Cycles of destruction

28th January 2016

Photograph of the Ouarkziz Impact Crater in Algeria.

The Ouarkziz Impact Crater in Algeria, which was formed by a meteor impact less than 70 million years ago, when dinosaurs still walked the Earth. Image credit: NASA/Public domain.

1. Impact and extinction cycles

Over 180 impact craters have been identified on Earth, and most of these were discovered in the first half of the 20th century[1].

Impacts have been associated with mass extinction events since the 1980s[2], with strong evidence coming from the Chicxulub Crater, which was linked to the extinction of the dinosaurs about 65 million years ago[3].

Mass extinction events were soon shown to be periodic[4], and this led astronomers to search for periodicities in impact events[5].

It’s difficult to determine if either of these events are periodic because both datasets are incomplete, and contain both periodic and non-periodic data. Timescales are often only approximate, and results are biased by the fact that more recent events are easier to identify.

This has led to inconsistent results, with some studies finding no evidence of a periodicity in mass extinctions[6] or impact events[7], and some reporting periods ranging from 25-30 million years for mass extinction events[8], and 25-35 million years for impact events in the last 260 million years[9]. This is around the time of the Permian mass extinction, when about 96% of species on Earth were wiped out[10].

Painting of an asteroid entering the Earth’s atmosphere.

Artist’s impression of an impact event on Earth. Image credit: NASA/Don Davis/Public domain.

2. Modern results

In a recent paper published in Monthly Notices of the Royal Astronomical Society, Professor of Biology Michael R. Rampino and Climate Scientist Ken Caldeira provide further evidence for a 26 million year impact event cycle using data from the Earth Impact Database 2015[11].

Rampino and Caldeira considered all impact craters from the last 260 million years, with ages that have error bars of less than ±10 million years. They also discounted craters younger than 5 million years in order to prevent the relatively large number of recent craters from skewing the results. This left 37 impact craters.

Rampino and Caldeira then used a circular method of spectral analysis, which ‘wraps‘ the time series in a circle with a circumference equal to the trial period. If a correct period is found, then, in a perfect dataset, all of the data will be in the same place on the circle. If there is no period, then they will be distributed randomly.

Their results show a period of 25.8±0.6 million years. The latest impact event occurred 11.8±1 million years ago, and so if this cycle is correct, there should be another impact event in 14±1.6 million years.

Rampino and Caldeira use the same method to determine a period of 27.0±0.7 million years for mass extinction events in the past 260 years. The latest mass extinction event occurred 16.0±1.3 million years ago, which means we are due another in 11±2 million years.

There are 11 main impact events, and 10 mass extinction events, where six of these appear to correlate.

3. Causes

It is still not known why impact events appear to be cyclical. The most likely suggestions are that they are caused by a massive undetected object in the Solar System, or by the Sun’s movement through the Galaxy.

3.1 Planet X

In the 1980s, it was suggested that impact events occur in cycles because of a massive undetected object in the Solar System that periodically passes close to the Oort Cloud, the sphere of comets that orbit the Sun[12]. This may cause comet showers throughout the Solar system.

This object was first thought to be another star, referred to as ‘Nemesis’ or the ‘Death Star’, which would make the Sun part of a binary system.

This has since been disproved, however it’s still possible that there’s a planet-sized object in the Oort Cloud or Kuiper Belt that periodically causes impact events.

The Kuiper Belt is a belt of asteroids that orbits the Sun from beyond Neptune, and so this theory predicts that most impact events are caused by asteroids and not comets. It has been suggested that most large craters are caused by comets[13], however results have been mixed[14].

Diagram showing the Oort Cloud, the Kuiper Belt is deep inside, and the planets orbit the Sun from within the Kuiper Belt.

Image credit: NASA/Public domain.

A potential massive planet in the Kuiper Belt is referred to as Planet X. It was first suggested that Planet X would have to be about five times the mass of the Earth, which is about 1/3 the mass of Uranus, the least massive of the outer planets[15]. However, it is now thought that it could be around the same mass as the Earth if it happens to be in the right place[16].

Astronomer Mike Brown and planetary scientist Konstantin Batygin have recently shown that there might be a planet at the edge of the Kuiper Belt that is around 10 times the mass of the Earth[17], and astronomers are now looking for evidence of this.

3.2 The Sun’s orbit through the Galaxy

Other early suggestions involved the Sun’s path through the Galaxy. The Sun does not orbit the Galactic centre in a flat plane, the way that planets orbit the Sun. Instead, it’s thought to move up and down, in a wave pattern.

Diagram showing that the Sun moves up and down with respect to the Galactic disc.

Diagram showing the orbit of the Sun through the Milky Way (approximate and exaggerated for clarity). Image credit: modified by Helen Klus, original image by NASA/CXC/M.Weiss/Public domain.

The Sun takes about 230 million years to orbit the Galaxy, travelling at about 250 km/s (just over half a million miles per hour). It passes through the vertical gravitational centre of the Galactic disc once every 30 million years or so[18a].

The Sun moves up and down because it is pulled by gravity. It is pulled towards the centre of gravity, overshoots slightly, and is then pulled back up or down. It moves about 200 light-years from the centre of the 1000 light-year-wide disc at each maximum or minimum. The Sun passes through the centre about 8 times with every Galactic orbit.

