The closest planet to the Sun, Mercury, is a world of intense extremes. According to a new study, those extremes could be responsible for some of the water ice locked up in dark craters at its poles.
Nearly 30 years ago, astronomers discovered something amazing. Using the Arecibo Radio Observatory in Puerto Rico, they bounced radio waves off the surface of Mercury. When they received the radar signatures back, the astronomers found that some regions were showing up extremely bright. In fact, it was the same kind of exceptionally bright reflectivity we expect to see from frozen water!
Twenty years later, when NASA's MESSENGER spacecraft arrived at Mercury, it began sending back high-resolution images of the surface. Lining these new pictures up with that old data revealed that those bright radar signatures detected by Arecibo lined up perfectly with regions of Mercury's surface that were in perpetual darkness.
This composite image shows an oblique view of Mercury's north pole with radar data shown in yellow (top), with overhead views of the north pole showing the Arecibo radar reflections of water ice (bottom left), how they line up with the terrain (bottom centre) and how this corresponds with perpetual shadows (bottom right). Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory
The dayside of Mercury continuously bakes under intense heat from the Sun, reaching temperatures of around 400°C. Out of direct view of the Sun, however, temperatures plunge 600 degrees, down to -200°C. That is certainly cold enough for water ice to survive, but the big question is, how did that water ice get there in the first place?
Some of that water undoubtedly came from asteroids and comets. That is likely for all of the rocky planets in the inner solar system. Still, in a new study published in Astrophysical Journal Letters, researchers from the Georgia Institute of Technology have found that some fairly simple chemistry could be producing water ice out of thin air on Mercury, which then freezes solid in the sheltered darkness of these craters.
This simple chemistry begins with molecules of hydrogen and oxygen, known as hydroxyl groups (OH), which are produced as protons from the solar wind bombard Mercury's surface. The extreme heat on the sunward side of the planet then energizes these OH molecules enough that when they smash into each other, it results in them combining to produce a water molecule (H2O).
This diagram shows the molecular structure of a mineral on Mercury, comprised of hydrogen, magnesium, oxygen and silicon. Hydroxyl groups break off the end due to solar wind bombardment, where they combine to form water. Credit: Georgia Tech/Orlando/Jones
Plenty of these water molecules do not survive for very long, as they're broken apart by solar wind particles. Those that manage to find their way into shady areas, the researchers wrote, quickly condense and then freeze into ice.
"This is not some strange, out-of-left-field idea," Brant Jones, coauthor of the study from Georgia Tech's School of Chemistry and Biochemistry, said in a press release. "The basic chemical mechanism has been observed dozens of times in studies since the late 1960s, but that was on well-defined surfaces. Applying that chemistry to complicated surfaces like those on a planet is groundbreaking research."
This artist's impression shows NASA's MESSENGER spacecraft in orbit around Mercury. Credit: NASA/JHUAPL
According to the study, over a period of just three million years, this chemical production of water could accumulate over 11 billion tonnes of ice in Mercury's shadowed craters.
"The process could easily account for up to 10 percent of Mercury's total ice," Jones said.
Once sheltered in perpetual darkness, this ice could persist indefinitely, due to Mercury's extremely thin atmosphere. On Venus, Earth and Mars, their atmospheres are thick enough that air molecules can bump up against one another and transfer heat in the process. Mercury's exosphere is so thin that the molecules whizzing around in it would hardly ever encounter each other.
Thus, without direct sunlight shining on the ice, there would be no way for it to melt or sublimate.
"It's a little like the song Hotel California," Thomas Orlando, the study's principal investigator and co-founder the Georgia Tech Center for Space Technology and Research, said in the statement. "The water molecules can check in to the shadows, but they can never leave."