NASA's Phoenix lander watched some bright nuggets, exposed in a trench it had dug near the martian north pole, disappear over the course of four days last week.

That disappearance, mission officials say, means that the material must be made of ice and not the other alternative, salt. The ice sublimated away — that is, turned directly to vapour. “Salt can't do that,” says Peter Smith of the University of Arizona in Tucson, principal investigator for the mission.

The lander is now digging other trenches, as well as scooping soil for chemical analysis.