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Why Scientists Sank a Telescope Into the Deepest Parts of the Ocean

The ANTARES underwater observatory is looking for particles from outer space, but it’s also contributing to marine science.

An illustration of the Antares neutrino detector.
An illustration of the Antares neutrino detector. François Montanet/CC BY-SA 2.0 FR

In 2006, scientists working on the ANTARES neutrino telescope found themselves facing a problem: their brand-new instruments were observing way more light than they had expected. This was particularly confusing given that ANTARES is sunk in pitch-black water, two and a half kilometers below the Mediterranean Sea.

You see, this is no ordinary star-scanning telescope. Rather than hunt for distant cosmic objects, the ANTARES observatory uses highly light-sensitive devices to detect something from space right here on earth: the light emitted by a cosmic neutrino—a subatomic particle, much like a charge-less electron—as it passes through seawater.

 The ANTARES physicists knew when they started that they might see a bit of light from other sources, like deep-sea fishes. But the glow they were seeing was intense enough to hinder their work. Where could it be coming from?

Luckily, unlike most observatories, ANTARES employes oceanographers, geologists, marine biologists, and climatologists in addition to astrophysicists. With further study, these experts discovered that the light was coming from a massive bloom of deep-sea bioluminescent bacteria, stirred into action by water that was being transported from the surface into the deep.

What had first appeared to be a roadblock turned out to be a breakthrough: for their studies of the deep-sea bloom, the ANTARES collaboration was awarded a special prize by the science magazine La Recherche, which awards exemplary scientific research annually, in 2014. Theirs was the longest-ever observation of such a bloom in the deep sea, and without ANTARES, scientists likely would have missed it.

ANTARES is located in the Mediterranean Sea off the coast of Toulon, shown here.
ANTARES is located in the Mediterranean Sea off the coast of Toulon, shown here. David.Monniaux/CC BY-SA 3.0

This is what makes this observatory so extraordinary; it is a true interdisciplinary collaboration, providing data for a group of fields that might otherwise seem wildly unconnected.

“That’s the funny part of this work, because when talk about my project with oceanographers, they say, ‘a telescope, really?’” says Severine Martini, a researcher at the Monterey Bay Research Institute who has used ANTARES for her studies of bioluminescent bacteria. She was among the group that studied the deep-sea bacterial bloom. “Meanwhile, when we talk about bioluminescence and oceanographic phenomena to astrophysicists, we get sort of blank looks. But that’s the good part of working with different fields. We are trying to solve different problems, but we are working together.”

The bottom of the ocean might seem like a strange choice of location for a telescope. But when you’re looking for neutrinos, the deep sea is actually the ideal place from which to search.

Astrophysicists are intrigued by cosmic neutrinos, believed to be produced by high-energy and violent occurrences in space; the only two confirmed sources are the sun and a distant supernova, but physicists are eager to discover where else these particles are coming from. The problem is that neutrinos are only one of the many particles constantly bombarding the earth—and unlike many of these other particles, like x-rays or cosmic rays, neutrinos only interact very weakly with matter, making them particularly hard to detect among the noise.

To make matters worse, neutrinos are also produced in the atmosphere, and can be hard to distinguish from their cosmic counterparts.

ANTARES has been used for the study of bioluminescent bacteria.
ANTARES has been used for the study of bioluminescent bacteria. Severine Martini

Here’s where the ocean comes in. Neutrinos are the only particles that can pass straight through earth itself, so ANTARES uses the earth as a shield, searching for “upward-going” muons–particles very similar to electrons, but without mass–that neutrinos produce as they pass up through the earth. To detect these muons, the observatory’s photomultipliers look for a tiny burst of light called Cherenkov radiation, produced when a charged particle is moving faster than the speed of light in water.

Positioning a neutrino telescope at the bottom of the sea is therefore like positioning it between two strainers, which filter out only what it is interested in recording.

