Jupiter’s moon Europa is almost five times as far away from the sun as Earth is, with surface temperatures that don’t rise above minus 260 degrees Fahrenheit. It’s slightly smaller than our moon and orbits but 400,000 miles from the solar system’s largest planet, which it takes but 3.5 Earth days to orbit. As a result it is tidally locked, always showing the same face to Jupiter.
When it comes to potentially habitable objects in our solar system, Europa would not seem to be a terribly likely possibility.
But, of course, it is. And in three years NASA’s Europa Clipper mission will launch to explore what would appear to be one of the most unlikely yet possible places in our solar system to find potential signs of life.
The reason why is that scientists are almost certain that under Europa ‘s 10-to 15 mile ice covering is a deep, global ocean of salty water.
The size of the ocean has not been well determined yet, with estimates of between 40 and 100 miles of depth. But a consensus has been reached that the ocean is likely to be global, and contains two to three times as much liquid water as found on Earth.
This then raises a question with great significance for Europa, other moons in the solar system and quite likely planets and moons well beyond us: How can there be so much liquid water inside such frigid places?
There are numerous possible answers to that question, and it’s likely that all or most played some role. These hypotheses include the long-ago presence inside Europa of radioactive elements — such as uranium, thorium and relatively short-lived radioactive aluminum-26 and iron-60 — that gave off enormous heat. Even greater heat also produced by the tidal friction generated by gravitational tugs between Europa and Jupiter and other nearby moons. Scientists infer that temperatures had to rise above 2750 degrees Fahrenheit, the melting point of iron, because of the presence of an iron-nickel core.
But heat alone does not an ocean make.
Recent papers in the journal Geophysical Research Letters offer two more explanations for how the Europan ocean formed and remains liquid. They assume the thermal pathways of intense radioactive and gravitational heating but also focus on volcanoes and the metamorphic process through which rocks — when they are exposed to extremely high temperatures and high pressures — change and lose the water, carbon dioxide and other “volatile” elements inside them.
“We know from studies on Earth that minerals lose their volatiles at certain temperatures, and we know indirectly that the interior of Europa reached those temperatures,” said Mohit Melwani Daswani, a research scientist at NASA’s Jet Propulsion Lab and lead author of the metamorphic Europa paper.
“Since we also know the amount of those volatiles in the original material that formed Europa — the building blocks that sometimes come to us as meteorites — we can then calculate how much water or carbon dioxide was released when the heating was at its strongest.”
“And our analysis is that more than enough water was released to form Europa’s global ocean, and that very large amounts of carbon dioxide were released as well.”
Melwani Daswani said he and colleagues are also studying whether and how the heated water associated with the metamorphic process in interior Europa may be playing a role in keeping the ocean liquid.
While the waters below the icy surface in the presumed Europa global ocean are thought to be generally tranquil, the forces playing on the moon are definitely not. How could they be when Jupiter has 318 times as much matter as the Earth (its mass) and is so close?
Of special importance is that Europa’s orbit is elliptical (slightly stretched out from circular) and so its distance from Jupiter varies, with the moon’s near side experiencing Jupiter’s gravity more strongly than its far side. The magnitude of this difference changes as Europa orbits, creating additional tides that stretch and relax the moon’s surface.
That stretching and flexing creates enormous heat, enough to produce a molten core of iron and nickel, as inferred from previous findings of the Galileo mission. And that in turn melts metamorphic rocks, as the latest Geophysical Research paper describes.
An earlier paper in Geophysical Research Letters took the heat story further by theorizing that volcanoes may well erupt at the seafloor of Europa’s ocean. The interior of Europa is made up largely of silicates and they expand and lose their water and gases under extreme heat.
That process may still be going on, said study lead author Marie Běhounková, of Charles University in the Czech Republic, and may help explain why the global ocean appears to have remained liquid or mushy for billions of years.
Such a ready heat source, if present as a volcano or a hydrothermal vent, could provide the energy that Europa would need to support life — as happens on Earth.
“Our findings provide additional evidence that Europa’s subsurface ocean may be an environment suitable for the emergence of life,” Běhounková said in a statement.
“Europa is one of the rare planetary bodies that might have maintained volcanic activity over billions of years, and possibly the only one beyond Earth that has large water reservoirs and a long-lived source of energy.”
Běhounková and her colleagues modeled in detail how Europa’s interior stretches and flexes as the moon is tugged by Jupiter’s powerful gravity. But one of the co-authors, Melwani Daswani again, said the team did not analyze what he considers to be an important caveat: That the physics of the ocean could keep some of the volcanoes from ever erupting. They would instead remain, he said, as concentrations of super-hot rock below the sea floor.
Interest in Europa has been strong ever since the twin Voyager missions reached it in 1979 and took the first flyby photos that showed it to be ice-covered and missing the many asteroid craters found on almost all other moons and rocky planets. That told scientists that some type of geological process was erasing the craters, and that subsurface water might be involved.
The scientific interest flowing from this new view of Europa was popularized in 1982 by science fiction writer par excellence Arthur C. Clarke in “2010: Odyssey Two.” In that book, astronauts setting out to explore the solar system are famously told “All these worlds are yours except Europa. Attempt no landing there.” Clarke often said he considered Europa to be the most likely object in our solar system to harbor extraterrestrial life, and many scientists have agreed.
It took four decades, but now there is a NASA mission to Europa scheduled for 2024 and designed, in the words of JPL astrobiologist and geophysicist Steven Vance, “to investigate and understand Europa’s habitability.”
When it reaches the neighborhood of Jupiter around 2030, the Europa Clipper will orbit the planet and conduct 45 flybys of the moon, some as close as 25 kilometers (16 miles) above its surface. On board will be ground-penetrating radar, an infrared spectrometer, magnetometers, and high-resolution imagers.
Vance, who is NASA co-lead of the Europa science teams habitability assessment group, said to understand whether Europa is habitable will require a firm confirmation that the ocean is present, to characterize the size and constituents of the ocean, and “especially to understand the sources of physical and chemical energy that might be present and how they might support life.”
That energy will be in the form of heat and also in the nature of the “redox flux” — how molecules give up and received energy-carrying electrons — that scientists hope to measure. Global fluxes of oxygen and hydrogen provide a measure of chemical disequilibria, a key requirement for life. Both the theorized metamorphic processes on Europa and its possible vulcanism on the seafloor would both release the molecules that would form the redox ladder.
“If we find lots of hydrogen and methane generated at the sea floor, we could then estimate the chemical gradient, the redox flux, in the ocean,” Vance said. “That would tell us a lot about habitability and about what types of metabolisms could be supported.”
NASA isn’t the only space agency planning to fly to Europa.
The European Space Agency (ESA) is also planning to launch a mission to the moons of Jupiter in 2022. Called JUICE (the Jupiter Icy Moons Explorer,) it would arrive at the Jupiter system in 2030. JUICE will carry instruments similar to those planned for the Europa Clipper mission, but JUICE is not designed to study Europa along. Its mission includes a detailed study of Jupiter itself as well as Europa’s two sister moons, Callisto and Ganymede.
Both Callisto and Ganymede are now thought to also have subterranean salty oceans, though perhaps not as likely to support life as Europa’s. These missions all reflect the realization in recent decades that many bodies in the solar system and beyond may well have these sub-surface oceans.