets have many moons, terrestrial planets have few, if any. Earth has our one Moon, Mercury and Venus have none, and Mars has just two. The two moons of Mars, Phobos and Deimos, are quite unlike Earth's Moon. They are small and irregularly shaped. Phobos has a diameter of just 22.2 kilometers (13.8 miles), and Deimos has a diameter of just 12.6 kilometers (7.8 miles).

Our Moon takes a month to orbit, while the orbital period of Phobos is just 7.66 hours, and that of Diemos is just 30.35 hours. In keeping with tradition, Phobos and Diemos were named after mythological beings associated with Mars (Greek god, Ares). Phobos (fear) and Deimos (dread) were the sons of Ares who accompanied him into battle.

Since Phobos and Diemos are so small, they weren't discovered until 1877. The discovery was made by Asaph Hall (1829-1907) of the US Naval Observatory during a deliberate search for Martian moons. Like our own Moon, the Martian moons are tidally locked, showing the same face towards their host planet. Since these moons are so unusual, their possible origin is still controversial. They appear to be much like carbonaceous C-type asteroids, so they might be captured asteroids. These moons could have coalesced from an orbital debris cloud after Mars formed, this debris cloud forming by a collision of Mars with a protoplanet.

In 2015, scientists from NASA's Goddard Space Flight Center (Greenbelt, Maryland), found evidence that the long grooves on the surface of Phobos, believed to be the result of the impact that caused its main feature, Stickney crater, are signs of structural failure that will ultimately destroy this moon.[1] The impact that formed this crater was so powerful, it nearly shattered Phobos, and modeling by the Goddard scientists indicates that the grooves resemble stretching marks caused by deformation from tidal forces.[1] It's thought that the interior of Phobos is just a rubble pile that's barely held together.[1] It's been determined that some grooves are younger than others, which is indicative of an ongoing process.[1]

A color image of Matian moon Phobos

A color image of Phobos, imaged by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter, March 23, 2008. Stickney crater, with a diameter of 9 kilometers (5.6 miles), can be seen at the lower right.

(NASA/JPL-Caltech/University of Arizona image via Wikimedia Commons. Click for larger image.)

Christopher Edwards, an assistant professor in the Department of Astronomy and Planetary Science of Northern Arizona University, and scientists at NASA’s Jet Propulsion Laboratory and Arizona State University, performed a recent thermal imaging study of Phobos to help determine whether Phobos is a captured asteroid, or a fragment of Mars that was ejected by impact of a meteorite.[2] They used the Thermal Emission Imaging System (THEMIS) of the Mars Odyssey orbiter to capture the images during the moon's different phases.[2] The Mars Odyssey orbiter has been observing Mars for more than 18 years.[2] These images are a marked improvement over previous studies.[3]

Thermal images of Phobos during its different phases

Thermal images of Phobos during its different phases, waxing, waning and full. These include a December 9, 2019, image of Phobos at its maximum temperature (27 degrees Celsius), an image of February 25, 2020, in which Mars' shadow completely blocked sunlight from its surface (-123 degrees Celsius), and a March 27, 2020, image where Phobos was exiting an eclipse and just warming.[2]. (NASA/JPL-Caltech/ASU/NAU image from Northern Arizona University (NAU), also available here)

The thermal imaging revealed that the surface of Phobos is relatively uniform and composed of very fine-grained, mostly basaltic, materials.[2] While these images don't provide a definitive answer to the question of Phobos' origin, they're an important piece of evidence. The Japanese Space Agency (JAXA) has planned a Martian Moons Exploration (MMX) mission to Phobos and Diemos. MMX will land and collect samples from Phobos, and the thermal imaging study has provided reconnaissance data for that mission.[2]


  1. Elizabeth Zubritsky, "Mars' Moon Phobos is Slowly Falling Apart," NASA's Goddard Space Flight Center Press Release, November 10, 2015.
  2. NAU planetary scientist captures new images of Martian moon Phobos to help determine its origins, Northern Arizona University Press Release, June 3, 2020.
  3. Martian Moon Phobos in Thermal Infrared Images, NASA Jet Propulsion Laboratory Website, October 4, 2017.

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