September 14, 2018 | David F. Coppedge

Pluto Rivals Earth in Geological Complexity

Call it what you will, Pluto is a planet of surprises. Its active geology is second only to Earth’s, say two planetary scientists.

What were they thinking? (A-hyulk)

There’s another push to re-classify Pluto as a planet. According to Science Daily, Philip Metzger of the University of Central Florida thinks the IAU (International Astronomical Union) thinks they acted a bit goofy when they downgraded Pluto to “dwarf planet” in 2006. They used a “sloppy definition” that a body must “clear its orbit” to be considered a true planet. “They didn’t say what they meant by clearing their orbit,” Metzger retorts. “If you take that literally, then there are no planets, because no planet clears its orbit.”

But Metzger’s preferred definition – a body that is large enough to become spherical in shape – opens the door to classifying all kinds of moons as planets. Surprisingly, that’s OK with Metzger and his colleague Kirby Runyon of Johns Hopkins University. He thinks calling Titan and Ganymede “planets” is “functionally useful,” and has historical precedent, too.

This academic debate means little in the long run. Bodies are what they are, regardless of what humans call them. It just goes to illustrate a philosopher’s critique of taxonomy in science: Do scientists really carve nature at its joints? How arbitrary are human classification schemes? What are the costs and benefits of lumpers and splitters? Do names of reference confuse or enlighten? Is one culture’s classification scheme “better” than another’s?

Pluto’s Value Beyond Its Classification

What’s much more interesting about Pluto than its name or classification is its geology. Listen to what Metzger and Runyon have to say about that:

  • Dunes, glaciers, and convection cells in Pluto’s Sputnik Planitia

    Denying planetary status to Pluto “would leave out the second-most complex, interesting planet in our solar system.”

  • “Metzger said that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.”
  • Because Pluto is spherical, it has “active geology in the body.”
  • “Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he said.”

Pluto is so interesting, Metzger adds, that it is “more dynamic and alive than Mars.” The variety of land forms and processes going on right now in Pluto and its largest moon Charon left planetary scientists excited, bewildered, and baffled when the New Horizons spacecraft took its historic pictures and collected data in 2015, as we reported (26 July 2018). They did not expect such a cold and distant body to have volcanoes, glaciers, and a possible subsurface ocean.

Metzger ended with a dramatic flourish: “The only planet that has more complex geology is the Earth.”

We agree that Pluto should be promoted to planet status again. That way, think of all the textbooks that won’t have to be rewritten. But we also think that the idea of a young solar system should be promoted back to respectability, too. Pluto does not fit the expectations of the moyboys. It looks young. So does Titan, Enceladus, Earth, Mercury, Venus, Io, Miranda, Triton, Saturn, the moon, Mars… We’ll leave it at that because it’s hard to name everything in the Solar System.

Watch for New Horizons’ next encounter this New Year’s Day, January 1, 2019. Should we make a prediction? Will the next target, Ultima Thule, look young as well? It’s a good bet, based on previous patterns of discovery in planetary science.


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