Mississauga Centre RASC

84th Meeting

                                                                                                 Speakers’ Night       

 

 

Day:                Friday May 11, 2007

 

Chair:                Randy Attwood

 

Speaker:          Dr. Peter Brown

                        

 

 

The Fall and the Recovery of the Tagish Lake Meteorite

 

Dr. Peter Brown from the University of Western Ontario studies meteorites, comets, asteroids and small solar system bodies. He is especially interested in meteors before they fall to the ground and hence the importance of the Tagish Lake (TL) meteorite, a unique fall in Tagish Lake on the Yukon/BC border on January 18, 2000. 

 

The TL fireball was seen by numerous witnesses and was reported to be brighter than the noonday sun, i.e. very, very bright.  Satellites observed it as well and had never seen such a bright fireball in 30 years.  In addition, a loud sonic boom shook Whitehorse and northern B.C.  12 hours before impact, the meteoroid passed within the orbit of the Moon and would have been detectable in large telescopes. At 8:43, a very bright fireball was seen and was painted by a graphic artist. Video at Whitehorse airport showed brightening caused by the passage.   The object was in flight in the atmosphere for 20 seconds, weighed 60,000 kg and had a diameter of 4 meters.  It entered at a very shallow angle fragmenting repeatedly over the Yukon/B.C. border.  It was the largest meteoroid impact over land in the last decade and statistically the largest object to hit the Earth in 1 year. Seismographs in Whitehorse detected the starburst from the sonic boom as a small earthquake.  With fragmentation, 97% turned to dust. US defense satellites detected the dust trail in the atmosphere. It was the first time since Tunguska that dust could be seen from so far away. The dust settled over western Canada and produced “noctilucent clouds” in Edmonton – very unusual in winter. Using all this information, Dr. Brown was able to reconstruct the trajectory of the object and work out that it has an asteroid-type orbit from the outer asteroid belt.

 

On January 25, Jim Brooks, an amateur geologist found meteorites on the ice of Taku arm of Tagish Lake and collected about 50 pieces on the 25th and 26th, the largest 176 grams. The next day, snow covered the remaining fall. The pieces were fragile, friable and porous.  He then contacted Dr. Brown.  From February 16 to 26 a one week search of the lake involving shoveling off of snow and using RCMP dogs came up empty.  The next expedition under Dr. Brown took place from April 6 to May 11 when the snow on the lake surface had melted from the ice surface.  Snowmobiles were used in a grid pattern. This time a total of 500 meteorites were recovered, many of them liberated from the ice with chainsaws. The largest piece weighed 3 kg but it broke apart like shrapnel. Meteorites were also found later by others on the beaches and in the woods surrounding the lake.

 

Tagish Lake has a great significance being the most studied meteorite in Canada with 150 papers published in 7 years.  It is the most primitive (least altered) material of the solar system, linked to the D-class asteroids by having the same spectral type and thought to be of the same material that made up Jupiter, 5% carbon by weight with primitive and “boring” chemistry.  Differing from other carbonaceous chondrites, it has the highest concentration of stellar dust grains at 0.2%.  TL has a density of 1.63 gm/cc with a porosity of 40% implying that no large empty spaces are required in asteroids (the porosity is microscopic not macroscopic).  Of the amino acid composition, most is glycine. Alanine makes up 600 ppm.  In fact, the organic material in very insoluble. The pre-solar organic globules in TL formed in giant molecular clouds or the outer proto-solar nebula and are ready-made material for pre-biotic activity.  

 

Of the total mass of 90 tons of TL, 10 kg of material were recovered and a large amount of carbon was deposited.  TL-type objects could have deposited a significant amount of organic material onto the Earth over its lifetime. Trace elements and isotopes in the meteorite are different from other carbonaceous chondrites.  Desiccation veins suggest water percolation 4 ˝ billion years ago in asteroids. However, our knowledge of asteroid geology isn’t comprehensive yet. We do know that Tagish Lake is a very rare type of meteorite.

 

 

Submitted by Chris Malicki, Secretary  Chris Malicki, Secretary                               back to Miss Centre. meeting reports page
Mississauga Centre RASC