Uncovering the Secrets of the Mariana Trench

Hundreds of people climb Mount Everest every year, and 27 astronauts have been to the moon, yet only 3 people have ever visited the lowest point on earth. This point, known as Challenger Deep, rests 36,070 feet below sea level at the bottom of the Mariana Trench in the western Pacific Ocean [1]. In March 2012, Avatar director James Cameron made history by making the first solo dive to the bottom of the trench and by using the second manned craft to ever reach the bottom. His vessel, Deepsea Challenger, took seven years to build and was designed specifically to reach the bottom of the Mariana Trench. Cameron’s purpose was not to set records but to document and analyze the conditions in the deepest parts of the ocean using modern technology. In recent years these marine trenches have come under increased scientific interest due to the presence of unique deep-sea life forms and the major role sea trenches play in the redistribution of the earth’s crust. Building dive craft that can travel to the sea floor and study these phenomena is no easy task though since the conditions that exist a mere 6.8 miles below the surface can crush all but the toughest of submersibles.

Diving to the deepest part of the ocean might seem like a pointless achievement, but exploring the deep ocean floor is beneficial to society since it will help scientists to better understand the world we live in. Water covers more than 70% of the planet’s surface, yet to date we have explored only 5% of the space taken up by oceans [2]. In particular, deep-sea trenches interest many scientists in the fields of geology and marine biology. The movement of the earth’s tectonic plates forms peaks and valleys in the earth. Areas such as the Mariana Trench are formed when one plate slides beneath a second tectonic plate in a process known as subduction (see Fig. 1).

Subduction zones are responsible for most of the active volcanoes on earth as well as most major earthquakes and tsunamis. In fact, 9 of the 10 largest earthquakes of the past century have occurred in subduction zones, as did the earthquake and tsunami that devastated Japan in March 2011 [3]. By exploring these fault lines and understanding how the plates interact with each other, geologists hope to better understand these natural disasters and more accurately model fault movement in subduction zones. The sea floor is also of great interest to biologists due to the unique life forms that live in the dark depths of the ocean. The average depth of the ocean floor is 12,200 feet, with sunlight penetrating only 1,000 feet below the waves [4]. Consequently, organisms living near the sea floor cannot use the sun as a source of energy and have developed unique alternatives to sustain themselves. For instance, while exploring geothermal vents in the earth, scientists discovered chemosynthetic bacteria that live off of the hydrogen sulfide exhausted from these deep-sea hydrothermal vents. These bacteria provide an energy source for other life forms and allow a unique ecosystem to thrive near these deep-ocean vents [5]. Scientists hope to analyze the DNA of these organisms and other sea creatures in order to understand how they function. Furthermore, since the sea floor contains large deposits of oil and minerals, finding a way to collect them safely and economically would prove to be very profitable.

Although exploring the ocean floor would greatly benefit society, charting the depths of the sea is no easy task. A deep-sea submersible faces the same problems as any other conventional submarine. For example, electrical fires, loss of power, or mechanical failures could leave the pilot trapped in the near-freezing depths of the ocean without light or much oxygen. However, the largest problem facing deep-sea submersibles is the incredible pressure at the bottom of the ocean, known as hydrostatic pressure. Through hydrostatic pressure, water pressure increases in proportion to the height of the water in the column, and the water at the bottom has to support the weight of the incompressible fluid. At the average ocean floor depth of 12,200 feet, there is a column of water more than 2 miles high pressing down on the sea floor and any submarine that travels there. At this depth, the pressure is 5,540 pounds per square inch (psi), or 377 times sea level pressure. Thus, in order to reach the bottom of the Mariana Trench, a vessel such as the Deepsea Challenger needs to be able to withstand over 16,000 psi of pressure to avoid being crushed like a tin can [6]. The high pressures at the ocean floor also make it difficult for deep-sea submersibles to dive and resurface.

The basic physical principle allowing a submersible to function is buoyancy. A conventional submarine has ballast tanks that can fill with either air or water, changing the density of the craft and allowing the submarine to either dive or surface. In a modern submarine, compressed air is used to flush seawater out of the ballast tanks and allow the submarine to surface. Most modern submarines have maximum depth ratings of less than 2,000 feet; this means that at that depth, the pressure of the water exceeds the pressure of the compressed air, preventing the water from being removed and causing the submarine to sink. For deep-diving vessels such as the Trieste and Deepsea Challenger, the dive mechanism is very simple. Massive weights are attached to bottom of the craft to weigh it down while the craft itself it is designed to float. With all of the weights attached, the dive vessel sinks to the bottom. When the submersible wants to remain stationary, it drops some of the weights until it stops moving in depth. When the vessel wants to surface, it simply drops the rest of the weights on the ocean floor and floats back up. However, the weights must be precisely calculated based on the maximum depth that the craft wants to achieve, because the density of the craft will increase as the pressure increases [6]. Even though this method requires refitting the weights after each dive, it remains the fastest and most economical way to reach the sea floor.

