Holographic Imaging Could Be Used to Detect Signs of Life in Space
Engineers explore ways to sample and identify living microbes in the outer
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
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
"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
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,
"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
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