Planetary scientists are studying the cumulative effects of small impacts on the surface of Jupiter’s moon Europa as they prepare to explore the icy moon with NASA’s Europa Clipper mission.
The surface of Europa looms large in this newly-reprocessed color view; image scale is 1.6 km per pixel; north on Europa is at right. Image credit: NASA / JPL-Caltech / SETI Institute.
“Jupiter’s moon Europa has perhaps the highest potential of any extraterrestrial body in our Solar System to host extant biological life,” said Dr. Emily Costello, a planetary scientist at the University of Hawaii at Manoa, and her colleagues.
“A subsurface ocean with uniquely hospitable chemistry may be communicating material to the surface through plume activity or through other forms of upwelling.”
“If life exists in Europa’s ocean, evidence may be available for discovery at the surface.”
“However, because Europa’s orbit is within Jupiter’s magnetosphere, biosignatures that are exposed to space will be destroyed by high-energy electron radiation.”
“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.”
In the new research, they estimated how far down the surface of Europa is disturbed by ‘impact gardening,’ the process by which small impacts churn the surface of airless planetary bodies.
“Impact gardening is the process by which frequent small impacts stochastically churn the uppermost regolith,” they explained.
“The physical and chemical effects of gardening are observable in the Apollo drill cores that were returned from Earth’s Moon.”
“The surfaces of other airless solar system bodies, such as Europa, must be similar with respect to gardening.”
“Although the thermal and ionic agents that effect change at the surface are different from those on the Moon, impact gardening on Europa exposes material that was previously out of reach of radiation.”
And, if it is sufficiently vigorous, gardening may be more important than radiation-penetration depth in controlling the depth distribution of surface-correlated irradiated material.”
“Knowing the depth of the gardened zone is critical for the exploration of Europa as a potentially habitable world.”
This image of an area called Chaos Transition shows blocks that have moved and ridges possibly related to how the crust fractures from the force of Jupiter’s gravity. Image credit: NASA / JPL-Caltech / SETI Institute.
The team estimates that the surface of Europa has been churned by small impacts to an average depth of about 30 cm (12 inches) over tens of millions of years.
And any molecules that might qualify as potential biosignatures, which include chemical signs of life, could be affected at that depth.
“If we hope to find pristine, chemical biosignatures, we will have to look below the zone where impacts have been gardening,” Dr. Costello said.
“Chemical biosignatures in areas shallower than that zone may have been exposed to destructive radiation.”
“This work broadens our understanding of the fundamental processes on surfaces across the Solar System,” said co-author Dr. Cynthia Phillips, a Europa scientist at NASA’s Jet Propulsion Laboratory.
“If we want to understand the physical characteristics and how planets in general evolve, we need to understand the role impact gardening has in reshaping them.”
The team’s results were published this week in the journal Nature Astronomy.
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E.S. Costello et al. Impact gardening on Europa and repercussions for possible biosignatures. Nat Astron, published online July 12, 2021; doi: 10.1038/s41550-021-01393-1
