“On Earth, ice ages take hold when the polar regions and high latitudes become cooler than average for thousands of years, causing glaciers to grow toward the mid-latitudes,” said NASA. “In contrast, the Martian variety occurs when — as a result of the planet’s increased tilt — its poles become warmer than lower latitudes.”
Water vapor moves toward the planet’s equator and forms ice and glaciers at mid-latitudes, said NASA.
“As the warm polar period ends, polar ice begins accumulating again, while ice is lost from mid-latitudes. This retreat and regrowth of polar ice is exactly what Smith and colleagues see in the record revealed by the [Shallow Subsurface Radar] images,” said NASA.