Enhanced-color Cassini spacecraft view of southern latitudes on Enceladus.
Credits: NASA/JPL-Caltech/Space Science Institute

Use of digital holographic microscopy, using lasers to record 3-D images, may be our best bet for spotting extraterrestrial microbes.

The technique is being advocated by Caltech’s Jay Nadeau and colleagues as a way to sample and identify living microbes in the outer solar system.

The work has been published this month in the journal Astrobiology within a special issue dedicated to the search for signs of life on Saturn’s icy moon Enceladus.

Self-directed motion

“It’s harder to distinguish between a microbe and a speck of dust than you’d think,” says Caltech’s Nadeau, research professor of medical engineering and aerospace in the Division of Engineering and Applied Science.

Narrow jets of gas and icy particles erupt from the south polar region of Enceladus, contributing to the moon’s giant plume. A cycle of activity in these small-scale jets may be periodically lofting extra particles into space, causing the overall plume to brighten dramatically.
Credits: NASA/JPL/Space Science Institute

“You have to differentiate between Brownian motion, which is the random motion of matter, and the intentional, self-directed motion of a living organism,” Nadeau said in a press statement.

To study the motion of potential microbes from Enceladus’s plumes, Nadeau proposes using an instrument called a digital holographic microscope that has been modified specifically for astrobiology.

3-D imaging

In digital holographic microscopy, an object is illuminated with a laser and the light that bounces off the object and back to a detector is measured. This scattered light contains information about the amplitude (the intensity) of the scattered light, and about its phase (a separate property that can be used to tell how far the light traveled after it scattered).

With the two types of information, a computer can reconstruct a 3-D image of the object—one that can show motion through all three dimensions.

“Digital holographic microscopy allows you to see and track even the tiniest of motions,” Nadeau says. Furthermore, by tagging potential microbes with fluorescent dyes that bind to broad classes of molecules that are likely to be indicators of life—proteins, sugars, lipids, and nucleic acids—”you can tell what the microbes are made of,” she says.

This view looks toward the Saturn-facing hemisphere of Enceladus (313 miles or 504 kilometers across). North is up. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 13, 2017.
Credits: NASA/JPL-Caltech/Space Science Institute

Extreme environments

To study the technology’s potential utility for analyzing extraterrestrial samples, Nadeau and her colleagues obtained samples of frigid water from the Arctic, which is sparsely populated with bacteria; those that are present are rendered sluggish by the cold temperatures.

With holographic microscopy, Nadeau was able to identify organisms with population densities of just 1,000 cells per milliliter of volume, similar to what exists in some of the most extreme environments on Earth, such as subglacial lakes.

Next, the team will attempt to replicate their results using samples from other microbe-poor regions on Earth, such as Antarctica.

Nadeau collaborated with Caltech graduate student Manuel Bedrossian and Chris Lindensmith of the Jet Propulsion Laboratory in Pasadena, California.

Resources

For more information, go to:

Digital Holographic Microscopy, a Method for Detection of Microorganisms in Plume Samples from Enceladus and Other Icy Worlds

http://resolver.caltech.edu/CaltechAUTHORS:20170707-104246206

Also, go to this video detailing Nadeau’s work and proposal to use new microscopes on spacecraft that could visit the icy moons of Enceladus (Saturn) and Europa (Jupiter) to collect and search water samples for life. Go to:

https://www.youtube.com/watch?time_continue=9&v=Q36_chQ45JQ

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