Mercury's Polar Ice Defies the Odds

Scientists with NASA's Messenger mission have announced that they've confirmed the existence of water ice in permanently shadowed craters near Mercury's poles.

Today three teams of planetary scientists announced a truly amazing result, something I've been hoping to hear for more than 20 years and expected to hear about 8 months ago. The hellish planet Mercury harbors substantial amounts of water ice — ice! — on the walls and floors of craters near its poles that are never exposed to sunlight.

This stunning revelation is detailed in a trio of reports published online today in Science Express. All are based on results from NASA's Messenger spacecraft, which has been orbiting the innermost planet for about 1½ years. I'll delve into those in a moment, but first let's step into the Wayback Machine.

In late 1991, I first got word that a team of astronomers had used one of NASA's big guns, the 230-foot (70-m) tracking antenna at Goldstone, California, to continuously illuminate Mercury with radio energy. Then they listened with the Very Large Array in New Mexico for faint radar echoes, with which they'd create a radar map of the planet.

To their complete surprise, the planet's north pole veritably lit up like a beacon. Water ice was the most likely compound to create such a strong radar echo, but how could that possibly be? "Even the youngest of budding astronomers knows that Mercury is one hot planet," wrote I in Sky & Telescope's January 1992 issue. "Mercury's midday temperatures soar to 825° Kelvin — hot enough to make molten puddles of lead, tin, and zinc."

And yet, simple geometry argued that the idea of ice at the Mercurian poles was not crazy at all but actually quite plausible. The innermost planet is so powerfully locked in the Sun's gravitational grip that the angular tilt of its spin axis, relative to its orbit, is virtually zero (just ¹⁄₃₀°). So, in principle, any ice that found its way to the floors of deep craters near the poles would never be exposed to sunlight and might well remain for a very long time.

Over the years, support for this notion of snowballs in hell only grew stronger, and Messenger's arrival in March 2011 finally gave scientists the tools they needed to prove it.

One method is to keep count of the neutrons reaching the spacecraft from Mercury. These are created when high-energy cosmic rays strike the surface and interact with atoms in rocky minerals. But hydrogen atoms gobble up slower-moving neutrons like so many atomic sponges, and a drop-off in the count rate would imply the plentiful presence of hydrogen — predominantly in H₂O — near the planet's surface.

That's exactly what early results from Messenger's neutron spectrometer were hinting to its science team (as reported here last March).

Now, after many more months of slowly amassing more neutron counts and also adjusting Messenger's orbit to come somewhat closer to the planet's north pole, the presence of water ice seems inescapable. Although the neutron spectrometer's maps are too crude to resolve individual craters, there's a clear enhancement due to hydrogen in the polar regions, according to lead scientist David Lawrence of Johns Hopkins' Applied Physics Laboratory. "We can prove the water's there," he says, and measurements are consistent with the shadowed regions being filled with water ice.

So how much might be sequestered in the darkness, out of sight from the spacecraft's cameras? Lawrence says the layer of ice is at least 20 inches (50 cm) deep and could be much deeper. All told, Mercury's polar regions might hold 100 billion to a trillion tons of ice — somewhere between a Lake Tahoe and a Lake Erie's worth.

A caveat: the neutron-spectrometer doesn't detect water directly; it indirectly detects hydrogen. So might some other hydrogen-rich substance be masquerading as water ice? Unlikely, according to additional results from Messenger's laser altimeter. It sends pulses of near-infrared light toward the surface eight times per second, then carefully computes the round-trip travel time of each reflection to determine the elevation of whatever its passing over. The instrument's science team uses these to build a topographic map of the planet over time. But the laser pulses also illuminate the ground below, and the strength of the reflections reveals how reflective the surface material is — even in the darkest shadows.

One such deep shadow lies in the 70-mile-wide crater Prokofiev, which lies with 100 miles of the planet's north pole. According to investigator Gregory Neumann (NASA/Goddard Space Flight Center), a large crescent-shaped area of the crater's poleward-facing inner rim is two to four times brighter than its surroundings, meaning is topped with something as reflective as ice. The real surprise, adds Neumann, is that the icy exposures in Prokofiev and elsewhere are surrounded by very dark material dark. He says these are likely thin blankets of material that lie atop layers of ice.

To make the case ice-tight, researchers led by David Paige (University of California, Los Angeles) used the newfound knowledge of highs and lows in the polar regions to calculate which portions of the polar regions are sometimes roasted by intense sunlight and which are not. They conclude that the floors and inner walls of many polar craters are so deeply shadowed that they never get warmer than -370°F (50 K). Any water that makes its way into these super-cold enclaves could remain there stably for a very long time.

Comets are the most likely source of all this water. Whenever one strikes Mercury, a cloud of water vapor, organic compounds, and other volatiles briefly enshrouds the planet. Some of these compounds migrate to the polar shadows and become "cold trapped" onto the surface. Paige suggests that, over time, space radiation causes the topmost layer of organics to transform into pitch-black goo, which eventually gets thick enough to insulate the pristine deposit beneath it.

Determining whether ice exists at Mercury's poles was actually a high priority for Messenger and its science team, notes principal investigator Sean Solomon (now at Columbia's Lamont-Doherty Earth Observatory). He explains that the water-ice hypothesis has now been subjected to three stringent tests — neutron emission, topographic plausibility, and thermal modeling — and it's passed all three. "We know of no other compound that matches [these three] characteristics," he says.

So while it might be a very long while before your local travel agent can book you a skiing vacation on Mercury's polar slopes, it's mind-expanding to think about how all that water ice got there and how its existence alters our view of solar-system evolution. For example, in answer to one rather obvious question at today's press briefing, Solomon rejected the idea that there might be life on the innermost planet. But, he allowed, "Mercury is becoming an object of astrobiological interest.