A recent study by a UA scientist provides new evidence that Jupiter’s moon Europa has a thin crust of ice over a vast ocean.
NASA included money to plan a mission to Europa in its budget this week, agreeing with Europa researchers at the University of Arizona and elsewhere that Jupiter’s smallest moon is a good place to search for forms of life.
Veronica Bray, an associate staff scientist in the UA Lunar and Planetary Laboratory, began her study of Europa for her doctorate in 2004 while focusing on impact cratering of icy moons.
She was attracted to Europa because of the “gigantic controversy about how thick the ice shell is.”
The debate has carried on for many years because the overall thickness determines whether life can exist in the liquid-water ocean under Europa’s ice surface, she said. In order for life to exist, the ice crust must be thinner than 10 kilometers, or about 6 miles thick, she said.
If the crust is thicker, the ocean would not be able to receive the necessary ingredients for life such as energy and food, said Richard Greenberg, a UA professor of planetary sciences and the author of two books about Europa.
Greenberg came to the conclusion that the crust was thin in the mid-1990s when he was studying various crack patterns in Europa’s ice crust, he said.
“There’s evidence that those cracks actually penetrated to the water layer below the ice,” he said. In order for that to happen, “the crust can’t be too thick.”
That’s when “it became clear to me that the ice had to be thinner” than 10 kilometers, he said.
Subsequent observations and research backed up Greenberg’s theories, including a study last year of Hubble Space Telescope images that captured what appears to be water vapor erupting from the moon’s surface.
Bray approached the problem by comparing “hydrocode” computer simulations of comets hitting Europa’s ice crust with images of the aftermath of those impacts taken during NASA flybys. With these models, she suggests the ice is about 7 kilometers thick.
“The ice crust thickness affects what a crater looks like,” Bray said. “If you had a big asteroid impact into very thick, cold ice, you’d get this very deep crater. If you’re in shallower, warmer crust, you’ll get a shallower crater.”
Bray completed hydrocode models for about 60 craters on Europa and compared them with actual topographic profiles of Europa created by Paul Schenk, a staff scientist at the Lunar and Planetary Institute.
Schenk used data from NASA’s Galileo Spacecraft, which studied Jupiter and its moons from 1995 to 2003, to create the digital terrain models of Europa, Bray said.
By comparing her simulations to the terrain models, she created the “most suitable, most realistic computer simulation” for the overall thickness of Europa’s ice crust, she said.
“Whenever we get a simulated crater shape that is the same as reality, we can say that it’s our most realistic cratering simulation,” she said.
“I also fitted my simulations to the global trend of Europa, which means my estimate of ice thickness is globally averaged and representative of the ice thickness over the last 60 million years.” She said the thickness has changed throughout the course of Europa’s existence.
Bray said it’s possible there’s life in Europa’s ocean, but she hesitates to say it’s probable.
“Just because you have the ingredients for life doesn’t mean you’ve got it,” she said.
She said it’s also important to remember that when scientists talk about extraterrestrial life, they’re usually not talking about something with legs and a brain but rather some form of bacteria that can survive extreme conditions.
Bray said she hopes her research will spark public interest and bolster the argument for going to Europa.
“NASA won’t get the money to send a mission to Europa unless the public is interested in it,” she said.
NASA program scientist Curt Niebur said, “I’ve been working on getting a mission out to Europa for almost a decade.”
Chances are looking up, he said, with the NASA announcement that it has allocated money in next year’s budget to begin formulating a mission to the icy moon.
“If there is life beyond Earth in our solar system, Europa is the best place to find it,” Niebur said.
According to Robert Pappalardo, a senior research scientist at NASA’s Jet Propulsion Laboratory, “it’s a big deal for Europa that NASA is acknowledging the priority of Europa exploration.” Pappalardo is the study scientist for Europa mission concepts at JPL, which means that he’s been looking at various options for Europa missions.
He’s responsible for “ensuring that the mission concepts can achieve the (goals) that the science community wants to achieve.”
If a mission does take place, Pappalardo would become the project scientist for the mission.
The main goal for a mission would be to “understand whether Europa is a habitable environment,” he said. “If we found another example of life in our solar system, it would revolutionize our understanding of biology and the chemistry of life.”
Niebur said Bray’s “work will have an incredibly huge impact” on a Europa mission. Bray “is a top-notch scientist.”
“The thickness of the ice will determine what instruments we bring to Europa,” he said.
Bray said, “Europa has the capability for more interesting, more developed life than Mars does.”
Greenberg said he believes it’s possible that complex life exists in Europa’s ocean.
“Most people who talk about life on Europa say, ‘Well if there’s life, it will be very simple,’” he said. “But that’s not necessarily true — especially if there’s a lot of oxygen in the ocean — then you have the possibility of more complex forms of life.”
Greenberg advocates sending a lander to Europa.
“If we could put something on the surface of Europa, there’s a good chance we’d find some really dramatic stuff because there’s so many direct connections between the ocean and the surface,” he said.