Searching for life in the icy crusts of ocean worlds

Searching for life in the icy crusts of ocean worlds

Long before NASA’s Perseverance rover landed on the Red Planet on February 18, one of the highest-level mission goals had already been set: looking for signs of ancient life on the Martian surface. In fact, the techniques used by one of the science instruments aboard the rover could have applications on the moons of Saturn, Enceladus and Titan, as well as on Jupiter’s moon Europa.

“Perseverance will look for a shopping list of minerals, organics and other chemical compounds that can reveal the microbial life that once thrived on Mars,” said Luther Beegle, principal investigator of the Mars 2020 exploration of habitable environments with Raman and luminescence for compounds. organic and chemical. (SHERLOC) instrument. “But the technology behind SHERLOC that will search for past life in Martian rocks is highly adaptive and can also be used to search for live microbes and the chemicals for life in the deep ice of the moons of Saturn and Jupiter.”

Enceladus, Europa, and even the nebula moon Titan are believed to hide vast oceans of liquid water containing chemical compounds associated with biological processes beneath their thick icy exteriors – environments very different from modern Mars. If microbial life exists in those waters, scientists can also find evidence for it in the ice. But how do you find that evidence if it’s locked deep in the ice?

Enter WATSON. Short for Wireline Analysis Tool for subsurface observation of northern ice sheets, the 3.9-foot-long (1.2-meter-long) tube-shaped prototype is being developed at the Jet Propulsion Laboratory of the NASA in Southern California. It has been mated to Honeybee Robotics’ planetary deep drill, and this combination was successfully tested in the extreme cold of the Greenland ice.

A smaller version of WATSON could one day travel aboard a future robotic mission to explore the habitability potential of one of these enigmatic moons. The instrument would scan the ice for biosignatures, organic molecules created by biological processes. If it detects any, a future version of WATSON, with the added ability to collect ice from the borehole wall, could collect samples for further study.

By using deep ultraviolet laser Raman spectroscopy to analyze the materials where they are found, rather than immediately retrieving ice samples and then studying them on the moon’s surface, the instrument would provide scientists with additional information about these samples by studying where they are located. found in context. of its surroundings.

“It would be great if we first studied what these samples look like in their natural environment before collecting them and mixing them into suspension for testing,” said Mike Malaska, a JPL astrobiologist and WATSON principal scientist.

“That is why we are developing this non-invasive instrument for use in icy environments: to get a deep look at the ice and identify groups of organic compounds, perhaps even microbes, so they can be studied before further analysis and lose yours. native context or modification of its structure “.

Although WATSON uses the same technique as SHERLOC from Perseverance, there are differences. On the one hand, SHERLOC will analyze Martian rocks and sediments for signs of past microbial life that can be collected and returned to Earth by future missions for further study. And SHERLOC doesn’t drill holes. A separate tool does that.

But both are based on a spectrometer and a deep ultraviolet laser, and where the WATSON ice instrument has an imager to look at the texture and particles in the ice wall, Perseverance’s SHERLOC is paired with a high-resolution camera. to take close-up rock pictures. textures to support your observations.

That camera shares the same name as the ice exploration prototype: WATSON. In this case, however, the acronym stands for Wide Angle Survey Sensor for Operations and Engineering. (After all, any instrument with a name inspired by the famous fictional detective Sherlock Holmes is sure to inspire references to its partner.)

Enceladus on Earth
Just as SHERLOC underwent extensive testing on Earth before going to Mars, so must WATSON before being sent to the outer solar system. To see how the instrument might work in the icy crust of Enceladus and the extremely low temperatures of the moon, the WATSON team chose Greenland as an “analog of Earth” for field testing of the prototype during a 2019 campaign.

Earth analogs share similar characteristics with other locations in our solar system. In the case of Greenland, the environment near the center of the island’s ice sheet and away from the coast approximates the surface of Enceladus, where oceanic materials gush from the prolific vents of the small moon and rain below. Meanwhile, the shattered ice on the edge of Greenland’s glaciers near the coast may serve as an analog to the deep and warped icy crust of Europa.

During the campaign to explore an existing well near Summit Station, a high-altitude remote observing station in Greenland, the instrument was put to the test. While descending more than 330 feet (100 meters), WATSON used its UV laser to illuminate the ice walls, causing some molecules to glow. The spectrometer then measured its faint brightness to give the team an idea of ​​its structure and composition.

While finding biological signatures in the Greenland ice sheet was not a surprise (the tests were conducted on Earth after all), mapping their distribution along the walls of the deep well raised new questions about how they are. features got to where they are. The team found that microbes deep in the ice tend to clump together in patches, not in layers as they originally expected.

“We created maps while WATSON scanned the sides of the well and the clustered blue, green and red hot spots, all representing different types of organic material,” Malaska said. “And what was interesting to me was that the layout of these access points was pretty much the same everywhere we looked – it doesn’t matter if the map was created at 10 or 100 meters [33 or 330 feet] deep down, these little compact spots were there. “

By measuring the spectral signatures of these hot spots, the team identified colors consistent with aromatic hydrocarbons (some that may have their origin in air pollution), lignins (compounds that help build cell walls in plants), and other biologically produced materials ( as complex organic compounds). acids that are also found in soils). In addition, the instrument recorded signatures similar to the glow produced by groups of microbes.

There is more testing to be done, ideally, on other Earth analogs that approximate the conditions of other icy moons, but the team were encouraged by WATSON’s sensitivity to such a wide variety of biosignatures. This high sensitivity would be useful on missions to ocean worlds, where the distribution and density of potential biosignatures is unknown, said Rohit Bhartia, principal investigator at WATSON and associate principal investigator for SHERLOC, of ​​Photon Systems in Covina, California.

“If we were to collect a random sample, we would likely miss something very interesting, but through our first field tests, we can better understand the distribution of organic substances and microbes in land ice that could help us by drilling into the crust of Enceladus “.

Research work

related links

Living Environment Scanning with Raman and Luminescence for Organic Chemicals (SHERLOC)

Lands Beyond Beyond – extrasolar planets – news and science
Life beyond Earth

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