• Holographic Imaging Could Be Used to Detect Signs of Life in Space

    From baalke@earthlink.net@21:1/5 to All on Thu Aug 24 23:39:39 2017
    https://www.caltech.edu/news/holographic-imaging-could-be-used-detect-signs-life-space-78931

    Holographic Imaging Could Be Used to Detect Signs of Life in Space

    Engineers explore ways to sample and identify living microbes in the outer solar system

    Caltech
    July 20, 2017

    We may be capable of finding microbes in space - but if we did, could
    we tell what they were, and that they were alive?

    This month the journal Astrobiology is publishing a special issue dedicated
    to the search for signs of life on Saturn's icy moon Enceladus. Included
    is a paper from Caltech's Jay Nadeau and colleagues offering evidence
    that a technique called digital holographic microscopy, which uses lasers
    to record 3-D images, may be our best bet for spotting extraterrestrial microbes.

    No probe since NASA's Viking program in the late 1970s has explicitly
    searched for extraterrestrial life - that is, for actual living organisms. Rather, the focus has been on finding water. Enceladus has a lot of water - an ocean's worth, hidden beneath an icy shell that coats the entire surface.
    But even if life does exist there in some microbial fashion, the difficulty
    for scientists on Earth is identifying those microbes from 790 million
    miles away.

    "It's harder to distinguish between a microbe and a speck of dust than
    you'd think," says Nadeau, research professor of medical engineering and aerospace in the Division of Engineering and Applied Science. "You have
    to differentiate between Brownian motion, which is the random motion of
    matter, and the intentional, self-directed motion of a living organism."

    Enceladus is the sixth-largest moon of Saturn, and is 100,000 times less massive than Earth. As such, Enceladus has an escape velocity - the minimum speed needed for an object on the moon to escape its surface - of just
    239 meters per second. That is a fraction of Earth's, which is a little
    over 11,000 meters per second.

    Enceladus's minuscule escape velocity allows for an unusual phenomenon: enormous geysers, venting water vapor through cracks in the moon's icy
    shell, regularly jet out into space. When the Saturn probe Cassini flew
    by Enceladus in 2005, it spotted water vapor plumes in the south polar
    region blasting icy particles at nearly 2,000 kilometers per hour to an altitude of nearly 500 kilometers above the surface. Scientists calculated
    that as much as 250 kilograms of water vapor were released every second
    in each plume. Since those first observations, more than a hundred geysers
    have been spotted. This water is thought to replenish Saturn's diaphanous
    E ring, which would otherwise dissipate quickly, and was the subject of
    a recent announcement by NASA describing Enceladus as an "ocean world"
    that is the closest NASA has come to finding a place with the necessary ingredients for habitability.

    Water blasting out into space offers a rare opportunity, says Nadeau.
    While landing on a foreign body is difficult and costly, a cheaper and
    easier option might be to send a probe to Enceladus and pass it through
    the jets, where it would collect water samples that could possibly contain microbes.

    Assuming a probe were to do so, it would open up a few questions for engineers like Nadeau, who studies microbes in extreme environments. Could microbes survive a journey in one of those jets? If so, how could a probe collect samples without destroying those microbes? And if samples are collected,
    how could they be identified as living cells?

    The problem with searching for microbes in a sample of water is that they
    can be difficult to identify. "The hardest thing about bacteria is that
    they just don't have a lot of cellular features," Nadeau says. Bacteria
    are usually blob-shaped and always tiny - smaller in diameter than a strand
    of hair. "Sometimes telling the difference between them and sand grains
    is very difficult," Nadeau says.

    Some strategies for demonstrating that a microscopic speck is actually
    a living microbe involve searching for patterns in its structure or studying its specific chemical composition. While these methods are useful, they
    should be used in conjunction with direct observations of potential microbes, 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," she says. In earlier work, Nadeau suggested that the movement exhibited by many living organisms could potentially be used as a robust, chemistry-independent biosignature for extraterrestrial life. The motion
    of living organisms can also be triggered or enhanced by "feeding" the
    microbes electrons and watching them grow more active.

    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.

    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.

    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. For comparison, the open ocean contains about 10,000 cells per milliliter and a typical pond might have 1-10 million cells
    per milliliter. That low threshold for detection, coupled with the system's ability to test a lot of samples quickly (at a rate of about one milliliter
    per hour) and its few moving parts, makes it ideal for astrobiology, Nadeau says.

    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 JPL.


    Written by Robert Perkins

    Contact:
    Robert Perkins
    (626) 395-1862
    rperkins@caltech.edu

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