It was first suggested that the Oort Cloud was affected as the Sun moves through the gravitational centre of the Galactic disc[19]. In 1996, Rampino referred to this idea as the Shiva Hypothesis, after the Hindu god of destruction[20].

One problem with this idea, however, is that scientists think we last moved though the centre of the Galactic disc about 1 million years ago[21], yet the last impact event was about 11 million years ago.

This idea has since been extended by physicists Lisa Randall and Matthew Reece, who show that the Oort Cloud may be affected in a similar way if it passes through a thin disc of dark matter[22].

If the Earth passes through a dark matter disc, then it might capture some of the dark matter. Dark matter could form a ball at the centre of the Earth’s core, causing the core to increase in temperature[18b].

This may lead to an increase in volcanic activity and earthquakes, and could even cause the Earth’s magnetic poles to reverse, so that the North Magnetic Pole becomes the South Magnetic Pole, and vice versa.

There's currently no strong evidence for a dark matter disc around the Milky Way, however Randall and Reece’s predictions can be tested by the ESA's Gaia satellite. Gaia is currently mapping the gravitational field of the Galaxy, and the first set of results should be released later this year.

If a dark matter disc is detected, then it would mean that the geological and biological evolution that has taken place on Earth is directly linked to the distribution of matter in the Galaxy.

4. References

  1. Planetary and Space Science Centre, 'Earth Impact Database', last accessed 15-02-16.

  2. Smit, J. and Klaver, G., 1981, 'Sanidine spherules at the Cretaceous–Tertiary boundary indicate a large impact event', Nature, 292, pp.47-49.

  3. Schulte, P., et al, 2010, 'The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary', Science, 327, pp.1214-1218.

  4. Raup, D. M. and Sepkoski, J. J., 1984, 'Periodicity of extinctions in the geologic past', Proceedings of the National Academy of Sciences, 81, pp.801-805.

  5. Rampino, M. R. and Stothers, R. B., 1984, 'Terrestrial mass extinctions, cometary impacts and the Sun's motion perpendicular to the galactic plane', Nature, 308, pp.709-712.

  6. Bailer-Jones, C. A. L., 2011, 'Erratum: Bayesian time series analysis of terrestrial impact cratering', Monthly Notices of the Royal Astronomical Society, 418, pp.2111-2112.

  7. Bailer-Jones, C. A. L., 2009, 'The evidence for and against astronomical impacts on climate change and mass extinctions: a review', International Journal of Astrobiology, 8, pp.213-219.

  8. Stothers, R. B., 1989, 'Structure and dating errors in the geologic time scale and periodicity in mass extinctions', Geophysical Research Letters, 16, pp.119-122.

  9. Wickramasinghe, J. T. and Napier, W. M., 2008, 'Impact cratering and the Oort cloud', Monthly Notices of the Royal Astronomical Society, 387, pp.153-157.

  10. Barnosky, A. D., et al, 2011, 'Has the Earth's sixth mass extinction already arrived?', Nature, 471, pp.51-57.

  11. Rampino, M. R. and Caldeira, K., 2015, 'Periodic impact cratering and extinction events over the last 260 million years', Monthly Notices of the Royal Astronomical Society, 454, pp.3480-3484.

  12. Whitmire, D. P. and Jackson, A. A., 1984, 'Are periodic mass extinctions driven by a distant solar companion?', Nature, 308, pp.713-715.

  13. Shoemaker, E. M., 1998, 'Impact cratering through geologic time', Journal of the Royal Astronomical Society of Canada, 92, pp.297-309.

  14. Mukhopadhyay, S., Farley, K. A., and Montanari, A., 2001, 'A short duration of the Cretaceous-Tertiary boundary event: Evidence from extraterrestrial helium-3', Science, 291, pp.1952-1955.

  15. Whitmire, D. P. and Matese, J. J., 1985, 'Periodic comet showers and planet X', Nature, 313, pp.36-38.

  16. Whitmire, D. P., 2016, 'Periodic mass extinctions and the Planet X model reconsidered', Monthly Notices of the Royal Astronomical Society: Letters, 455, pp.114-117.

  17. Batygin, K. and Brown, M. E., 2016, 'Evidence for a Distant Giant Planet in the Solar System', The Astronomical Journal, 151, pp.22.

  18. (a, b) Rampino, M. R., 2015, 'Disc dark matter in the Galaxy and potential cycles of extraterrestrial impacts, mass extinctions and geological events', Monthly Notices of the Royal Astronomical Society, 448, pp.1816-1820.

  19. Schwartz, R. D. and James, P. B., 1984, 'Periodic mass extinctions and the Sun's oscillation about the galactic plane', Nature, 308, pp.712-713.

  20. Rampino, M. R. and Haggerty, B. M., 1996, 'The 'Shiva Hypothesis': Impacts, mass extinctions, and the galaxy', Earth, Moon and Planets, 72, pp.441-460.

  21. Gies, D. R. and Helsel, J. W., 2005, 'Ice age epochs and the Sun’s path through the Galaxy', The Astrophysical Journal, 626, pp.844.

  22. Randall, L. and Reece, M., 2014, 'Dark matter as a trigger for periodic comet impacts', Physical review letters, 112, pp.161301.

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