H.M.S. Challenger and the cosmic dust

Press, 15 October 1982, Page 18

H.M.S. Challenger and the cosmic dust

When H.M.S. Challenger docked in New Zealand on its round-the-world trip of scientific discovery in 1874 no-one could have guessed that it would revolutionise astronomers’ theories of the origin of the solar system. DUNCAN STEEL recalls the occasion.

In the early 1870 s. the Royal Society of London was keen to mount another transglobal scientific expedition, hoping to repeat the success of the Beagles voyage 40 years before.

Zealand 10 days later it was to stumble upon a vast, unexpected source of meteorites, a reservoir of extraterrestrial material unequalled in importance until the Apollo astronauts returned with samples from the Moon almost a century later. And it is still the cause of debate among astronomers and space scientists the world over.

The Beagle had been blessed, somewhat fortuitously. with a .young Cambridge biologist by the name of Charles Darwin. His diligent observations had led to the synthesis of his theory of evolution. The Royal Society wanted to see whether similar scientific leaps would result from a new, wellequipped expedition. On June, 28, 1874. H.M.S. Challenger berthed in Wellington after a fortnight's voyage from Sydney. The crew were glad to get ashore. They had spent two days at anchor in the lee of D'Urville Island waiting for a gale to abate before Challenger was able to pass through the Cook Strait. Incredible though it may seem, when the ship left New

After much persuasion the Royal Navy supplied a suitable vessel, and the government paid for the modifications and specialised equipment which were needed for the purpose in hand. H.M.S. Challenger was a three-masted corvette with auxiliary steam power, and a crew of 243. After refurbishment as a floating laboratory, including a dark-room for the youthful science of photography, she sailed from Portsmouth in December. 1872.

Over the next three years and a half the vessel spent a

thousand days at sea. covered 100.000 kilometres, and crossed the equator eight times on its round-the-world trip.

impenetrable bush and dense forest which surrounded Wellington.

One final comment of Moseley's is particularly interesting: "In the poulterer's shop the curious parrot, or Kaka, is hung up for sale." Challenger had earlier crossed the Antarctic Circle. In the next phase of the expedition the ship went north to Fiji, the New Hebrides. New Guinea, the Philippines, Japan, and then east to Hawaii. Tahiti, and Chile before rounding the Horn and sailing home to Britain.

Although the ship was under the over-all command of Captain George Nares, the contingent of scientists were the responsibility of Sir Charles Wyville Thomson, from the University of Edinburgh. Staff funded by the government included two naturalists, Henry Nottidge Moseley and John Murray, and a physicist-chemist, John Young Buchanon.

These three were each on a salary of 200 pounds a year, but were probably somewhat chagrined that the expedition's artist, a Mr J. J. Wild, received twice that sum.

Charting the ocean bottom between Sydney and Wellington had not been difficult as the vessel was equipped with special apparatus for dragging the depths of the sea. The huge dredge net had already been used to return samples from the ocean floor more than 3000 fathoms below the surface. Many strange, exotic fish and invertebrates had been plucked from their lairs, most of them previously unknown to science.

On their route to the Pacific, via the Cape of Good Hope, the Indian Ocean, and the southern coast of Australia. the Challenger’s scientists examined and classified flora and fauna of the marine environment, and also the environs of their ports of call. After a stay in Sydney, Challenger crossed the Tasman on a slow, storm-tossed leg of its voyage during which the mariners continuously took soundings of the ocean floor. This was to ascertain whether it would be possible to lay a submarine cable to link Australia and New Zealand.

Although raising the dredge from the depths was a strenuous task, actually sinking it in the first place was also a trial. It apparently took more than three hours to feed out the cables as the net gradually settled to the sea-bottom.

The sediments pulled to the surface were termed by the Challenger’s naturalists as being “abyssal” or “pelagic” deposits. In the Pacific basin the sediment was largely found to be made up of red clay, but several unexpected items were found.

