Space probes with digital holographic microscopes could be used to detect alien organisms

Satrun’s icy moon Enceladus has a global subsurface ocean with the **necessary ingredients to harbour life** as we know it on Earth. **Cassini** , the constant companion of Saturn for over thirteen years observed plumes of water erupting from a region near the south pole of Enceladus, known as the tiger stripes. Cassini, unfortunately did not have the equipment necessary to directly look for life when it made a close flyby over the icy moon. In fact, no probe since NASA’s Viking program in the late 1970s has actively looked for living organisms. [caption id=“attachment_371759” align=“aligncenter” width=“640”]  The evidence from Cassini shows the water beneath the plumes is interacting with the rock beneath the ocean to produce the kind of environment that will be habitable to microbes. Image: NASA.[/caption] Scientists at Caltech are suggesting that an approach that uses lasers to capture holographic microscopic images is the best bet for gathering evidence of alien life from a distance. The technique could be used on the plumes of water on Enceladus. The jets of water rush upwards at 2,000 kilometers per hour, and reach an altitude of nearly 500 kilometers above the surface of the moon, with each plume releasing as much as 250 kg of water vapour every second. [caption id=“attachment_350889” align=“aligncenter” width=“640”]  Autonomous drones being tested in the oceans of the Earth. Image: Nasa/JPL-Caltech[/caption] The water plumes make it easier for probes to search for life, as against drilling beneath the ice cap and **using submarine drones** , an approach NASA is exploring as an option to look for life on another icy world, Europa. A probe sent to Enceladus could sniff the jets of water, and potentially directly collect samples of alien microbes. If scientists were to do this, they would have to figure out how to identify the organisms, and if they are alive, how to collect the samples without killing the life forms. [caption id=“attachment_371769” align=“aligncenter” width=“640”]  Cassini observing the plumes of water erupting from Enceladus. Image: Nasa.[/caption] Chemical analysis of the sample is useful, but observing the motion of the organisms would allow for a deeper scientific understanding. However, to achieve this, the imaging instrument has to be capable of distinguishing between the brownian motion of particles of dust, and the purposeful motion of living organisms. This is where the holographic imaging technique comes into play.  Caltech’s Jay Nadeau says, “Looking at patterns and chemistry is useful, but I think we need to take a step back and look for more general characteristics of living things, like the presence of motion. That is, if you see an E. coli, you know that it is alive—and not, say, a grain of sand—because of the way it is moving. Digital holographic microscopy allows you to see and track even the tiniest of motions.”

To demonstrate the technique, Nadeau used samples of water from the cold waters of the Arctic. The 3D imaging technique allowed the researchers to detect organisms even though they were present in very low densities, in one of the most extreme environments on Earth known to support life. The findings of the research will be in this month’s edition of the scientific journal Astrobiology, which is a special issue dedicated to the various approaches that can be used to detect signs of life on Enceladus.