The Arctic Ocean is one of the most remote and inhospitable places on Earth and, as a result, is the last major ocean basin on Earth to be explored for seafloor fluid flow (hydrothermal vents and cold seeps) and any unique life that might be found nearby. A key scientific goal for this expedition will be to locate and explore sites of hydrothermal venting on the Karasik Seamount about 200 miles from the North Pole. In addition, the cruise will serve as a test for methods and techniques that may help NASA define its procedures to identify signs of biological activity on other ocean worlds in our solar system and beyond, starting with Jupiter’s moon Europa during an orbital mission to the Jovian moon planned for launch in the 2020’s.
Up until the late 1970s, scientists thought all life on Earth was supported by photosynthesis, that is, every living organism on the planet could trace the origin of its food chain back to plants and algae that convert sunlight and carbon dioxide into oxygen and simple carbohydrates. But expeditions to the mid-ocean ridge, beginning in the Pacific Ocean near the Galapagos in 1977 found a surprising diversity of life around places where hot, mineral-laden water was pouring from the seafloor. Not only did the discovery demonstrate that life could exist far from the reach of sunlight, it also revealed a previously unknown process known as chemosynthesis, in which, like photosynthesis, organisms at the base of the food chain (in this case bacteria) convert inorganic chemicals such as hydrogen sulfide (H2S) and methane (CH4) to drive their metabolic processes.
These bacteria either become food or provide food for other, larger animals on the seafloor. After 40 years, the majority of the globe-spanning mid-ocean ridge remains unexplored, but scientists have still found dozens of vents sites and more than 600 previously unknown species—a rate of discovery of one new species every two weeks.
But early searches for hydrothermal vents focused on the most volcanically active and fastest-spreading (10-20cm/yr) segments of the mid-ocean ridge because, it was thought, venting was dependent on volcanic eruptions and magma upwelling beneath the seafloor. The slow and ultra-slow ridges (1-2cm/yr), where repeat episodes of eruptions are few and far between and ocean spreading is driven primarily by tectonic forces pulling plates on either side of the ridge apart, were thought to be too “cold” to support active venting.
In the mid-1990s, however, WHOI’s Chris German (a co-principal investigator of this cruise) lead his first expedition to conduct a careful, systematic search for hydrothermal venting along a section of the Mid-Atlantic Ridge and found evidence for twice as much vent activity as had been predicted by a model that assumed that spreading-rate (and therefore volcanic activity) was the master variable that controlled venting. The key turned out to be that venting doesn’t only occur where there is fresh volcanism. It can also occur wherever deep cracks in the crust allow seawater to penetrate deep beneath the seafloor and into the hot rocks found in Earth’s interior.
This opens the possibility for heated seawater to come into contact with different types of rock and to extract a much wider range of chemicals that, in turn, drives a wider and more diverse range of chemical reactions (and microbiological processes) when this fluid returns to the surface. By the early 2000s, evidence existed for venting on even the planet’s slowest spreading ridges—the Southwest Indian Ridge in the Indian Ocean and the Gakkel Ridge in Antarctica. However, because of the remote locations of each, it took another decade until the first vents on the Southwest Indian Ridge were documented as part of a U.S.-China expedition using WHOI’s first deep-diving autonomous underwater vehicle ABE on the Chinese ship Da Yang Yi Hao in 2007 to photograph, but not to sample, the new vent sites and what was living there.
Building on from that, however, a series of expeditions to the equally ultra-slow spreading Mid-Cayman Rise in the Caribbean, conducted between 2009 and 2013, have now helped set the stage for exploration of the Arctic’s Gakkel Ridge by enabling engineers to come up with new tools and techniques that allow scientists to conduct systematic surveys of deep vent sites while remaining at the mercy of sea ice movement on the surface. They did this by developing the hybrid remotely operated vehicle (HROV) Nereus, which was linked to the surface via a lightweight fiber optic tether that allowed it to travel further from the ship or, as would be needed in the Arctic, to work at one location on the seafloor while the ship moves elsewhere, pushed around by the drifting sea-ice.
In 2014, Chris German and a team that worked on Nereus traveled to the Arctic to conduct sea trails of a new vehicle, Nereid Under Ice (NUI), that—like Nereus—is also equipped with an optical fiber tether that allows the vehicle to travel far from its support ship when in remotely operated mode. In addition, NUI is designed to be able to look up and work at the underside of sea ice where biological processes are an important part of the Arctic Ocean ecosystem as well as to work close to the rugged seafloor.
This sets the stage for the current expedition—the hunt for hydrothermal vents in the last unexplored ocean basin on Earth and the one that may very well be best placed to help set the stage for the search for life beyond our planet.
Life on Ocean Worlds
While scientists on the cruise have their eyes open for hydrothermal vents in the Arctic, they will have their minds on other, more distant places. NASA has identified up to ten planetary bodies in our solar system alone —including several of the moons of Jupiter and Saturn—that have globe-spanning, salty oceans trapped beneath a thick sheet of ice. Two—Jupiter’s moon Europa and Saturn’s moon Enceladus, are even known to have a rocky seafloor at the base of those salt-water oceans. This sets up the potential to satisfy the basic conditions for life as we know it: liquid water, a plausible source of metabolic energy in organisms, nutrients provided by water-rock interactions at the seafloor, and—assuming those moons are long-lived—the time for life to become established.
In the Arctic, the team will use NUI first in autonomous (free-swimming) mode to hunt for chemical signs in the seawater of venting deep below. The vehicle will then dive close to the seafloor to map the terrain and then photograph promising sites for further exploration. Then, in remotely operated mode, with video feeds returned in real time to the pilots and scientists aboard ship, the team will send NUI down to collect samples and document the chemical and biological makeup of what’s living on the seafloor, including any vent communities they discover. Finally, they will follow the chemical and biological trail from any vents they find back up through the water—potentially all the way to the underside of the sea ice—where another team will set out from the ship onto the top of the floating ice to collect cores that they hope will contain signs of life from the ocean floor, far below. What they discover could help NASA decide how to conduct future exploration of other icy ocean worlds in our outer solar system, starting with a planned fly-by mission to Jupiter’s moon Europa—another world with an icy crust and a liquid saltwater ocean in contact with a rocky seafloor, much like those in the Arctic.