You may know that much of the climate change on earth over the last two million years--the coming and going of ice ages--is caused by the "orbital geometry" of the planet. The amount of planetary tilt and the time of year the tilt occurs change over time. When the Northern Hemisphere is less tilted towards the sun on June 21st, and at the same time the Earth is as far from the sun in its elliptical orbit as it ever gets, ice age conditions prevail. This makes ice ages on Earth pretty regular, cyclic, events.
You also may know that a big chunk of Earth's water is frozen into the ice caps.
You also may know that the history of Earth climate is preserved, in part, in changes in the ice in those ice caps.
Well, same for Mars!
Previously developed climate models suggested that the last 300,000 years of Martian history experienced low-level swings in climate, while the prior 600,000 years experienced more severe swings, owing to differences in the tilt of the planet. Most of the water we know about on Mars is in the Martian polar caps. And now, we can see, using radar, evidence of climate change reflected in that ice. From NASA:
New, three-dimensional imaging of Martian north-polar ice layers by a radar instrument on NASA's Mars Reconnaissance Orbiter is consistent with theoretical models of Martian climate swings during the past few million years.
Alignment of the layering patterns with the modeled climate cycles provides insight about how the layers accumulated. These ice-rich, layered deposits cover an area one-third larger than Texas and form a stack up to 2 kilometers (1.2 miles) thick atop a basal deposit with additional ice.
"Contrast in electrical properties between layers is what provides the reflectivity we observe with the radar," said Nathaniel Putzig..., 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."
Essentially, the radar detects different amounts and/or kinds of dirt, and the ice is dirty in different ways. These vastly different climate periods (of more vs. less severe oscillation in climate change) probably leave behind different amounts of dirt in the ice. The radar can penetrate the ice and "see" these differences, with one period having more dirt than another.
There are two distinct models for how the dirt gets concentrated in the ice enough to be distinguished by the radar. One is that ice evaporates away more during some periods than others, leaving behind more dirt when the ice disappears, like the dirty snow during the late winter in northern cities. The other model simply has more dust in the atmosphere, and thus more dust falling on the ice, during certain periods. The present study supports the later model (more dust = dirtier ice). The radar reflectivity signal observed in this study is probably too coarse to link specific features of the signals with specific Martian "ice ages" so far.
"The radar has been giving us spectacular results," said Jeffrey Plaut of NASA's Jet Propulsion Laboratory, Pasadena, Calif., a co-author of the paper. "We have mapped continuous underground layers in three dimensions across a vast area."
The other images are different views of the polar cap using the radar images, and are explained in great detail on NASA's site.