Hottest planet cold enough for ice
Mercury, the innermost planet in the Solar System, is like a small rock orbiting the Sun, continuously assaulted by the star’s heat and radiation. It would have to be the last place to look for water.
However, observations of NASA’s MESSENGER spacecraft indicate that Mercury seems to harbour enough water-ice to fill 20 billion Olympic skating rinks.
On November 29, during a televised press conference, NASA announced that data recorded since March 2011 by MESSENGER’s onboard instruments hinted that large quantities of water ice were stowed in the shadows of craters around the planet's North Pole.
Unlike Earth, Mercury’s rotation is not tilted about an axis. This means one side of the planet permanently faces the sun, becoming hot enough to melt lead. The other side, however, constantly faces away from the sun, and is extremely cold.
This characteristic allows the insides of craters to maintain low temperatures for millions of years, and capable of storing water-ice. But then, where is the water coming from?
Bright spots were identified by MESSENGER’s infrared laser fired from orbit into nine craters around the North Pole. The spots lined up perfectly with a thermal model of ultra-cold spots on the planet that would never be warmer than -170 degrees centigrade.
These icy spots are surrounded by darker terrain that receives a bit more sunlight and heat.
Measurements by the neutron spectrometer aboard MESSENGER suggest that this darker area is a layer of material about 10 cm thick that lies on top of more ice, insulating it.
Dr. David Paige, a planetary scientist at the University of California, Los Angeles, and lead author of one of three papers in Science that indicate the craters might contain ice, said, “The darker material around the bright spots may be made up of complex hydrocarbons expelled from comet or asteroid impacts.” Such compounds must not be mistaken as signs of life since they can be produced by simple chemical reactions as well.
The water-ice could also have been derived from crashing comets, the study by Paige and his team concludes.
Finding water on the system’s hottest planet changes the way scientists perceive the Solar System’s formation.
Indeed, in the mid-1990s, strong radar signals were fired from the US Arecibo radar dish in Puerto Rico, aimed at Mercury’s poles. Bright radar reflections were seen from crater-like regions, which were indicative of water-ice.
“However, other substances might also reflect radar in a similar manner, like sulphur or cold silicate materials,” says David J. Lawrence, a physicist from the Johns Hopkins University Applied Physics Laboratory and lead author of the neutron spectrometer study.
Lawrence and his team observed particles called neutrons bouncing and ricocheting off the planet via a spectrometer aboard MESSENGER. As high-energy cosmic rays from outer space bombarded into atoms on the planet, debris of particles, including neutrons, was the result.
However, hydrogen atoms in the path of neutrons can hold the speeding particles almost completely as both weigh about the same.
Since water molecules contain two hydrogen atoms each, areas that could contain water-ice will show a suppressed count of neutrons in the space above them.
Because scientists have been living with the idea of Mercury containing water for the last couple decades, the find by MESSENGER is not likely to be revolutionary. However, it bolsters an exciting idea. As Lawrence says, “I think this discovery reinforces the reality that water is able to find its way to many places in the Solar System, and this fact should be kept in mind when studying the system and its history.”