When we consider if Europa is suitable for life, there are so many questions about what the heck could survive the harsh environment near Jupiter. The moon only has a tenuous atmosphere. It's baked in radiation. If there is an ocean underneath its icy shell, there could be a very thick barrier (miles) between surface and liquid. But how are we going to know unless we go there?MORE: How Tiny Satellites are Invading the Solar System
Last October and following a competition
, NASA invited 10 universities to submit feasibility tiny-satellite studies to hitchhike with its proposed Europa mission that would launch in the 2020s. The universities are now working hard on their concepts for CubeSats, tiny satellites that so far have only been used in low Earth orbit. This is what five of those universities have so far. Bear in mind the concepts are preliminary and may not necessarily be selected by NASA.
NASA/JPL/Johns Hopkins University Applied Physics Laboratory
Defying the "2010: Odyssey Two" movie command not to land at Europa, this concept would measure the radiation and dust as observed about 62 miles (100 kilometers) above the surface. It will last about 20 hours, taking a short of shallow path before making a grazing impact with the surface. The dust it sees forms a sort-of survey of the surface since presumably, the dust would be kicked up there by meteorites hitting the ground.MORE: Cubesats May Hitchhike on Mission to Europa
While planetary protection guidelines will be considered for this, graduate student Siddarth Krishnamoorthy (at Stanford University) said this won't pose much of a threat. "It's small enough and it’s going fast, but it doesn’t have the energy to blow through a very thick ice crust."
Image: A cloud of charged particles surrounding Jupiter in this picture from the Cassini spacecraft, showing wavelengths invisible to the human eye.
There's still lots to learn about Europa's environment, and this proposal plans to do as much as possible in a short mission. The CubeSat will "aerobrake" by butting against Europa's thin atmosphere, allowing it to slow down for a little more time to gather information. It also will include an energetic ion detector on board to see how charged particles behave near the surface.MORE: NASA Releases 'Remastered' View of Enigmatic Europa
"One of of team members has been involved in some of the aerobraking effort at Mars; he brought that expertise to bear," said Nancy Chanover, an astronomer and the mission's principal investigator at New Mexico State University.
Image: Europa seen from above by the Voyager 2 spacecraft.
NASA/JPL/University of Arizona
In a daring twist for CubeSats, this proposal would have one land on the surface and do simple measurements there. Previous landers on other solar system bodies were more complex. This one would use a new, miniature instrument called a "nanowire" to do a preliminary look at the surface for other missions.MORE: Diving on Europa? Here's How We Could Penetrate That Ice
"As shown by the Rosetta/Philae lander, landing on a comet or a small asteroid is extremely difficult and risky," wrote principal investigator Joseph Wang, an astronautical engineer at the University of Southern California, in an e-mail to Discovery News. "If the miniature instrument ... proves to be successful, then we would be able to design a CubeSat based 'nanolander' and landing methods that are entirely different from what have been used."
Image: A portion of Europa's icy, cracked surface seen in close-up (left).
Europa's magnetic field provides a back door to take a look at the ocean. By looking at fluctuations in the field as Europa moves around Jupiter, researchers get a sense of how deep and salty the ocean is. That's because these properties play into how magnetic fields are conducted. "The use of smaller satellites presents an interesting alternative to traditional spacecraft which have visited the icy moons of Jupiter and Saturn," wrote Casey Steuer, a graduate student who studies space instruments at the University of Michigan. (The principal science investigator is faculty member Xianzhe Jia.)MORE: Tiny Robotic Cubes Could Rule the Solar System
"The expansive instrument suites on-board the Cassini and Galileo missions have, without a doubt, blown open the door for planetary scientists in a number of areas. However, these icy moons are begging to be investigated with orbiter's and landers, and not just during flyby missions."
Image: A crescent view of Europa seen by Voyager 2.
While this university has not decided what to do yet, they know that they want to make their mission very complementary to the work of the parent Europa spacecraft. "Radiation hardening" (a specialty of the university) of the electronics will also be included, to let them survive in the harsh environment. But there's no final decision on what science will be performed or what instruments will be included -- many kinds are being considered. "The interaction of Europa as a body with the Jupiter magnetic field is quite different," said Jekan Thanga, an assistant professor who is leading the Arizona State University team. "Europa does not have a magnetic field, but it induces one. The water could have an effect, too There's a lot of description about that."MORE: NASA's Europa Mission to Hunt Down Life's Niches
Image: Artist's impression of NASA's Europa Clipper spacecraft at Europa.
