Pluto tends to hog all of the spotlight, but it’s not the only distant world floating through space at 4.5 billion miles from Earth.
Meet Pluto’s bizarre moon, Nix. Named for the Greek goddess of night, this tiny Plutonian moon exhibits some of the craziest behaviour that planetary scientists have ever seen. It’s not just the fact that it’s shaped like an egg (as shown in the animation below), that’s weird:
There are plenty of odd-shaped moons in the solar system such as the moons of Mars (Phobos and Deimos) or Saturn’s sponge-shaped moon Hyperion. But no moon in the entire solar system spins quite like Nix and some of Pluto’s other moons.
Case in point, Nix not only wobbles back and forth, it also tilts up and down:
“If you lived on Nix, you would not know if the sun is coming up tomorrow; it is that extreme,” Mark Showalter, who is a senior research scientist at SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California, told Space.com. “You’d have days where the sun rises in the east and sets in the north.”
Not only that, the length of a day is also constantly changing.
Showalter is lead author of a paper published in Nature on June 4 that reports the bizarre behaviour of Nix as well as some other tantalising mysteries regarding the entire Plutonian system.
Pluto, in fact, harbours five moons. Their names — all of which are references to places or characters of the Greek underworld — are Styx, Nix, Hydra, Kerberos, and Charon:
Between 2005 and 2012 the Hubble Space Telescope snapped a series of photos of Pluto’s two tiny moons Nix and Hydra. From the images, Showalter and University of Maryland in College Park planetary astronomer Douglas Hamilton, measured the moons’ brightness, which depended on the amount of sunlight that was reflecting off of the moons at the time Hubble took the photo.
By looking at the different ways that light bounced off of Nix and Hydra, the two astronomers mapped the rotational and orbital behaviour of the moon, and discovered a cosmic dance unlike anything they’d ever seen. Below is a series of computer-generated images of Nix based on the astronomers’ brightness measurements.
Although they are still trying to determine the cause of Nix’s odd rotational behaviour, Showalter and Douglas suspect it has much to do with Pluto’s largest moon, Charon.
Charon is about 100 times larger than any of Pluto’s other four moons, which means it has the most gravitational influence of any other body in the Plutonian system (besides Pluto, of course). As Charon and Pluto orbit each other, their gravitational pull on the other four moons strengthens and weakens as they move respectively closer and farther apart.
A similar phenomenon is responsible for the tides we have Earth: tides grow and fall depending on the strength of the gravitational tug that the sun and Moon exert on our watery, blue planet. When the Earth, Sun, and Moon are lined up, we experience maximum tides.
It’s this dynamic gravitational environment established by the Pluto-Charon duo that Showater and Hamilton attribute to the whacky rotational orbit of not only Nix but Pluto’s other egg-shaped moon Hydra as well.
What makes this rotation particularly bizarre — aside from the wobbling and tilting — is its speed.
Most moons in the solar system, including ours, are in what astronomers call synchronous rotation, a phenomenon in which an object rotates on its axis at the same speed it takes to complete one orbit. That’s why we always see the same face of our Moon.
“Prior to the Hubble observations, nobody appreciated the intricate dynamics of the Pluto system,” Showalter said in a statement released by the European Space Agency.
Here’s what the phenomenon looks like (the red half is the side we always see):
But Nix and Hydra are not locked in step with their parent companions Pluto and Charon. Instead, these two moons exhibit what scientists call chaotic rotation, meaning the side that faces Pluto and Charon is always changing. Here’s what it might look like if the Moon had chaotic rotation:
Showater and Hamilton suspect that Pluto’s other two tiny moons, Kerberos and Styx, also exhibit chaotic rotation, but there are not enough measurements to determine if that’s the case.
That could soon change, however, with NASA’s New Horizons spacecraft, which will fly through the Pluto system in July of this year.
“Pluto will continue to surprise us when New Horizons flies past it in July,” Showalter said in a NASA press release. “Our work with the Hubble telescope just gives us a foretaste of what’s in store.”
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