The irregular shapes and orbits of Mars’s two small moons, Phobos and Deimos, have long been a subject of scientific curiosity. Recent research suggests that these moons may have originated from captured asteroids. A supercomputer simulation released in November 2024 posits that these moons formed from debris after Mars gravitationally disrupted a passing body. In contrast, a December 2024 study proposes that Mars tore apart a passing asteroid to form these moons. A rare photo of Mars alongside Phobos and Deimos, taken by a NASA spacecraft in July 2025, offers fresh visual evidence to test these theories.
Discovery and Early Observations of Phobos and Deimos
Asaph Hall discovered Phobos and Deimos in 1877. Their potato-like shapes and close orbits to Mars were key early facts that challenged formation theories. Observations from 20th-century telescopic and spacecraft, including the Viking missions, revealed that these moons had low densities and dark surfaces resembling carbonaceous asteroids. This led to the hypothesis that they might be captured asteroids. A 2023 exploration revisited these observations to question whether the moons co-formed with Mars or arrived later.
Physical Characteristics of Mars’s Moons
Phobos, the larger of the two moons, is about 22 kilometers across and orbits Mars three times a day. It has a massive impact crater called Stickney, which covers nearly half its surface. Deimos is smaller, at 12 kilometers wide, and has a more circular orbit farther from Mars. Both moons exhibit rubble-pile structures, suggesting they are not solid bodies. Spectral data indicates compositions similar to outer solar system asteroids, supporting the idea of external capture rather than in-situ formation.
The Captured Asteroid Hypothesis
The classic theory suggests that Phobos and Deimos were captured asteroids, pulled into orbit by Mars’s gravity during its early history. This theory is based on their asteroid-like appearances. However, this idea faces challenges, such as the moons’ nearly circular and equatorial orbits, which are unlikely for random captures without atmospheric drag or other mechanisms. A 2023 analysis weighed this hypothesis against alternatives like giant impacts.
Supercomputer Simulations of Moon Formation
A new supercomputer simulation from November 2024 models Mars’s gravity disrupting a large, passing asteroid to create a debris disk that coalesces into Phobos and Deimos. The simulation accounts for the moons’ current orbits by incorporating Mars’s early atmospheric effects to circularize the paths over time. This simulation also provides insights into why Mars lacks a large moon like Earth’s, as it shows limited debris from the event compared to Earth’s impact history.
The Ripped-Apart Asteroid Theory
A December 2024 study proposes that Mars ripped apart a passing asteroid, with fragments reassembling into Phobos and Deimos after tidal forces tore the body during a close encounter. This theory explains the moons’ irregular shapes and compositions, drawing parallels to similar processes observed in other solar system bodies. It resolves discrepancies in capture theories by emphasizing a violent, one-time disruption event.
Recent NASA Imagery and Its Insights
A NASA spacecraft photo taken in July 2025 captures Mars with Phobos and Deimos in frame, providing high-resolution views that confirm their positions relative to the planet. This imagery aids in testing origin theories by revealing surface details like Phobos’s grooves, potentially from tidal stresses during capture or formation. Observations from missions like the Mars Reconnaissance Orbiter contribute to ongoing debates without resolving them definitively.
More from MorningOverview