The latest images sent to Earth reveal tantalising glimpses of what looks like frozen water.

Scientists think the Phoenix Mars lander's descent engine may have blown away a layer of dirt, exposing the ice.

The craft's robotic arm reached out and touched the soil for the first time, leaving behind a striking, footprint-like impression, they said on Sunday.

The robotic arm was making a test run, just one week after the landing.

"This first touch allows us to utilise the robotic arm accurately," said David Spencer, Phoenix's surface mission manager at Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California.

"We are in a good situation" for future testing, he said.

The spacecraft, which has its own laboratory onboard, will soon start digging for soil and ice.

Any dirt and ice it scoops up will be shovelled into several small ovens to be heated. The resulting gases will be analysed by various instruments.

The mission's main goal is to investigate the planet's geological history and search for the chemical building blocks of life.

It is being led by the University of Arizona in Tucson, and managed by JPL.