The 'shrinking planet'

Until plate tectonics came along, the most widely accepted theory to explain the Earth's mountains was shrinkage.

The idea was that the Earth was once molten, and is now solidifying, and like most things in nature, as it cools and solidifies it shrinks, causing the crust to get wrinkled into mountains. We see this happen when an apple or tomato dries up. The inside loses moisture and shrinks. The skin does not shrink as much and goes all wrinkly as it tries to cover a smaller object. Today we know the Earth's mountains are due to plate tectonics, not shrinkage.

However, the planet Mercury is another matter. It really looks as though that planet has mountain ranges that seem to be due to the planet getting smaller.

Mercury is the closest known planet to the sun. It orbits at only 39% of the Earth's distance from the sun, so the solar heat is nearly seven times greater. It takes about 88 days to complete an orbit around the sun and rotates very slowly.

A Mercurian day is 58.6 times longer than one of ours. Since the planet has effectively no atmosphere, there is no greenhouse effect to keep in the heat, and the long days and nights mean that during the day there is plenty of time to heat up and plenty of time to cool off. On the planet's equator, temperatures range from a torrid 430 C at noon, to a frigid -170 C during the night. In the planet's polar regions, the noon temperatures only reach a still uncomfortable 110 C, falling to -190 C after sunset. The temperature variations, together with the intensity of the solar X-rays and ultraviolet radiation, means a manned mission to that planet is unlikely to happen any time soon.

The continuous recycling of its surface by erosion and plate tectonics has erased much of the Earth's early geological history. However, this is not true of the moon. With no weather or plate tectonics, its billions of years of history are in plain view. We see great lava plains and countless impact craters. In many places, craters have partially or totally obliterated other craters. With no plate collisions, the moon's mountains are mostly the rims of ancient, huge craters, often partially covered by huge, ancient lava flows.

When space probes gave us our first ever close-up views of Mercury, the first impression was that Mercury looks very like the moon, with countless craters, suggesting a similar history. However, closer examination shows mountains and other features, such as places where the surface has cracked, and the material on one side pushed up and over the material on the other. In geology these are referred to as overthrust faults. Some of these features are thousands of kilometres long and three kilometres high. There are some examples of this on the moon, but not many, and they are old. On Mercury, the geological processes building the mountains and geological faults are still active. We can tell because they are distorting even young craters. The mountains of Mercury, together with the geological faults suggest strongly that they are being formed by the planet's continuing shrinkage.

Mars seems to be completely solid, so is not shrinking. With Earth's active plate tectonics, any evidence of shrinkage gets rapidly erased. Venus does not seem to have any plate motions. However, the planet is highly volcanic and thanks to a runaway greenhouse effect, far hotter than Mercury. The high temperatures render the surface rocks rather plastic, making them easily slump and flow. Its surface is geologically young. 

Mercury has a large, liquid core, which shrinks as it cools, and small bodies cool faster than big ones. There is no evidence of plate motions, which leaves the story of the "shrinking planet" clearly evident for us all to see.

  • Venus shines brightly in the west after sunset.
  • Left to right, Mars, Saturn and Jupiter lie close together low in the southeast before dawn.
  • The moon will be new 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]

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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