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The Earth's orbit, and that of every other planet in the Solar System, goes through cycles. We understand recent patterns well, but have been hazy on most of our planet's history. New research shows ancient lake sediments can fill in a little of this picture, telling us not only what was happening to Earth's orbit 200 million years ago, but also what Mercury, Venus, Mars, and Jupiter were doing at the time.
With so many objects having sufficient gravitational pull to influence each other, it is effectively impossible to model the inner Solar System's orbits before 60 million years ago. Every 10 million years we go back before then the uncertainty rises by roughly a factor of 10. Average orbital distances have not changed, but elongation and resonances between planets have. Today, for example, Venus has an almost perfectly circular orbit, while Mars is more stretched, and Earth's shifts between over time.
Professor Paul Olsen of Columbia University collected cores from what were once tropical lakes in modern New Jersey and Arizona. The sediments were laid down between 223 and 199 million years ago, spanning the Triassic-Jurassic boundary. The lakes' depths were determined by wet and dry eras. Just as before humans intervened, the timing of recent ice ages was determined by cycles of the Earth's tilt and the shape of its orbit, Olsen concluded orbital changes were responsible for the patterns he found.
Last year, Olsen reported a 405,000-year cycle in the sediments, known as the McLaughlin cycle, which matches one seen today driven by the gravity of Jupiter and Venus.
Now in Proceedings of the National Academy of Sciences, Olsen reports that, just as music is made up of many different frequencies, the sediments also record other cycles lasting different periods. Unlike the McLaughlin, most of these were of different lengths in the Triassic to today. The McLaughlin cycle acts as a measuring stick, for example indicating that Axial precession, which now operates on a period of 25,772 years, took just 20,000 years back then.
To be able to read Earth's movements in 200-million-year-old mud is remarkable enough, but Olsen and co-authors have extended this to other worlds by calculating the orbital dynamics of other planets needed to explain what he found. For example, a “Grand Cycle” Earth-Mars resonance, which today lasts 2.4 million years, had a period of 1.75 million years in the late Triassic, revealing the Red Planet's orbit as well as Earth's.
Olsen argues his work could help reveal Martian and terrestrial paleoclimates, particularly if he can expand it with higher latitude data. It could even be used to test ideas such as the theory the Solar System regularly passes through a plane of concentrated dark matter, influencing planetary orbits.
