The idea of finding ice on Mercury seems about as plausible as finding snow on the ground in July. And yet, results from NASA’s MESSENGER mission, managed and operated by the Hopkins Applied Physics Laboratory (APL) since 2004, have confirmed the long-held hypothesis that ice can be found on the poles of this scorcher of a planet.
“[It] is crazy. That’s one of the fascinating things about this,” David Lawrence, a MESSENGER team member and researcher for the APL, said of the investigation. Yet, a very calculated set of data supports the claim and was published by Science Express in a series of three papers on Nov. 29.
The first paper, lead authored by Lawrence, discusses the use of the neutron spectrometer located on MESSENGER to measure the hydrogen concentration on Mercury. As a water molecule contains two hydrogens, a high concentration would indicate the presence of water ice.
“We fly the spacecraft all around the planet and we look for places where the number of neutrons go down, and from that we can tell where there’s a lot of hydrogen,” Lawrence said.
Neutrons are continuously generated when cosmic rays collide with Mercury’s surface and have a very similar mass to hydrogen. During a collision between the two molecules, therefore, the hydrogen protons can practically absorb the neutrons, stopping them much in the same way a cue ball stops after impacting another ball on a pool table.
It turns out that the hydrogen concentrations were particularly high at the poles, suggesting that the ice is located in those areas. In the second study, Gregory Neumann of the NASA Goddard Space Flight Center used the Mercury Laser Altimeter (MLA) to form a topographic map, which showed the reflectivity of the planet and the fact that polar regions contained areas of both bright and very dark components. The bright components indicate ice on the planet’s surface.
David Paige, a professor at UCLA and lead author on the third paper, explained the dark component findings by showing that in regions covered by permanent shadow, water ice was buried under 10 to 20 centimeters of dark material. The deposit contains “organics” in its composition and may have been delivered by comets.
The recent MESSENGER findings were not the first to point to the presence of water ice on Mercury. “It’s more of a confirmation than a discovery,” Sean Solomon, principal investigator for the MESSENGER mission and director of the Lamont-Doherty Earth Observatory at Columbia University, said.
Solomon refers to the 1970s crater images recorded by the Mariner 10, which later proved to match up with radar-bright areas found by Puerto Rico’s Arecibo radio telescope in 1991. Bright reflections would suggest the presence of water ice, but until MESSENGER, there was no way to confirm the type of material.
Mercury is the closest planet to the Sun, and at its hottest points can reach about 800 degrees fahrenheit. How then could ice possibly exist? Unlike Earth, Mercury’s axis of rotation is directed North so the poles, where the ice has been found, are never hit by sunlight. Mercury also has no atmosphere so heat is not able to diffuse around the planet.
“It’s one thing to say that it’s cold enough for Mercury to have ice, but it’s another thing to say there’s actually ice and the obvious question is where does the water come from?” Paige said. The researchers believe that the ice arrived when comets that contained the substance hit Mercury and formed craters at the poles.
As with any discovery of water or ice, the idea that life might be supported on another planet arises. “I personally don’t want to speculate... It’d be great to send a spacecraft back there, land on the surface, and dig it up and measure the details. We don’t think we’re going to find any life, but you see all the building blocks potentially...and that’s exciting in its own right,” Lawrence said.
The mission is still ongoing and continues to collect data. The researchers hope to one day collect sample material from the dark deposits and analyze the contents, though the development of a technology able to overcome orbital dynamics and probe Mercury’s surface currently stands in the way. A study of this sort could lead the team to understand where the organic material originated from, perhaps providing some clues as to where the materials for life on Earth originated as well.
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