杏吧原创

A MAN with a Stetson perched on his head reclines in his chair, an assortment of rocks displayed in front of him. A second man in a fedora browses the collection, pausing over one specimen. The size of a chocolate bar, the silvery rock is inlaid with a mosaic of grainy grey shapes.

鈥淲hat are you asking for that one?鈥� asks the fedora.

鈥淥h, somewhere around five thousand,鈥� replies the Stetson.

It鈥檚 a routine exchange at the annual Tucson Gem, Mineral & Fossil Showcase in Arizona, a marketplace for international collectors of petrified wood, dinosaur bones, gold and more. Except there鈥檚 something special about this rock: it came from space.

The man in the Stetson, Marvin Killgore, hunts and trades meteorites. It鈥檚 not an easy living. Rocks from space fall anywhere, any time, and many look unremarkable to the untrained eye. Killgore honed his skills in more than 40 countries over 27 years. Catch him at an idle moment, and his eyes are trained on the ground. 鈥淚鈥檓 always prospecting,鈥� he says. 鈥淚 found a meteorite in the parking lot here last year.鈥�

鈥淥ften just homely grey lumps, meteorites are scientific treasure troves鈥�

Collectors aren鈥檛 the only ones interested in his wares. Meteorites are objects of great scientific interest 鈥� they are time capsules from the solar system鈥檚 birth, encoding clues about how our cosmic neighbourhood came to be, and maybe about why life blossomed in at least one part of it. To piece together the full picture, we need more of them. Killgore and his ilk鈥檚 trained eyes won鈥檛 be enough. It鈥檚 time for some cleverer ways of finding space rocks.

Across town from the gem show, some of Killgore鈥檚 prize specimens are on display at the University of Arizona. The collection of space shrapnel there comes in a bewildering array of varieties: silvery iron-nickel asteroid cores, a grey-pink lunar rock, an olive-faced boulder the size of a newborn calf.

But such flashy rocks are rare. About 85 per cent of meteorites, space rocks that actually reach Earth鈥檚 surface, are of a type called ordinary chondrites. Often homely grey lumps that won鈥檛 earn you much cash, these are scientific treasure troves. Each is packed with spheres called chondrules, ranging in size from pepper flakes to marbles. They formed when molten rock droplets cooled 4.6 billion years ago, long before any planets existed in the solar system. 鈥淲ith a chondrite, you have the oldest rock you鈥檒l ever hold in your hand,鈥� says planetary scientist Dolores Hill, who leads tours of the exhibition.

Meteorites like this could help us figure out some of our cosmic backyard鈥檚 deepest mysteries, such as why the planets exist in a neat arrangement of four rocky worlds followed by four gas giants. In the burgeoning number of other planetary systems we now know of, it鈥檚 far more common to see a mixed line-up with gas giants that have migrated inwards to mingle with their rocky cousins. Why are we so different? One proposal is that Jupiter may have barrelled inwards before retreating to its current position.

Another theory says there was no water on the young Earth, in which case it was probably delivered later on by collisions with other bodies like asteroids or comets. Jupiter鈥檚 gravity might have helped slingshot these bodies towards Earth 鈥� depending on where it was.

Chondrites and other meteorites mainly come from the asteroid belt, a repository of material from the solar system鈥檚 early days that sits between the orbits of Mars and Jupiter. Examining the chemical composition of any fragments that come our way can tell us where they ultimately came from. We could then run computer simulations exploring how the gas giants might have moved around in the early solar system, and how this could have kicked asteroids from where they were born into the orbits they ended up in.

In particular, the farther out the asteroid material was when it formed from the cloud of dust and gas surrounding the early sun, the more laden it would have been with deuterium, a heavy isotope of hydrogen that has a neutron in its atomic nucleus. So analyse the ratio of hydrogen to deuterium in meteorites and you can tell roughly where their parent rock was born.

A neat idea 鈥� but there are a few stumbling blocks. First, there鈥檚 what keeps people like Killgore in business: the sheer rarity of meteorite finds on Earth. Second, there鈥檚 the fact we have a biased sample, consisting only of the sorts of rocks that cross Earth鈥檚 orbit. Finally, although analysing isotopes can point to where a rock originally formed, it doesn鈥檛 reveal its most recent orbit, limiting the accuracy of any simulations.

of the University of Arizona is pursuing one obvious solution: get space rocks of known providence by grabbing them where they lie. He leads NASA鈥檚 OSIRIS-REx mission, which plans to take samples from a 500-metre-diameter asteroid called Bennu in 2022.

