A building-sized rock in orbit
Near the edge of the largest crater on Phobos stands a single, enormous boulder. This object, known as the Phobos monolith, is a geological feature about 85 meters (279 feet) across and 90 meters (300 feet) tall. Its sharp, angular appearance and the long, dark shadow it casts across the dusty landscape make it a prominent feature on this tiny moon. The monolith was first identified in 1998 from images taken by the Mars Orbiter Camera on the Mars Global Surveyor spacecraft. Its discovery was made by Efrain Palermo, who analyzed extensive imagery from Martian probes, and was later confirmed by Lan Fleming, an imaging subcontractor at NASA's Johnson Space Center.
The most accepted geological explanation is that the monolith is a piece of impact ejecta. Phobos's surface is dominated by the Stickney crater, a giant impact basin 9 kilometers (5.6 miles) in diameter that covers a substantial portion of the moon's surface. The impact that formed Stickney nearly shattered Phobos, and it would have thrown vast quantities of rock across the moon. The monolith is likely one such boulder, a single massive piece of rock displaced by this or another impact event. Its location near the rim of Stickney supports this theory. The surface of Phobos is covered in many boulders, but this one is distinguished by its sheer size and isolated position.
A candidate for exploration
The composition of Phobos itself is a subject of scientific interest. Spectral observations suggest it is made of C-type rock, similar to dark carbonaceous chondrite asteroids, with a very low reflectivity, or albedo, of only 0.071. This composition, combined with its low density, indicates that Phobos is not a solid body but likely a "rubble pile"—a loose collection of rock and dust held together by gravity, with significant internal porosity. The monolith, as a potentially solid fragment of this material, is an opportunity for study.
This unusual rock has been a point of interest for conspiracy theorists and scientists. Former astronaut Buzz Aldrin famously spoke in favor of a mission to study the object. The monolith's vicinity has been formally proposed as a landing site for a Canadian robotic mission concept called Phobos Reconnaissance and International Mars Exploration (PRIME). Such a mission would involve an orbiter and a lander, equipped with instruments to study the moon's geology in detail. Analyzing the monolith up close could provide direct evidence of the moon's composition and internal structure, helping to finally determine if Phobos is a captured asteroid or was formed from debris orbiting Mars. Japan's Martian Moons eXploration (MMX) mission, scheduled to launch in 2026, plans to collect samples from Phobos, which could indirectly explain the nature of features like the monolith.