Download Astronomy and the Coal Age of Alabama

Astronomy and the Coal Age of
Alabama
Ron Buta
University of Alabama
Dept. of Physics and Astronomy1
and
Alabama Paleontological Society2
1www.astr.ua.edu
2www.alabamapaleo.org
Deep Time
“The story of life on Earth unfurls against a backdrop of time deep time: the length of time the universe has existed, the length
of time the Earth has been a planet, the length of time life has
been on Earth. We are better at understanding things we can have
some experience of, but we have, and can have, no experience of
deep time.” Eugenie C. Scott, in Evolution vs. Creationism: An
Introduction, 2004
The Coal Age: 323-290 Myr ago, a time of swamp forests, dense foliage,
amphibians, primitive reptiles, and large insects
Much of the world’s
coal reserves were
laid down during this
period, called the
Pennsylvanian
subdivision of the
Carboniferous.
Artwork due to Jim Lacefield
Coal Age plants left vast reserves of coal in Walker County,
Alabama, and treasure troves of Pennsylvanian-era fossils. The
Steven C. Minkin Paleozoic Footprint Site is one such treasure
trove, which has been dated at 310 Myr old.
“By quality, by
quantity, and by
geologic age, it is the
most important
discovery of
Carboniferous tracks
hitherto known.” H.
Haubold
The Minkin Site is part of the
Pottsville Formation. The 310 Myr
age is based on marine index fossils.
The Pottsville is defined by 8
stratigraphic clusters of coal beds.
Upper Pottsville cycles represent
<~500,000 years of deposition each.
J. Pashin, 2005
It was a time of great upheaval….
The Appalachian Mountains were at least as high as the modern Andes.
The area we now call Alabama was south of the equator, and there
was a large inland sea, the Warrior Basin.
There was considerable
sedimentation from the
Appalachians into the
Warrior Basin. Over
deep time, the mountains
eroded.
Artwork due to Jim Lacefield
The Appalachian Mountains are now only a shadow of what
they once were.
The Minkin site is famous for the abundant fossil trackways of
early tetrapods that have been found there. But most important is
the glimpse it gives us of terrestrial animal life 60-80 Myr before
there were dinosaurs.
Tiktaalik - transition animal
380 Myr ago
The Minkin site was once a tidal mud flat with sky above …. and
maybe star-gazing amphibians leaving their footprints?
What could they
have seen in their
sky?
Mobile Bay
The Minkin site may have at one time looked like this...
Deep time has
erased this world
completely.
Artwork due to Jim Lacefield
Here, an animal walked on the mudflat and left these tracks of
mystery. What else was different about this time apart from the
drastic ecological differences we know about?
Cincosaurus cobbi
Doing what any animal would have done: a tetrapod walking around an obstacle.
Things like this make the Minkin site extremely interesting.
But just as interesting is the site’s age.
Just how long a period of time is 310 Myr?
In a “geological clock”, it
is only 7% the age of the
Earth. But how does it
compare to astronomical
timescales?
For example, where was
the Sun 310 Myr ago?
ForTheScience.org
The Sun orbits the center of our Galaxy, also known as the
Milky Way. It is a highly flattened system that looks like this
from our vantage point.
The Milky Way has a bright center, but that center is obscured by thick dust clouds.
If it could be viewed from outside,
the Milky Way would look a little
like the galaxy at left, a spiral
galaxy.
The Sun orbits far from the
center: ~25,000 light years,
<------------------------------------------like out here
at a speed of ~210 km/sec.
At this speed, it takes 226
Myr for the Sun to orbit
once!
NGC 3953
This galaxy is a tilted pinwheel of stars, gas, and dust.
This is called a “galactic year”.
Since it formed, the Sun has gone around only 20 times. At the
time the Minkin site animals were alive, the Sun had orbited a
little over 18 times, still a teenager!
What is significant about 226 Myr?
This was close to the beginning of the
Mesozoic Era, the middle life era
dominated by dinosaurs
Thus, ~1 galactic year ago, the age of
dinosaurs began!
www.ucmp.berkeley.edu/help/timeform.html
Late Triassic dinosaur Coelophysis
(220-200 Ma)
What a difference 1 galactic year makes!
Age of dinosaurs------------>>-----------Age of Man----------->>>>
1 more galactic year---->>
--------->>>>> ????????
