An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time
THE CHANGING UNIVERSE: EVOLUTION HAPPENS!
a) Changes in the Solar System - Continued
At the beginning, there were many more chunks of rock and ice around, but as our system has evolved, many of those chunks have either hit the planets and moons or have been flung out of the system by the influence of a large planet's gravity. By the way, we can observe the impacts of smaller chunks with the Earth today, and observe "near misses" by larger objects. In this way, we can determine the current rate of impacts. This provides another measure of the great age of the surfaces of the Moon and the solid planets.
Robotic spacecraft orbiting Mars have found many dry river-beds there. But Mars is too cold today for water to exist in liquid form. Furthermore, the planet's atmosphere is so thin that any liquid water would rapidly evaporate away. Yet the river-beds are clear evidence that in the distant past Mars had liquid water flowing on its surface. We conclude that Mars too has evolved. It was warmer and had a thicker atmosphere billions of years ago, but because of its lower gravity, has now lost much of its sheltering air.
These and many other lines of evidence reveal that the planets of the solar system have changed over time. By studying these changes, we can gain insight into Earth's past and perhaps its future.
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An Ancient Universe - Table of Contents
Home | Introduction | The Universe: An Overview | The Process of Science | The Ancient Universe - The Age of the Expanding Universe - The Age of the Oldest Stars - The Age of Light From Distant Galaxies - The Age of the Chemical Elements | The Changing Universe - Changes in the Solar System - Changes in Stars - Changes in the Universe | Science and Religion | Resource Guide | Activities
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