Crater From 1908 Russian Space Impact Found

Almost a century after a mysterious explosion in Russia flattened a huge swath of Siberian forest, scientists have found what they believe is a crater made by the cosmic object that made the blast.

The crater was discovered under a lake near the Podkamennaya Tunguska River in western Siberia, where the cataclysm, known as the Tunguska event, took place on June 30, 1908, a ball of fire exploded about 6 miles (10 kilometers) above the ground in the sparsely populated region, scientists say. The blast released 15 megatons of energy—about a thousand times that of the atomic bomb dropped on Hiroshima—and flattened 770 square miles (2,000 square kilometers) of forest. Since then many teams of scientists have combed the site, but none was able to find any fragments of an object, like a rocky asteroid or a comet, that might have caused the event. In their new study, a team of Italian scientists used acoustic imagery to investigate the bottom of Lake Cheko, about five miles (eight kilometers) north of the explosion's suspected epicenter. "When our expedition [was at] Tunguska, we didn't have a clue that Lake Cheko might fill a crater," said Luca Gasperini, a geologist with the Marine Science Institute in Bologna who led the study. "We searched its bottom looking for extraterrestrial particles trapped in the mud. We mapped the basin and took samples. As we examined the data, we couldn't believe what they were suggesting. "The funnel-like shape of the basin and samples from its sedimentary deposits suggest that the lake fills an impact crater," Gasperini said.

A "Soft Crash"

The basin of Lake Cheko is not circular, deep, and steep like a typical impact crater, the scientists say. Instead it's elongated and shallow, about 1,640 feet (500 meters) long with a maximum depth of only 165 feet (50 meters). Gasperini's team says that the basin's unusual shape is the result of a fragment thrown from the Tunguska explosion that plowed into the ground, leaving a long, trenchlike depression. "We suggest that a 10-meter-wide [33-foot-wide] fragment of the object escaped the explosion and kept going in the same direction. It was relatively slow, about 1 kilometer a second [0.6 mile a second]," Gasperini said. The lake is located along the most probable track of the cosmic body, he added, which likely made a "soft crash" in the marshy terrain. "It splashed on the soft, swampy soil and melted the underlying permafrost layer, releasing CO2 [carbon dioxide], water vapor, and methane that broadened the hole, hence the shape and size of the basin, unusual for an impact crater. "Our hypothesis is the only one that accounts for the funnel-like morphology of Lake Cheko's bottom," he added. In a previous expedition, Russian scientists studied Lake Cheko and concluded that it had formed before 1908, indicating that it was not formed by the Tunguska event. The team had measured sediments on the bottom of the lake and determined that the deposits were accumulating there at about 0.4 inch (1 centimeter) a year. This suggested that Lake Cheko was several centuries old. But Gasperini's team argues that the older deposits found by the Russians were already there when the explosion took place. "We found evidence that only the topmost, one-meter-deep [three-foot-deep] layer of debris actually came from the inflowing river," Gasperini said. "[The] deeper sediments are deposits that predate 1908. They were the target over which the impact took place, so Lake Cheko is only one century old." The team's findings are based on a 1999 expedition to Tunguska and appeared in the August issue of the journal Terra Nova.

Asteroid or Comet?

William Hartmann, senior scientist of the Planetary Science Institute in Tucson, Arizona, said the new findings are compelling but do not address all of the lingering questions about the event. "It's an exciting result that might shed new light on the Tunguska explosion," he said. "Certainly it warrants new studies of the area. "But it raises a question in my mind: If one large fragment hit the ground, we would normally expect thousands of smaller fragments also to hit the ground along the path, and many searches have failed to find such meteorite fragments. So, why no smaller pieces?" Finding fragments from the explosion is considered key to determining what kind of object made the impact. An asteroid would probably leave some remains, while a comet might be annihilated in the blast, Hartmann said. "Our crater hypothesis is consistent with both possibilities," Gasperini said. "If the body was an asteroid, a surviving fragment may be buried beneath the lake. If it was a comet, its chemical signature should be found in the deepest layers of sediments." Gasperini and his colleagues are planning to go back to Siberia next year to search for more, and perhaps more conclusive, clues to the century-old puzzle. "We want to dig deeply in the bottom of the lake to definitively test our hypothesis and try to solve the Tunguska mystery," he stated.