Age of Man ------------------------------------Age of ??????????????
The Minkin site is older than 1 galactic year. What
does the Sun’s orbit in the Milky Way look like?
Answer: the orbit is approximately circular but is not
closed. The Sun moves slowly in and out as it orbits,
varying its distance from the center by as much as 3,000
light years.
This is the kind of orbit a planet follows
around the Sun. It is not quite the kind of
orbit the Sun itself follows around the
galactic center.
The parameters of the Sun’s
orbit are known well enough to
extrapolate it back in time.
This shows the orbit carried
back to the Devonian.
310 Myr ago, the Sun was located
about 38,000 light years (point
“SCM/UCM”) from where it is
today (“Now”).
This would change our perspective
on the rest of the Universe since we
must view it through the “lens” of our
own galaxy.
What impact would this greatly different location have on the
appearance of the night sky at the Minkin site?
First, imagine the stargazing tetrapods of the past!
Constellation Orion with a meteor
Would this tetrapod have been able to
see Orion or the Big Dipper?
No, but for two separate reasons.
Representative tetrapod from Clack (2002)
The first reason is that the Milky Way does not rotate as a solid body
would. Relative patterns on a solid object are maintained as the
object rotates. In the Milky Way, stars farther from the center take
longer to go around than stars closer in. Also, orbits are generally not
closed. This changes all relative patterns, like constellations.
The Big Dipper would have looked virtually the same to the ancient
Egyptians, but 100,000 years ago it looked different. It will further lose its
familiar shape in the future. Its 7 bright stars are not all at the same distance.
This kind of thing can’t be extrapolated too far backward or forward in time.
The second reason is the relatively short lifetimes of massive stars.
Our familiar constellations are dominated by such stars.
Stars that are powered by hydrogen fusion in the center are called main
sequence stars.
Our Sun is a main sequence star.
It is probably 1-2% brighter
today than it was 310 Myr ago.
The main sequence lifetime of any star is strongly dependent on its
mass. The more massive the star, the shorter its lifetime.
Hot stars more massive than 15 solar masses shine for only a few
million years.
Mass
lifetime
A 3.5 solar mass star has a main sequence
lifetime approximately equal to the age of the
Minkin site.
What this means is that any currently visible star
more massive than 3.5 solar masses did not exist
310 Myr ago!
<-----the Sun
This is true of virtually all the bright stars in the constellation
Orion: all are likely more massive than 3.5 solar masses
Betelgeuse
Bellatrix
The red supergiant Betelgeuse is
probably the oldest of them all: 10
million years!
Thus, not only would Orion not have
been visible 310 Myr ago because of
the non-solid-body rotation of the
Milky Way, none of its main stars
even existed!
Mintaka
Alnilam
Alnitak
Orion’s stars take us back no
further than the Miocene.
Saiph
Rigel
Enjoy them while they last!
What about bright stars whose main sequence lifetimes are much
longer than the age of the Minkin Site? Where were they 310 Myr
ago?
Answer: Again, the Milky Way’s non-solid body rotation means
relative positions even of older stars would change. These changes
are imperceptible over hundreds or thousands of years, but over
310 Myr, the changes would be large enough to completely change
all patterns of older, lower mass stars.
Example: Alpha Centauri is currently the nearest star (a system of 3) to the Sun. It is 4.3
light years away and is the third brightest star in the night sky. It is also thought to be 5-6
billion years old. Where was it 310 Myr ago?
Alpha Cen
Southern Cross
Answer: Although Alpha Cen’s orbit is close to ours, its distance from us and its
direction in the sky vary considerably over time. 310 Myr ago, it was 2,600 light
years away from us and was not visible to the naked eye!
Extrapolating back to the Devonian,
at no time was Alpha Cen ever as
close to us as it is now.
Alpha Cen is a fine binary star but would
not be seen as such from 2,600 light years
Also interesting from this perspective are galactic star clusters
Open cluster M38
From studies of brightnesses and star colors, we can estimate the age of a cluster
like this. This one has a few red giants and an age equal to that of the Minkin site.
Our Sun could have been born in a cluster like this. When very
young, these clusters show glowing gas and dust.
The Lagoon Nebula 7 Myr old
Pink nebulous clusters like the Lagoon Nebula line the spiral arms of this
distant galaxy, but none of those seen in the picture existed 310 Myr ago.
