Long-time Centauri Dreams readers already know of my admiration for Richard Greenberg’s work on Europa, admirably summarized in his 2008 title Unmasking Europa: The Search for Life on Jupiter’s Ocean Moon (Copernicus). It’s a lively and challenging book, one which Greenberg used to take sharp issue with many of his colleagues, and although he played this aspect of the work down in a phone conversation when I reviewed the book, the animated back and forth makes for a fascinating look at how planetary science gets done.
In his book, Greenberg argues forcefully that the thickness of Europa’s ice is unlikely to be more than a few kilometers, and that its active resurfacing would make it possible for life-forms below the ice to occasionally be carried above it. That would be good news for our hopes of detecting life, of course, for it would obviate the need to drill through the ice sheet. A spacecraft’s electronics might not last long given radiation levels this close to Jupiter, but probably long enough to make that kind of identification, if such is indeed possible.
Image: View of a small region of the thin, disrupted, ice crust in the Conamara region of Jupiter’s moon Europa showing the interplay of surface color with ice structures. The white and blue colors outline areas that have been blanketed by a fine dust of ice particles ejected at the time of formation of the large (26 kilometer in diameter) crater Pwyll some 1000 kilometers to the south. A few small craters of less than 500 meters or 547 yards in diameter can be seen associated with these regions. These were probably formed, at the same time as the blanketing occurred, by large, intact, blocks of ice thrown up in the impact explosion that formed Pwyll. The unblanketed surface has a reddish brown color that has been painted by mineral contaminants carried and spread by water vapor released from below the crust when it was disrupted. The original color of the icy surface was probably a deep blue color seen in large areas elsewhere on the moon. The colors in this picture have been enhanced for visibility. Credit: NASA/JPL/University of Arizona.
The tortured ice surrounding a distant Europan ridge, then, could be the venue for our first discovery of non-terrestrial life. The larger question is whether the ocean floor on Europa actually provides the conditions for life. Here the answer also ties in to that active resurfacing, one that leaves few impact craters intact and suggests that what we see from a spacecraft could be no older than 50 million years or so. Greenberg notes at the ongoing Division of Planetary Sciences meeting in Puerto Rico this week that cracks on the surface continually fill with fresh ice, while surface areas already in place are gradually replaced.
Add in mechanisms for gradually adding fresh material to the surface and you’ve exhausted the possibilities for resurfacing. But they’re all Greenberg needs to estimate that the delivery rate of oxidizers into the ocean is fast, so fast that the oxygen concentration of this sub-surface ocean could exceed that of Earth’s oceans in just a few million years. The upshot: This is enough oxygen to support not just micro-organisms but larger creatures, macrofauna whose metabolisms demand more oxygen.
And this is intriguing: Greenberg argues that it would have taken a couple of billion years for the first oxygen to reach the ocean. That delay could be crucial, for early organic structures could be disrupted by oxidation. On Earth, oxygen’s late arrival allowed life to go from pre-biotic chemistry to organisms that evolved to manage oxygen’s damaging effects. The same mechanism might have allowed creatures to emerge in Europa’s ocean.
We’re talking, remember, about a global body of water, one containing about twice the liquid water of all Earth’s oceans combined. If Greenberg is right, that ocean contains a hundred times more oxygen than previously estimated, allowing roughly 3 billion kilograms of macrofauna to subsist if their need for oxygen is roughly the same as we find in terrestrial fish.
Read Unmasking Europa for a close look at the Europan surface as seen through Voyager and Galileo imagery, the latter unfortunately compromised by the failure of the spacecraft’s high-gain antenna. Thin ice is a model supported by the imagery, and Greenberg goes on to discuss it in terms of the tidal forces acting on the moon. The case for thin ice is powerful, but we need new missions to nail it down, the first of which, the Europa Jupiter System Mission, won’t arrive any earlier than 2026.