Ice in the shadows of Mercury

Look closer

Mercury’s polar craters offer a surprising contrast between brightly lit rims and persistently shaded interiors. MESSENGER observations connect this geometry with evidence for polar deposits.

Follow the evidence

These products combine imaging with radar, temperature and illumination information. Their palettes describe different measurements rather than natural surface colours. Compare what each map measures before connecting a coloured region with evidence for ice.

The shadow is part of the measurement

Arecibo radar observations in 1991 identified bright polar patches, but Mariner 10 had not mapped enough of Mercury to establish a complete terrain comparison. MESSENGER’s 2011–2012 images placed the radar-bright regions inside shadowed terrain. Mercury’s small axial tilt allows some polar crater interiors to escape sunlight throughout the year. This is why the collection pairs a radar overlay with temperature and illumination maps: each addresses a different part of the ice interpretation, rather than providing three photographs of the same visible substance.

Exposed ice and a darker cover

The mission’s November 2012 synthesis supported water ice as the principal component of the northern deposits. In the coldest locations, ice can remain at the surface; across much of the deposit area it is inferred beneath a dark covering because the surface is too warm for exposed ice to persist. The team proposed impact-delivered complex organic compounds as a likely explanation for that covering. This proposed delivery history is less direct than the observed association with shadow. A yellow pixel in the gallery is a radar mapping choice, not a natural-colour view of yellow ice.

Sources & further reading

• Water Ice on Mercury — NASA Image and Video Library / JPL
• Hot and Cold — NASA Image and Video Library / JPL
• Shadows in the South — NASA Image and Video Library / JPL
• NASA Spacecraft Finds New Mercury Water Ice Evidence — NASA/JPL