During the last week of August in 1835 readers of the New York Sun were treated to one of the great scientific discoveries of all time. A series of articles reported that British astronomer Sir John Herschel had developed a ridiculous new telescope that had given him glimpses of planets in other solar systems, close views of comets and – just to put a nice bow on things – he had discovered life on the moon. Of course, this all turned out to be a hoax (John Herschel had no part in it), but it got people thinking about life on the worlds that orbit other worlds and we call moons. Now, in the second week of March 2015, a couple major discoveries could reignite that curiosity.
On March 11th and 12th two teams of scientists reported in the journal Nature and in a NASA teleconference respectively that they had serious reason to believe that they had discovered two brand new oceans right here in our own solar system. The first on Enceladus, an icy moon orbiting Saturn, and the second on Ganymede, the largest moon in our solar system (it’s almost the size of Mars) which orbits Jupiter. Along with Europa, another Jovian (Jupiter-orbiting) satellite, these icy encrusted water worlds are the best candidates we’ve so far discovered for alien life.
Europa has long been a hot topic at astronomical dinner parties. Back in the 70’s when Pioneer spacecraft 11 and 12 flew by Jupiter they took the first pictures Europa’s surface and shocked the world when they revealed long dark channels on an otherwise smooth icy surface along with a stunning lack of craters. A lack of craters is interesting because it suggests that some force exists on the moon that can erase surface features. The Earth doesn’t have many craters because they erode. Our moon has lots, because not a lot happens up there.
When the Galileo mission returned to Jupiter in the 90’s, photos revealed that Europa’s surface channels appeared to be the result of the cracking that was later filled in by some flowing material (warmer ice or liquid water). Analysis of Jupiter’s magnetic field around Europa also provided evidence that some conductive material (probably salt water) exists beneath the ice.
If you’re wondering how moons so far out in the solar system could have liquid water beneath thick, sun-blocking ice, the answer is a process called tidal heating. See, as moons orbit planets the gravity of their hosts stretches them every which way, producing heat. If you’ve ever played squash or messed around with modeling clay you’ve seen similar forced at work. As a material gets stretched (be it clay, rubber or moon) it generates heat and becomes more elastic. On the scale of moons the heating can be kind of nuts. The ocean on Saturn’s moon Enceladus is thought to be as hot as 90 degrees Celsius (200 F)!
That heat is actually what led to its discovery. Back in 2005 the spacecraft Cassini saw massive plumes of water vapor shooting out of the moon’s south pole. Analysis of particles found in the vapor revealed silicon, which on Earth is generally created around hot ocean vents. That means that Enceladus is geologically active, and that is thought to be a prerequisite for life.
The ocean on Ganymede (back in the Jupiter system) is equally interesting but less likely to be home to any freaky fish-type things. That is because the liquid water probably doesn’t touch a rocky surface like on Enceladus. If you were to start digging on Ganymede you would burrow through 100 miles of ice before hitting water. When you sunk to the bottom of the ocean you would find yourself on more ice beneath which lies the rocky mantle and finally the moon’s iron core. The only reason we know there is water down there is because the auroras around the moon’s magnetic poles (caused by its iron core, just like Earth) are more stable than they would be if there was no water.
To sum things up, it has been a pretty crazy week in space exploration. It seems that will our focus on all the planets outside the solar system that may harbour life, we might have missed some supremely weird ice-fishing opportunities closer to home.