Some icy moons in the outer Solar System likely contain an ocean beneath their ice shells. Jupiter’s moon Europa is thought to have an ocean beneath its ice shell and may be especially suitable for life if radiolytic oxidants generated at its surface travel efficiently through the ice. A research team headed by University of Texas at Austin’s Dr. Marc Hesse proposes that oxidants can be transported through the ice by drainage of salt water, or brines, generated during the formation of Europa’s chaotic terrains — landscapes made up of cracks, ridges and ice blocks that cover a quarter of the icy world. These enigmatic surface features are thought to require the formation of large volumes of near-surface brine. The authors show that the brines drain through the underlying ice before re-freezing and transport oxidants in pulses of melt called porosity waves. These pulses may propagate through the ice on timescales of 20,000 years.
Artist’s concept of ocean on Jupiter’s moon Europa. Image credit: NASA / JPL-Caltech.
Europa is a top spot to look for alien life because astronomers have detected signs of oxygen and water, along with chemicals that could serve as nutrients.
However, the moon’s ice shell — which is estimated to be about 15 miles thick — serves as a barrier between water and oxygen, which is generated by sunlight and charged particles from Jupiter striking the icy surface.
If life as we know it exists in the ocean, there needs to be a way for oxygen to get to it.
“The most plausible scenario based on the available evidence is for the oxygen to be carried by salt water, or brine,” Dr. Hesse said.
Scientists think that chaos terrains form above regions where Europa’s ice shell partially melts to form brine, which can mix with oxygen from the surface.
The computer model created by Dr. Hesse and colleagues showed what happens to the brine after the formation of the chaos terrain.
The model showed the brine draining in a distinct manner, taking the form of a porosity wave that causes pores in the ice to momentarily widen — allowing the brine to pass through before sealing back up.
The researchers compare the process to the classic cartoon gag of a bulge of water making its way down a garden hose.
This mode of transport appears to be an effective way to bring oxygen through the ice, with 86% of the oxygen taken up at the surface riding the wave all the way to the ocean.
But the available data allows for a wide range of oxygen levels delivered to Europa’s ocean over its history — with estimates ranging by a factor of 10,000.
“The highest estimate would make the oxygen levels in Europa’s ocean similar to those in Earth’s oceans — which raises hope about the potential for that oxygen to support life in the hidden sea,” said Dr. Steven Vance, a researcher at NASA’s Jet Propulsion Laboratory.
“It’s enticing to think of some kind of aerobic organisms living just under the ice.”
“NASA’s upcoming 2024 Europa Clipper mission may help improve estimates for oxygen and other ingredients for life on the icy moon.”
“The study presents a compelling explanation for oxygen transport on Europa,” said Dr. Kevin Hand, also from NASA’s Jet Propulsion Laboratory.
“We know that Europa has useful compounds like oxygen on its surface, but do those make it down into the ocean below, where life can use them? In this work, the answer seems to be yes.”
The team’s results were published in the journal Geophysical Research Letters.
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Marc A. Hesse et al. Downward Oxidant Transport Through Europa’s Ice Shell by Density-Driven Brine Percolation. Geophysical Research Letters, published online February 10, 2022; doi: 10.1029/2021GL095416
