Mercury may be a scorching hunk of rock just next door to the sun, but planetary scientists have discovered nearly pure frozen water and even some organic material in the planet’s frigid polar regions.
The findings from the Messenger spacecraft orbiting the planet cap the decades-long search for water on the second-hottest planet in the solar system and may help scientists better understand the origins of the molecular building blocks for life on Earth.
The new research “doesn’t mean we have life on Mercury,” said UCLA planetary scientist David Paige, lead author of one of three papers published Thursday by the journal Science. “But it is relevant for the question of life in the solar system in general.”
As much as 1.1 trillion tons of ice could lie on or just beneath Mercury’s surface in the nooks and crannies of craters that never see sunlight, according to scientists working on the Messenger mission. Much of that ice may be protected by a dark layer of carbon-rich organic material several inches thick, they said.
Before the Messenger spacecraft dropped into orbit in March 2011, Mercury remained something of an enigma. The Arecibo radio telescope in Puerto Rico detected bright, shiny spots on the planet’s surface in 1991, which scientists interpreted as a strong sign of frozen water. These spots seemed to map well with some of the shadowy parts of polar craters that were glimpsed in the 1970s by the Mariner 10 spacecraft, which saw only about half of the planet’s surface.
Researchers had calculated that because Mercury’s axis is tilted less than 1 degree, there are regions near its poles that never see the sun. Though surface temperatures can hit a broiling 800 degrees, the permanently shadowed regions could dip to minus 370 degrees.
With an X-ray spectrometer, magnetometer and topography-measuring laser altimeter among the gadgets in its high-tech tool belt, Messenger was prepared to solve Mercury’s long-standing mysteries.
One group of scientists used the altimeter to look for light and dark deposits in the polar regions. The bright material, they surmised, was typical of water ice on the surface, while the dark regions matched well with a layer of organic material darkened by bombardment from space radiation.
Then Paige’s team used topographic data from the altimeter to construct a heat map of the surface. They found that the coldest regions were also the ones that shone brightly, further evidence of surface ice. In slightly warmer areas where ice would have to be buried to endure, the researchers saw the dark patches that were distinct from the signature of Mercury’s soil.
A third collaboration looked for water by searching for a main ingredient: hydrogen. Using the spacecraft’s neutron spectrometer, they looked for a decrease in a certain population of neutrons, a sign that they had run smack into a hydrogen atom. In fact, they found that the dark patches often hid deposits that were so chock-full of hydrogen that they must have contained nearly pure water beneath the surface.
The three independent lines of evidence make a very strong case that there’s water on Mercury, said Martin Slade, a planetary scientist at the Jet Propulsion Laboratory in La Cañada Flintridge, who was not involved in any of the studies. “Personally, I didn’t see any alternative,” he said.
The water and organic material probably aren’t native to Mercury; it could have been delivered by icy comets as they smashed into the surface. It’s widely believed that this is how organic molecules made their way to Earth as well, but the theory can’t be tested because geologic forces have churned up the evidence.
Mercury, then, offers a glimpse of a possible early stage before the development of life as we know it, Paige said.