Images captured by the orbiting Mars Global Surveyor spacecraft show the surface of Phobos has been pounded into powder by meteors.

Some of the meteoroids caused landslides on steep crater slopes, leaving light and dark streaks on the surface.

Using temperature readings, Nasa scientists say the surface must be composed largely of finely ground powder at least one metre thick.

Phobos, the Greek word for fear, is about 16 miles wide. The other moon, Deimos, Greek for terror, is about 9 miles wide.

Short days, cold nights

Surface pitted by meteoroids

The new information has helped researchers paint a picture of the moon.

They have found that daytime temperatures are similar to a winter's day on earth at around -4ºC.

But the day only lasts around 7 hours and at night it dips to -76.67ºC, colder than an Antarctic night.

"The infrared data tells us that Phobos, which does not have an atmosphere to hold heat in during the night, probably has a surface composed of very small particles that lose their heat rapidly once the Sun has set," said Dr Philip Christensen of Arizona State University, Tempe.

"This has to be an incredibly fine powder formed from impacts over millions of years, and it looks like the whole surface is made up of fine dust," said Dr Christensen, the principal investigator for the experiment on the Mars Global Surveyor spacecraft.

On Monday, the spacecraft starts the second phase of aerobraking, a process in which it dips into the fringes of the Martian atmosphere to trim its elliptical orbit gradually and make it circular.

Over the next four-and-a-half months, the probe slowly descend towards Mars, until it is circling around the polar caps.

The magnetometer and thermal spectrometer will be turned on in December to gather data each time the spacecraft passes closest to Mars' surface.