Knocking the World on Its Side

moon surface

Gregory Jenkins remembers looking at the surface of the moon through his son's telescope, his eyes tracing the craters and crevices. "I was thinking, 'Man, things were really violent early on,'" says Jenkins. "If objects this big were slamming into the moon, than larger things must have been slamming into the Earth."

Large enough to knock the planet on its side? Jenkins, assistant professor of meteorology at Penn State, thinks so. He believes that a large planetoid—the same one that fractured to create the moon—crashed into the Earth four and a half billion years ago, tilting it 70 degrees from vertical.

A tilted planet could explain two very different climate paradoxes: why the Earth's environment during the early Precambrian was warm, despite a weak sun; and why the Earth may have later become a giant snowball.

The first paradox, called the Faint Young Sun problem, arises because 3.8 to 2.5 billion years ago, during the Archean period, the sun was only about 75 percent as bright as it is today. With such a weak sun, the Earth should have been much too cold to foster the beginning of life.

But the geologic record shows that the planet then was actually warmer than it is today. Most scientists, says Jenkins, explain the Faint Young Sun problem by adding massive amounts of the greenhouse gas carbon dioxide to their computer models of the early climate. But in his climate model, Jenkins gives the Archean atmosphere only slightly elevated levels of carbon dioxide, while tilting the planet 70 degrees. "Tilt makes sense to me," he explains. "It changes the amount of solar radiation distributed at the various latitudes."

In Jenkins' model, tilting the Earth 70 degrees gives large parts of each hemisphere 24 hours of sunlight for three months. If this sunlight were to fall on land, temperatures would be hotter than 125 degrees Fahrenheit in summer, then plummet during the three-month, 24-hour night. However, the Earth was 95 percent water during the Archean. Because water has a higher specific heat than land, it doesn't get as hot during the three-month sunbath, but also it never cools below the freezing point of ocean water. Jenkins' model shows that a 70-degree tilt could produce the warmer temperatures expected during the Archean.

The second problem, that of a Snowball Earth, could also be solved by a 70-degree tilt. During the Proterozoic 2.5 billion to 544 million years ago, evidence exists for glaciation at the equator. Researchers suggest that total global glaciation occurred at least three different times.

"Three glaciations would mean three extinctions for large numbers of species," says Jenkins. "To suggest that life made it through really hard times three times is really hard for me to believe." In his model for this period, Jenkins uses both the 70-degree tilt and somewhat more land mass to reflect the geography of the time. "If Earth had a high obliquity, than the smallest amount of sun would hit the equator making it the most likely place for glaciation," he says. The model suggests that while glaciation does occur at the equators, the poles remain ice free. This scenario, where some ocean and land remain unfrozen, would allow species to survive.

While an Earth on its side may provide explanations for the Faint Young Sun problem and the Snowball Earth, another question remains: How did the Earth go from a tilt of 70 degrees to today's tilt of 23 degrees from vertical?

"George Williams, department of geology and geophysics, University of Adelaide, originally proposed the tilt and suggested that if a large enough mass built up on the pole, it could untilt," Jenkins explains. "At the end of the Precambrian, most of the Earth's land mass was centered around the South Pole."

Recent studies conducted by geoscientists Darren Williams and Jim Kasting of Penn State suggest that with enough mass, it would take about 100 million years to return the Earth to a tilt between 20 and 30 degrees. "Based on what we know about the climate then, this is a viable solution," Jenkins says.

Gregory Jenkins, Ph.D., is assistant professor of meteorology in the College of Earth and Mineral Sciences, 524 Walker Building, University Park, PA 16802; 814-865-0478; gsj1@psu.edu. His research is funded by NASA and NSF. Reported by A'ndrea Messer.

Last Updated May 01, 2001