Society for Popular Astronomy

Dear al(L)
New Scientist 7 April suggests "Ages of oldest rocks are off by millions of years"
'Two ofthe solar system's best time keepers have been caught misbehaving, suggesting the accepted ages for the oldest known rocks are wrong ..................
using radioactive samarium-146 .........converge' on a half life of 103 million years ,plus minus 5 million.........................' using a newer,more accurate technique, M Paul, Hebrew University in Jerusalemfound the figure is just 68 million ............................that means some Martian meteorites - formed 20 million to 80 m years earlier than thought......................
A second hiccup, concerns a pair of uranium isotopes,which decay into two lead isotopes...................................
Now Joe Hies of the British Geological survey & colleagues have made the widest ranging study of uranium isotope ratios yet .................................
That led them to ..................shifts the ages of the Earth's oldest rocks thought to be 4.4 billion years old are now younger by 700,000 years.
""If the Earth was only 18 years old, we would have to take 1 day off the life of its oldet materials...........
it might seem small but ........................If a mass extinction occurred just before a meteorite struck, that paints a very different picturethan if the meteorite hit first..............."
Best wishes from Cliff

Is there any evidence, Cliff, to show that impacts alone were responsible for any mass extinctions?
It's become a cliche to say that, for example, the Chixulub event wiped out the dinosaurs, but they were already on their knees due to supervolcanism: many dinosaur species had already disappeared.
Chixulub seems to have been the final stress point that tipped the scales for the dinosaurs and, of course, for other fauna.
There are large impact craters, e.g in the Canadian Shield, that don't appear to coincide with times of mass extinction. Our planet seems to recover quickly from these impact events.

Dear Brian
I must admit I personally am nothing like as confident as I used to be in the claims that an impact wiped out the dinosaurs as I used to be ie when i think the idea was first mooted. It seems a very neat idea but I must admit I now only think of it as a possibilty -not proven fact.
Indeed I must confess that I cann't help wondering if some "scientists" now tend to rather rush into accepting good ideas as proven facts a bit prematurely. I wonder if the need\desire to get funding for science projects is a factor ?
Best wishes from Cliff