Although the basic physical principles used to reach Challenger Deep have not changed in the past 60 years, the technology used in deep-sea diving has allowed the submersibles to become practical scientific instruments. The first craft to ever reach the Mariana Trench was the Bathyscaphe Trieste, which was built in 1948 and seated two people [7]. It weighed 150 tons and consisted of a giant float suspended over a thick metal diving sphere where the crew was confined (see Fig. 2). The float was filled with 22,000 gallons of gasoline, which is incompressible and lighter than water, and seawater could be taken in to lower the density as needed [8].

Although the Trieste’s dive was a major feat of engineering, this craft was not a practical ship to use for scientific exploration. The Trieste had few lights, no video cameras, no way to collect samples from the sea floor, and only spent less than half an hour at Challenger Deep. An ABC report of the dive stated, “They [Walsh and Piccard] said at the time that their sub kicked up so much muck that there was almost nothing visible through their thick viewing ports” [9]. However, the development of syntactic foam in the 1960s led to a new building material for diving vessels that were both buoyant and durable, eliminating the need for a massive gasoline-filled float. The new material allowed for the development of much smaller craft, outfitted with more modern technology to allow for economical scientific exploration. In addition, advancements in robotics allowed deep-sea craft to be fitted with mechanical arms, enabling them to interact with the environment and collect samples for research. These new compact submersibles, such as the DSV Alvin, have been used in a number of tasks, ranging from exploring the Titanic to disarming nuclear warheads on the ocean floor.

Although data gathered by Alvin has been referenced in over 2,000 scientific papers, the submersible can only dive to a depth of 14,800 feet and thus can only reach 63% of the ocean floor [10]. James Cameron’s Deepsea Challenger builds upon the previous designs of syntactic foam-based craft by using an improved foam that can resist the extreme pressures of the Mariana Trench and allow it to travel anywhere in the ocean. Cameron’s submersible has a sleek aerodynamic design allowing it to descend and resurface much faster than the Trieste, allowing for more time spent on the sea floor. Like Alvin and similar dive robots, the Deepsea Challenger is equipped with a robot arm to collect samples from the Mariana Trench. What makes the submersible unique are the massive light and camera arrays attached to the midsection of the craft (see Fig. 3). Cameron wanted to capture the environment with high-definition 3D video cameras, so 5 cameras were custom built and attached to the sub so Cameron could bring stunning footage of the Mariana Trench back to the surface. Cameron hopes to use these cameras to both entertain audiences and to enhance scientific research.

Looking to the Future

The information gathered from the Deepsea Challenger and similar dive crafts has a myriad of possible applications. For instance, NASA astrobiologist Kevin Hand is currently reviewing James Cameron’s dive footage to assess the possibility of life in the deep oceans of Jupiter’s moon Europa [11]. Furthermore, since life on earth began in the oceans, understanding how organisms survive and replicate in the harsh conditions of the ocean floor will further scientists’ knowledge of how life developed on earth. Governments have also discussed the prospect of disposing radioactive waste or excess carbon dioxide by embedding it in the sea floor [12]. If this could be done with minimal environmental impact, this disposal would temporarily solve many of our hazardous waste problems. Geologists could also use the craft to study underwater subduction faults to possibly predict future earthquakes and tsunamis, which could save countless lives.

Since so much of the ocean remains a mystery, the possibilities are almost endless. Moreover, exploring the ocean is more financially feasible than one might expect. Former director of the National Oceanic and Atmospheric Association Dr. Sylvia Earle recently said, “A fraction of what we invest going skyward would answer some major questions about this part of the Solar System [the ocean]” [1]. Outer space may be the final frontier, but we must first find out what opportunities are hiding beneath the waves. Cameron’s visit to Challenger Deep may have only been the second manned trip in history, but it will not be his last. According to Cameron, “It’s not a one-time deal and then moving on. This is the beginning of opening up this new frontier” [9].