Mercury is a world of extremes. Daytime temperature on the planet closest to the sun can soar as high as 400 degrees Celsius near the equator—hot enough to melt lead. When day turns to night, the planet’s surface temperature plunges to below –150 degrees C.

But some places on Mercury are slightly more stable.Inside polar craters on the dim­inutive planet are regions that never see the light of day, shaded as they are by the cra­­ters’ rims. The temperature there remains cold throughout the Mercury day. Now new data from NASA’s MESSENGER satellite, which were presented in March at the annual Lunar and Planetary Science Conference, corroborate a long-held hypothesis that Mercury has squirreled away pockets of water ice in those shadowy craters, despite the sun’s proximity.

Since 2011 MESSENGER has orbited the innermost planet, charting Mercury’s surface in unprecedented detail. MESSENGER’s maps of polar craters match up nicely with earlier imagery of the poles, taken by Earth-based radars, which showed anomalously bright features—patches that reflected radio waves much better than the surrounding terrain, just as ice does.

But the radar hotspots also line smaller craters and those at lower latitudes that would have less ice-friendly temperatures across the crater floor. These ice deposits would likely require a thin insulating blanket, perhaps a layer of fine-grained surface material, or regolith, to keep it from sublimating away.

In fact, MESSENGER’s data seem to confirm that some insulating material blankets whatever ice may line the craters. The temperatures inside the shadowed craters are just right for ice deposits blanketed by regolith darkened by organic compounds, explained David Paige of the University of California, Los Angeles.

The new look at features spotted long ago by Earth-based radars, Paige said, shows “fairly conclusively that they are predominantly composed of thermally stable water ice.”

This article was published in print as "Fire and Water."

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