Chemistry

A Single Grain of Ice Could Hold Evidence of Life on Europa and Enceladus

The Solar System’s icy ocean moons are primary targets in our search for life. Missions to Europa and Enceladus will explore these moons from orbit, improving our understanding of them and their potential to support life. Both worlds emit plumes of water from their internal oceans, and the spacecraft sent to both worlds will examine those plumes and even sample them.

New research suggests that evidence of life in the moons’ oceans could be present in just a single grain of ice, and our spacecraft can detect it.

It’s all because of improvements to scientific instruments, particularly the mass spectrometer. Mass spectrometers can identify unknown chemical compounds by their molecular weights and can also quantify known compounds. These instruments are now powerful enough to detect a tiny amount of cellular material.

“For the first time, we have shown that even a tiny fraction of cellular material could be identified by a mass spectrometer onboard a spacecraft,” said Fabian Klenner, a University of Washington postdoctoral researcher in Earth and space sciences. Klenner is also the lead author of a new paper in the journal Science Advances. “Our results give us more confidence that using upcoming instruments, we will be able to detect lifeforms similar to those on Earth, which we increasingly believe could be present on ocean-bearing moons.”

The new research is “How to identify cell material in a single ice grain emitted from Enceladus or Europa.

Mass spectrometers have been around for decades but have improved rapidly in recent years. Researchers working on developing more powerful mass spectrometry have won two Nobel Prizes: one for Physics in 1989 and one for Chemistry in 2002. The 2002 prize is of particular interest in this research because it was awarded for the development of techniques that allowed mass spectrometers to detect biological macromolecules, including proteins.

Now, spacecraft and rovers often have mass spectrometers in their suite of instruments. NASA’s Curiosity rover has one, and so will the Europa Clipper, which will be sent on its way to Europa in October 2024. It’ll arrive there in 2030, so this research makes its anticipated arrival even more intriguing.

We know that Enceladus and Europa emit cryovolcanic plumes of material from their concealed oceans. The Cassini mission observed these eruptions coming from Enceladus’ south-polar region. Eventually, the spacecraft came within 50 km of the icy moon and passed directly through the plumes. Using its mass spectrometer, it detected carbon dioxide, water, various hydrocarbons, and organic chemicals.

A false-colour image of the plumes erupting from Enceladus. Image Credit: NASA/ESA

“Enceladus has got warmth, water and organic chemicals, some of the essential building blocks needed for life,” said Dennis Matson in 2008, a Cassini project scientist at NASA’s JPL at the time.

Europa also has cryovolcanic plumes. The Hubble Space Telescope spotted them in 2012, and then scientists working with data from the Galileo mission said that data supported the discovery.

This composite image shows suspected plumes of water vapour erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The plumes, photographed by Hubble’s Imaging Spectrograph, were seen in silhouette as the moon passed in front of Jupiter. Hubble’s ultraviolet sensitivity allowed for the features, rising over 160 kilometres above Europa’s icy surface, to be discerned. The Hubble data were taken on January 26, 2014. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. Image Credit: NASA/HST/STScI

When the Europa Clipper reaches its destination in 2030, it’ll employ an instrument called SUDA, the SUrface Dust Analyzer. SUDA will use mass spectrometry to detect chemicals in Europa’s plumes. This research suggests that SUDA should be able to detect cellular material on a single ice grain if it’s there.

This artist’s illustration shows what Europa might be like. Warm water containing organic material could make its way from the ocean, through cracks in the ice, out into space on ice grains via cryovolcanic plumes. Image Credit: NASA

This research is based on a common bacterium found in Alaskan waters. It’s called Sphingopyxis alaskensis, and the researchers chose it because it’s so small. It also lives in cold environments and can survive on few nutrients. It’s possible that its small size and other attributes make it an analogue for any life that may exist in Europa’s ocean.

In their experiments, the researchers simulated how mass spectrometry could detect organic material in a tiny ice grain. The results showed that along with detecting expected non-organic chemicals, mass spectrometry also detected amino acids from Sphingopyxis alaskensis.

“They are extremely small, so they are, in theory, capable of fitting into ice grains that are emitted from an ocean world like Enceladus or Europa,” Klenner said.

This figure from the research shows the cationic mass spectrum of the cell material equivalent to one S. alaskensis cell in a 15-?m-diameter H2O droplet. Although the mass spectrum is dominated by water, sodium-water, potassium-water, and ammonium-water clusters, amino acids, together with other metabolic intermediates from the S. alaskensis cell, can be identified. The spectrum is an average of 224 individual spectra. Image Credit: Klenner et al. 2024.

The search for life at Europa may come down to individual grains of ice. That’s partly because different molecules end up in different ice grains. If biological material is concentrated in ice grains, then it makes sense to detect individual ones rather than averaging results over a larger sample of ice.

But will there actually be biological material in ice grains? How would it get there?

On Earth, bacterial cells are encased in protective lipid membranes. That means that they sometimes form a surface layer on the ocean or other bodies of water. If the same is true of any life that may exist on Europa or Enceladus, then these bacteria can form a skin on the surface of the ocean. On these icy moons, gas bubbles that rise from the ocean and burst at the surface could incorporate cellular matter from the bacteria into the plumes.

The drawing on the left shows Enceladus and its ice-covered ocean, with cracks near the south pole that are believed to penetrate through the icy crust. The middle panel shows where life could thrive: at the top of the water, in a proposed thin layer (shown yellow) like on Earth’s oceans. The right panel shows that as gas bubbles rise and pop, bacterial cells could get lofted into space with droplets that then become the ice grains that were detected by Cassini. A mass spectrometer should be able to detect cellular matter on a single ice grain. Image Credit: European Space Agency

“We here describe a plausible scenario for how bacterial cells can, in theory, be incorporated into icy material that is formed from liquid water on Enceladus or Europa and then gets emitted into space,” Klenner said.

This is where mass spectrometry and SUDA come in. SUDA is much more powerful than earlier mass spectrometers, and has the capability to detect the fatty acids and lipids that may be launched into the plumes. While detecting actual DNA might seem like the holy grail, Klenner disagrees.

“For me, it is even more exciting to look for lipids, or for fatty acids, than to look for building blocks of DNA, and the reason is because fatty acids appear to be more stable,” Klenner said.

In their paper, the researchers state their results clearly. “Our experiments show that even if only 1% of a cell’s constituents are contained in a 15-micrometre ice grain (or one cell in a 70-micrometre-diameter grain), the bacterial signatures would be apparent in the spectral data,” they explain.

This is good news for the Europa Clipper and its SUDA instrument.

“With suitable instrumentation, such as the SUrface Dust Analyzer on NASA’s Europa Clipper space probe, it might be easier than we thought to find life, or traces of it, on icy moons,” said senior author Frank Postberg, a professor of planetary sciences at the Freie Universität Berlin. “If life is present there, of course, and cares to be enclosed in ice grains originating from an environment such as a subsurface water reservoir.”

Evan Gough

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