NASA scientists released a statement yesterday reporting that their MESSENGER probe has discovered water ice on Mercury (the closest planet to our Sun), buried under a layer of dark material that appears to be composed of complex organic compounds — the basic building blocks of biological life.
"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," said Sean Solomon, principal investigator of the MESSENGER mission, in today's NASA statement. "MESSENGER has now supplied a unanimous affirmative verdict."
Given the proximity of Mercury to the Sun, it may seem impossible for a world that can reach daytime temperatures of over 400 degrees Celsius could have water ice, but with virtually no atmosphere to trap solar radiation, the only way that objects on Mercury can truly experience that heat is through direct exposure to sunlight. Mercury has only a 1° tilt to its polar axis (compared to the ~23 degree tilt of Earth), so the bottoms of some of the deep craters at the planet's poles have never been exposed to the light from the Sun, and the temperature there remains a chilly minus 173 degrees Celsius.
The idea that there were areas of Mercury where the Sun never shines has been around for years, ever since astronomers confirmed Mercury's minuscule axial tilt. This led some to speculate that water ice, possibly deposited by comets, may be found at the bottom of polar craters. Observations in 1991 by the Arecibo radio telescope, in Puerto Rico, provided support for this idea when the data showed unusually bright patches at the poles that were consistent with what radar reflection from water ice looked like.
The MESSENGER probe, which arrived at its destination last year, has been scanning Mercury with its Gamma-Ray and Neutron Spectrometer (GRNS), mapping out the composition of the planet's surface.
"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 centimetres thick that is less rich in hydrogen," wrote David Lawrence, a MESSENGER Participating Scientist from The Johns Hopkins University Applied Physics Laboratory who is the lead author of one of the papers. "The buried layer has a hydrogen content consistent with nearly pure water ice."
Estimates put the amount of ice at between 100 billion and 1 trillion tonnes, so this is no small amount, and this 'surficial layer' is of particular importance as well. According to one of the papers written from this data, this dark material shows evidence of organic compounds, which were likely deposited on the planet by comet and meteorite impacts. Organic compounds are the basic building blocks of life. This doesn't necessarily mean that there is definitely life on Mercury, but according to Sean Solomon, it does raise some questions.
"Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions."