Rings around the Earth: A clue to climate change?
While most of us know about rings around Saturn and Jupiter, some scientists believe there once were rings of rock debris around our own planet. Two scientists Peter J. Fawcett, of the University of New Mexico, and Mark B.E. Boslough, of the U.S. Department of Energy's Sandia National Laboratories have suggested that "a geologically recent" collision (about 35 million years ago) may have caused such a temporary debris ring.
The two also suggest that such temporary rings lasting from 100,000 to a few millions of years may explain some patterns of climate change observed in the earth's geological record. These conclusions are spelled out in an article in the Journal of Geophysical Research, Atmospheres, August 16 edition.
Lore of the Rings
"One way to get a ring," says Sandia's Boslough, "is with an impact." There is a growing body of evidence showing that the earth has been subjected to numerous impacts by comets and asteroids throughout its history. Among these impacts are the Meteor Crater, in Arizona, the buried Chixulub crater, in the Yucatan Peninsula of Mexico, and a chain of at least five craters spread across several continents.
Several studies, both theoretical and with laboratory data, suggest that some large impacts are capable of ejecting material into space in the form of debris rings, if the mechanics of the impact meet certain requirements. The authors conclude that the mostly likely scenario for ring creation is a low-angle impact by a large asteroid. Some earth materials and melted meteoric debris, called "tektites" would form the ring materials.
Boslough describes an impact where the collision object ricochets back into the atmosphere. The ricochet becomes part of an expanding vapor cloud, setting up an interaction that allows some of the debris to attain orbit velocity. The orbiting debris will collapse into a single plane by the same mechanics that led to the rings of Saturn and other planets, Boslough explains. Such a ring would most likely form near the equator, because of the dynamics involved with the moon and the earth's equatorial bulge.
Speculation on climates past
The effects of the larger impact events on earth's environment and climate have been the subjects of much speculation and research over the past two decades. "Clearly, large impacts have affected the evolution of the earth, life on it and its atmospheric environment," says Fawcett.
Much of the work has focused on the Cretaceous-Tertiary (K-T) boundary event, which marked a mass extinction and the end of the age of the dinosaurs about 65 million years ago. A number of these studies suggest an impact resulting in the suspension of a layer of dust in the upper atmosphere blocking sunlight and cooling the earth. The two researchers asked could other impacts result in different atmosphere-altering phenomena?
An equatorial ring would cast a shadow primarily in the tropics, as it does for Saturn. Depending on location, surface area, and darkness of the ring shadow, the amount of incoming solar warmth, or insolation, could be significantly altered, the two authors conclude. To test their theory, the two assumed an opaque ring, like Saturn's B-ring, scaled to earth-size and tested global climate affects using a climate model.
The model selected and modified for the simulation was developed by the National Center for Atmospheric Research (NCAR.) The Center's "Genesis" climate model includes atmospheric circulation information and layers of vegetation, soil, snow, sea temperature and land ice data. The goals of the internally funded project were for Sandia to adapt a popular climate code to run on distributed-memory parallel computers and to establish relationships with the climate change research community, Boslough explained. The Labs made use of its Sandia University Research Program to fund Fawcett's efforts to analyze the data from the adapted code.
A Ring World
The equatorial debris ring has a profound effect on climate, because it "reflects a significant fraction of tropical insolation back to space before it can interact with the atmosphere," the authors conclude. Surface and atmospheric temperatures, changes in temperature ranges from equator to poles, circulation patterns and the rain and snow cycles were all impacted by the ring, the model shows.
The two scientists looked at changes shown in the model to predict changes that might be found in the earth's geologic record as a way to test their work. In addition to the K-T boundary event, they looked at a more recent impacts and a much older one.
The most recent event about 35 million years ago is identified by an iridium layer (often associated with meteors) and two pronounced mico-tektite fields, where these melted meteor-related materials have been found and dated. Climatic records from sedimentary materials just above the iridium/micro-tektite interval indicate a 100,000-year cooling interval. Orbiting debris in a ring, casting its shadow in the subtropics could have sustained such a cooling trend, the authors suggest.
The K-T boundary impact about 65 million years ago was much larger than the more recent impact and had a much larger immediate effect on the environment as measured by extinctions and atmospheric changes. But there were no long-term effects on the climate, leading the authors to conclude the event probably did not generate a debris ring.
Another interesting aspect of the modeling work is its implications for the so-called "Snowball Earth" theory. This theory holds that the earth was completely frozen over at the surface as many as four times in the neoproterozoic period 750 to 580 million years ago. While much remains to be learned about the geologic evidence for this theory, "an opaque ring could have acted as the trigger to at least one episode of global glaciation," the two researchers say. This would address one difficult question for the theorists: how did earth come to be frozen?