The Incredible Shrinking Planet

SCIENCE

Like a raisin spinning around the sun, Mercury is shrinking and wrinkling. The planet is now about 14 kilometers (8.6 miles) smaller in diameter than it was nearly four billion years ago. (National Geographic News)

Use our resources to learn more about Mercury and the other planets in our solar system.

This breathtaking image of Mercury is not quite what the planet would look like to the human eye. In the image, colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet's surface. Young crater rays, extending radially from fresh impact craters, appear light blue or white. Medium- and dark-blue areas are a geologic unit of Mercury's crust known as the "low-reflectance material", thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. The giant Caloris basin is the large circular tan feature located just to the upper right of center of the image. Image by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

This breathtaking image (click to enlarge!!) of Mercury is not quite what the planet would look like to the human eye. In the image, colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Young crater rays, extending radially from fresh impact craters, appear light blue or white. Medium- and dark-blue areas are a geologic unit of Mercury’s crust known as the “low-reflectance material”, thought to be rich in a dark, opaque mineral. Tan areas are plains formed by eruption of highly fluid lavas. The giant Caloris basin is the large circular tan feature located just to the upper right of center of the image.
Image by NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Discussion Ideas

  • The Nat Geo News article mentions scarps and rupes—”Mercury’s version of a mountain belt.” Look at our lovely photo gallery of our solar system’s planets. Where else in the solar system would you expect to find planetary geology such as scarps and rupes? Where in our solar system would you not expect to find these features?
    • Mountain ranges are found on all the rocky planets and moons in our solar system. The Nat Geo News article mentions Mercury, Mars, and our own moon (as well as Earth, of course)! But mountains, valleys, and cliffs can also be found on Venus, and the rocky moons of Jupiter (Io, Europa, Ganymede, and Callisto are the big ones), Saturn (such as Titan and Mimas), and Uranus (Miranda and Titania).
    • You would not expect to find mountain ranges on our “gas giant” planets (Jupiter, Saturn, Uranus, and Neptune). As their name implies, these big, beautiful planets are mostly made of swirling storms of gas and don’t have rocky surfaces that warp and crater.
  • Compare these beautiful cutaway drawings of the interiors of Mercury and Earth. (Click to enlarge)
  • What is the major difference? What do you think Mercury’s big iron core has to do with its shrinking size?
    • Earth’s interior has more defined layers—the solid inner core (yellow), molten outer core (orange), mantle (red) and thin rocky crust. Mercury, on the other hand, has a huge partly-molten iron core (orange) covered by a thin rocky crust.
    • Mercury’s metal core has everything to do with its shrinking state. To quote a terrific BBC article on the subject, “[a]lthough some of [Mercury's] core must still be liquid, part of it will have cooled and solidified, losing volume as a result. This will have scrunched the thin, overlying layer of rock.”
  • According to NASA, Mercury is 14 kilometers (8.6 miles) smaller in diameter than it was about four billion years ago. Eventually, do you think Mercury will shrink away to nothing?
    • No, for two major reasons.
      • 1. Eventually, all that liquid or molten iron in Mercury’s core would solidify completely, shrinking Mercury to a stable size.
      • 2. Mercury will never get the chance to shrink completely—it will be obliterated and absorbed by the sun. In about five billion years, Mercury’s neighbor, the sun, will stop being a nice main-sequence star and become a rabid red giant. In its red giant phase, the sun will swell up to envelop the orbits of Mercury and Venus—and possibly Earth. In any case, the solar system (and life) as we know it will end. Ho hum.

One response to “The Incredible Shrinking Planet

  1. Pingback: Astronaut Wants Our Help! | Nat Geo Education Blog·

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