Message #203885

[Rebecca]

Phobos was discovered, along with Mars's smaller moon Deimos, in 1877 by American astronomer Asaph Hall at the US Naval Observatory in Washington DC. For most of their subsequent history, the moons' diminutive size has relegated them to mere footnotes in the astronomical textbooks. Phobos is an irregularly shaped rock just less than 28 kilometres across, while Deimos is even smaller (see diagram). So they were dismissed as being small space rocks that wandered too close to Mars and were unlucky enough to be captured by its gravity.

This view was bolstered by the first measurements of Phobos's composition, taken by the spacecraft Mariner 9 and Vikings 1 and 2 in the 1970s (see Missions to Phobos). Sunlight reflecting from the surface showed that Phobos was dark, absorbing more than 90 per cent of the incoming sunlight and resembling the meteorites known as carbonaceous chondrites. These ancient celestial objects are thought to originate in the furthest parts of the asteroid belt, twice as far from the sun as Mars itself. The most recent measurements of Phobos revealed a closer resemblance to even older asteroids found only in the outer solar system beyond the main belt. The same is true for Deimos.