Water is the foundation of biological life. That's why it was so exciting, in 2019, when two independent teams of astronomers, using Hubble Space Telescope data, observed transiting exoplanet K2-18b with water vapor in its atmosphere. Maybe a little, maybe a lot. Of the thousands of exoplanets observed in recent years, this was the first promising candidate for harboring life.
And yet, even as we send telescopes into deep space to investigate the mysteries of galactic water, there's a surprising amount we don't know about our own terrestrial water. Only 20 percent of the oceans have been mapped, and marine biologists estimate that as much as 90 percent of oceanic species are yet to be discovered.
The plants and animals that have been discovered are astounding. See, for example, the giant clam and its unique symbiotic relationship with algal cells, which it farms at high densities, millimeters deep in its mantle tissue in neat, ordered rows. Vast numbers of oceanic species are thought to utilize bioluminescence to attract food, find mates, and evade predators. Others, like the self-evidently named fangtooth fish, have evolved to become "ultra-black;" that is, they completely absorb photons, usually those emitted from predators on the hunt or just-consumed prey.
But just as the Earth's oceans are a frontier for discovery, they're also full of untapped resources, including metal-rich layers of ore on the ocean floor known as polymetallic nodules. These nodules contain nickel, cobalt, titanium, tellurium, and yttrium—metals that are hard to mine on land, and yet crucial to a green-energy future. Tellurium is a component in thin-film solar panels, and electric vehicles rely on batteries containing cobalt and nickel. Yttrium is a common element in solid-state lasers used in modern manufacturing, dentistry, and medicine.
There are a lot of these nodules on the seafloor. Tons of them. Enough that several companies have applied for permits to begin mining them at large scale—a process that's bound to disturb, displace, and even destroy oceanic habitats and species.
The heat is on to learn as much as we can about the oceans. Fiber optic cables that crisscross the ocean floor are ushering in a new era of ocean science that includes measuring earthquakes thousands of miles away. Underwater imaging, first popularized by Jacques Cousteau in the 1950s, has matured into an alliance between scientists and cinematographers to develop underwater imaging instruments with unique requirements for pressure, refraction, and lighting. These modern sensors, along with advanced robotics and unmanned underwater vehicles, may yet reveal unimagined mysteries in the deep.
After all, we never would have known that the fangtooth fish absorbed more than 99 percent of photons if someone hadn't tried to take a picture of it—with a flash.
Gwen Weerts, Photonics Focus Editor-In-Chief
