Antarctic: solved the riddle of missing iron meteorites

Just below the surface, a rich treasure trove could be hidden

Search for meteorites in the Antarctic as part of the NASA-funded ANSMET program. © Linda Martel / ANSMET, NASA
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Hidden under the ice? Just below the ice surface of the Antarctic, a whole layer of iron meteorites could wait for their discovery. Because, as researchers have now discovered, these chunks of iron remain trapped in the ice rather than being carried up. The reason: When sunlight falls into the ice, it heats up more than the rocks and therefore sinks, as the scientists report in the journal "Nature Communications".

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The Antarctic is a worthwhile find for meteorites, because on the bare, vegetation-free ice can make even small chunks well. No wonder that two-thirds of the approximately 35, 000 meteorites found so far have been discovered there. But one thing is strange: Although the probability of hit is the same everywhere in the world, in the Antarctic there seem to be less iron meteorites than anywhere else.

"Data show that the proportion of Antarctic iron-bearing meteorites, at just 0.7 percent of all finds, is significantly lower than in the rest of the world, accounting for 5.5 percent, " report Geoffrey Evatt of the University of Manchester and his colleagues, In the Antarctic La Paz ice field even 0.3 percent of the finds are made of iron, the rest are stone meteorites. But this lack of iron meteorites can not have a cosmic cause - it must be geological.

Is the thermal conductivity to blame?

Where the missing meteorites could be, Evatt and his colleagues have now investigated. Your guess: The different material of the meteorites could affect their behavior in the ice. Because iron conducts heat better than stone, ferrous meteorites could get hotter as the sun falls through the ice. But they could melt deeper into the ice and therefore remain hidden. display

Whether this hypothesis is correct, the researchers first tested in a laboratory experiment. For this they froze a 15-millimeter piece of an iron meteorite and an equal of a stone meteorite in an ice block and irradiated the whole thing with a sunlight lamp. The result: "Both types of meteorites warmed up enough to melt their surroundings and sink in the ice block, " the researchers report. However, the iron meteorite dropped significantly faster at 2.4 millimeters per hour than the stone meteorite at just 1.5 millimeters per hour.

Caught in the ice

But are these differences sufficient to explain the strange lack of iron meteorites on the Antarctic ice surface? For this to be the case, erosion and ice currents in the so-called meteorite traps of the Antarctic would just be sufficient to bring the stone chondrites up in spite of this tendency to sink, but to act too slowly for the iron meteorites.

When the researchers checked this with the aid of a geophysical model, the simulation results were surprisingly well in line with the observations in the Antarctic: iron-containing meteorites with a correspondingly higher thermal conductivity remained underwent ice surface, while stone meteorites surfaced over time due to erosion and ice flow.

"A whole layer of meteorites"

"This suggests that there is a layer of widely distributed iron meteorites beneath the ice surface, " say Evatt and his colleagues. In the La Paz Icefield alone, they protect the density of iron meteorites hidden beneath the ice to about one per square kilometer. From the simulation also shows that these "hidden" chunks are not very deep, probably only a few tens of centimeters.

"Even if the density of 'missing' iron meteorites is low, targeted meteorite search programs would be quite feasible, " say the researchers. The advantage lies in their opinion obvious: Especially the rare iron and iron-containing stone meteorites provide valuable information about our solar system in itself.

"Each new sample of such a meteorite has the potential to be at the core of a unique asteroid, giving us insights into the number, diversity, and evolution of planetary building blocks in the early solar system, " says Evatt and his colleagues. In addition, new discoveries could fill important gaps in our knowledge of the relationship between the various types of meterorites. (Nature Communications, 2016; doi: 10.1038 / ncomms10679)

(Nature, 17.02.2016 - NPO)