Taken from the November 2017 issue of Physics World

In the deep, dark depths of the ocean, where chimneys spout hot black clouds of particles, there is life. Astronomer Jon Willis sets out on a voyage to understand these harsh environments and search for clues about whether life could also survive on watery moons in our solar system

Friday 16 June 2017, 8.30 p.m., 260 km off the west coast of Vancouver Island in the north-east Pacific Ocean. I’m in a darkened control room on the exploration ship Nautilus. Roaming below, remotely operated vehicles (ROVs) Argus and Hercules are diving into the ocean depths, while their pilots and navigators sit beside me with rapt faces illuminated in the glow of giant HD monitors.

Staring at the screens, our 10-strong exploration team is gripped by a scene enacted from a science-fiction movie: an alien landscape of sculpted and forbidding towers is emerging from the dark abyss. Black, mineral-rich clouds are billowing from a panorama of irregular chimneys created by fissures in the Earth’s surface called hydrothermal vents. Some of these obelisks are giants, such as the aptly named Godzilla, which grew to a height of 45 m before collapsing under its own weight. Others feature delicate fluted branches and fans like underwater candelabra. And in addition to these giant smoking chimneys there is a multitude of smaller vents, some like miniature engines with spewing exhausts, others, emitting cooler, clearer water, like shimmering mirages in a scorched desert.

It is to this seemingly industrial landscape engineered by heat and chemistry that I have come to glimpse the future, to imagine echoes of the technology before me applied to the vast ocean worlds of our solar system where, on moons such as Jupiter’s Europa, we may one day look for new life.

Astronomer turned seafarer

My background, my day job if you like, is astronomy. Based at the University of Victoria in Canada, I use large telescopes and space-based observatories to study the universe at its largest scales, looking at clusters of galaxies and their distribution over the sky. However, I also have a keen interest in astrobiology – the search for life beyond Earth. As that search takes its first, faltering steps, we still have much to learn from our own planet about life in extreme and novel environments. This is why I have come to see the deep-sea hydrothermal vents, for the geochemical energy coursing through the multitude of chimneys and towers also supports truly unique ecosystems in conditions that many believe may exist within the ice moons of Jupiter and Saturn.

I have joined the Nautilus for two weeks as a science communication fellow, one of a group of formal and informal educators with a passion for outreach, selected by the Ocean Exploration Trust (OET) – a non-profit, US-based society that seeks to bring ocean discoveries to an enthused and interested public. Its president and founder is Robert Ballard, a marine geologist who has spent a lifetime exploring the world’s oceans with innovative technology and is best known for discovering the Titanic shipwreck in 1985.

This Nautilus voyage is part of Ocean Networks Canada’s (ONC) Expedition 2017 – Wiring the Abyss. ONC, an initiative of the University of Victoria, operates deep-sea observatories off the coasts of Canada and in the Arctic, and our two-week voyage will be taking us to numerous locations to perform a variety of tasks. Currently though, we are floating above one of their deepest sites – Endeavour. Located at depths of 2200–2600 m, Endeavour is part of the Juan de Fuca (JdF) Ridge – a boundary between the Pacific and JdF tectonic plates. The site is one of five connected via the North East Pacific Time-series Underwater Networked Experiments (NEPTUNE) observatory – an 840 km loop of power and fibre-optic cable that links a network of instruments on the sea floor to scientists ashore. NEPTUNE spans the JdF plate and offers researchers access to a wide range of deep ocean geological environments within a relatively small geographical area.

NEPTUNE was switched on in 2009 and, in many ways, it functions like the local power network in your own town or city. At intervals along the cabled system are clusters of instruments around sites of scientific interest. At these “nodes” the main line voltage of 10,000 V is stepped down to 400 V by transformers and directed to junction boxes, each functioning like the circuit panel in your own home. From here, power at voltages between 15 and 48 V is distributed via extension cables to individual instruments installed on the sea floor. Fibre-optic cables run through the entire network and allow scientists ashore to gather data in real time – with 500 terabytes of data and counting all stored on ONC’s servers.

In case you were thinking that all of this sounds fairly straightforward, just remember that all of this hardware has to be installed and operated at depths of up to 2600 m below sea level and up to 260 km offshore. The water temperature is just 2 °C above freezing, the pressure of overlying water is up to 260 atmospheres (about 26,350,000 Pa) and it is very, very dark.

This is where the Nautilus comes in. Named after Captain Nemo’s ship in Jules Verne’s novel Twenty Thousand Leagues Under the Sea, this 64 m long ship offers a comfortable, cosy home for 17 professional crew and 31 scientists, engineers and educators. My own personal piece of real estate while aboard is a bunk in cabin 81. Located in the deepest, darkest section of the vessel, the cabin might seem a poor deal, designed for those on the lowest rung of the crew ladder. However, I soon discovered that being close to the ship’s roll axis means that heavy seas disturb me less than my crew mates higher up in the ship (literally and figuratively). The lack of a porthole in my cabin is also a welcome blessing when I try to nap during the middle of the day (a regular occurrence given that my watches are 4–8 a.m. and 4–8 p.m.). As a science-communication fellow, my job while on watch is to occupy the comms seat in the control van – acting as the human link between the watch crew and the public ashore following us on nautiluslive.org. I am afforded a ringside seat, my only responsibility being to convey the interest and excitement of deep-ocean exploration to our online audience.

