In its ongoing search for life on other worlds, NASA’s maxim has long been “follow the water.” Water is essential for life. At least it is on Earth, which happens to be the only planet we’ve confirmed has life. But water isn’t unique to Earth. It’s actually pretty common throughout the solar system, and as of yesterday we can officially add Mercury, that little planet that orbits nice and close to the Sun, to the list soggy bodies.
Liquid water and life share a biochemical connection. The reactions that give rise to and sustain life need a fluid in which to occur. Water is ideal. In water, molecules can dissolve to facilitate these reactions. And because liquid water is always in flux, it’s an effective medium to transfer vital metabolites and nutrients from one place to another, a property as important on a cellular level as it is on a global scale. With this emphasis on flux and transportation, liquid water is the type scientists are after. Moving vital molecules in solid ice is a little tricky, and vapour-based life would, well, evaporate.
There are a few theories about how life-giving water came to Earth. One theory suggests that the hydrogen and oxygen in the early Earth’s environment combined chemically below the surface and was shot out as water through an eruption. Another theory posits that water molecules adhered to the interstellar dust grains that accreted to form the planets of the solar system, making water an original feature on all early planets. Then there are two different impact theories where water rich asteroids and meteorites as well as comets slammed into the Earth and brought water to the surface.
The comet theory is increasingly gaining support as scientists find further evidence that these distant travelers hold the same type and composition of water as the Earth (part normal and part heavy – a water molecule made with the hydrogen isotope deuterium). Comets originate in the far ends of the solar system and hit a lot of things all the time, so its feasible that if they did bring water to Earth they brought water to other bodies as well.
Moving from the outer solar system to the inner, Neptune’s mantle, the region below its thick atmosphere, appears to be water rich with traces of methane, ammonia, and other elements all under high pressure. Uranus has the same arrangement. Saturn is far less wet with onle a thin layer on top of its core dominated by a soupy liquid mixture of water, methane, and ammonia under high temperatures and pressures. Jupiter has the same structure, though it does have some water vapour in its ammonia- and hydrogen sulfide-rich cloud layer.
The outer moons are a little more uniform where water is concerned. Neptune’s Triton, Uranus’ Titania and Oberon, Saturn’s Enceladus, and Jupiter’s Europa, Callisto, and Ganymede are all covered in ice; Jupiter’s moons are all thought to have significant subsurface oceans as well. Saturn’s moon Titan, which is covered with liquid methane seas, hosts a subsurface ocean while Rhea is almost three-quarters made of ice. Saturn’s rings are also thought to hold water in a frozen state.
Water exists in the bodies of the asteroid belt. In 1998, the Mars Phoenix Lander found frozen water at the planet’s poles and NASA subsequently confirmed the red planet has some liquid water beneath its surface and found evidence of past flowing water. Frozen water also exists deep in the Moon’s craters and within its mantle. And now ice deposits have been confirmed to exist in the deep craters on Mercury poles.
Mercury orbits incredibly close to the Sun; its mean distance is just 35,983,606 miles, compared to the Earth’s 92,960,000 miles. The tiny planet has an average temperature of 332º Fahrenheit. On the day (or Sun-facing side) temperatures can reach up to 801º F, while the night side can fall to -279º F. But that’s the surface. Not every spot on Mercury goes through that day-night warming-cooling cycle. Mercury sits basically vertical – its axis is tilted less than one degree – meaning the base of the craters at the poles never see the Sun. Despite being the closest planet to the Sun, these craters are some of the coldest spots in the solar system. And they’re the perfect environment for water ice to settle.
The possibility that ice existed in these craters has been floating around for a while, and it’s one of the things NASA’s MESSENGER spacecraft (an awkward acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging) hoped to confirm.
Since it launched on August 3, 2004, MESSENGER has found compelling evidence consistent with the frozen water theory. It found hydrogen at Mercury's north pole and an area of high reflectivity that highly suggested ice and some odd darker regions. The most recent data is lending support to the idea that water ice is the major constituent of these polar deposits. There are also mysterious dark spots in regions where it’s cold (we’re still in the craters) but where organic material can survive. So these volatiles are organic material mixing with the ice.
Organic materials are life's ingredients, but this discovery doesn’t mean there’s life on Mercury, or that life will be possible on Mercury, or even that Mercury can support life. While the permanently shadowed craters do contain water ice, it's not at the surface, it’s about six inches below. And water ice isn’t the best for life anyways.
So don’t go getting all excited about the next habitable planet just yet. The odds of life springing up on Mercury are slim. The planet has no atmosphere, so even if some ice were uncovered in a temperature region where liquid water can exist, it would evaporate pretty quickly. That said, confirmation of water ice on Mercury has made the planet a point of astrobiological interest. MESSENGER scientists said the next step would be to actually land on the little planet and sample the ice to see just what, if anything, is inside the deposits. But a Mercury lander is certainly something a ways off in our interplanetary future.