Discovery of 200-km-high (124-mi.) ice geysers above the southern hemisphere of Europa has raised hopes that a flyby mission already in the works may raise the near-term chances of finding life in the global ocean beneath the frozen surface of Jupiter's big moon.
Scientists have long believed that Europa's ocean is one of the few places in the Solar System where life might have evolved, but mission-concept studies to date have focused on penetrating kilometers of ice to find out. Discovery of the geysers by astronomers using the Hubble Space Telescope offers another option.
“If we can actually fly at 10 kilometers above Europa through a plume, we're sampling the subsurface oceans, and if we bring something like the organic analyzers that are built by the mass spectrometry group that built the sample analysis at Mars, and detect organic molecules, that would be pretty phenomenal,” says John Grunsfeld, an astronomer,associate administrator for science and three-time Hubble-servicing astronaut.
Europa has long had a congressional champion in Rep. John Culbertson (R-Texas), who uses his seat on the House Appropriations Committee to funnel funds—$80 million this year and a like amount the year before—for advance work on a “Europa Clipper” mission to the icy moon. With the geyser's discovery, Jim Green, the Planetary Science Division director atheadquarters, has ordered additional studies at the to determine just what instruments might be added to Europa Clipper to take advantage of them.
“Immediately, I asked them to re-look at the payload to determine that they can take advantage of the plumes,” Green says. “They've already seen from the orbit trajectory that they can fly through the plumes, that it is indeed possible and would be planned, and therefore we want to make sure we have the right instrumentation.”
Scientists also want to make sure they actually have seen water. The original observation, announced in a paper published by Science on Dec. 12, 2013, detected very faint ultraviolet emissions of hydrogen and oxygen above the moon's south pole (see illustration) that scientists interpreted as water plumes erupting through cracks in the ice (AW&ST Jan. 13, p. 44).
“We pushed Hubble to its limits to see this very faint emission,” says Joachim Saur of the University of Cologne, one of the Science article coauthors. “These could be stealth plumes, because they might be tenuous and difficult to observe in the visible light.”
Additional observations are in the works, tricky because the intermittent aurora-like emissions are linked to Jupiter's magnetic field and must be observed through the same ultraviolet signature generated by the magnetic field of Earth, Green says.
The Cassini mission has repeatedly imaged ice geysers at the south pole of Saturn's moon Enceladus (AW&ST July 4, 2011, p. 19), but Europa is a much better candidate for life because of its size—3,100 km dia., or about the same as Earth's Moon. By comparison, Enceladus measures only 500 km in diameter, and the northern hemisphere is cratered. That suggests the ice spewing from the “Tiger Stripes” cracks at the opposite pole “geologically may be a recent event,” says Green.
“Europa's probably been like that for 5 billion years, with a lot of opportunity for evolution not only of the body geologically but . . . even the potential evolution of life,” he says.
Still to be determined is precisely what lies beneath Europa's icy surface. Although planetary scientists agree that the uncratered surface indicates Europa is geologically active and the ice covers an ocean kept liquid by the heat of tidal squeezing from its massive planet's gravity, they are divided over how thick the ice is. Until the discovery of the water plumes, which appear to be part of the “communication” between the liquid ocean and the frozen surface that also includes blocky “chaos” regions, the Europa instrumentation studies focused on penetrating radars to solve the thickness question.
Also of interest are instruments that can analyze the brownish “gunk” that coats Europa's surface in different terrains and evaluate the best way through the high-radiation zone around Jupiter for an eventual robotic landing. One solution may be small cubesat-like spacecraft that could be dropped on the surface, taking radiation measurements all the way down and relaying them to Earth through the primary spacecraft.
Sending a robotic spacecraft through the water plumes above the ice would allow scientists to examine the subsurface ocean without drilling through the ice.
“We would want to have some really sensitive mass spectrometers that would be able to take the material in and then disassociate it and get its elemental composition—is it water, is it a carbon, sulfur, you know, what are the minor constituents?” says Green. “Is there ammonia in it? Those kinds of things are important to us for an understanding of what's in the ocean.”
So far, NASA has selected about a dozen potential instruments for more work, using $15 million of the funding Culbertson has made available. But it is a long way to the $4.7 billion estimate for a Europa orbiter, or the $2 billion cost to send a Europa Clipper through a series of flybys. To date, the closest to Europa scientists have sent their instruments were the 11 flybys conducted in the 1990s by the Galileo Jupiter probe, and NASA managers concede that under present circumstances, doing more will be difficult.
“All I can do is work with the community, try to do what's in the decadal [to] the best of my ability under the financial constraints and the fiscal environment that we have,” says Green, who also has an expensive Mars program to conduct. “So we're always looking for creative ways to make that happen.”