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Skywatching

Mountains on other worlds

Our world has lots of mountain ranges. Even a small telescope will show mountains on the moon. There are mountains on Venus and Mars. This might sound like a "so what?" issue, but actually it is more intriguing than it might seem.

The Earth's crust is divided into a number of huge tectonic plates, which slowly move around, driven by the convective rise and fall of magma from the Earth's mantle. They collide, slide under one another, grind together, join together into supercontinents, and then move apart again. All this happens at about the speed fingernails grow. When plates collide, parts of one plate get scraped off by the other plate, accumulating as a crumpled mass along the edge of the upper plate. In addition, the upper plate is crumpled by the impact. These crumples are mountain ranges.

The ones down the West Coast of North America are due to the North American Plate colliding with the Pacific Plate. Once mountains form, erosion by ice, water, temperature changes and wind get to work, wearing them down. The Himalayas are also young mountains, formed by India crashing into the underbelly of Asia, and still growing. The plate collisions that made mountains on the eastern side of North America are long over, and the mountains have been eroded into low hills.

There are mountains on most solid objects in the solar system. Do they have plate tectonics too?

After Earth, Mars must now be the most explored body in the solar system. Apart from some of the biggest volcanoes in the solar system, the martian mountains are low and rounded, shaped by millions or billions of years of erosion. It is possible that in the distant past, when Mars was a warmer, wetter world, its surface could have had moving plates. Being a smaller world than ours, its mantle and probably the core cooled to the point where they are solid, and the fluid motions driving plate tectonics stopped. Another clue that the recycling of the surface has stopped is the large number of impact craters we see on the surface. In contrast, over the whole of North America only about 60 impact craters have been found. The rest have been erased by erosion and plate motions.

Venus, the next planet in toward the sun, is about the same size as the Earth and has long thought to be the Earth's twin. In some ways it might be, but in other important ways it definitely is not. A runaway greenhouse effect makes its surface hot enough to melt lead and tin. Images of its surface show lots of volcanoes but very few impact craters. Since Venus must have been hit as many times as Mars and Earth have, this suggests that like Earth, Venus' surface is being recycled.

The surface of Venus shows no signs of tectonic plates, but there is evidence of the surface being divided into smaller blocks, which rise and fall. One possibility is that we have plate motions because we have oceans. This leads to some areas of the Earth's crust being much warmer than others, enhancing large-scale convective flows of material from the Earth's mantle. Venus has no oceans. It is far too hot. Mars, on the other hand, bears extensive evidence that there was once a lot of surface water, maybe enough for plate motions.

The moon shows no traces of plate motions. However, even a pair of binoculars will show there are many mountainous areas on its surface. A careful look through a telescope will show that many of the mountain ranges are curved. In most cases the mountains we see are the edges of craters sticking up above the surface of lava flows. It seems that for billions of years the Moon's geology has been driven by impacts.

It looks as though having lots of water played a big role in making our planet what it is. The intriguing question is how we managed to get so much.

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  • Venus lies very low in the sunset glow.
  • Jupiter and Saturn rise in the southeast soon after dark.
  • The moon will be full on the 22nd.


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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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