 Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal.The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin.Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars.