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SYFY WIRE Bad Astronomy

Just Another Day on Pluto

By Phil Plait

Pluto is an odd little beast.

You knew this, if you’ve been paying attention at all these past few months. But one thing that struck me recently is its 6.4-day rotation rate. That’s weird; most free bodies in the solar system spin either much faster than Earth or much slower. Jupiter, Saturn, Uranus, and Neptune have days that are shorter than Earth’s, ranging from 10 to 17 or so hours. But Mercury takes 60 days, Venus a whopping 243 days. Mars spins at almost the same rate as Earth, with a day 24 hours 37 minutes long.

So Pluto’s weeklong spin is an oddity. If you could watch it, what would it look like?

Well, it would look like the photo at the top of this post. Those images were taken by the New Horizons probe as it passed Pluto in July. As the spacecraft approached, it took a lot of snapshots of the little world, and over time was able to see it at various points in its day. These were collected, scaled to the same size, and arrayed to show a day in the life of the frozen ice ball.

Each shot shows the highest-resolution image taken for that particular point in the day. The shot at the 3 o’clock position was taken from farthest away (about 8 million kilometers), and the one at the 6 o’clock position was the hemisphere seen by the probe when it passed a mere 12,500 km over the surface.

You can see the familiar heart-shaped Tombaugh Regio, a flatter region that is covered in nitrogen ice (the left lobe, Sputnik Planum, is much smoother than the right lobe). I like how you can see different resolutions blurring the features at different times; it is physically showing you what can and cannot be seen clearly during various points in the flyby.

Charon, Pluto’s outsized biggest moon, orbits the world in that same 6.4-day period, and also spins at the same rate (this is not a coincidence). The New Horizons team also put together a similar mosaic for Charon:

I’ll be honest: Despite the somewhat lower contrast and details, I like this one better. I think it’s because the north pole of Charon is so clearly marked by the dark spot nicknamed Mordor Macula. It’s not clear why Mordor is dark (and red), but it may have to do with organic molecules from Pluto raining down on it.

Geez. Pluto and Charon are weird.

Which brings me back to the length of the day/month there. Charon and the littler moons may have formed by a huge impact; a big chunk of ice and rock smacked Pluto hard long ago, and the debris coalesced to form those moons. The impact could have changed Pluto’s rotation rate, and then tidal interactions with the newly born Charon would have modified it as well. Once everything locked into place, we got this weeklong spin of the three systems (Pluto itself, Charon itself, and the two together orbiting each other).

Most small objects in the solar system have had their spin rate modified. Earth spun much faster before our own Moon formed in much the same way as Charon. Venus spins slowly and backward, so it may have been flipped over by an impact (or some other process we haven’t quite figured out). Mercury’s proximity to the Sun played havoc with its spin. Asteroids spin up due to sunlight.

So perhaps Pluto isn’t all that odd after all. It’s actually quite normal in its weirdness … or, at least, that’s my spin on it.

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