'Gardened Zones' on Europa Could Be the Key to Finding Life, Study Says

When it comes to the search for alien life within our own solar system, few worlds are more tantalizing than Jupiter’s moon Europa. This small moon contains a voluminous ocean of liquid water under its icy crust that could potentially host extraterrestrial organisms, which is why NASA and the European Space Agency (ESA) are developing space probes that will get a closer look at it.

But as evidence builds that Europa could be habitable under its crust, a problem remains: the intense radiation that Jupiter emits likely annihilates any signs of life, known as biosignatures, that upwell onto the moon’s surface, presenting a challenge to future missions that aim to detect life with Europa landers.

Now, a team of researchers led by Emily Costello, a postdoctoral researcher at the Hawaiʻi Institute of Geophysics and Planetology, have shed new light on this obstacle by examining the role of “impact gardening” in the search for life on Europa. Impact gardening occurs when rocks collide with a planetary body without an atmosphere, causing a mechanical churn that continually exposes new layers of the surface, known as the “gardened zone,” to all the erosive effects of space, according to a study published on Monday in Nature Astronomy.

“A key challenge in the search for signs of life on Europa is to identify the most likely places in which to discover biosignatures that have not been destroyed by surface exposure,” Costello and her colleagues said in their study.

“Knowing the depth of the gardened zone is critical for the exploration of Europa as a potentially habitable world,” the researchers added. “We will need to sample material below the gardened zone if we wish to discover biomolecules that have never been exposed to hazardous radiation at the surface.”

Of course, that raises the question: just how deep is Europa’s gardened zone? To provide an answer, the team produced the first comprehensive models of impact gardening on Europa, with the help of Moon rocks returned from the Apollo program that also show a distinct gardened zone.

This approach yielded good news and bad news. The bad news is that the models suggest that impact gardening exposes the top 30 centimeters (12 inches) of Europa’s global surface to radiation, on average. Contrary to previous studies that proposed the possible presence of juicy biosignatures only a few centimeters under the moon’s surface, the new study finds that signs of life would be embedded much deeper in the ice.

“Even at higher latitudes, at which biosignatures may be preserved against radiation at centimetre depths, impact gardening cycles material upwards to the surface and may thwart the hopes of preservation and sampling at shallow depths,” Costello and her colleagues said in the study.

That said, the good news is that pristine material from Europa’s ocean could be sampled at shallower depths in rare circumstances, such as in the fallout of recent landslides or fresh meteorite impacts. These natural processes can excavate layers of ice from below the gardened zone and position them within centimeters of the surface. Looking for recent examples of such disturbances could reveal samples that have not experienced the damaging long-term effects of radiation yet.

“These regions should be targets for exploration” and “preferred locations for the search for life on Europa,” the team said. 

Fortunately, scientists will soon benefit from close-up observations of Europa from ESA’s Jupiter Icy Worlds Explorer (JUICE) and NASA’s Europa Clipper, both scheduled to launch in the 2020s. These spacecraft will conduct intimate flybys of Europa, and they may be able to spot regions with freshly excavated material on the surface that would be prime destinations for future lander missions.