bristle worm

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Challenger Deep

The Challenger Deep is the deepest surveyed point in the oceans, with a depth of about 11,000 metres (about 36,000 feet). The exact depth is unknown. It is located in the Mariana Islands group at the southern end of the Mariana Trench. The closest land is Fais Island, one of the outer islands of Yap, 289 km (180 miles) southwest and Guam 306 km (190 miles) to the northeast. The point is named after the British Royal Navy survey ship HMS Challenger of 1872-76.

Surveys and descents

The maximum surveyed depth of the Challenger Deep is 10,923 meters (35,838 ft) or 6.7875 miles. (National Geographic puts the depth at 10,920.07 meters (35,827 ft) below sea level.) The pressure at this depth is approximately 1,095 times that at the surface, or 110 MPa.

The HMS Challenger Expedition (December 1872 – May 1876) first sounded the depths now known as the Challenger Deep. This first sounding was made on 23 March, 1875 at station 225 The reported depth was 4,475 fathoms (8,184 m, 26,850 ft), based on two separate soundings.

A 1912 book, The Depths of the Ocean by Sir John Murray, records the depth of the Challenger Deep as 31,614 feet (9,636 m). Sir John was one of the Expedition scientists, a young man at the time. Page 131 of Murray's book refers to the Challenger Deep. All of the original reports of the Challenger expedition can be viewed on the web at the Challenger Library

In 1951, about 75 years after its original discovery, the entire Mariana Trench was surveyed by a second Royal Navy vessel, captained by George Stephen Ritchie (later Rear Admiral Ritchie); this vessel was also named HMS Challenger after the original expedition ship. During this survey, the deepest part of the trench was recorded using echo sounding, a much more precise and vastly easier way to measure depth than the sounding equipment and drag lines used in the original expedition. HMS Challenger measured a depth of 5,960 fathoms (10,900 m, 35,760 ft) at .

On 23 January 1960, the Swiss-built Bathyscaphe Trieste, acquired by the U.S. Navy, descended to the ocean floor in the trench manned by Jacques Piccard (who co-designed the submersible along with his father, Auguste Piccard) and USN Lieutenant Don Walsh. The descent took almost five hours and the two men spent barely twenty minutes on the ocean floor before undertaking the three-hour-and-fifteen-minute ascent. They measured the depth as 10,916 metres (35,813 ft).

In 1984, a Japanese survey vessel using a narrow, multi-beam echo sounder took a measurement of 10,924 meters (35,838 ft).

On 24 March, 1995 the Japanese robotic deep-sea probe Kaiko broke the depth record for uncrewed probes when it reached close to the surveyed bottom of the Challenger Deep. Created by the Japan Marine Science and Technology Center (JAMSTEC) it was one of few uncrewed deep-sea probes in operation that could dive deeper than 6,000 metres (19,680 ft). Its recorded depth of 10,911 m (35,797 ft) for the Challenger Deep is believed to be the most accurate measurement taken yet. Kaiko also collected a sediment core from the bottom of the deep.

The Woods Hole Oceanographic Institution is planning to send its Nereus hybrid remotely operated vehicle (HROV) to the Challenger Deep some time in 2008.

Lifeforms

On their 1960 descent, the crew of the Trieste noted that the floor consisted of diatomaceous ooze and reported observing "some type of flatfish, resembling a sole, about long and across" lying on the seabed. The report has since been questioned, with suggestions that it may have been a sea cucumber. The video camera on board the Kaiko probe spotted a sea cucumber, a scale worm (a type of bristle worm) and a shrimp at the bottom.

An analysis of the sediment samples collected by Kaiko found large numbers of simple organisms at 10,900 metres water depth. While similar lifeforms have been known to exist in shallower ocean trenches (>7,000 m) and on the abyssal plain, the lifeforms discovered in the Challenger Deep possibly represent taxa distinct from those in shallower ecosystems.

The overwhelming majority of the 432 organisms collected were simple, soft-shelled foraminifera, with four of the others representing species of the complex, multi-chambered genera Leptohalysis and Reophax. 85% of the specimens were organic soft-shelled allogromids, which is unusual compared to samples of sediment-dwelling organisms from other deep-sea environments, where the percentage of organic-walled foraminifera ranges from 5% to 20%. As small organisms with hard calcated shells have trouble growing at extreme (10,000 m) depths because the water there is severely lacking in calcium carbonate, scientists theorize that the preponderance of soft-shelled organisms at the Challenger Deep may have resulted from the typical biosphere present when the Challenger Deep was shallower than it is now. Over the course of six to nine million years, as the Challenger Deep grew to its present depth, many of the species present in the sediment died out or were unable to adapt to the increasing water pressure and changing environment. The remaining species may have been the ancestors of the Challenger Deep's current denizens.