We don't always remember that we are whizzing around at 1000
miles an hour as we read the Advocate, whizzing in order to get round 24 000 miles (the earth's circumference) in 24 hours. Luckily the atmosphere is mostly keeping up with our frightful speed. As well as that circular whizz,
we are belting around the sun at about 67 000 mph. However you may have forgotten that the whole Solar System is also shambling around the Milky Way at a pretty good clip. It is that third variety of motion which interests
me today, because it may be related to evolution, to the extinction of the dinosaurs, and to the possibility that Homo sapiens may suddenly become extinct.
That great cosmic bounce of 65 million years ago, which ended the
dominance of the dinosaurs and gave us mammals our chance, was a great piece of luck for us, but it was only one of the many great extinctions which occurred in the last 300 million years. There were ten of them, occurring
surprisingly regularly about every 27 or 30 million years.
It is quite possible that all were due to a big comet or asteroid hitting the earth, maybe to two or three smaller ones. The extinction of 65 million years ago almost
certainly was due to one or possibly two.
But the next question is whatever could cause a regular series of big bounces, apart from that hoary old cause, the Will of God? What other, more physical reason could be found, provided
with better evidence?
When a big flying object hits the earth it makes a big crater. These impact craters can easily be distinguished from volcanic ones, and their ages can nowadays be rather accurately assessed. Small craters,
less than 5 kilometers across, get eroded away rapidly on geological time scales, but bigger ones last longer and some 140 of these have now been found on land, often in desert areas. Craters in the sea floor (70% of the
earth's surface) are permanently lost to science.
Putting this information together there seems to be a periodicity of 27 or 30 million years over the last 300 million. What could be the astronomical pacemaker?
Outside the most distant planets are two clouds or belts of smallish objects in space, the Oort cloud and the Kuiper belt, containing lots of potential comets or asteroids. Might something bigger pass through them regularly and
stir them up to produce showers of cosmic missiles which might, by pure chance batter the earth?
There are two local candidates: a so-far-undiscovered Planet X, far outside the planets we know, and a more amusing suggestion that
our sun may be part of a binary star system, the sun's partner being a small and inconspicuous brown dwarf star which is now over a light-year away from us and, naturally, very difficult to see, being small and brown, not at all
bright and rather boring. Astronomers at Berkeley are looking for the little star. It has already been given two names, the Nemesis or death-star, or the name I prefer, Siva. Siva is the Hindu god of destruction, depicted
with four arms, usually dancing a sinister dance. With Brahma the creator and Vishnu the preserver Siva forms a non-Christian heavenly trinity. An elegant conceit.
The other idea is the one in my title, As the sun and its solar
system amble around the Milky Way it is said to weave drunkenly up and down through the horizontal plane of the galaxy, passing through the denser levels of the plane about every 30 million years. on its way up or down. The
original idea was that the solar system would encounter more stars in the plane than it usually does and that these stars would disturb those Oort clouds. But when they did the mathematics the stars turned out to be too few
and too far.
So the astronomers had another think and came up with another suggestion. In our galaxy, as in others, there are enormous clouds of gas and dust which may, one millenium, condense into stars or may stay as they are
for millions and millions of years. Those clouds are much bigger and more massive than a measly star or two, even though you would hardly notice one if you passed through it. And their gravity has the physical ability to
disturb the trajectories of orbiting comets, possibly shoving some into the inner solar system where we have our own agreeable residence.
Another intriguing suggestion turned up. These giant clouds consist mostly of hydrogen,
and. as you know hydrogen likes to combine with oxygen to make water. Scientists have found that the amount of oxygen in our ancient atmosphere fell from 35% to 28% in the million years before that great Big Bounce, 65 million
years ago. That oxygen drop could be the signature of a Giant Molecular Cloud of hydrogen. And if the GMC's hydrogen combined with the earth's atmospheric oxygen to make lovely water, a meter's depth of water (so they say)
would have fallen on our planet. What a lovely bit of fantastical Sherlock Holmes deduction! The dinosaurs need never have barked!
So you need not be ashamed of the little stumbles our Solar System has made on its pilgrimage
around the Milky Way. It might have been just those inelegancies which accounted for the fact that you are here, reading the Advocate, and enjoying, on a Sunday, the sure and certain hope of eternal life. Some hope.
Not mine!