It is to explore the ocean at great depth that the Nautilus deploys its two ROVs Hercules and Argus. The Argus of antiquity was a giant commanded by Hera to watch over the nymph Io with his 100 eyes, and the current-day ROV Argus lives up to the name by acting as a chaperone to Hercules. The two ROVs are linked together by a 50 m neutrally buoyant cable, and Argus is then attached to the Nautilus via a 4 km-long cable that provides power and fibre-optic connectivity. The ROV-babysitter, which can go to depths of 6 km, serves as both a watcher, with HD cameras keeping an eye on Hercules, and a shock absorber, preventing the rolling surface motion of the Nautilus from affecting its charge. Meanwhile, Hercules does the research to depths of up to 4 km – it operates as a completely stable work platform and, with two manipulator arms, a suite of thrusters and HD cameras, it provides its pilot aboard the Nautilus with an immersive sense of presence on the sea floor.

Underwater chimneys

The hydrothermal vent systems we have come to explore are associated with tectonic boundaries where weakness in the Earth’s crust results in the magma of the liquid mantle approaching far closer to the surface than anywhere else – to within 1 km in some locations, compared to 5–10 km in most other areas on the ocean crust. Seawater percolating down through faults and fissures is heated near the magma and this abundance of energy powers aqueous reactions with rocks that saturate the superheated water with dissolved minerals. Chief among these are sulphides of iron, copper and zinc.

As water is heated it becomes less dense and therefore more buoyant than the seawater above it. The faulted and fractured rocks overlying tectonic plate boundaries are therefore like a complex plumbing system, with cold seawater descending and hot mineral-rich water ascending though the rock layers. Upon reaching the sea floor, these plumes of hot water, which can be up to 400 °C, encounter seawater with an ambient temperature of 2–4 °C. The crushing pressure at Endeavour maintains the superheated water in a liquid state yet contact with colder water causes dissolved minerals inside it to precipitate. In particular, as the iron sulphide precipitates it forms dense hazes of fine black particulates which, when viewed underwater, appear as belching clouds and give these vent systems their common name – black smokers.

The temperature gradients around the vent systems are immense and test the nerve of any submersible or ROV pilot with the temerity to explore them. Water escaping at the very base of a vent may be as hot as 400 °C and rises in a vertical, expanding plume. However, a probe 1 m off to the side will still register the temperature of ambient sea water, say 3 °C in this case. Even as the probe is inched closer, to within a few centimetres of the plume base, it will only register a temperature of 20 °C or so. When black smokers were first discovered, the pilots of the submersible Alvin used a manipulator arm to insert a temperature probe directly into water escaping from the base of a vent. The probe, constructed of the same perspex material as the viewing ports, promptly melted and the pilots beat a measured yet deliberate retreat. Indeed, one treasured memento from my visit to Endeavour is a section of three-quarter inch electrical cable in a thick plastic sheath. It had the misfortune to fall across the opening of a seemingly innocuous vent and, in the space of about 1 s, it melted and parted – now appearing like an oversized bright-green stalk of asparagus.

One of our first tasks when we arrived at Endeavour was to install a probe into the water emerging from a specific vent location and determine the concentration of dissolved ions. Known as a Benthic and Resistivity Sensor (BARS), the device consists of a custom-built titanium pressure cylinder housing the electronics needed to measure temperature and resistivity. Set on short stubby legs about 15 cm long, the BARS receives data from a ceramic probe inserted into the base of a nearby vent system. This is one example where extended time series data provide critical insight as the temperature and chemistry of the emerging vent water can vary rapidly in response to changing conditions in the reaction zone several hundreds of metres below. However, to be of value, the water must be sampled as it emerges from the very base of the vent, right on top of the fissure in the volcanic rock.

I was therefore shocked that my first encounter with a hydrothermal vent would be to topple a 2 m tower to expose the base of the vent. Had I come all this way just to nuke the chimney? I need not have worried as vent chimneys have been observed to grow at prodigious rates – up to 30 cm in a day and up to 5 m over the span of a year. BARS devices have themselves been embedded within tall chimneys that have enveloped them during their time on the sea floor. On a personal note, any chagrin I felt at my involvement in toppling the chimney was replaced with scientific glee as I was later able to handle a sizeable chunk of the vent material – the grey, friable rock has the strange consistency of highly compressed cigarette ash. Sadly, my prized sample will dry and crumble with time, the particles of iron sulphide steadily rusting as they are oxidized in our atmosphere. Vent chimneys, and the hydrothermal activity that creates them, are transient phenomena.