If alien life swims in the ocean beneath Europa's icy surface, it might have got its start from comets cracking the icy shell to deliver vital pre-life ingredients, say researchers.
New simulations show that a specific family of comets have the mass, velocity and opportunity to do the job -- penetrating the full range of likely Europan ice thicknesses.
“It's one of the best candidates for an ecosystem,” said Rónadh Cox of Williams College, Mass., regarding Europa's ocean. “But how do you get biological precursers into the ocean?”
To find out if icy, chemical-rich comets could do it, she and her student, Aaron Bauer, modeled impacts by the full range of comets that have been influenced by Jupiter, and brought into relatively short orbits around the sun -- so-called Jupiter Family Comets. They simulated the collisions of these known comets into a range of ice thicknesses. Their results were surprising and led to a new insights into the moon's actual ice crust thickness.
“It turns out it doesn't matter how thick the crust is,” said Cox, the lead author of a paper about the research in the latest issue of Journal of Geophysical Research-Planets. That's because it's all a matter of the frequency of impacts over the very long window of opportunity -- in this case the almost five billion-years that the solar system has been around.
If Europa's ice, for instance, is at the very thick end of the spectrum -- 40 kilometers (25 miles) deep -- it would require a 5 to 7-kilometer (3-5 mile) diameter comet to breach it, she said.
They found that the odds of that happening with a Jupiter Family Comet is once in 100 million years. That's a virtual certainty over five billion years.
The odds of getting through the ice get a lot better if the ice is thinner. Thinner ice would allow the more numerous smaller comets to break through the ice once every 10 million years, Cox explained.
“It means the crust is penetrated frequently in geological time,” said Cox.
It also means that the two dozen impact craters seen on Europa today can be compared to the simulated impact craters to see what size their impactors were. Those that left craters on Europa are failures, because if they had breached the ice they would have probably been flooded with fresh ice. So they provide a clue about how the thick the ice really is.
“We got the best match if ice was 10 to 15 kilometers (6 to 9 miles) thick,” said Cox. This ice thickness agrees with other studies that estimate Europa's ice thickness by entirely different methods, she said.
Could the surface features in Europa's crust reveal where comets have impacted and penetrated through the ice?NASA/JPL-Caltech/SETI Institute
“I think that this is the most complete and careful work yet on the relation between the Europan ice shell thickness and the size and depth of impact craters on its surface,” said Purdue University's Jay Melosh, an expert on planetary impacts. “I am particularly impressed that the authors can match the observed crater depth-diameter relation over the entire range of crater sizes on Europa and get an excellent fit for a 10 kilometer (6.2 mile) thick ice shell. There has been a long-standing controversy over whether the ice on Europa is thin (that is, 7 to 15 km) or thick (around 40 km).”
As for those comets that broke the ice, Cox says the evidence for those is there on Europa's surface as well, although it's harder to be certain, as there are a lot more analogues for studying impact craters than for ice-penetrating impacts.
“If impactors are going through the ice there has to be a scar,” Cox said. “Geomorphically, we don't know what that is. What do those look like now that they are frozen?”
They might look like the patchy, complicated Europan icescape called chaos terrain. Very similar looking surface patterns have been created by re-freezing of ice after explosives have been used to break through sea ice on Earth, Cox said. And that's about the only analogue she has seen for what has happened on Europa.
“We advocate that these are sites of impactors that penetrated the ice,” said Cox.
If so, and the ice is on the thin side, it's good news for life on Europa and our chances of finding it.
“Cox and Bauer’s paper strongly supports the thin-shell side,” said Melosh. “This is good news for the astrobiology community because it means that exchange of material between the surface and underlying ocean is relatively easy, so that nutrients for a putative Europan biosphere can get in and samples of that biosphere may be ejected to the surface, within reach of future sample return missions.”
Note: Independent science writer Larry O’Hanlon is also the blogs and social media manager for the American Geophysical Union, which publishes Journal of Geophysical Research-Planets.