In the month of June, 1908, a vast section of Siberia was devastated by a massive explosion, but what caused it? Dr William Revilleinquires.
ONE HUNDRED years ago, June 30th, 1908, the biggest collision of an extraterrestrial object with earth in modern human history occurred in a remote region of Siberia. Something exploded 8km above the Podkamennaya Tunguska river devastating 2,150 sq km of forest and flattening 80 million trees. Was it an asteroid, a comet, or something else? Recent work described by L Gasperini, E Bonatti and G Longo in Scientific American(June, 2008) is shedding light on this question.
The effects of the Tunguska event were witnessed over much of northern Europe in the form of silvery clouds, vivid coloured sunsets or luminous night skys - Londoners read newspapers at midnight without artificial light.
It is important to study such events because collisions of earth with extraterrestrial bodies can have huge consequences for life on earth. The example most often cited is the collision of earth with a massive asteroid 65 million years ago that caused the extinction of the dinosaurs. Large-scale impacts are rare, estimated to occur no more frequently than once every 100,000 years. Tunguska-like events are estimated to occur once every 200-1,000 years and it is not at all unlikely that another such event could soon occur again. If such an explosion took place over Dublin it would destroy the city. Understanding Tunguska might help us to take steps to avoid the occurrence of such events.
Was Tunguska caused by a comet or an asteroid? As Gasperini and colleagues explain, comets move far away from the sun on long-period orbits before returning, whereas asteroids orbit the sun more closely and have short-period orbits. A comet would impact the earth at a much greater velocity than an asteroid and therefore a small comet would release the same energy on impact as a larger asteroid.
We know that asteroids hit the earth but there is no confirmed case of a comet impact. The Tunguska incident was more likely caused by an asteroid, but, if so, why has nobody found asteroid fragments?
Tunguska wasn't investigated scientifically for nearly 20 years after the incident because of the political turmoil in Russia and the physical isolation of the site. In 1927, Leonid Kulik, from the Russian Academy of Sciences, led the first of three field trips to investigate Tunguska. He found no impact crater or fragments from whatever hit the area. Consequently, the theory of an airburst of a stony asteroid or a small comet gained popularity with the explosion occurring 5km to 10km above the surface. In 1975, it was estimated that the explosion released energy equivalent to 1,000 Hiroshima atomic bombs. The most recent simulations indicate that the object that exploded was much smaller than earlier estimated but its destructive potential was aggravated by the fireball that erupted when it exploded.
A small lake (350m by 500m), Lake Cheko, is located about 8km from the suspected epicentre of the Tunguska explosion. Might this be an impact crater, made perhaps by a metre-sized fragment of asteroid that survived the explosion? Russian work in 1960 rejected this idea after finding loose sediments as thick as seven metres on the lake bottom. A few years ago a team of Italian researchers, headed by the authors of the Scientific Americanarticle, went to Lake Cheko in order to study the sediments. They found sediments 10m deep on the lake bottom and noted that most of the current sedimentation is carried in by the little Kimchu River that feeds the lake. Based on the estimate that this type of sedimentation would not grow more than a few cm per year, it looked like the lake existed before 1908.
But, the lake basin has steep slopes and overall has the shape of an inverted funnel. If the lake were thousands of years old it would be expected to have a flat bottom, as sediment gradually filled it. Most other Siberian lakes have flat bottoms. In fact, the shape of Lake Cheko's floor is similar to known impact craters of similar size.
But how to explain the sediment? The Italian team found that the sediment is layered, with a thin layer, 1m thick, of fine sediment, typical of quiet deposition processes, overlying a lower layer of chaotic deposits. It looks like the true sedimentary deposits are only 1m thick, consistent with a very young age for the lake. The deeper sediment seems to represent buried forest - buried tree trunks have been identified by the Italians. Furthermore, acoustic and magnetic data detected a metre-sized rocky object several metres below the deepest point of the lake. Is this a fragment of the body that exploded overhead?
The Italian team is continuing its investigation and hopes to solve the Tunguska mystery soon.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC.(http:// understandingscience. ucc.ie.)