New observations by the Johns Hopkins-built NASA Messenger spacecraft support a long-held hypothesis that Mercury harbors abundant water ice in its permanently shadowed polar craters.
The findings, supported by three independent lines of evidence, were announced this afternoon during a NASA news conference.
Given that Mercury is the closet planet to the sun, it would seem to be an unlikely place to find ice. But the tilt of Mercury's rotational axis is less than one degree (almost zero), so there are pockets at the planet's poles that are never exposed to sunlight. Scientists suggested decades ago that there might be water ice and other frozen volatiles trapped at Mercury's poles.
New data from Messenger strongly indicate that water ice is the major constituent of Mercury's north polar deposits, that ice is exposed at the surface in the coldest of those deposits, but that the ice is buried beneath an unusually dark material across most of the deposits.
Messenger uses neutron spectroscopy to measure average hydrogen concentrations within Mercury's radar-bright regions. Water-ice concentrations are derived from the hydrogen measurements.
"The neutron data indicate that Mercury's radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 20 centimeters thick that is less rich in hydrogen," writes David Lawrence, a Messenger Participating Scientist based at The Johns Hopkins University Applied Physics Laboratory and the lead author of one of three papers on the findings published today by Science. "The buried layer has a hydrogen content consistent with nearly pure water ice."
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