They were photographed in September 1998 in Chasma Boreale, a giant trough at Mars' north polar cap.

CNN: Scientists debate implications of Mars pictures

But the volume of the Martian north polar cap is less than half that of the Greenland ice cap, and about four percent of the Antarctic ice sheet.

BBC: By BBC News Online Science Editor Dr David Whitehouse

The estimated volume of the north ice cap is about 10 times less than the minimum volume of an ancient ocean that some scientists believe once existed on Mars.

BBC: By BBC News Online Science Editor Dr David Whitehouse