Astronomy and the Coal Age of Alabama

1. 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

2. 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

3. 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

4. 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

5. 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

6. It was a time of great upheaval…. The Appalachian Mountains were at least as high as the modern Andes.

7. 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

8. The Appalachian Mountains are now only a shadow of what they once were.

9. 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

10. 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

11. The Minkin site may have at one time looked like this... Deep time has erased this world completely. Artwork due to Jim Lacefield

12. 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

13. 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.

14. 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

15. 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.

16. 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 is called a “galactic year”. This galaxy is a tilted pinwheel of stars, gas, and dust.

17. 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!

18. 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)

19. What a difference 1 galactic year makes! Age of dinosaurs------------>>-----------Age of Man----------->>>> 1 more galactic year---->> --------->>>>> ???????? Age of Man ------------------------------------Age of ??????????????

20. 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.

21. 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.

22. 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)

23. 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.

24. 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.

25. 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.

26. 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. A 3.5 solar mass star has a main sequence lifetime approximately equal to the age of the Minkin site. Mass lifetime What this means is that any currently visible star more massive than 3.5 solar masses did not exist 310 Myr ago! <-----the Sun

27. This is true of virtually all the bright stars in the constellation Orion: all are likely more massive than 3.5 solar masses 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! Betelgeuse Bellatrix Mintaka Alnilam Alnitak Orion’s stars take us back no further than the Miocene. Saiph Rigel Enjoy them while they last!

28. 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.

29. 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

30. 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

31. 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.

32. 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

33. 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!

34. 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.

35. 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.

36. 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?

37. 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 andthe rest of the crew at the Minkin site. Two giant elliptical galaxies and attendant satellites

38. 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!

39. 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.

40. 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

41. What were the Moon and planets like 310 Myr ago?

42. 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.

43. 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.

44. 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.

45. 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.

46. 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.

47. What about the Earth?

48. 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.

49. 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.

50. 440 Myr from now, an Earth-based scene like this will no longer be possible.