Evidence of frozen water discovered on Mercury
Sen—NASA's Messenger spacecraft has found compelling evidence of water ice in the permanently shadowed craters around Mercury's poles.
Scientists reached the conclusion that the closest planet to the Sun harbours frozen water from evidence gathered by the orbiting Messenger (MErcury Surface, Space Environment, GEochemistry, and Ranging) spacecraft. Three separate papers on the findings have been published in Science Express.
Messenger's Gamma-Ray and Neutron Spectrometer, which detects elements on Mercury’s surface via the gamma rays emitted after the surface is struck with cosmic rays, found excess hydrogen at Mercury's north pole. The amount of hydrogen detected has led scientists to conclude there are concentrations of water ice at the north pole. David Lawrence, a scientist at The John Hopkins University Applied Physics Laboratory, and the lead author of the paper on the Neutron Spectrometer findings, said: "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. The buried layer has a hydrogen content consistent with nearly pure water ice."
Further evidence was captured by the probe's Mercury Laser Altimeter (MLA) which uses lasers to map the surface features by measuring the amount of time it takes the lasers to reflect off the planet's surface. The MLA has fired more than 10 million laser pulses at Mercury to provide detailed maps of its surface. The MLA showed dark and bright deposits at near infrared wavelength near Mercury's north pole. The measurements of the reflectance of these deposits are evidence that the bright deposits are water ice on the surface whilst the darker deposits, with less reflectance, are where the ice is covered by a thermally insulating layer. Gregory Neumann of the NASA Goddard Space Flight Center, and one of the author's of a paper on the MLA findings, said: "Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice."
A third paper, led by David Paige of the University of California, provides detailed models of the surface temperatures of Mercury's north polar region. The temperature model is based on the actual topography of Mercury's surface based on measurements from the probe's MLA. Paige and his colleagues conclude that the darker deposits are likely to be a mix of complex organic compounds delivered to Mercury by comets and asteroids.
Mercury, the smallest planet in the Solar System and the closest planet to the Sun, is titled on its axis by only one degree so there is hardly any sunlight at the poles. Scientists have long held the view that the shadowy craters in the polar regions could harbour water ice. In 1991 the Arecibo radio telescope in Puerto Rico detected bright patches at Mercury's poles which reflected radio waves in the way one would expect if they were deposits of water ice. The bright patches appeared to be around the large impact craters mapped by Mariner 10 in the 1970s. Messenger's detailed mapping and instruments appear to have found the evidence that reinforces the view that water ice exists in the shadowy craters of Mercury's poles.
Shown in red are areas of Mercury’s north polar region that are in shadow in all images acquired by MESSENGER to date. The polar deposits imaged by Earth-based radar are in yellow and the background image is the mosaic of MESSENGER images. 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. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory
The Messenger probe was launched on 3 August 2004 and was inserted into Mercury’s orbit in March 2011. When Mercury is at its closest to the Sun, the craft experiences temperatures of up to 370 degrees Celsius. A reflective sunshade is used to protect its sensitive equipment and to keep them operating at much lower temperatures. Messenger's solar panels harness the Sun's energy to power the probe.
Messenger is only the second space probe to visit Mercury after NASA's Mariner 10 in 1974-5 which captured the first close up images of the planet. However, Mariner 10 captured less than half of the surface area whilst Messenger has exceeded this and only a tiny percentage of the surface remains to be imaged.
Two more probes are due to be put into Mercury's orbit by the BepiColombo mission, a joint enterprise between the European Space Agency (ESA) and the Japanese space agency (JAXA). The BepiColombo mission is due to launch in 2015 atop an Ariane 5 rocket. The two probes will be placed into polar orbits around Mercury when they arrive in January 2022.