Beneath a not-so-distant world's icy, brown shell, scientists believe there is a massive ocean that could be teeming with life much like that found in the blackness of Earth's most extreme depths.
But it's not one of the possibly Earth-like planets astronomers are trying to study in the Milky Way light years away — it's in our own solar system.
Europa, one of Jupiter's largest moons, is thought to be one of the best known candidates to sustain life. While its environment is extreme relative to ours, data from several previous missions to distant planets suggests it may contain all the necessities for life: water, nutrients and a source of energy.
NASA is expected to decide next week whether to proceed with developing a roughly $2 billion mission to explore just how hospitable Europa might be. Scientists, including local teams from the Johns Hopkins Applied Physics Laboratory in Laurel, have proposed sending a craft carrying a suite of instruments to orbit Jupiter and survey the contents and depth of Europa's icy crust and ocean in the 2020s.
It could provide answers to decades-old questions about Europa, but is likely to raise new ones, too.
"The tricky thing about it is, detecting life is very, very difficult," said Curt Niebur, a program scientist working on the mission at NASA headquarters in Washington. "We don't have an instrument that's a life detector that can spit out an answer and say, 'Yes, that's alive.'"
Knowledge about Europa comes from eight spacecraft, including NASA's Voyager and Galileo missions. But only Galileo has made multiple flybys, from 1989 through 2003, and its images are grainy and data otherwise limited.
Here's what we know: Europa is 1,900 miles wide, slightly smaller than Earth's moon. Its entire surface is covered in ice and the warmest it gets is 260 degrees below zero. It is one of Jupiter's four largest moons, known as the Galilean satellites because of their discovery by Galileo Galilei in 1610. Europa orbits the gaseous giant once every 85 hours.
But the moon's other characteristics are less certain. Astronomers don't know what makes up the brown streaks and lies in cracks riddling Europa's crust, or what creates mysterious "plumes" of gaseous matter that often spew from its surface hundreds of kilometers into space.
Images from the Galileo mission suggest the shell of ice around Europa is active and shifting, possibly cycling matter from the surface down into an ocean below, Niebur said. And while scientists hypothesize that the ocean holds twice as much water as the Earth's surface does, they don't know how deep it is, nor how salty.
Jupiter's massive and powerful magnetic field also bombards Europa and the other moons with energetic particles and radiation.
"The way Jupiter works is something not familiar to the standard human experience," said Joseph Westlake, a planetary scientist at the Hopkins lab.
But scientists nonetheless believe Europa could have some similarities to Earth. Though the sun is our planet's key source of energy, some life forms in the oceans' deepest trenches instead get their energy by breaking down chemicals emitted from volcanic vents. Scientists suspect similar conditions could exist at the bottom of Europa's ocean.
Proving whether life exists on Europa is, for now, an impossible task, Niebur said.
"The only sure way is to bring a sample back to a lab on Earth," he said. "That's hideously complicated and expensive."
Instead, the Europa mission seeks to at least prove that the moon is indeed habitable, Niebur said. The mission could determine whether it's worth going back to examine it further, he added.
To answer that question, scientists have proposed sending a suite of nine instruments there to gather images and other data. NASA selected the projects last month, and top space agency officials are scheduled to issue a decision on whether to formally adopt the mission plans Thursday.
Two of the instruments are being developed at Hopkins' Applied Physics Lab, while scientists there also are collaborating on projects being led elsewhere.
Westlake is the principal investigator on the Plasma Instrument for Magnetic Sounding, or PIMS, which will help determine the thickness of the moon's ice shell and the depth and salinity of its ocean. The instrument will filter through all the noise created by Jupiter's magnetic field and a highly charged plasma cloud that surrounds the massive planet to zero in on how Europa's water and ice conduct electrical currents, and thus determine more about their characteristics.
Planetary scientist Elizabeth Turtle is principal investigator on the Europa Imaging System, a set of two cameras planned to capture high-resolution images of more than 90 percent of the moon's surface. So far, about 14 percent of Europa has been captured at a resolution of 500 meters per pixel, but the proposed mission's cameras would improve that resolution tenfold to 50 meters per pixel, and expand it to map most of the planet.
Other Hopkins researchers will help lead development of instruments using spectroscopy, which examines how light interacts with matter, and radar to study Europa's composition and crust.
But significant work remains to be done in planning the spacecraft that would make the nearly 400 million-mile journey. The mission's $2 billion estimated cost would be spread over about 20 years, with a launch date scheduled for the early 2020s and then three to seven years before the mission reaches Europa.
It would spend two years making about 45 passes by Europa, with the possibility of doubling that in an extended mission, Niebur said.
Neither NASA nor the scientists have determined a name for the mission, but they said they are excited to begin the journey.
"This has been a high priority for a long time," Niebur said. "What a thrill it is to get it this far."