The Universe in the Classroom

An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time


a) Changes in the Solar System - Continued

The Cratered Face of the Moon
The Cratered Face of the Moon

Impact craters on the Earth, Moon, and other worlds are formed by the bombardment of chunks of rock and ice from space. By studying these craters, we can learn how common these impacts have been. The Moon is a good place to do this, because its craters have not been eroded away, as they have on the active Earth. The Moon turns out to contain many old craters, and fewer young ones. So we conclude that the solar system experienced many more impacts in the distant past than today.

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.

We're grateful that the number of impacts has been decreasing, since large impacts can have devastating effects on the Earth. There is strong evidence that 65 million years ago, a chunk about 10 km across hit what is now Mexico. The resulting explosion raised so much dust and smoke that the entire Earth experienced a long dark period. The lack of sunlight and warmth killed off much vegetation, and many animals, perhaps including the dinosaurs. When geologists dig in layers of rock from 65 million years ago, when the fossil record shows this "great dying", they find higher traces of elements that are rare on Earth, but more common in these rocks from space. The huge mass of debris from the impact was carried by our planet's winds all over the Earth and is now part of the rocks from that time.

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.

An Old Riverbed on Mars
Old Riverbed on Mars

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

© Copyright 2001, American Astronomical Society. Permission to reproduce in its entirety for any non-profit, educational purpose is hereby granted. For all other uses contact the publisher: Astronomical Society of the Pacific, 390 Ashton Ave., San Francisco, CA 94112.

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