Life in the icy crusts of ocean worlds

Long before NASA’s Perseverance Rover hit the Red Planet on February 18, one of its mission goals at the highest level had already been set: to search for signs of ancient life on the Martian surface. The techniques used by one of the scientific instruments aboard the Rover can be used on Saturn’s moons Enceladus and Titan as well as Jupiter’s moon Europe.

“Perseverance is looking for a shopping list of minerals, organics and other chemicals that can reveal microbial life once it thrives on Mars,” said Luther Beegle, lead researcher for Mars 2020’s Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals, said. (SHERLOC) instrument. “But the technology behind SHERLOC that goes after the previous life in Mars rocks is very adaptable and can also be used to search for living microbes and the chemical building blocks for life in the deep ice of the moons of Saturn and Jupiter.”

Enceladus, Europe and even the hazy moon Titan are thought to contain large oceans of liquid water containing chemical compounds associated with biological processes beneath their thick icy exterior – much different environments than modern Mars. If microbial life exists in those waters, scientists can also find evidence of it in the ice. But how do you find the evidence when it’s locked deep in the ice?

Enter WATSON. The 3.9-foot-long (1.2-meter-long) tubular prototype is an acronym for Wired Line Analysis Instrument for Underground Observation of Northern Ice Plates, and is currently being developed at NASA’s Jet Propulsion Laboratory in Southern California. It has been linked to Honeybee Robotics’ Planetary Deep Drill, and this combination has been successfully tested in the extreme cold of Greenland’s ice.

A smaller version of WATSON may one day ride aboard a future robotic mission to explore the habitability potential of one of these enigmatic moons. The instrument scans the ice in search of bio-signatures – organic molecules created by biological processes. If spotted, a future version of WATSON, with the added ability to trap ice from the borehole wall, could collect samples for further study.

Using the deep ultraviolet laser Raman spectroscopy to analyze the materials where they occur, rather than picking up ice samples immediately and then studying them on the lunar surface, the instrument will provide scientists with additional information on these samples by examine where it is in context. of their environment.

“It would be great if we first studied what these specimens looked like in their natural environment, before we picked them up and mixed them into a mixture to test them,” said Mike Malaska, an astrobiologist at JPL and chief scientist at WATSON. . “That’s why we are developing this non-invasive tool for use in icy environments: to look deep into the ice and identify clusters of organic compounds – perhaps even microbes – so that they can be studied before we further analyze and change or change their original context. their structure. “

Although WATSON uses the same technique as SHERLOC from Perseverance, there are differences. First, SHERLOC will analyze Mars rocks and sediment to look for signs of microbial life that may be collected in the future and returned to Earth by future missions. And SHERLOC does not drill holes. A separate tool does this.

But both rely on a deep ultraviolet laser and spectrometer, and where the WATSON ice instrument has an imager to observe the texture and particles in the ice wall, Perseverance’s SHERLOC is connected to a high-resolution camera to capture rock photos of to take near. textures to support its observations. The camera happens to share the same name as the prototype examining the ice: WATSON. In this case, however, the acronym stands for Wide Angle Topographic Sensor for Operations and eNgineering. (After all, any instrument with a name inspired by the famous fictional detective Sherlock Holmes is likely to inspire references to his mate.)

Enceladus on earth

Just as SHERLOC tested extensively on Earth before going to Mars, so must WATSON before it is sent to the outer solar system. To see how the instrument can perform in the icy crust of Enceladus and the extraordinarily low temperatures of the moon, the WATSON team selected Greenland as an ‘earth analogue’ for prototype field tests during a 2019 campaign.

Earth analogues have similar properties to other places in our solar system. In the case of Greenland, the environment near the center of the island’s ice sheet and off the coast approaches the surface of Enceladus, where the ocean material from the small moon’s fertile openings erupts and rains. The broken ice at the edge of the glaciers of Greenland near the coast can meanwhile serve as an analogue to Europe’s deep, icy crust.

During the campaign to explore an existing borehole near Summit Station, a large-distance remote sensing station in Greenland, the instrument was placed through the barrel. As it descended more than 100 meters, WATSON used its UV laser to illuminate the ice walls, causing some molecules to glow. The spectrometer then measured their dim glow to give the team insight into their structure and composition.

While finding biosignatures in Greenland’s ice pack was not a surprise – after all, the tests were on Earth – mapping the distribution along the walls of the deep borehole has raised new questions about how these properties came where they are. The team discovered that microbes deep in the ice tend to clump together in stains, not in layers as they originally expected.

“We made maps when WATSON scanned the sides of the borehole and the groupings of hotspots of blue vegetables and reds – all representing different types of organic matter,” Malaska said. “And what was interesting to me was that the distribution of these hotspots just about everywhere we looked was the same: no matter if the map was made at 10 or 100 meters [33 or 330 feet] in depth, these compact little skins were there. ‘

By measuring the spectral signatures of these hotspots, the team identified colors corresponding to aromatic hydrocarbons (some may be derived from air pollution), lignins (compounds that help build cell walls in plants) and other biologically produced materials (such as complex organic materials) acids that also occur in soils). In addition, the instrument recorded signatures similar to the glow produced by clusters of microbes.

More needs to be tested – ideally, in other Earth analogues approaching the conditions of other icy moons – but the team was encouraged by how sensitive WATSON was to such a wide range of bio-signatures. This high sensitivity will be useful on missions to ocean worlds, where the distribution and density of any potential biosignatures are unknown, said Rohit Bhartia, chief investigator for WATSON and deputy chief investigator for SHERLOC, of ​​Photon Systems in Covina, California. “If we were to collect a random sample, we would probably miss something very interesting, but through our first field tests we can better understand the distribution of organic and microbes in terrestrial ice that can help us if we are in the crust of Enceladus. ‘

The results of the field test were published in the journal Astrobiology in the fall of 2020 and presented at the American Geophysical Union Fall Meeting 2020 on December 11th.