Hiding from Jupiter’s Radiation

The radiation belts around Jupiter as seen by NASA’s Cassini spacecraft, as it flew by in January 2001. The three radar images show changes in the belts over the course of Jupiter’s 10-hour rotation.
Credit: NASA/JPL

It’s dangerous to remain too long inside the radiation belts of Jupiter. The high-energy particles can damage space probes, and they also can destroy biological molecules or other signatures of life that might exist on inner moons like Europa. A new study plans to determine just how hazardous an impact the radiation belts have on the Jovian system.

"Our goal is to find some areas that might be interesting for a future mission to explore," says Wes Patterson of Johns Hopkins University’s Applied Physics Laboratory.

Patterson and his colleagues are building a detailed map of the surface of Europa and another map of its sister moon Ganymede. The project—led by Louise Prockter of John Hopkins University as part of NASA’s Exobiology and Evolutionary Biology program—will identify dead zones where radiation would likely fry any interesting chemical compounds, as well as possible safe havens that might harbor material expelled from the ocean below.

The work should provide targets for follow-up characterization by the next mission to Europa, which will probably be an orbiter. However, Patterson says it will be hard to directly address the question of life from orbit, so he and his colleagues envision their radiation maps guiding subsequent missions that will presumably land on the surface.

"This should help ensure that we get the best bang for the buck from a lander," Patterson says.

Letting the ocean come to us

A "family portrait" of Jupiter with four of its moons. From top to bottom: Io, Europa, Ganymede and Callisto.
Credit: NASA.

It is presumed that both Europa and Ganymede have oceans beneath their icy crusts. Europa is the more interesting of the two because there are indications that the ocean occasionally wells up from below and washes over the surface. This connection between the underlying ocean and the surface makes the moon a more geologically and chemically dynamic place, and therefore more likely to harbor life.

"In the near future there is very little chance we will get to the ocean, but geology tells us that there are pathways of communication between the surface and the ocean," Patterson says.

These potential pathways appear on the Europan surface as ridges and coarse patches of terrain. The ridges may be fractures in the ice that extend down to the ocean below. The coarse patches (called chaos) may be where the ice has melted through, or where convection is bringing up warm ice that once had contact with the ocean below.

The ice surrounding some of these geologic features appears to have its own unique chemical signatures.

The surface of Europa shows signs of cracks that could allow material to pass to and fro between the ocean below and the irradiated surface.
Credit: Calvin J. Hamilton, Voyager Project, JPL, NASA

"This material, if from the ocean, may contain molecular evidence for life in the ocean," says Bob Carlson from the Jet Propulsion Laboratory, who is not a part of this project.

Of particular interest are molecules—such as hydrogen sulfide, methane and formaldehyde – that provide energy for organisms living in Earth’s extreme environments. These same nutrients may play a role in Europa’s deep cold ocean.

Radiation menace

Assuming the ocean contains biologically-useful compounds and that some of these find their way to the surface, they may not survive very long there.

"It is unlikely that any big molecules will be found in exposed regions," says Chris Paranicas, also from Johns Hopkins University.

Radiation in the form of high-energy electrons and ions continuously bombard the top layers of Europa’s icy crust. This deadly dose is due to the fact that Europa—along with the three other Galilean moons (Io, Ganymede and Callisto)—orbits within Jupiter’s radiation belts.

These belts are much like Earth’s Van Allen belts but bigger, since Jupiter’s magnetic field is ten times stronger than Earth’s. Electrons and ions from the solar wind become trapped in the magnetic field and spiral down onto Jupiter’s poles to create impressive auroras.

The radiation in Jupiter’s belts is a million times more intense than in Earth’s belts. For this reason, spacecraft—such as the Galileo orbiter—have typically tried to spend as little time as possible inside the belts. Although the radiation is generally well-understood, no one has yet figured out precisely what the effect is on Jupiter’s moons.

An aurora on Jupiter’s seen in UV light by the Hubble Space Telescope.
Credit: John T. Clarke/Univ. of Michigan, ESA, NASA

"We want to give the radiation ‘weather’ on the surface," Paranicas says.

The researchers will incorporate data from the Voyager and Galileo missions in order to estimate how the radiation dose varies across the icy outer layers of Europa and Ganymede. Neither moon has much of an atmosphere, so particles stream down to the surface unimpeded and barrel into the ice.

The ions from the belt can penetrate about a millimeter into the ice. Electrons reach roughly a centimeter down, but they also emit high-energy photons that can go as far as a meter deep. Regardless of the type of radiation, these high-energy particles will rip electrons off of molecules, thereby "oxidizing" everything on the surface.

This chemical destruction ironically could be vital to any potential life below. The ocean is believed to contain little to no dissolved oxygen (known as a "reducing environment"). In such an oxygen-poor bath, life could harvest chemical energy by combining surface oxidants with chemicals in the ocean that react with them (so-called "reductants").

For such mixing to happen, the same geological process that brings oceanic material up to the surface must also be shuttling surface material down to the ocean.

Safe havens

This drawing illustrates how flows of electrons steered by Jupiter’s magnetic field connect the upper atmosphere near Jupiter’s north and south poles to three of Jupiter’s large moons: Io (left), Europa (right) and Ganymede (center).
Credit: NASA/John Spencer, Lowell Observatory and John Clarke, Boston University

If this complicated procedure was taking place on Europa, it would be hard to verify since the radiation would destroy all evidence for it on the surface of the moon.

However, all is not lost.

The radiation belts are rotating around Jupiter faster than Europa does. This results in the radiation predominantly striking the trailing hemisphere of the moon—which is always the same portion of the moon since Europa is locked in a synchronous orbit around Jupiter. Moreover, there is a constant stream of micrometeorites landing on the leading hemisphere. These tiny rocks form a regolith layer up to 3 meters thick that could protect oceanic material from incoming radiation.

"Optimizing the search for interesting [molecular] species requires looking for young areas that aren’t heavily irradiated," Carlson says. "The leading hemisphere is still heavily bombarded and won’t provide pristine samples by itself, but some regions may provide effective topographic shielding."

One of the goals of the project is to pinpoint these protected regions. Patterson thinks the boundary between trailing and leading hemispheres may be a good target. Here, enough radiation may hit to see some of the positive effects, while enough regolith may be present to shield out most of the negative effects.