Snow stuck to the ground for the first time yesterday evening. After work I reached for the shovel and removed an inch of white frosting from the deck and walkway outside my home. So begins the transition from an earthy world to an icy one.
Liquid water exposed on the surface of planets and moons is exceedingly rare in the solar system, but ice is everywhere – Mercury’s polar regions, comets, Saturn’s rings, and it’s a key ingredient in the composition of many of the moons of the giant planets. One of the iciest is Jupiter’s moon Europa, which is visible in little more than a pair of binoculars when it’s suitably positioned. Being nearly five times farther from the sun than Earth and without an atmosphere, the temperature on Europa’s surface ranges from -260 F at the equator to -370 F at the poles. Ice there is as hard as granite.
Europa, like most moons and planets, is built of concentric layers like a candy jawbreaker. At center is a core of iron, wrapped in thick shell of rock and covered by an icy crust tens of miles deep. Tucked between the rock and icy surface there is excellent evidence for a salty ocean some 60 miles deep. You wouldn’t think liquid water possible in such extreme cold, but thanks to something called tidal heating, the interior of Europa basks in toasty warmth.
Jupiter is more than 300 times more massive than Earth, and the innermost moons Io and Europa feel its fierce gravitational tug most strongly. Jupiter’s gravity combined with the moons’ elliptical orbits causes them to flex and stretch as they orbit the giant planet. Io’s rocky surface bulges up and down up to 328 feet during its 1.8 day orbit!
This ‘tidal flexing’ heats Io’s interior to melting the same way that kneading dough over and over warms it up. Molten rock inside the moon vents to the surface in a dazzling array of sulfur-spewing volcanoes, making Io the most volcanically active body in the solar system.
The same type of flexing happens to Europa though to a lesser degree, heating its interior enough to melt the thick inner shell of ice to liquid. Water at Europa’s surface is frozen into gigantic blocks and plates of jumbled ice resembling Arctic pack ice. Looking at the photo, it’s obvious that some of the blocks have broken apart and then “rafted” into new positions only to refreeze.
This week scientists studying data from NASA’s Galileo probe have discovered what appears to be a body of liquid water as big as Lake Superior locked inside the icy shell of Jupiter’s moon Europa.
Focusing on bumpy, roughly circular patches of crust called chaos terrains, scientists found evidence of the ice layer melting all the way to the surface possibly from heat and water transferred by a lake from below. Their conclusions are based on studies of the behavior of Earth’s ice shelves and glaciers.
What’s exciting about this prospect is that Europian lakes may open a portal to transferring materials and solar energy back and forth from the surface to the interior.
“One opinion in the scientific community has been if the ice shell is thick, that’s bad for biology. That might mean the surface isn’t communicating with the underlying ocean,” said University of Texas researcher Britney Schmidt, lead author of the recent Nature paper on the topic. A ‘lakes link’ makes prospects are brighter for potential alien life in Europa’s dark ocean. Sulfates from Io as well as oxygen and minerals from the surface could mix with the waters of the salty lake providing essential nutrients and energy sources for hardy microbes.
Thera Macula, under which the lake is suspected, is a vast depression of chaotic terrain 1,300 feet lower than the surrounding surface. Blocks of ice inside appear to have been broken off the edges of the chaos region, while curved fractures along Thera’s perimeter hint that collapse may have been involved in its formation.
Heat or warmer water currents from the hidden lake water may have warmed the surface enough to create fractures, causing the ice to collapse and mix with the water below. While it may not be have been quite the “warm little pond” Darwin envisioned, – a reference he made to the origin of life - Thera Macula and its ilk may provide a way for potential life to sustain itself.