Ice on Mercury. Mercury?!?
by Emily Lakdawalla on November 29, 2012
Ice and organics on another planet! Mars? No–Mercury!
In a week when the Internet rumor mill has been churning out increasingly sensational (and, as it turns out, wrong) speculation about organic materials on Mars, NASA has reported the discovery of both water and organic materials on a different planet entirely. The location will come as a surprise to many of you: it’s Mercury, the planet closest to the Sun.
It doesn’t seem like it should be possible for there to be ice on Mercury. It orbits the Sun at a distance only a third that of Earth, which means that the Sun is about ten times more powerful there (that’s the inverse square law at work). Mercury has no atmosphere to protect it or to even out its temperatures, and it’s locked in a spin-orbit resonance such that it rotates exactly three times for every two Mercury years.
Because spin and revolution have the same sense of rotation, Mercury’s solar days — the time from sunrise to sunrise — last twice as long as its years. The Sun blazes down on any one spot on Mercury’s surface for 176 Earth days, heating some places above 550 Kelvins (that’s 280°C or 540°F). That’s hot enough to clean your oven; it doesn’t seem like any water, or organic material, for that matter, should be able to survive on Mercury’s surface.
But there’s something else a little unusual about Mercury: it doesn’t have seasons. Its axis is tilted by a negligible 2 degrees. So there are a lot of places near Mercury’s poles where there are deep holes (craters) and permanently shadowed regions, where the Sun never rises above the horizon. These places in permanent darkness exposed to the blackness of space can get quite cold, down to 50 Kelvins (-220°C or -360°F).
It has long been theorized that these permanently dark, cold places could “trap” volatile molecules. Here’s the way that works: Mercury gets hit all the time by comets and asteroids. All comets, and some asteroids, contain quite a bit of water as a proportion of their mass. When they slam into Mercury, they largely vaporize. That vapor goes into the sky, bound by Mercury’s gravity. Eventually, most of it escapes, blown away by the solar wind. But that doesn’t happen immediately. The molecules do what gas molecules anywhere do — they diffuse. They bounce around, running into each other, adsorbing onto a rock, then vaporizing again. If a molecule just happens to stick to a rock in one of those permanently shadowed places, though, it doesn’t leave. It’s so cold that the gas molecule becomes trapped; it doesn’t have the energy to launch upward from the rock again. This is not a fast process, but given geologic time, permanently shadowed regions at Mercury’s poles should wind up accumulating substantial deposits of water ice and other volatile materials — other gases like carbon dioxide, and organic materials.
The first confirmation of this story came decades ago, when the great Arecibo radio telescope — the largest in the world — was used to broadcast signals at Mercury and listen for the echoes. The radar reflected particularly strongly from certain areas near the North Pole. One material that reflects radar in that way is ice. But there are minerals that do the same thing, so while this was consistent with the ice story, it wasn’t proof.
What was needed was to show that the stuff that Arecibo saw as being bright was located in permanently shadowed places, and only in permanently shadowed places. But we didn’t know at the time where those places were on Mercury, because the only spacecraft ever to visit Mercury was in 1974 and 1975, and Mariner 10 did a lot for its time, but it didn’t see the poles very well, and it didn’t get any topographic data.
That’s where MESSENGER came in. MESSENGER is a small mission, one of NASA’s least-expensive Discovery line of planetary missions. It is Mercury’s very first (and only) orbiter; MESSENGER arrived at Mercury in March of 2011 and began mapping Mercury with several instruments (key ones for this story are its laser altimeter and its neutron spectrometer).
The laser altimeter made a detailed map of the planet’s topography. Using that map, scientists simulated years of Mercury’s orbit to locate places that never saw sunlight. Then they overlaid the Arecibo map. The results were pretty stunning, actually. It’s very rare that you get this good agreement between any two data sets acquired by completely different instruments in completely different ways.
That pretty clearly proves that the radar-bright material is associated with permanently shadowed areas. But it doesn’t prove that it’s water. For that, the laser altimeter team looked at their data a different way. A laser altimeter is a rangefinder that bounces a laser signal off a surface and watches for the return pulse. You get topography from the timing of the return pulse, but you can also get information from the brightness of the return pulse. Unlike MESSENGER’s camera, which depends on sunlight to illuminate the surface, so can’t see into the permanently shadowed regions, the laser altimeter is like shining a flashlight into those places. If it’s water ice, you’d expect the reflection to be brighter than expected.
It wasn’t brighter in those places. It was actually darker than the rest of Mercury, as dark as coal. Huh?
This was a setback. It couldn’t be ice. So what was it?
Well, in the meantime, there was also data coming back from the neutron spectrometer. A neutron spectrometer senses neutrons that come out of Mercury when galactic cosmic rays slam into it and do nasty things to other atoms. But hydrogen acts as a barrier to these atoms, so, in any place where there is a lot of hydrogen buried in the subsurface, you get a place where the neutron spectrometer sees relatively few neutrons. Now, neutron spectrometers are pretty low-resolution things. The neutron spectrometer can’t tell you anything about whether the permanently shadowed areas are places where there are low neutrons. But it did show that there was a definite low in the neutron counts localized right around the north pole. The neutron spectrometer team did some calculations, and found their measurements to be exactly consistent with the presence of water ice in all the places that Arecibo said there was water ice.
So what, then, was the deal with the laser altimeter data showing this stuff to be dark? The team came up with an explanation: they were seeing organic materials at the surface. Elsewhere in the solar system, wherever there is ice that has been exposed for a long time, its surface is often very dark. It’s a lag deposit; ice gets dust in it, and some water molecules manage to escape, but the organic-rich dust is less volatile and doesn’t escape. The dark dust, being dark, is also warmer than the bright ice, so its presence actually encourages ice to vaporize faster than it might otherwise do. So what the MESSENGER team was seeing was entirely consistent with there being thick deposits of water ice in these permanently shadowed regions, mantled with some organic material.
To perform a last check on this, they modified the way they used the spacecraft after their primary mission of one Mercury solar day (or two Mercury years) ended last fall, and they went into their extended mission. In their primary mission, the laser altimeter never actually looked at the surface very close to the pole. They had to slew the spacecraft sideways a little bit in order to do that. During the extended mission, when the laser altimeter looked at craters quite close to the pole, which had the darkest, coldest regions, it finally started seeing bright reflections of water ice.
Et voilà. They’d finished making their case. They wrote it all up, and their papers were published today by the prestigious journal Science.
Want to learn more? I get a bit deeper into the science here (I show more pictures there, too). Watch my blog, too, if you’re also curious about that what-did-Curiosity-find-on-Mars-story that I alluded to at the start of the post!
These results only came about because NASA funded MESSENGER’s extended mission. But NASA’s budget is really tight. Nearly all of our wonderful planetary missions are long past the ends of their primary missions, and every one of them faces cutbacks every budget cycle. The current budget in front of congress cuts NASA’s planetary exploration program by 300 million dollars. They may have to shut down the Opportunity rover, or other lovable missions. NASA’s budget cuts prevents us from planning more missions of any kind for years. If you don’t think that’s the right direction, make your voice heard: Save our science missions!