Conceptual illustration of Enceladus, moon of planet Saturn, with water geysers and water vapor plumes.
Is there life on other planets? From Orson Welles infamous War of the Worlds radio broadcast in 1938 to Netflix’s recent Three-Body Problem, creatives and audiences imagined dramatic, terrifying, and sometimes gruesome scenarios of extraterrestrials landing on earth.
Scientists, meanwhile, are looking in the other direction, sending spacecraft to different planets to take photos and gather samples that suggest the possibility of life there. Since Italian astronomer Giovanni Schiaparelli observed what he called canali (translated as canals) on the surface of Mars in 1877, much of the search for extraterrestrial life has been nearly synonymous with the search for water. That, says Portland State University professor of physics Jay Nadeau, has shifted interest away from Mars as subsurface global oceans have been discovered on multiple moons of the outer planets, particularly those of Jupiter and Saturn. Many in the astrobiology world are now focusing on these so-called Ocean Worlds, particularly Europa (a moon of Jupiter) and Enceladus (a moon of Saturn).
The perennial search for life will get a huge boost in 2025, when NASA will send a spacecraft to Enceladus, the sixth largest moon of Saturn – and holography will play an important role, in the form of a digital holography microscope built by Nadeau and her team.
Enceladus sparked interest as a destination after NASA’s Cassini spacecraft, in a series of flybys from 2005 to 2015, detected “plumes of salty water and organic molecules spewing from fractures known as ‘tiger stripes’,” suggesting a potentially habitable environment, and one that may be sampled without the need to land. Concepts for Enceladus missions that involve orbital flybys of the plumes, landers, or both, have been proposed to multiple agencies. The European Space Agency designated Enceladus as its number one priority in its long-term plan called Voyage 2050. Private missions to Enceladus have also captured the imagination of funders, such as the Yuri Milner-founded Breakthrough Enceladus concept.
If there is microbial life on Enceladus, ice grains from the plumes may contain intact or even living organisms. The challenge for scientists, says Nadeau, is “identifying those microbes from 790 million miles away.” She explains that microbes can be challenging to identify because they “don’t have a lot of cellular features.” “Sometimes telling the difference between them and sand grains is very difficult,” she says.
Using digital holographic microscopy to aid in the detection of life was the goal Nadeau began to address when she was at McGill University in Montreal and later as a Caltech research professor, between 2015 and 2017. The idea was that digital holographic microscopy could be used to identify movement. To understand how it works and the role holography plays, we need to go back to 1892, when two scientists – Ludwig Mach and Ludwig Zehnder – invented a device that would split a light into two parallel waves and then recombine them by interference, which adds the amplitudes of the individual waves at each point affected by more than one wave.The technology was useful for all kinds of scientific inquiry, but holography wasn’t invented until 1948, when physicist/engineer Dennis Gabor did so, using a Mach-Zehnder interferometer based on an electron microscope and a heavily filtered mercury arc light source to create a three-dimensional image. The first visual hologram, using lasers, appeared in 1964, but computers couldn’t keep up with the data demands of digital holography until the mid-1990s, and it wasn’t until the early 2000s that digital holographic microscopy became a reality.
At Caltech, Nadeau, who was a research professor of medical engineering and aerospace in the Division of Engineering and Applied Science, and her NASA/JPL collaborators were interested in customizing a digital holography microscope that could help search for signs of life in outer space. The challenges were immense. At the time, Nadeau observed that, “You have to differentiate between Brownian motion, which is the random motion of matter, and the intentional, self-directed motion of a living organism. Digital holographic microscopy allows you to see and track even the tiniest of motions. With holographic capabilities, the computer can reconstruct a 3D image of the scattered light that shows motion in three dimensions.”
But getting the funds to build the microscope wasn’t easy, as proposal reviewers noted that Mach-Zehnder devices are very sensitive to vibration and movement, in general, and that if it was even a fraction out of alignment, the device would stop working. To that end, Nadeau’s team developed an optical train where all the elements were parallel and couldn’t be knocked out of alignment.
The project to create this customized interferometer was approved in 2014; by 2015, the team had its first prototype. Dubbed SHAMU (Submersible Holographic Astrobiology Microscope with Ultraresolution), the team – composed of optical engineers and biologists from Caltech, JPL and the University of Washington – first went to Greenland in Spring 2015 to test the prototype in an extreme environment. They used snowmobiles to move their scientific equipment to various spots on frozen fjords. At each site, they drilled a hole into the sea ice, submerged the microscope to where salty water (called brine) was in the ice and collected the resultant holographic images.
The team also brought its samples to a lab in Nuuk, Greenland where they soon learned that warming them and feeding them a bacterial growth medium to replicate “the standard conditions under which microorganisms from the sea were typically studied” was hugely successful. This first field trip was validation that digital holographic microscopy had the potential to perform as needed on a mission to outer space. “We know from this that we can tell that things are alive when you take them straight out of ice,” says Nadeau. “If we can see life in there on Earth, then it’s possible there might be living organisms that are flash-frozen as they emerge from the plumes of Enceladus. There might be life in pockets of ice on Mars as well – perhaps you don’t have to have a big liquid ocean to find living organisms.”
Nadeau stressed the importance of holography to a space flight. “You get a 3D image from a single TIFF,” she says, “which gives built-in data compression for the low data budgets of space missions.” Another advantage of the digital holography microscope was that “the focusing is done digitally offline, so you don’t have someone in space twiddling focus knobs.”
After the expedition to Greenland, the team was scheduled to do a similar real-world proof of concept in Iceland – then COVID-19 hit. Instead, they went to local spots including the Salton Sea, Mammoth Lakes, and Death Valley National Park in California; Barrow, Alaska in the Arctic; Ash Meadows National Wildlife Refuge in Armargosa Valley, Nevada; and The Cedars Ecological Preserve in Sonoma County, California, which featured hyper-alkaline pools. Closer to Portland, says Nadeau, they also tested the device at the Mount St. Helens crater.
“We saw microbes swimming just about everywhere we went,” says Nadeau. “We learned that, in general, if you scoop up sea water, to see the bacteria, you need to stimulate them by warming them up and feeding them. L-Serine is an amino acid they particularly like.” It is likely that microbes elsewhere in the solar system will use the same amino acids as those on Earth since these identical molecules have been found on asteroids and meteorites and in interstellar space. However, whether alien microbes will use the L- or D-form of the amino acid is unknown and may help distinguish extraterrestrial from Earth life.
The team is now preparing the microscope for a flight to the International Space Station in 2025, where it will be used to study swimming behavior of microorganisms in microgravity. The first flight will largely be to qualify the instrument for missions of exploration elsewhere, but the team also hopes that once the instrument is on the Space Station, others may propose more experiments using it. Professor Cheryl Nickerson and her colleagues from Arizona State University have done a great deal of work on bacteria in space and have expressed interest in imaging cells as they experience microgravity. Nadeau adds that she would like to incorporate fluorescent tagging to help identify bacteria in Space Station experiments and missions.
The mission to further explore the possibility of life in outer space relies on a toolset based on decades of scientific research and testing. Digital holography plays a singular role that enables the process to work for this specific use case. The astronauts can quickly and easily acquire the holographic imagery, with a much lighter data load than other potential image capture systems. The result is a fully 3D volume in a single image, and that holographic image makes it possible for scientists to discern cellular life among random motion of other microscopic activity. Holography has already been used successfully for a variety of scientific, industrial and artistic purposes, and now its use as a key technology in the search for life in outer space is a spectacular step forward.