Who says Mercury’s too hot to be really cool? Even three times closer to the Sun than we are, lacking atmosphere and with scorching daytime temperatures of 425 ºC (800 ºF), Mercury still has places more than cold enough to hide ice. This is the most recent announcement from the MESSENGER mission team: (very nearly) confirmed ice on the first rock from the Sun!
I could describe it myself, but instead I’ll let you read what the MESSENGER team released on November 29:
New observations by the MESSENGER spacecraft provide compelling support for the long-held hypothesis that Mercury harbors abundant water ice and other frozen volatile materials in its permanently shadowed polar craters.
Three independent lines of evidence support this conclusion: the first measurements of excess hydrogen at Mercury’s north pole with MESSENGER’s Neutron Spectrometer, the first measurements of the reflectance of Mercury’s polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter (MLA), and the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions that utilize the actual topography of Mercury’s surface measured by MLA. These findings are presented in three papers published online today in Science Express.
Given its proximity to the Sun, Mercury would seem to be an unlikely place to find ice. But the tilt of Mercury’s rotational axis is almost zero — less than one degree — so there are pockets at the planet’s poles that never see sunlight. Scientists suggested decades ago that there might be water ice and other frozen volatiles trapped at Mercury’s poles.
The idea received a boost in 1991, when the Arecibo radio telescope in Puerto Rico detected unusually radar-bright patches at Mercury’s poles, spots that reflected radio waves in the way one would expect if there were water ice. Many of these patches corresponded to the location of large impact craters mapped by the Mariner 10 spacecraft in the 1970s. But because Mariner saw less than 50 percent of the planet, planetary scientists lacked a complete diagram of the poles to compare with the images.
MESSENGER’s arrival at Mercury last year changed that. Images from the spacecraft’s Mercury Dual Imaging System taken in 2011 and earlier this year confirmed that radar-bright features at Mercury’s north and south poles are within shadowed regions on Mercury’s surface, findings that are consistent with the water-ice hypothesis.
Now the newest 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, areas where temperatures are a bit too warm for ice to be stable at the surface itself.
“For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict.”
– Sean Solomon, principal investigator of the MESSENGER mission
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 the papers. “The buried layer has a hydrogen content consistent with nearly pure water ice.”
Data from MESSENGER’s Mercury Laser Altimeter (MLA) — which has fired more than 10 million laser pulses at Mercury to make detailed maps of the planet’s topography — corroborate the radar results and Neutron Spectrometer measurements of Mercury’s polar region, writes Gregory Neumann of the NASA Goddard Flight Center. In a second paper, Neumann and his colleagues report that the first MLA measurements of the shadowed north polar regions reveal irregular dark and bright deposits at near-infrared wavelength near Mercury’s north pole.
“These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice,” Neumann writes. “Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice.”
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and entered orbit about Mercury on March 17, 2011 (March 18, 2011 UTC), to begin a yearlong study of its target planet. MESSENGER’s extended mission began on March 18, 2012.
Image credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory