[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Changes in gravity? and mosasaurs in Alabama

This may be a bit late, but I'll chime in with some comments on
whether gravity could have been weaker in the past, accounting for
the size of the largest sauropods (for example).

Limits on changes in the Newtonian gravitational constant G must have
been extremely small (if even non-zero) overthe whole of terrestrial history.
We can tell from observation that the same value hold across our Galaxy
and its neighbors, most clearly from double stars yielding consistent
masses as determined from their orbits and their temperatures and
luminosities. Changes with time are strongly restricted by the fact
that we see familiar kinds of galaxy dynamics and stellar populations in
distant galaxies. With Hubble, it's not particularly hard to look at the
structure of galaxies as they were before the Earth was formed, and in 
combination with big ground-based instruments we can also look at their
mix of stars. Both these things tell us about G. The galaxy's structure
and size are obviously sensitive to the strength of gravity. In a more
subtle way, so is the mix of stars (which is of course younger at that
epoch, one reason to be interested in such observations). The luminosity
of a star of particular mass is controlled by the interaction of its
self-gravity with the nuclear "thermostat" in the core. Change G,
you change the whole structure and lifetime of the star, and every star
in a galaxy.

Properties of the Earth and Moon also limit the history of G. The fact that
we have a Moon, and that its hemispheric asymmetry in crustal thickness
and basalt flows, tells us that the Earth's mass hasn't changed 
significantly since the Mesozoic (and suggest that the Moon has
been here since before the _maria_ lavas were erupted, which finished
over 3 Gyr ago). The Moon is, in a sense, just barely
bound to the Earth, and its orbital evolution hasn't made much difference
in this time span.

If G hasn't changed over the relevant time, the only remaining logical
possibility is a change in the Earth's radius. I'll leave a geophysicist
to comment in detail, but a shrinking Earth is about the only possibility
that hasn't been proposed.

And, just to stay on topic with content on Mesozoic reptiles:

A mosasaur skeleton (specifically a juenile tylosaur) has just gone on
display at the Alabama Museum of Natural History in Tuscaloosa, for
about a month. The beastie, named "Bossie" in honor of a cowbell found
near the discovery site, was first found by Randy Comer and Chuck Parker
of the US Army Corps of Engineers along the banks of the Tombigbee
River in Greene County, Alabama. Something like 2000 hours of (mostly volunteer)
preparation work went into getting the skeleton into display shape. 
A cast of the original skull pieces shows how crushed it was as found -
it is pretty impressive to an outsider how much of it was reconstructed
from the pieces. The skull, with 13 teeth in place, and (by my count) 80 spinal 
bones were found (not sure whether those were all vertebrae, since I can't tell 
where the tail starts or whether there's a distinction between vertebrae
and tail bones...). The flippers and associated bone girdles were not found.

Bossie's skeleton as displayed is 20 feet (6 m) long, with an estimate that
the whole animal was a meter longer. Apparently they'd expect an adult to
stretch to almost 10 meters, accounting for the "juvenile" description. 

For those who want to have a look, museum information can be found at

Bill Keel
Astronomy, University of Alabama