Three-dimensional radar imaging of Martian north-polar ice layers courtesy of NASA’s Mars Reconnaissance Orbiter matches climate swing predictions for the last few million years.

The ice-rich layers cover an area larger than Texas, form a stack up to two kilometres thick and hold about one-third as much water as the Greenland ice sheet on Earth.

Scientists already knew from previous radar observations that the north-polar layered deposits are composed mostly of ice, and that radar contrasts between different layers in the deposits represent varying amounts of dust. The new radar results provide a cross-sectional view of layering and show that high-reflectivity zones with multiple contrasting layers alternate with more-homogenous zones of lower reflectivity.

“Contrast in electrical properties between layers is what provides the reflectivity we observe with the radar,” says Nathaniel Putzig of Southwest Research Institute, a member of the science team for the Shallow Radar instrument on the orbiter. “The pattern of reflectivity tells us about the pattern of material variations within the layers.”

The way in which these two zones alternate correlate with models of Mars' climate in recent times, that is, in the past four million years. Specifically, the layers relate to changes in the tilt of Mars on its axis.

The most recently deposited upper layers have a high reflectivity; the researchers propose that such high-contrast layering corresponds to periods of relatively small swings in the planet’s tilt. Indeed, the most recent 300,000 years of Martian history represent a period of less dramatic swings in the planet’s tilt than during the preceding 600,000 years.

Putzig and colleagues also propose a mechanism for how those contrasting layers form, which contradicts an earlier interpretation that suggests that the dustier layers in those are formed during high-tilt periods when sunshine on the polar region sublimates some of the top layer’s ice and concentrates the dust left behind. Putzig's team favour the alternative interpretation that the dustier layers are simply deposited during periods when the atmosphere is dustier.

The new mapping also provides evidence that the geographical centre of ice deposition probably shifted by 400 kilometres or more during the past few million years.

“The radar has been giving us spectacular results,” says Jeffrey Plaut of NASA’s Jet Propulsion Laboratory. “We have mapped continuous underground layers in three dimensions across a vast area.”