This is the way the galaxy looked
31,000,000 years ago. If you could
see it as it looks now, most of the
pink nebulae will have faded and
even disappeared. Newer ones
would be seen.
None of these pink nebulae could have been
seen by a telescopically-inclined Coal Age
amphibian!
The Pleiades star cluster visible to the naked eye in Taurus is only 100 Myr old. It
was not shining in anybody’s sky 310 Myr ago.
If the Sun was born in a cluster, where are its “siblings”?
Answer: most of these clusters disperse after several tens of
millions of years. Very few hang together more than a billion
years. It is likely the Sun had long since left its cluster by the
time of the Minkin site.
M67- this cluster is 2.6 billion
years old and is still hanging
together.
Time and Distance
The sky is like a time machine. Light travels in vacuum at finite
speed, 186,272 miles per second. In a year, a beam of light travels
one light year, or 6 trillion miles. This means that the farther out in
space we look, the farther back in time we see.
The Minkin site is 310 Myr old. Sunlight that reflected off the
trackmakers on a sunny day now is 310 million light years away
from Earth. Is there anything interesting at that distance?
Yes! The Coma galaxy cluster, Abell 1656, is 321 million light years away. It
is early Pennsylvanian light reaching us today. Within the uncertainty of the
distance, the light we are receiving from these galaxies left them when the
Minkin site trackmakers were alive!
If aliens in these galaxies
could see the Earth, they
would see Cincosaurus
cobbi and the rest of the
crew at the Minkin site.
Two giant elliptical galaxies and attendant satellites
The Great Spiral in Andromeda is 2.9 million light years from
Earth. The spiral is falling towards our Galaxy at 300 km/sec.
Since the Minkin site trackmakers were alive, the Andromeda
Galaxy has moved 313,000 light years closer to us. This is 2-3
times the size of the Milky Way!
Another issue is precession of the Earth’s rotation axis. Owing to the
gravitational influence of the Moon and Sun, the Earth’s poles
execute a circle on the sky. This circle takes 26,000 years to
complete.
Polaris, the current “north star,” will reprise its role again in 26,000
years. But 310 Myr is ~12,000 precession periods!
Polaris could not have
been the north star 310
Myr ago. It has a mass
of 5 solar masses. It
didn’t exist!
Constellations near the north polar precession circle
What were the Moon and planets like 310 Myr ago?
By and large, the Moon would have looked the same 310 Myr ago.
|
Tycho
But the lunar impact crater Tycho wasn’t there. It has an estimated age of only 108
Myr. Its absence would slightly change the naked eye appearance of the Moon.
The Moon was already tidally-locked by Pennsylvanian times.
The latest studies suggest that Saturn’s rings are very old. But
they are constantly changing and kept fresh by interactions with
the planet’s many moons. They could have looked different 310
Myr ago.
The rings were once thought
to be only 100 Myr old.
What about Jupiter’s Great Red Spot?
The spot has persisted for 350 years. Because Jupiter has a completely liquid
surface, there is nothing to weaken the storm. Would the spot have existed 310
Myr ago? Smaller storms come and go regularly.
Mars has a thin atmosphere and regular weathering, but it
would have been just as cold and dry 310 Myr ago as it is today.
Venus probably evolved rapidly after forming, but 310 Myr ago it
was already in a very slow phase of evolution. Thus, it would have
been very similar to what it is like today. It may have had a
completely molten surface 700 Myr ago.
What about the Earth?
Tidal forces in the Earth-Moon system slow the rotation of the Earth by 1.7
milliseconds per century. 310 Myr ago, the “day” was 22.5 current hours long.
The same tidal forces have been pushing the Moon farther away from Earth at
2.2 cm/year. 310 Myr ago, the Moon was 6,700 km closer to us. Still not close
enough to see craters with the naked eye.
440 Myr from now, an Earth-based scene like this will no longer be
possible.
Eclipses like this will last longer 440 Myr from now.
Conclusions
-the heavens are not immutable!
- many stars shining in our current sky did not exist 310 Myr ago
- even if they did exist, none of our current constellation patterns
would have been seen, owing to the non-solid-body rotation of the
Milky Way
- Pennsylvanian light from Earth is currently passing through the
Coma galaxy cluster
- of all the main planets, Earth probably would have looked the
most different