But such missions are expensive, so we need better ways to bulk up the harvest of space shrapnel. One well-worn method is to head to Antarctica. Meteorites that fall on the continent鈥檚 high interior get buried in the ice and carried towards the coast as the ice slowly slips towards the sea. But then they meet the rising underlying terrain of the Transantarctic Mountains, where they can be forced upwards to the surface. For the past 40 years, the US government has sponsored hunts along the base of the mountain range as part of the . Researchers combing the ice on snowmobiles have now found more than 21,000 objects, including meteorites from Mars and the moon.

of the Carnegie Institution of Washington and his colleagues have found another source to harvest in Antarctica: space dust. This is a mixture of material, some of it shed from comets, some of it that has just never coalesced into larger rocks and has been hanging around in space since the early days of the solar system.

There鈥檚 no hope of distinguishing this dust from the grains made on Earth 鈥� at least not in most places. 鈥淭he air at the South Pole is so clean that there鈥檚 very little terrestrial dust,鈥� Alexander says. 鈥淢ost of the dust, hopefully, is from outer space.鈥� To sample it, the team recently installed a 6-metre-high 鈥渧acuum cleaner鈥� with an inlet tube that sticks out like a trunk. Alexander expects that much of the dust has had a very different life from your average meteorite. His preliminary analysis suggest that some of it is very old, and along with the chondrules possibly represents the first solids formed in the solar system.

More samples from different sources help ease the first and second problems of meteorite hunting, but there is still the crucial third problem of pinpointing where the material came from. 鈥淎ll we can do,鈥� says Hill, 鈥渋s analyse the specimens, group them together, and say that maybe these were formed in the same region or from the same object.鈥�

Getting around this last problem means knowing not just where a meteorite fell, but how it fell, in the hope of reconstructing its trajectory and so its origin. But most meteorites are like Killgore鈥檚 parking lot rock: a lucky find without context. Killgore sometimes locates meteorites in a more systematic way by tracing how they disrupted weather radar. But much of the search is by walking and looking, often using a metal detector or a walking stick with a magnet stuck on the end.

Ten years ago Phil Bland of Curtin University in Perth, Australia, started experimenting with a smarter way. He and his colleagues created the , made up of 50 cameras spread across the desert of southern and western Australia. Each captures night-long exposures of the sky, including the luminous path of any meteors. A fireball鈥檚 size reveals how large the rock is and whether it will burn up in the atmosphere. Bland鈥檚 team measures its trajectory on multiple cameras and calculates where the meteorite landed. Then they drive into the desert.

Their first hunt began on the night of 20 July 2007 with a bright white fireball that had ripped through Earth鈥檚 atmosphere at 13 kilometres per second. After months of work, the team traced the fall to a spot in the Nullarbor plain.

The following year, Bland and seven others set out in a truck and three cars carrying water supplies for two weeks of camping. 鈥淚t wasn鈥檛 great searching country,鈥� Bland says. But combing through clusters of short, hardy shrubs, and marking the area they covered on GPS devices, they found a meteorite on their first day, within 100 metres of their prediction. Bland later found another chunk, making 324 grams of rock by the end of the trip. They christened their find the Bunburra Rockhole meteorite, after a nearby cave. 鈥淚t was a very nice way to start,鈥� says Bland.

Since then they have recovered three more meteorites, convincingly better than similar camera networks in North America and Europe that cover areas rich in vegetation where the rocks are trickier to spot. 鈥淢ost of them have only delivered one meteorite each over 10 years,鈥� Bland says.

His team now has a different problem: having estimated the locations of 15 further meteorites, they鈥檙e struggling to recruit enough people who know what a meteorite looks like and are willing to sleep in the middle of nowhere for weeks to bring them in.

Crucially, trajectory mapping from the camera networks can point to where a meteorite came from. 鈥淵ou can get the entire orbital history of this rock,鈥� Bland says. That is the first step to a more detailed chemical map of the entire asteroid belt, and perhaps some answers as to what happened in the early solar system.

The Bunburra rock itself demonstrates some of this promise. It turns out to be a type called a eucrite, which lacks the internal chondrules of a chondrite. But it鈥檚 an unusual sort, originating not from the asteroid belt, but from an orbit almost entirely contained within Earth鈥檚.

鈥淢eteorite hunters often use just a walking stick with a magnet stuck on the end鈥�

All this, along with what we鈥檙e learning about other planetary systems (see 鈥�Exoasteroids鈥�), is revealing ever more secrets buried in space rocks. But nothing will replace old-fashioned prospecting, says Lauretta, largely because camera networks like Bland鈥檚 cover only a small fraction of land. Places like the Sahara and Atacama deserts will continue to be the territory of collectors like Killgore.

Back at the meteorite booth in Tucson, the man in the fedora decides not to buy the rock. Killgore says that no longer bothers him much. Since he started sharing his samples with scientists, he has come to appreciate his wares for the information they contain. It鈥檚 just one more reason to keep collecting these postcards from the solar system.

This article appeared in print under the headline 鈥淩ocks of ages鈥