NASA's Dawn deep-space probe has discovered ice inside the eternally shadowed craters of the polar regions of the dwarf planet Ceres. Discovered using onboard cameras built by the Max Planck Institute for Solar System Research (MPS), the water ice survives in the crater interiors thanks to the extreme cold there.

According to the MPS team, the ice was discovered using Dawn's Framing Cameras, which are used for navigation as well as clear and spectral-selective imaging. Built in Göttingen, the cameras allowed the team to peer into the craters near the north pole of Ceres. Because the planetoid only tilts 4.028°, as opposed to the Earth's 23.5°, some of the polar craters are never exposed to sunlight and remain at a temperature of -163° C (-261° F).

This eternal darkness would seem to make them impossible to study, but the team says that small amounts of light scatter from the crater rims – providing enough for the Framing Cameras to look for bright deposits of surface ice. Out of 634 identified dark craters, 10 have shown bright spots and one young crater's spot is occasionally lit by the sun at its edges.

Crater No. 1, whose interior has a large region in permanent darkness (a), in the weak scattered light, the framing cameras can make out bright deposits of ice (b), Crater No. 2 with its dark region is shown in Figures (c) to (e), the ice shown in (d) extends into the region with direct illumination (e)(Credit: Nature Astronomy)

"This offers the opportunity to analyze the light reflected from there with Dawn's Visible and IR Spectrometer onboard instrument, which was supplied by the Italian space agency," says Andreas Nathues, who leads the Framing Camera experiment at the MPS. "We can clearly see the spectral signature of water ice, but were unable to find other frozen gases."

Earlier observations by Dawn already indicate that there are ice deposits in the interior of Ceres at a density of 2.1621 g/cm3. However, much of this ice is unstable over geological time scales and, because the planetoid has no atmosphere, any ice exposed to the surface flashes into water vapor when heated by sunlight.

Because of the darkness and extreme cold, the scientists believe that the crater ice is much longer lived, but that the surface deposits there are younger than on the rest of Ceres. They believe this is because the deposits were created as fallout from an ancient meteor impact that blew subsurface ice from the lower latitudes into the polar regions.

Launched from Cape Canaveral Air Force Station in Florida on September 27, 2007, the Dawn probe went into orbit around Ceres in March 2015 after a journey using its ion drive system to visit the dwarf planet Vesta in the asteroid belt. It's the first NASA deep space spacecraft to use ion propulsion and the first mission to visit multiple targets while under power thanks to the ability of the ion engine to fire at low thrust for years at a time. The spacecraft is now in permanent orbit around Ceres and is expected to operate until 2017.

The research was published in Nature Astronomy.

The video below shows a simulation of the impact that created the polar ice deposits on Ceres.

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