Perhaps it was partly due to the unpleasantness of their trip across the Tasman that the crew were not impressed by the young colony. The naturalist Moseley said of Wellington that “the town necessarily contrasts unfavourably in appearance to Sydney,” and noted that all of the houses were constructed of wood. In his diary of the trip, Moseley described the Maori carvings which he saw, and also the

John Murray and a guest explorer from the University of Ghent in Belgium, the Abbe Renard, noted first of all the presence of nodules of manganese typically spherical and a few centimetres in

diameter. These are now known to be widely strewn about the ocean floor and will probably be a major world source of the ore over the next few decades.

Murraj' had a surprise, however.' when he sieved out the tiny grains of sand and grit, and passed a magnet over the top of them. He found that hundreds of black globules were attracted by the magnet, having a core of metallic iron. When he analysed their chemical composition Murray found in addition that unusual combinations of other minerals, such as cobalt and nickel, were present in the globules. Intrigued by his discovery, Murray started to sift through other portions of the sediment under a microscope, and found many other grains of types with which he was not familiar—certainly they did not occur in the mud, sludge, and. sand dragged up from shallower waters.

Some were globules of other metals, always less than a millimetre in diameter, and suggested to Murray that as some stage they had melted, formed into droplets, and then solidified, much like the manufacture of lead shot. Others were irregular in form with a bronze lustre; and still others looked like small splinters of brown stone.

At last Murray’s diligent sifting and sorting paid off. He found a type of sediment which was both unusual and identifiable. After checking and rechecking, Murray was convinced that he was peering down his microscope at samples which appear appeared identical to many meteorites, but in miniature. With this clue in mind, he compared the mineral constitution of the other odd samples (such as the iron globules) with examples of different types of meteorites known to geologists. Murray came up trumps. He found that all of his previously unidentifiable grains could now be paired with meteoritic classifications, and hence were extraterrestrial in origin. Murray christened these particles

"cosmic dust." a name which describes them perfectly. The next question needing to be answered was why this concentration of meteoritic material, totally unexpected by the Challenger's scientists, covered the ocean floor.

The solution was quite straightforward. Large meteorites, those a few kilograms or more in weight, survive entry to the earth's atmosphere and drop to the surface with only superficial damage. Much smaller particles (those less than a hundredth of a millimetre in diameter) burn up as they meet the upper atmospheric layers, causing the familiar “shooting stars."

But what of the meteors between these extremes of size? They are large enough to avoid being totally evaporated. even though they might melt, but are too small to be spotted as meteorites when they reach the ground. John Murray realised that these would be lost if they floated down on land or in coastal regions, because of the effects of tides and weather, but would slowly accumulate in the deep

ocean basins where the dust would lie undisturbed for thousands of years. In more recent years astronomers have recognized the importance of the cosmic dust which lines the ocean bed. Unlike rocks returned from the moon, they represent a "free” source of material from interplanetary space, which is of great use to physicists trying to explain how the earth, and the rest of the planets, were originally formed. By studying the isotopic composition of the dust, using radioactive techniques which were not available at the time of Challenger's voyage. researchers have proved conclusively that the grains came from outer space. Quick to recognize the reason why the dust accumulates in the oceans, scientists saw that there was another relatively undisturbed region on the earth—the polar ice caps. Skimming off the upper layers of snow and ice which have been laid down since the industrial revolution, and have been polluted with soot and other unnatural particles, researchers found that the underlying ice is full

of the cosmic dust—although tonnes of the ice have to be melted to sieve out even a few microscopic grains. Another place in which the dust has been found is high in the atmosphere, where it can be intercepted before floating down to the surface. The American N.A.S.A. space agency have used high flying U-2 spy-planes, their wings fitted with sticky plates, to collect samples in a pristine condition.

When Challenger sailed back to home waters in May, 1876. it had lost at least two of its crew members. George Nares had been transferred to the command of two ships in the Arctic Ocean the year before, a promotion as such. The other missing person was not so lucky. Once while dredging over a broken seabottom. the net had stuck and its rope snapped under the strain. The heavy block to which the tether was attached flew loose, dealing a young boy called Stokes a fatal blow.

In retrospect the unfortunate ship’s lad might be seen as the first victim of the modern age of space research.