Mars’s moons are named for fear and panic.

Phobos and Deimos connect planetary astronomy with Greek mythology. They were companions of Ares, the Greek war god associated with the Roman Mars.

Both moons are irregularly shaped, unlike the nearly spherical Moon seen from Earth. Their small masses mean gravity has not forced them into rounded planetary shapes.

They also follow very different orbital schedules. Phobos is the larger, closer moon and circles Mars much faster than the more distant Deimos, making the pair dynamically distinct.

American astronomer Asaph Hall discovered both Martian moons in 1877 using the large refracting telescope at the United States Naval Observatory. He identified Deimos first and Phobos several days later. Finding the faint companions required separating their light from the much brighter planet nearby, a challenge quite different from noticing Earth's prominent Moon.

The names follow a mythological connection rather than a description of their physical behavior. Phobos is commonly translated as fear, and Deimos as terror or panic. These figures accompanied Ares in Greek tradition. Their association with war linked them naturally to Mars, even though the planet's familiar name comes from Roman rather than Greek mythology.

Spacecraft later revealed worlds that look more like battered rocks than miniature versions of Earth's Moon. Phobos is roughly 22 kilometers across on average, while Deimos is about 12 kilometers across. Their irregular shapes make any single diameter an approximation, so dimensions along different axes can produce different figures without necessarily contradicting one another.

Both surfaces bear the marks of impacts and are covered by loose material. Phobos is especially recognizable for its large Stickney Crater and networks of grooves. Such features provide evidence about collisions, surface processes, and structural stresses, although scientists continue to investigate the origins of particular patterns rather than treating every groove as fully explained.

Phobos completes an orbit in about 7.7 hours, faster than Mars rotates. For an observer on much of the Martian surface, it therefore moves across the sky in the opposite direction from the usual daily motion of distant stars. Deimos takes about 30 hours to orbit, giving it a very different apparent rhythm.

The moons' origins remain debated. Their dark surfaces and irregular shapes encouraged the idea that they were captured asteroids. However, their nearly circular, near-equatorial orbits are difficult to explain through simple capture alone. Other scenarios involve formation from debris produced by a large impact on Mars. Similarity in appearance is a clue, not a complete origin story.

Tidal interactions are also changing the system. Phobos is gradually moving inward, and models predict eventual disruption or collision in the distant future. That is not an immediate threat to Mars missions. It illustrates that a moon's present orbit is one chapter in a longer evolution governed by gravity, tides, and the physical properties of the bodies involved.

Phobos and Deimos are small, irregular satellites rather than miniature versions of Earth’s round Moon. Their appearance can resemble asteroids, but resemblance alone does not settle origin. Understanding them requires combining shape and composition with the harder question of how their present orbits could have formed.

Hall found Deimos before Phobos during August 1877. Their discovery within the same month is why short summaries often give a single year for the pair.

No. Capture is one proposed explanation, but orbital properties complicate that picture. Formation from impact-generated debris is another major possibility, and the origin question is not settled by appearance alone.

Their gravity is too weak to overcome material strength enough to produce nearly spherical shapes. Larger bodies can deform toward rounded forms under their own gravity, while small bodies retain irregular outlines.