Scientists Sean C. Solomon of the Carnegie Institution of Washington's, Department of Terrestrial Magnetism in Washington, D.C., and Mark Bullock and David Grinspoon of the Department of Space Studies at the Southwest Research Institute in Boulder, Colorado, will report their findings in the October 1, 1999, issue of Science.

Although Venus is frequently referred to as Earth's sister, there are fundamental differences between the two planets. The climate and geological histories are two examples. On Earth, the mean surface temperature has changed over time by only a few degrees. In contrast, scientists now believe that the surface temperature on Venus may have varied by more than 100 K.

Researchers know that the Earth's surface is the product of active plate tectonics, a process in which nearly rigid plates are in steady relative motion and interact principally at their boundaries. However, Venus shows no evidence for global plate tectonics. So what gave rise to the tens of thousands of faults, known as wrinkle ridges, that deform the ridged plains?

The evidence indicates that the formation of the wrinkle ridges was triggered by the volcanic eruptions that produced the vast plains. The plains-forming event, which occurred about 500 million years ago, was the largest episode of volcanism in the planet's known history.

The estimated volume of this material is huge at least 100 to 200 million cubic kilometers enough to blanket the planet in a layer 500 meters thick. Evidence from impact craters suggests that the time it took from the emplacement of the lava flows to the deformation that produced the abundant wrinkles was quite short by geologic standards less than 100 million years.

The scientists modeled the climatic consequences of the large eruption. They determined that a large change in surface temperature resulted, which was transferred to the interior thereby causing the thermal stress that created the ridges.

The mechanism behind these results begins with the fact that Venus is now and has long been a greenhouse caldron. The surface temperature is about 740 K, and a global cloud layer and the thick atmosphere, consisting of 97% CO2, keeps the heat contained. Water and SO2 are important cloud constituents and also affect the radiative balance of the atmosphere.

According to the model, the release to the atmosphere of additional water and sulfur gases during the enormous volcanic eruption enhanced the greenhouse effect and heated the planet by about 60 K over the ensuing 100 million years. During this time, heating of the surface rocks in turn heated the planet's interior. The hot material expanded such that it compressed the surrounding area and thus created the abundant wrinkle ridges.

The model looked at the most significant episode of known volcanism on Venus. However, because so much of the planet's surface dates from this same period, the scientists know little about variations in climate before this event.

According to Sean Solomon, lead author of the study, "What we are suggesting by extrapolation is that there may have been many large volcanic events in the history of Venus, each of which led to climate instabilities and to episodes of heating and cooling. Much of the history of faulting on Venus may be linked to climate change driven by major volcanic eruptions. This strong coupling among volcanism, climate, and global-scale deformation can account for many of the differences between Venus and Earth."

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