Million Years Ago 65 Million Years Essay - 2,114 words

The "Nemesis Theory" was an outgrowth of the discovery of Alvarez et al. , that the impact of a large (10 km diameter) comet or asteroid was responsible for the great mass extinction that took place 65 million years ago. Studies of the fossil record by Dave Raup and Jack Sepkoski shows that this was not an isolated event, but one of several mass extinctions that appear to occur on a regular 26 million year cycle. Their original paper analyzed marine fossil families, and was published in the Proceedings of the National Academy of Science USA, vol 81, pages 801 - 805 (1984). The original extinction data of Raup and Sepkoski are replotted in the following figure. The vertical axis shows the "extinction rate. " This was taken from the values given by Raup and Sepkoski for the percent family extinctions at each geologic boundary. In order to take into account the uncertainty in the boundary ages, each data point was plotted as a Gaussian, with width equal to the uncertainty, and area equal to the extinction rate.


This plot thus represents a statistical estimate of the extinction rate vs. time. The individual Gaussian's for each stage boundary are shown as dotted lines. The extinction 65 million years ago is indicated with the little dinosaur icon. The peak near 11 Ma is real, but exaggerated by the requirement that the plot go to zero at the present. Arrows are plotted every 26 million years.


Note that many of these are close to the peaks in the extinction rate. This is the apparent 26 million year periodicity discovered by Raup and Sepkoski. There have been many statistical studies of these data. Although several studies indicate the periodicity is significant, not everyone agrees. I suggest that you decide for yourself. If you decide that the extinctions are not statistically significant, then there is no need for the Nemesis theory.


Additional work by Sepkoski shows that the periodicity is also present for fossil genera. His results were published in the Journal of the Geological Society of London, vol 146, pp 7 - 19 (1989). Figure 2 from this paper is shown below. Please note that the time axis has been reversed compared to that of the previous figure. Plotted is the per-genus extinction rate (in units of extinctions / genus /Myr) for 49 sampling intervals. The upper time series (labelled Total) is for Sepkoski's entire data set of 17, 500 genera, where the lower "filtered" time series is for a subset of 11, 000 from which genera confined to single stratigraphic intervals have been excluded.


The vertical lines are plotted at 26 Myr intervals. The Nemesis theory was devised to account for this regularity in the timing of the mass extinctions reported by Raup and Sepkoski. According to this model, a companion star orbiting the Sun perturbs the Oort comet cloud every 26 Myr causing comet showers in the inner solar system. One or more of these comets strike the Earth causing a mass extinction. The Nemesis theory was originally published in Nature by Davis, Hut, and Muller (vol 308, pp 715 - 717, 1984). A longer description of the work leading up to the theory was writen in book form: "Nemesis, " by Richard Muller (Weidenfeld & Nicolson, 1988).


You can read Chapter 1 Cosmic Terrorist. here. This book is out of print, but I have some extra copies. Contact me if you need a copy.


There is a great deal of confusion among astronomers about the stability of the Nemesis orbit. Even many theorists who should know better believe that the orbit is unstable, and that the original Nemesis paper was in error. However detailed calculations by Piet Hut at the Institute for Advanced Study in Princeton show that the original estimate about the orbit were correct. Hut's results were published in Nature, vol 311, pp. 636 - 640 (1984). In our original paper we had stated that the orbit presently has a stability time constant of approximately one billion years. Many people naively assumed that this was incompatible with the 4. 5 billion-year age of the solar system.


But unlike the lifetime of a radioactive element, the lifetime of the Nemesis orbit is not predicted to be constant with time. In fact, Hut has shown that the lifetime decreases linearly, not exponentially, with age. The expected orbit lifetime when the solar system was formed was (presumably) about 5. 5 billion years. When nearby stars pass the solar system, the orbit of Nemesis is given slight boosts in energy. The Nemesis orbit becomes larger and less stable. At present, the Nemesis orbit has a semi-major axis of about 1. 5 light-years, and the orbit is expected to remain bound to the sun for only another billion years.


Note that the Nemesis theory predicts that the periodicity should not be precise. Perturbations from passing stars are not sufficient to disrupt the orbit, but they are sufficient to cause a slight (a few Myr) jitter in the interval between extinction. Why do so many people think the orbit is unstable? The basic answer is that scientists often don't have time to read the literature, so they depend on the summaries of others. Click here for more details.


Nemesis is most likely a red dwarf star, magnitude between 7 and 12. Virtually all such stars have been catalogued, but very few of them have had their distance measured. It is likely that Nemesis, if it exists, will be visible with binoculars or a small telescope. We don't need a large telescope to find Nemesis. We need a small or medium telescope, and enough time to look at and analyze 3000 candidate stars. A series of images taken throughout the year should allow us to measure the large parallax of this star.


We are also eliminating the stars measured by the Hipparcos satellite. We began the search for Nemesis using the automated telescope at Leuschner Observatory. However this telescope was not designed for the heavy use it was receiving from this search and from our automated search for nearby supernova. So we are in the process of constructing a new telescope. It is now undergoing final testing, and it soon will be moved to a mountaintop near Monterey. Its primary use will be fo ...................................................................................................................................................................................................................................................................................................................................................................

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Essay Tags: billion years, solar system, mass extinctions, million years ago, 65 million years

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