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SYFY WIRE Bad Astronomy


By Phil Plait

Today, for the first time, we have received our first looks at Pluto and its moon Charon up close! And the pictures are truly amazing.

First, here’s Charon:

Wow! Charon is 1,207 kilometers (750 miles) in diameter, to give you a sense of scale. That’s roughly the distance from New York City to Atlanta.

The dark region at the north is nicknamed Mordor. If you look closely, there are craters in it that are bright. This means the dark coating is a thin veneer, thin enough that small impacts go right through it to the brighter material beneath.

Scientists have already speculated that the dark region may be a deposit of ice leaked from Pluto’s atmosphere very slowly over time. Pluto’s atmosphere is incredibly thin, but due to its low gravity the atmosphere there is pretty puffy, and easily lost at the top to flow over to Charon.

The southern region of Charon is far smoother, and therefore must be young: Over time, impacts would dot it with craters, so if there are few craters then Charon must have been resurfaced by activity at some recent time. It’s hard to say how old it is; these are still heavily compressed images sent back as a failsafe; higher resolution images are coming soon.

Of note is the gash at the upper right on the moon’s limb; that’s a canyon up to 10 kilometers deep. At the top of the canyon is a notch that is also quite deep.

At the moment there’s not more we can say; it’ll take a while to analyze this, and then we’ll be getting those better images over the next few days as well.

The star of the show, of course, is Pluto. First, the heart-shaped area has been informally renamed Tombaugh Regio, after Clyde Tombaugh who discovered Pluto. That’s nice.

New Horizons took a close-up shot of Pluto at the lower bit of the heart, just to the left of the cusp. This is what it saw:

WHOA. Those are mountains, and big ones, 3.5 kilometers (11,000 feet) high. Methane and nitrogen are frozen at Pluto’s temperature, but they aren’t stiff enough to support mountains that high. These features must be bedrock—actually, “bedice”—made of water ice, which is harder than rock at these temperatures. The methane and nitrogen detected must be thin coatings on top of the water ice underneath.

Amazingly, there are no craters at all in this image. None. That means the surface must be young, having been resurfaced quite recently. The scientists on New Horizons figure the terrain must be less than 100 million years old, but that’s a guess.

That’s amazingly young! And there is a strong implication here. This is the first time we’ve seen a world that is mostly composed of ice that is not orbiting a planet. In general, tides work to heat icy moons (see my episode of Crash Course Astronomy about tides for details), and that can play into smoothing the surface features.

But Pluto doesn’t have that! Pluto and Charon are tidally locked, spinning at the same rate they orbit each other, so there is no tidal stress. What could be heating Pluto so that its surface is young? Radioactivity from rocks beneath the surface? It’s hard to say just yet. We have a Plutonian mystery on our hands.

Also, if there’s a coating of nitrogen on the mountains, it must be dredged up from underground. What’s doing that? Cryovolcanoes (literally ice volcanoes), or gas vents like geysers? Hopefully the higher resolution images and spectra will gives us the evidence we need to figure that out.

Also, the lower regions are smooth, looking like they were filled in. Perhaps that’s from liquid flow (water laced with ammonia can stay liquid beneath the surface, so if it leaks out it could do this). The surface to the lower right is hummocky, hilly in waves. It almost looks like there’s a line through it too, like a fault line. Again, these are still low-res images. We’ll know more soon.

Note that Charon and Pluto are very different; Pluto is bright, Charon dark. Why? Perhaps atmospheric transport is behind this … or maybe something else. As new data come in we’ll learn more (and come up with even more questions, too).

Finally, here is Hydra, one of Pluto’s smaller moons:

I know, it’s very low-res, but it’s still interesting! It shows that Hydra is 45 x 30 kilometers (28 x 19 miles) in size, and very irregular. Like so many small bodies, it looks lumpy and potato-shaped. It’s very shiny, reflecting about 40 percent of the light that hits it, meaning the surface is probably slightly contaminated water ice.

This is all astounding. Astounding! Remember, Pluto is a staggering 5 billion kilometers (3 billion miles) from Earth. We humans sent New Horizons there a dozen times faster than a rifle bullet, let it travel for more than nine years, flew it past Jupiter to steal a little bit of energy and speed it up even more, and when the piano-size probe got there, its trajectory was accurate to about 70 kilometers! It passed 12,500 kilometers over the surface, and mind you was able to take sharp data at that speed despite sunlight that is weaker than what we see on Earth by a factor of 1,000!

This is what we humans do when we keep our heads up, when we cooperate, when we allow our curiosity to guide ourselves.

I almost wrote, “… when we let our curiosity get the better of us.” But you know what? Our curiosity is the better of us.

Look at what we did! And there is much, much more to come. We’ll be receiving flyby data literally for months, and each new image, each spectrum, each measurement will reveal something new about these worlds at the frontier of the solar system.

Stay tuned. There’s much more to explore.

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