Aliens Can Survive Trip to Earth

We could have alien origins, say scientists who sent fossilized microscopic life-forms into space and back inside an artificial meteorite.
The researchers attached the baseball-size rock to the outside of the European Space Agency's Foton M3 spacecraft to test whether biological material could survive the round-trip journey. Sculpted from stone from the Orkney Islands in northern Scotland, the rock containedfossilized microbes and the molecular signatures of microbes. The unmanned spacecraft was launched by rocket from Kazakhstan's Baikonur Cosmodrome carrying 43 experiments. The craft landed in Kazakhstan on September 26 after orbiting the planet for 12 days. "In the bit of rock we got back, some biological compounds have survived," said project leader John Parnell from the University of Aberdeen in Scotland. Preliminary findings suggest that it's possible simple organisms could arrive via meteorites, he said. The research also suggests that living microbes would likely have survived in a slightly bigger rock, he added. "This study of organic material is completely new," he said. Previous artificial meteorite experiments have examined only the degree to which rocks melt upon entering the atmosphere. The new experiment is part of European Space Agency's STONE program, which tests effects of reentry on artificial meteorites. "It was shielded when it went up into space but exposed when it came back," Parnell said. The rock had similar properties to a type of meteorite known as a carbonaceous chondrite. Such meteorites contain water and carbon compounds, both essential to life. "We wanted to see if a rock that was rich in carbon and water would suffer a lot of mass loss," Parnell said. "That was certainly the case. About three-quarters of the mass of our sample disappeared." Living microbes probably wouldn't have survived in a meteorite this size because it reached temperatures of about 392 degrees Fahrenheit (200 degrees Celsius), the project leader said. But "if our rock was bigger, say 20 centimeters (about 8 inches) across, then we can be quite confident that [the] temperature would not penetrate to the middle, so that if anything had been living there, it would have survived." A much larger meteorite, however, would completely melt and vaporize on impact, according to Parnell. "There's a sort of window of opportunity in terms of size, between being too small and too big," he added. Microbes are known to live deep inside rocks, and are found several kilometers down in Earth's crust, Parnell noted.

Mars Origins?

The theory that says interplanetary organisms seeded life on different planets, such as Earth, is known as panspermia. If panspermia explains the origins of life on Earth, astrobiologists believe that Mars is the most likely source. For instance, studies suggest about 5 percent of meteorites from Mars eventually end up hitting Earth.

"That journey can take anything up to 15 million years, but there are a few that will make it very quickly," Parnell said.

"A very few will make it in a year or so. Those are the ones which could conceivably bring something interesting with them."

"The surface of Mars is quite inhospitable, due to dryness and low temperature, but one could conceive of subsurface life still being on Mars," he added. In the experiment, microbes were also dried onto the undersides of several artificial meteorites. "This biological material didn't survive, but it may have been preserved, or its signatures may have been preserved," said STONE scientist Charles Cockell of the Open University in the United Kingdom. The rocks are still being analyzed, Cockell added."We know that life can make it from continent to continent, but what about from planet to planet?" he said. "Of course, at the moment we don't know of life on another planet, but this experiment is an intriguing test of an interplanetary version of an old ecological question." David Morrison is a senior scientist at the NASA Astrobiology Institute in Moffett Field, California. Parnell's project lends credibility to the idea that meteors from outer space can give rides to hitchhiking microbes, he told National Geographic News by email. Whether exchange of life has ever occurred following the meteorites' impact is a more complex question, but "we should be open to the possibility that there is microbial life on Mars that shares a common ancestor with Earth life," he said. "It may not be likely, but we cannot exclude the possibility that we are, in effect, all Martians."