Two recent discoveries on other planets in our solar system have led to great excitement in the scientific community. The Messenger spacecraft, in orbit around Mercury since March 2011, has confirmed the presence of water ice on Mercury. Less than four months after its arrival on Mars, Curiosity has found some interesting chemical compounds. What do these discoveries mean?
First, to Mercury. The innermost planet in our solar system, temperatures at Mercury’s equator can reach up to around 800°F during the day during its closest approaches to the sun. At night, however, temperatures at the poles can drop to -370°F. How is such a disparity possible? For one thing, the planet has virtually no atmosphere, so any heating that occurs during the day radiates out into space at night. The nights are also extremely long; Mercury’s rotation is in a 3:2 ratio with its orbit of 88 Earth days, so while the planet spins on its axis every 58.7 days, a solar day on the surface actually lasts two years, 176 days. The axis of rotation is almost perfectly perpendicular to its orbit, so sunlight reaches the polar regions at a virtually constant angle (unlike Earth, where our axial tilt of about 23.4° leads to constant sunlight during polar summers and constant darkness during polar winters).
What does any of this have to do with ice on Mercury? There isn’t enough atmosphere to deflect or burn up the meteors and comets that periodically cross Mercury’s orbit, so the surface is covered in impact craters. Some of the ones located near the poles are deep enough that the steep walls combined with the low angle of the sun leave part of the crater floor in constant darkness, and the craters and other surface features cast permanent shadows toward the poles as well. The thin atmosphere also means light isn’t even reflected down into these areas like it would be in deep mountain valleys on Earth.
We first found evidence of polar water ice on Mercury back in 1991. Radar images taken by the Arecibo radio telescope in Puerto Rico showed bright spikes near the poles consistant with the reflective properties of water ice, but there wasn’t enough data to give a definitive answer as to whether it was really ice or another shiny substance such as sulfur or silicate compounds. While in orbit, Messenger used a “neutron spectrometer” to measure the rate of neutrons bouncing off the planet’s surface. Hydrogen molecules absorb neutrons easily, so areas where fewer neutrons bounce back are good indicators of the presence of water with its two hydrogen molecules. Laser pulses fired by Messenger to map Mercury’s topography confirmed the ice and also detected that in regions where temperatures warm above -280°F, much of the ice’s surface is covered by a layer of dark material that is more than a foot thick in some areas. The material may consist of carbon-based organic compounds that, along with the water itself, was likely deposited on Mercury by comets that collided with the planet. The relative warmth in those areas may have allowed the top layer of ice to vaporize, leaving behind the other material. While this is a significant discovery, many questions still remain as to what kinds of compounds are present on the ice and whether liquid water exists anywhere on the planet.
On to Mars. Two weeks ago NASA announced that the Curiosity rover had found something “for the history books” but, citing the need to confirm the measurements, didn’t release the details until yesterday. Speculation was rampant, but now the wait is over. Analysis of the first soil samples obtained directly from the surface of Mars found compounds that contain water, sulfur, chlorine, and other ingredients. Carbon-based organic compounds have not yet been found, but given the tiny sample size, we certainly can’t rule out their presence. The chlorine found was in the form of perchlorates, confirming earlier discovery of the chemical by the Mars Phoenix Lander a few years ago. Perchlorates can destroy organic molecules, so this isn’t great news, but it isn’t unexpected either. There was also a very slight signal that might be from a very simple carbon compound, but there wasn’t enough evidence to confirm that it was actually a carbon compound or or a compound from Earth that hitchhiked on the rover despite the best efforts to decontaminate it.
So why all the excitement? Well, it can be put down partly to a misunderstanding from the original NPR reporter who got the quote. NASA’s John Grotzinger apparently meant that the entire mission would be of historic importance, not this particular dicovery. But it is still scientifically significant. The mere fact that we can figure out what compounds are in a tiny scoop of dirt on another world millions of miles away is pretty damn awesome. It confirms that the equipment is all working as planned; no small feat given the number of modules that had to work together to scoop and analyze the soil samples. And we need to remember that the surface mission isn’t even four months old yet, and Curiosity has only covered a small amount of territory within Gale Crater. The search goes on, and who knows what we’ll find next?
Mercury: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/National Astronomy and Ionosphere Center, Arecibo Observatory