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Location: Chemistry/Physics Building, MUN
Time: 8:00 pm
Present: 29
Doug gave the usual introduction.
Ben Llewellyn gave a review of some books for young people. There are a series of over 40 "Eyewitness Books" that can be found in the young readers area of Chapters. Three of these are directly of indirectly applicable to Astronomy: "Astronomy", "Time and Space", and "Chemistry". Ben gave us a brief guided tour of the contents of "Time and Space". These books are well illustrated, and probably suitable for High School students. They are $19.99 ea. at Chapters.
Chris Stevenson gave the introduction to Dr. Phil McCausland's talk, still a current member. Chris mentioned Phil is now at the University of Western Ontario and Curator of their meteorite collection. [He was curator of something.]
There are two main locations in the talk: Meteor Crater in Arizona and Carancas, Peru. Carancas is a crater created in living memory.
Phil gave us a tour of some confirmed impact craters on the earth, many very old (in human terms). There are a confirmed 174 impact craters. Some examples were:
In addition, there were pictures of some impact craters on asteroids.
Just as on these asteroids, the question arises on the earth as to the formation of these impact structures in relation to the size and type of meteoroid. The speed and angle of impact is also important. There are three main types of large craters seen:
Phil showed us some pictures from his visit to Meteor Crater. We got a sense of the scale of the site, and were shown some of the interesting spots and geological features (particularly evidence of strata tilting).
The event at Carancas occurred September 5, 2007 in southeast Peru. It was in the general area of Lake Titicaca. The event was observed by several infrasound and seismic stations.
Eyewitnesses related how the fireball seemed as bright as the Sun. A picture of the trail was taken by a quick-thinking boy. Phil noted that usually eyewitnesses are not individually reliable. Taken as a group, with very simple location and direction questions ("where were you?" "to the left or right?"), a track can be gotten with enough eyewitnesses. This happened with Tagish Lake. In this case the crater gave some idea of the direction and speed of impact, and one eyewitness managed to be right under the track. There was also data to be had from the infrasound and seismic stations.
At the site itself there were hundreds of ejecta. A building 1 km away had broken windows, presumably from overpressure (gives an idea of the energy of the impact). Some other reported examples by the locals:
Phil noted a common misconception is that the meteorite must be hot to cause the boiling water in the mud. Only the crust ever gets hot, and that is pretty cook by the time of landing. The heat comes from the enormous kinetic energy of the meteorite being dissipated after being brought to a stop.
Phil had a sample of this meteorite. It seems an ordinary chondritic, though what looks to be a fusion crust in this sample picture is probably a shock vein, caused by the impact.
Some groups have been working on interpreting the data from the stations. The only really useful data is from the infrasound station at La Paz, Bolivia. There were two events recorded, about 20 seconds apart. They were probably from the crater event itself, and the sonic boom caused by the speed of the meteorite through the atmosphere. From these events we get an azimuth between 55 and 85 degrees, with a steep trajectory. Breaking windows at a certain distance gives 3-5 kPa of overpressure -- you can determine some rough boundaries for the energy of the event.
The upper bound on speed would be 17 km/s, with the trajectory bounds given this would give an aphelion somewhat less than Jupiter's distance from the sun. The lower bound would be 12 km/s -- anything slower would have a long travel time through the atmosphere and would probably break up. With the altitude of 3.7 km this object didn't have time to slow down to terminal velocity. It probably hit the ground between 1-2 km/s, maybe as high as 3 km/s. Phil explained with some charts the typical velocity/height profile of most meteorite passages.
Likely characteristics:
This crater could be important in another respect. Could the strike in mud be an analog for an asteroid impact? If the asteroid is covered in regolith, the behaviour may be the same.
There were some questions afterword.
Phil was presented with a MUN mug as a thank you.
Robert passed around a handout on the sky over the next month or so. Separately, there was a handout on the lunar eclipse tonight. Robert kept it brief and reminded everyone interested in observing at the park over the summer should contact him via email or phone.
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