NEMO Neutrino Mission Makes Surprising Discovery

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Serendipity is an integral part of the scientific enterprise. Just ask the physicists working on the Neutrino Mediterranean Observatory (NEMO), an underwater neutrino experiment currently under construction, just off the coast of Catania, Italy.

NEMO is a small-scale prototype for the planned KM3NeT deep-sea neutrino telescope, to test the robustness of the design.

The folks at Symmetry Breaking recently reported that the international collaboration of NEMO scientists made a surprising discovery while measuring water currents and temperatures: unexpected patterns of water flow, called marine vortices, nearly two miles deep in the Ionian Sea.

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These are essentially small whirlpools, six miles or so across, that rotate roughly one inch per second. Ask an oceanographer, and s/he will tell you that these kinds of vortices aren’t supposed to form in closed basins like the Mediterranean.

At least that was the conventional wisdom.

Now the oceanographic community is reworking their understanding of these complex fluid dynamics. New simulations indicate that while the vortices might have formed locally, it’s also possible they traveled from the Afriatic or Aegean seas — a distance of several hundred miles.

Technically, NEMO is a precursor to KM3NeT, which will search for neutrinos from distant sources such as gamma ray bursts or supernovae, as well as the elusive dark matter.

Why do physicists think it’s a good idea to build a neutrino experiment underwater in the first place? For the same reason the folks at IceCube wanted to build a large array of detectors deep in the ice in Antarctica.

Neutrinos are extremely difficult to detect, because they very rarely interact with any type of matter, even though they’re the most abundant type of particle in the known universe. Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth. And it’s easy to confuse them with cosmic rays.

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The polar ice is very pure, optically transparent, and free of radioactivity, which can add a lot of extra background “noise” to the data. These qualities make the ice of Antarctica the ideal medium for neutrino detection. Similar properties can be found in the waters of the Mediterranean.

In fact, KM3NeT is intended to complement the ongoing IceCube experiment. IceCube is in the Southern Hemisphere and hence can only detect sources in the Northern sky. NEMO, and later KM3NeT, will be able to cover the rest of the sky.

Both IceCube and KM3NeT are designed to detect Cerenkov radiation as an indicator of a neutrino colliding with an atom in the ice, or sea water — the faint blue glow commonly emitted from radioactive materials when they’re immersed in water (as shown in the blue glow surrounding the Advanced Test Reactor, top).

The underlying principle behind Cerenkov radiation is similar to that of a sonic boom. If an aircraft is traveling faster than the speed of sound, the air flowing around its wings can’t move out of the way fast enough, creating a sudden pressure drop moving away from the wing at the speed of sound. And this pressure front, or shock wave, creates that loud boom you hear after an aircraft flies overhead.

With Cerenkov radiation, the same kind of shock wave is created, only with light instead of sound. Think of it as a “photonic boom.” It occurs when a charged particle moves through a medium faster than the speed of light and generates a similar shock wave.

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NEMO is one of three major pilot initiatives in the Mediterranean, aimed at deploying prototype telescopes at depths ranging from 2500 to 4500 meters underwater. (The others are NESTOR and ANTARES, currently the the largest neutrino telescope in the Northern hemisphere.)

It’s not a simple thing to set up such a large experimental array over a mile underwater in the ocean, which is why part of NEMO’s mission is to employ acoustic detectors as well to monitor the marine environment. That’s how they made their discovery of the marine vortices.

It’s not the first such discovery, either. Around five years ago, during an even earlier phase of the project, NEMO researchers discovered a surprising variety of marine life, more than a mile underwater.

They were studying noise in the ocean depths, the better to filter out the background signal when the actual neutrino detection begins. Among the many noise sources they measured were the distinctive “acoustic emissions” of sperm whales and dolphins, which they use to navigate and hunt.

The frequent presence of whales was especially surprising. The whale signatures were much more intense, indicating they roam at greater depths, closer to the acoustic sensors, compared to dolphins, which typically hang out a few hundred meters from the surface.

So NEMO is already making interesting contributions to science, even though it’s still getting its detectors in place. It bodes well for the future of deep-sea neutrino telescopes.

Image credits: NEMO, Idaho National Laboratory