Life in a dark, cold world

Geologically speaking, these vent systems were a revelation when discovered in 1977 – they provide the missing link in understanding both heat flow through Earth’s crust and the chemical composition of its oceans. What was completely unexpected, however, was that such environments, well below the reach of any sunlight, would host abundant and unique ecosystems.

When scientists visited the first of these black smoker vents, they were astounded with views of gardens of giant tube worms wafting in the turbulent currents at the periphery of the vents – their vivid red bodies and gills in stark contrast to their white, calcareous tubes. Ghostly white crabs prowl through this forest of tubes and frond-like gills, snipping off tasty morsels from any tube worm too hesitant in withdrawing. Gauzy mats of filamentous bacteria are harvested by hosts of eyeless shrimp, some of which transport their own travelling gardens of bacteria growing on their undersides.

The important question is what supports this complex ecosystem, where the pressure is as high as 260 atm (it increases by 1 atm for every 10 m in depth), there is no sunlight whatsoever and the temperature is near to freezing. The key resource turns out to be hydrogen sulphide (H 2 S) and oxygen (O 2 ) dissolved in the sea water. Combined with dissolved carbon dioxide (CO 2 ), these gases provide an abundant and constantly renewed source of geochemical energy for colonies of microbes. However, what was originally poorly understood at the time, yet has since been revealed in a number of elegant studies, is the extent of the symbiotic relationships between macrofauna and microbes.

The critical requirement for the tube worms is that they must simultaneously provide their symbiotic bacteria with both H2S from the vent fluid and O 2 from ambient seawater. Access to both occurs on the turbulent periphery of vent systems, in waters of temperature 2–60 °C, where partial mixing of vent and ambient seawater exposes organisms to both reservoirs of gas on timescales of tens of seconds or less. ONC’s infrastructure offers a simple, yet highly effective, way to study this mixing process by laying out a sensor mat over the garden of tube worms. Consisting of a wired grid of temperature and dissolved oxygen sensors, the mat allows competing flows of vent and ambient seawater to be monitored in real time and checked visually using a pan-and-tilt video camera equipped with powerful lights (though to avoid disrupting life at the vents the lights are used sparingly).

Upon seeing these ecosystems for the first time in 1979 the late biologist and oceanographer Holger Jannasch, from the Woods Hole Oceanographic Institution, made an immediate astrobiological connection. “We were struck by the thought, and its fundamental implications,” he once recalled, “that here solar energy, which is so prevalent in running life on our planet, appears to be largely replaced by terrestrial energy – chemolithoautotrophic bacteria taking over the role of green plants. This was a powerful new concept and, in my mind, one of the major biological discoveries of the 20th century.”

Deep-ocean, sulphide-oxidizing bacteria remain metabolically reliant on surface photosynthetic organisms (plants, algae and bacteria), which provide all of the dissolved oxygen in Earth’s oceans. Anaerobic chemosynthetic bacteria and archaea – microbes that can metabolize geochemical energy without using molecular oxygen – are present but rarer and less well studied than their oxygen-breathing brethren. Clearly there is still much to learn.

My watch is ending

Our time at Endeavour is nearly done. The four hours of my watch have passed in a shimmering blur and by the time I come back for my next shift on comms, the exploration group will have moved on to the next task of the voyage. While this particular dive has lasted nine hours, more demanding, complex dives with Hercules and Argus can go on for up to 72 hours. In using remotely operated yet highly capable vehicles controlled over a near-instantaneous link, each dive shows how it is feasible that such techniques may one day be used to explore oceans beneath the ice sheets of Europa and other moons in our solar system. By being present at the Endeavour hydrothermal vent system, I hoped to see vents as close-up as possible, yet also glimpse a more distant vision – of one possible future way to explore beyond our planet. Those were my hopes and I was not disappointed.

A lunar ocean on Europa?

Arriving at Jupiter in 1995, the NASA spacecraft Galileo began the first detailed exploration of our solar system’s largest planet and its retinue of moons. One of Galileo’s major discoveries was of a weak magnetic field emanating from Jupiter’s second closest moon, Europa. Oddly, this field rotated not once every 3.6 days, as Europa rotates, but every 9.8 hours, in step with Jupiter itself. The inference is that Europa’s magnetic field is induced by Jupiter’s: a physical effect that requires an electrically conductive layer beneath the surface ice sheet. The Galileo observations point to a 100 km deep, lunar-wide, salty, liquid water ocean – a volume of liquid water equal to two times that of planet Earth. Internal heat, caused by rhythmic tidal forces raised as it orbits Jupiter, maintain Europa’s ocean in a liquid state. Furthermore, the inferred presence of salt in Europa’s oceans suggests that liquid water is in contact with rock. On Earth, heat flow across rock–ocean boundaries occurs most intensely at deep ocean hydrothermal vents and the possibility that such vents may exist within Europa’s oceans is motivating a new generation of solar system oceanographers in their quest for new discoveries.

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