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A century ago on June 30, the Tunguska mystery was born.
It began as a massive explosion, something like the power of 1,000 atomic bombs going off together in the skies over isolated northeastern Siberia. It flattened the trees under the blast and people have been speculating ever since about what may have caused it.
I first learned of it in my teens as "proof" that UFOs exist and one had exploded. More reasonable explanations propose a comet or meteorite impact, but the mystery still persists.
The June 26 issue of the journal Nature and the June 28 issue of New Scientist both carry review articles evaluating the available data. That in New Scientist has the better graphics.
But there are too many other things of interest to spend an entire column on Tunguska. The above issue of Nature has a major section on cosmic impacts. Articles deal with the apparent huge impact "at the bottom of the Moon" and an apparent impact that has divided Mars into rather a "split" planet with one major part made of rough highlands and mountainous topography, the remainder relatively flat and featureless. A section of superb photographs of craters on Earth, Moon, Mars, and on moons of Mars, Jupiter and Saturn documents the fact that huge chunks of rock and ice are common in our solar system, in geologic time at least.
Arthur C. Clarke, the science fiction writer, used Tunguska to open his 1972 work "Rendezvous with Rama" (that it missed Moscow by three hours and 2,500 miles), and concluded his book with a warning that Earth certainly faces future impacts from asteroids or other space wanderers. He proposed a "Project Safeguard" by which Earth's scientists would keep an eye on the sky for those that may conceivably hit Earth. And indeed such a Safequard program exists today. Readers will remember publicity a few weeks ago how Curtis Craig and his students at American Fork High School have participated in this study and are credited with the discovery of four previously unknown asteroids. None of those, thankfully, seems to present any threat.
But on to other things. We recently discussed sea-floor spreading, the fact that the Atlantic (our main example) is steadily growing wider as fresh lava pours out from the Mid-Atlantic Ridge. The deep-water lava extrusions seem to be relatively mild rather than explosive. The June 26 issue of Nature considers the reasons for this, but also documents a spot in the Arctic Ocean (on Gakkel Ridge) where a genuine explosive event seems to have occurred more than 12,000 feet down.
Global warming, or climate change, gets a lot of media space these days. Debating whether it is just a natural cycle, or whether humans are contributing to or causing it, is beyond a simple newspaper column. But the June 20 issue of Science magazine traces the climate change in Greenland over the past million years (from pollen records in an oceanic sediment core). A companion article discusses the difficulties in accurately predicting accumulation of ice, and loss of ice, in both Greenland and Antarctica.
New Scientist for June 21 reviews evidence that many millions of years ago, Antarctica supported sub-tropical vegetation with a much warmer climate. The July 5 issue of Science News documents that the coniferous forests of northern latitudes are steadily moving northward and replacing tundra in the Arctic as the climate warms. And somewhere in the last two weeks (curse my lousy memory!) I've seen data that similar vegetation on our own mountains has been moving upward about 30 feet per decade into the treeless regions. |
