Permanently Shadowed Polar Craters
Shown in red are areas of Mercury’s north polar region that are in shadow in all images acquired by MESSENGER to date. Image coverage, and mapping of shadows, is incomplete near the pole. The polar deposits imaged by Earth-based radar are in yellow (from Image 2.1), and the background image is the mosaic of MESSENGER images from Image 2.2. This comparison indicates that all of the polar deposits imaged by Earth-based radar are located in areas of persistent shadow as documented by MESSENGER images. Updated from N. L. Chabot et al., Journal of Geophysical Research, 117, doi: 10.1029/2012JE004172 (2012).
Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory
New information from MESSENGER spacecraft shows there is frozen water on Mecury. Its onboard instruments have been studying Mercury in unprecedented detail since its historic arrival there in March 2011. Scientists are seeing clearly for the first time a chapter in the story of how the inner planets, including Earth, acquired their water and some of the chemical building blocks for life.
“The new data indicate the water ice in Mercury’s polar regions, if spread over an area the size of Washington, D.C., would be more than 2 miles thick,” said David Lawrence, a MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and lead author of one of three papers describing the findings. The papers were published online in Thursday’s edition of Science Express.
Spacecraft instruments completed the first measurements of excess hydrogen at Mercury’s north pole, made the first measurements of the reflectivity of Mercury’s polar deposits at near-infrared wavelengths, and enabled the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions.
Given its proximity to the sun, Mercury would seem to be an unlikely place to find ice. However, the tilt of Mercury’s rotational axis is less than 1 degree, and as a result, there are pockets at the planet’s poles that never see sunlight.
Time-Lapsed Animation of a Mercury Day
In this NASA Video you can see parts of ProNASA Spacecraft Finds New Evidence for Water Ice on Mercury WASHINGTON — A NASA spacecraft studying Mercury has provided compelling support for the long-held hypothesis the planet harbors abundant water ice and other frozen volatile materials within its permanently shadowed polar craters.
Scientists suggested decades ago 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 radar-bright patches at Mercury’s poles. Many of these patches corresponded to the locations of large impact craters mapped by NASA’s Mariner 10 spacecraft in the 1970s. However, because Mariner saw less than 50 percent of the planet, planetary scientists lacked a complete diagram of the poles to compare with the radar images.
Images from the spacecraft taken in 2011 and earlier this year confirmed all radar-bright features at Mercury’s north and south poles lie within shadowed regions on the planet’s surface. These findings are consistent with the water ice hypothesis.
The new observations from MESSENGER support the idea that ice is the major constituent of Mercury’s north polar deposits. These measurements also reveal ice is exposed at the surface in the coldest of those deposits, but buried beneath unusually dark material across most of the deposits. In the areas where ice is buried, temperatures at the surface are slightly too warm for ice to be stable.
MESSENGER’s neutron spectrometer provides a measure of average hydrogen concentrations within Mercury’s radar-bright regions. Water ice concentrations are derived from the hydrogen measurements.
“We estimate from our neutron measurements the water ice lies beneath a layer that has much less hydrogen. The surface layer is between 10 and 20 centimeters [4-8 inches] thick,” Lawrence said.
Additional data from detailed topography maps compiled by the spacecraft corroborate the radar results and neutron measurements of Mercury’s polar region. In a second paper by Gregory Neumann of NASA’s Goddard Flight Center in Greenbelt, Md., measurements of the shadowed north polar regions reveal irregular dark and bright deposits at near-infrared wavelength near Mercury’s north pole.
This video shows how the shape of Mecury is seen from orbit.
MESSENGER’s Mercury Laser Altimeter sends out laser pulses that hit the ground and return to the instrument. The amount of light that returns for each pulse gives the reflectance at that point on the surface. The amount of time it takes the pulse to make its trip indicates how far away that point on the surface is, allowing the topography to be mapped. Credit: NASA/Goddard Space Flight Center
“Nobody had seen these dark regions on Mercury before, so they were mysterious at first,” Neumann said.
The spacecraft recorded dark patches with diminished reflectance, consistent with the theory that ice in those areas is covered by a thermally insulating layer. Neumann suggests impacts of comets or volatile-rich asteroids could have provided both the dark and bright deposits, a finding corroborated in a third paper led by David Paige of the University of California at Los Angeles.
“The dark material is likely a mix of complex organic compounds delivered to Mercury by the impacts of comets and volatile-rich asteroids, the same objects that likely delivered water to the innermost planet,” Paige said.
This dark insulating material is a new wrinkle to the story, according to MESSENGER principal investigator Sean Solomon of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y.
“For more than 20 years, the jury has been deliberating whether the planet closest to the sun hosts abundant water ice in its permanently shadowed polar regions,” Solomon said. “MESSENGER now has supplied a unanimous affirmative verdict.”
