Radar measurements of Mars' polar ice caps reveal that the mostly dry, dusty planet is emerging from an ice age, following multiple rounds of climate change. Understanding the Martian climate will help determine when the planet was habitable in the past, how that changed, and may inform studies of climate change on Earth. Models have suggested that Mars has undergone ice ages in the past, but empirical data to confirm this has been sparse. Here, Isaac Smith and colleagues used radar to analyze layers of ice within the planet's polar ice caps, using the Shallow Radar instrument onboard the Mars Reconnaissance Orbiter spacecraft. As ice erodes, wind can create spiral troughs and other distinct features. Tracing the layers of these features within the ice can reveal changes in ice accumulation and flow - and thus changes in climate - in the past. While the southern ice cap is relatively small and altered by meteorite impacts, the researchers were able to trace the layers within the northern ice cap. They found layers and migration paths that increase in slope abruptly, reverse direction, or are completely buried. Their analysis suggests that the planet is currently emerging from an ice age, in a retreat that began approximately 370,000 years ago.