(See related photo: "Meteor Crater Mystery Solved" [March 10, 2005].)
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.
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