You were rewarded if you looked for meteors last Thursday night. From 9 p.m. on, you saw one every minute or two. And fully half of these Geminids were brilliant. It was our best shooting stars display since November 2001.
While watching the sky, you also noticed that one star stood out from all others. This is Jupiter, which remains at its most brilliant. It is this year’s Christmas or Holiday Star: the brightest “star” in all the heavens.
Using his first telescope, Galileo looked at Jupiter in 1610 when it was in the same constellation that it is in now: Taurus. He discovered four giant moons circling it, though it didn’t cross his mind that he might be gazing toward extraterrestrial life. Even as the centuries passed, the little dots offered no detail or information, except for their odd orbital basics – which are that innermost Io whizzes around Jupiter four times for every two orbits of Europa and one circuit of giant Ganymede. The moons are locked in perfect synch with each other like a marching band.
Occasionally, sharp-eyed youngsters would report seeing all four with just the naked eye. These claims were not disbelieved, since the moons are fifth-magnitude – well within the naked-eye limit – and the only real challenge is discerning them against the harsh glare of Jupiter itself. But all such observational niceties gave way to stunning images and a flood of information when the US sent a series of spacecraft to take close-up images of these large satellites. Pioneers 10 and 11 and then the two Voyagers in the 1970s, followed by the dedicated Galileo mission in the 1990s and the new Horizons flyby in the 2000s, transformed each moon into a unique living world.
The innermost two were clearly more “living” than the others. They changed. They altered their appearance from visit to visit. Io did this with nonstop volcanoes. Europa did the same, except now the animation mechanism was not fire but ice. Europa is covered with floating ice sheets. These rub against their neighbors, and liquid water from below oozes into the airless sky and settles on both sides of the crevices to form parallel frozen hills.
This is big. Here are the first new oceans since Balboa found the Pacific. Later orbiting spacecraft revealed the wispy signature of sodium atoms floating above Europa: The oceans are salt water.
The implications scarcely require a degree in planetary geology. Liquid water can only exist under pressure and in a narrow temperature range. On Earth, air supplies the pressure and the Sun bestows the warmth. But Europa has no air, and sunlight there is 27 times feebler than ours. That’s not enough heat.
The secret mechanism starts at Europa’s surface. The floating ice keeps the liquid under pressure, and also protects it from the vacuum of space. Meanwhile the real keeper-of-the-juice is ongoing warmth generated deep below. The machinery is tidal. Europa’s body is constantly twisted and distorted by the close presence of giant Jupiter, 318 times more massive than Earth. In addition, that locked-in orbital synchronization with the inner moon Io means that poor Europa is tugged with every orbit, tidally stretching its entire body. The net result is a continuous heat supply – more than enough to keep its oceans cozy and liquid.
Over the eons, Europa, like Jupiter and its other moons, gets bombarded by comets, helplessly pulled in by the intractable Jovian gravity. Such comets often contain complex amino acids: the building blocks of life. These are continually delivered to Earth, and probably supplied the initial steps toward fashioning life’s proteins. On Europa the process has been just as ongoing.
Since life began in Earth’s oceans, there is no reason why the same processes shouldn’t have been simultaneously occurring on Europa. Indeed, it would be more puzzling if we don’t find life there than if we do.
A proposed NASA/ESO mission, slated for a liftoff between 2016 and 2020, would send a probe to Europa. It will of course be unmanned. Europa’s surface gets a steady 540 rads of daily radiation: enough to kill a human in two days.
Although space enthusiasts had been hoping for a lander that could burn its way through the ice to probe the ocean below, budget considerations now make it appear that only an orbiter will be sent, which many regard as an essentially pointless exercise. But someday we’ll drill down and see what, if anything, swims in the warm Europan seas.