So putting the telescope at the bottom of the sea theoretically makes it more useful for observing outer space—but it also makes it a lot more useful for observing the bottom of the sea. “In a broad sense, we are opening a new window on the universe,” says Antoine Kouchner, a professor and researcher in cosmology at the Université Paris Diderot, and the spokesperson for the ANTARES collaboration. “But being an observatory cabled to shore enables real time data and monitoring. And that’s where it becomes interesting to sea science.”

Most information from the deep sea comes in small bites; usually, instruments or vehicles are sent to the bottom for only a few hours at a time. Information about the deep sea is therefore usually scattered and disconnected, both spatially and temporally.

A prototype of a KM3NeT DOM installed in the instrumentation line of the ANTARES neutrino telescope.
A prototype of a KM3NeT DOM installed in the instrumentation line of the ANTARES neutrino telescope. Edewolf/CC BY-SA 3.0

In contrast, the ANTARES observatory has been transmitting data constantly, day in and day out, over the course of years. This information could prove particularly relevant for scientists studying climate change, who need data sets that span many years in order to sift out what is changing in a warming ocean.

“Basically, having an electric plug and an Ethernet cable available at 2500m depth is a big step forward for the earth and sea science studies,” joked Paschal Coyle, particle astrophysicist and former spokesperson of the collaboration. “It was a surprise to me that this community was not already doing what we were doing.”

Thanks to the many sensors that ANTARES provides—monitoring oxygen, temperature, pressure, salinity, seismic activity, and much more—the scope of non-physics projects at ANTARES is now broad, from tracking sediment flow on the seafloor to recording sperm whale calls as they hunt in the deep.

And they’re still studying those tiny spots of light that were noise to the astrophysicists. During her Ph.D. at the Mediterranean Institute of Oceanography, Martini discovered a new form of bioluminescent bacteria that she has been observing almost continually in the deep using ANTARES. In her most recent paper, in November, Martini described the bioluminescence activity of these bacteria for a consecutive year using ANTARES data. She found that the bacteria emitted light even under stable conditions, and that bioluminescent bacteria were more active than bacteria as a whole—suggestion that light emission provides some sort of ecological benefit.

“I think it’s interesting because we discovered something nobody would have ever looked for,” said Martini. “The ecological role of bioluminescence is well described for a lot of macro-organisms, but for bacteria we still don’t really know why they are emitting light.” One theory suggests the bacterium glow when many of them attach to food particles, in hopes of attracting a larger, hungry animal like a fish. For a bacterium, being eaten is a good thing, as being excreted later on can help it spread to new environments.

An artist's impression of the new KM3NeT neutrino telescope, to be located in France, Sicily and Greece.
An artist’s impression of the new KM3NeT neutrino telescope, to be located in France, Sicily and Greece. Edewolf/CC BY-SA 3.0

The next step for ANTARES is a big one: the collaboration is building a new observatory, dubbed KM3NeT. The new observatory will be 50 times larger than ANTARES and located at three sites, in France, Sicily and Greece. Over its ten years of operation, ANTARES was unsuccessful at detecting cosmic neutrinos; with the new observatory, the collaborators hope to finally solve the puzzle of the particles’ origin, and to learn more about their fundamental properties.

Additionally, the marine and earth science community has been involved in the development of KM3NeT from the beginning. The new stations will host even more sensors, including a camera made for detecting life, radioactivity detectors, and a remotely-operated vehicle—which Coyle, who is serving as the spokesperson for the new observatory, compared to the Disney robot Wall-E—that will be able to explore the seafloor and film what it finds.

“The potential of interdisciplinary, synergetic science with cabled deep sea marine observatories is tremendous,” said Coyle. “ANTARES has pioneered the way, and I am sure many surprises on this front will be forthcoming.”

With the oceans absorbing  huge quantities of heat and acidifying carbon dioxide, changes seem almost certain. But with these collaborative observatories monitoring constantly—acting, as Coyle put it, as “the guardians of the abyss”—perhaps they won’t be so much of a surprise.