More than 100 geysers spray water into space from cracks in the icy surface of Enceladus, one of Saturn’s moons. A large subsurface ocean supplies these jets with water ice and organic compounds, including amino acids—critical elements of life-forms we might recognize. A new study shows there is hope for collecting these delicate molecules on a future mission that could fly through Enceladus’s icy plumes.
Amino acids might provide a clear signature of life—extinct or alive—on the planetary body. These molecules are chiral—they cannot be superimposed on their mirror image—and can form in either a right- or left-handed pattern. On Earth, life has evolved to use purely left-handed amino acids. Scientists believe living things elsewhere, including on Enceladus, would also leave traces of a preferred handedness, or chirality, in their environment.
“We’ve found plenty of ice in comets and asteroids, and we’ve seen evidence of water on Mars, but this could be the very first time we actually reach out and touch it,” said Anna Butterworth, a planetary scientist at the University of California, Berkeley, and lead author of the study.
However, gathering delicate organic samples with a spacecraft poses a steep challenge. Scientists have worried that current collection methods would fundamentally alter the amino acids, damaging hopes of finding life’s chemical imprints.
A mission might be decades away, but new research with a high-velocity laboratory gun suggests the chirality would, in fact, survive. Butterworth and her colleagues will present their research on 14 December at AGU’s Fall Meeting 2022.
Blasting Ice from a Gun, for Science
The ethereal plumes provide a unique opportunity to study the moon’s salty subsurface water without needing to drill through the thick crust of ice that encases the ocean world. NASA’s Cassini mission first discovered them in 2005, prompting planetary scientists to imagine ways to study the fascinating plumes more closely.
Amino acids are an enticing biosignature jackpot, but their chirality is fragile. Extreme heat and pressure can erase the natural handedness of the amino acids, leaving a 50:50 distribution representative of lifeless chemistry. A spacecraft sent to fetch Enceladus’s amino acids would likely gather molecules that strike collectors at high speed.
To test sampling methods that could preserve those signatures despite these intense impacts, Butterworth and her colleagues turned to a high-speed gas gun at the University of Kent in the United Kingdom.
The researchers fired bullets of ice laced with mixtures of the amino acids alanine and serine at speeds up to 2 kilometers per second at thin foils of aluminum and indium, simulating the conditions of a future fly-through mission. At such high speeds, the ice bullets shattered into plumelike jets and collided with the foils in blasts of tiny particles.
Using a method called capillary electrophoresis, a biochemical technique that separates microscopic molecules, the team measured the ratios of left- and right-handed amino acids in the collision debris left on the foils.
The results clearly showed that the amino acids in the ice bullets preserved their chirality after the smashups—even at velocities greater than that of typical satellite missions.
“The high-speed impacts allow for lots of room for mission planning and design,” said Butterworth. “We can be confident that if there is this biosignature, we’ll be able to detect it. We wouldn’t destroy it by just the energy of the impact.”
The metals used for the gas gun targets are soft enough to collect the particles without damaging their chemical structures. These results were significant for Christopher Glein, a geochemist at Southwest Research Institute in San Antonio, Texas, and an expert on Enceladus’s ocean chemistry.
“[The researchers’] findings are surprising, and that’s what makes it exciting,” Glein said. “These are ultralow concentrations of amino acids, and [their detection] is extremely impressive.”
Next, the team hopes to expand into an even broader set of amino acids and impact velocities, Butterworth said. They also want to experiment with different geometries of the collecting sheets, allowing for even more flexibility with mission design.
If a future spacecraft swoops through wisps of chiral amino acids from Enceladus, it could demonstrate the capabilities for life, past or present, beyond our pale blue dot.
—Isabel Swafford (@Isabel_Swafford), Science Writer