Sedimentary rocks laid down when a great seaway ran through the middle of North America have demonstrated that Mars has helped shape Earth's climate. A transition in the relationship between the orbits of Earth and Mars occurred while dinosaurs still dominated the Earth, providing support to a theory of the way planetary orbits shift.
Materials deposited on the floor of what is known as the North American Seaway provide a record of Earth's climate during the Cretaceous era. Depending on the conditions of the day the rocks were either shale or limestone.
Professor Stephen Meyers of the University of Wisconsin, Madison, carefully dated the alternating layers of each in the Niobrara Formation, Colorado, to provide a record of the period's changing climate. “Imagine a very warm and wet climate state that pumps clay into the seaway via rivers, producing a clay-rich rock or shale, alternating with a drier and cooler climate state which pumps less clay into the seaway and produces a calcium carbonate-rich rock or limestone,” Meyers said in a statement.
Meyers argues in Nature that the changes were so regular they must have been driven by alterations to Earth's orbit.
We know that orbital patterns known as Milankovitch cycles have been responsible for swings between glacial and interglacial conditions over the last few million years. These are a result of three things: Shifts between a more rounded and more elongated (or eccentric) orbit, the tilt of the Earth's axis, and the season in which Earth is closest to the Sun.
Meyers claims that during the Cretaceous, Martian gravity helped determine the first of these.
When looking at the orbits of moons around Jupiter and Saturn we often see what are known as resonances. For example, Jupiter's moon Europa takes twice as long to orbit as its neighbor Io, so that every second time Io gets back to the same spot, Europa is tugging at it. Cumulatively, this can change a moon's, or indeed a planet's, orbit.
Such ratios are not always permanent, however, and Meyers concludes that a change he found in the period of shifts between shale and clay is a result of a “resonance transition”, where Earth and Mars moved from one resonance pattern to another. At around 85-87 million years ago, Meyer found the cycle of Earth's orbital eccentricity shifted from 1.2 million years to 2.4 million years. Meanwhile, other orbital characteristics stayed the same. Such a change is the signature of a resonance transition with another planet.
Theoretical models of the evolution of the orbits of the planets suggest they should behave chaotically. This doesn't mean that the planets should bounce around like billiard balls after a break, but that small variations can lead to changes to their orbits that cannot be predicted over long periods of time, in this case anything over 50 million years. These models indicate that there should have been times where the planets moved from one resonance to another, with accompanying effects on the climate.
Although the idea of chaotic orbits dates back to 1989, previous geological evidence has been ambiguous. Meyer has claimed his findings, “Made possible by the availability of high-quality, radioisotopic dates and the strong astronomical signal preserved in the rocks" represent the clarity that has been needed to confirm the theory.