Today’s post is written by Simon Porter, a New Horizons postdoctoral researcher at the Southwest Research Institute in Boulder, Colorado. Simon’s work focuses on the small satellites of Pluto.
This week’s beautiful Charon images remind us that Pluto is not just one body; it’s a whole system of worlds.
Pluto and its largest moon Charon dance around each other, making circles around their common center of mass, which lies in an empty space between them. Around the dancing couple are four small moons. In order of increasing distance, their names are Styx (just beyond Charon), then Nix, Kerberos and Hydra. These tiny moons also orbit around the system’s center of mass. The orbits line up like a miniature solar system, except with a binary system at the center, similar to the planetary system around the star Kepler 47. All four of the small moons are less than about 30 miles (50 kilometers) in their longest dimension. Each has a lumpy shape because, unlike Pluto and Charon, they aren’t big enough for gravity to squish them into a ball.
Nix and Hydra were discovered in 2005, shortly before New Horizons launched in 2006, and their initials were a subtle nod to the New Horizons mission that started the search for them, just as the P and L in Pluto are a subtle nod to astronomer Percival Lowell, who began the search for Pluto.
Styx and Kerberos weren’t discovered until 2011 and 2012, well after the New Horizons spacecraft was on its way to Pluto. Although the mission’s observing plans were pretty well set by then, the New Horizons science team anticipated that new discoveries from other facilities might be made during the long cruise to Pluto and had left room for a handful of “TBD” observations, which became the only ones specifically devoted to Kerberos and Styx. That’s why New Horizons took many more pictures of Nix and Hydra than of Styx and Kerberos.
Nix is the second-largest of Pluto’s small moons and was the closest to New Horizons during the flyby, so we got better imaging of it than any of the other small moons. So far, we’ve been able to download close-up pictures of Nix taken at three different times by the Long Range Reconnaissance Imager (LORRI) high-resolution camera, but the best image is still on the spacecraft’s digital recorders waiting to come to Earth.
From looking at Nix as point of light with the Hubble Space Telescope and with New Horizons on approach, we knew that Nix’s brightness regularly changed over time, and therefore it was probably elongated. However, the first image (on the left) really surprised us, because Nix appeared to be round—not at all elongated.
Mark Showalter – who discovered Styx and Kerberos — pointed out that we were probably just looking down the long axis, and that the next images would look more “potato-ish.” Sure enough, the next image showed Nix looking far more elongated, but with one great surprise in it: a big crater! Nix isn’t very large, and there is a very fine line between an impact that will make a crater that big and one that will break Nix apart. So either Nix was very lucky in surviving that collision, or it’s a fragment of an older moon that was somehow destroyed.
The last Nix image we have so far was taken right after the spacecraft passed Pluto and started to look back on its crescent. Because Nix has no atmosphere, it isn’t as spectacular as the images looking back at Pluto, but measuring the brightness of that little crescent of light can help tell us about what the surface of Nix is made of, and whether its surface is smooth or covered in boulders.
What we do know about the big crater on Nix is that it appears to be a different color than the rest of the moon. The color image below was taken by New Horizons’ Ralph-Multispectral Visible Imaging Camera (MVIC) three minutes before the LORRI picture; MVIC has one-fourth of LORRI’s resolution, but it can see in four colors: blue, red, near-infrared, and methane. In this image, the RGB colors are mapped to near-IR, red and blue, just like the enhanced-color images of Pluto and Charon. While most of Nix is a neutral white, the crater and its ejecta blanket (the material thrown out by the crater) appear to be a much redder material. Craters excavate material from below and throw it on the surface. This tells us that under its white surface, Nix is probably made of much darker material. We don’t actually know what either the dark or the light material is, nor will we be able to tell until we download the Nix data from the Ralph-Linear Etalon Imaging Spectral Array (LEISA) composition mapping spectrometer.
New Horizons also imaged Hydra and has sent some of these images to Earth. Below is the best LORRI image of Hydra taken by New Horizons. Unfortunately, Hydra was on the opposite side of Pluto from New Horizons at closest approach, so the images of Hydra are from farther away and therefore are at lower resolution than the Nix images we have. Because Hydra’s orbit was still somewhat uncertain, the mission planners designed this observation to be a mosaic of six slightly-overlapping shots. As it turns out, we hit the jackpot and Hydra fell right at the intersection of four of those six shots, meaning we got two full and two half-views of Hydra for the price of one!
The composite of these images shows that Hydra has a much more complicated shape than Nix and looks a bit like a much bigger version of comet 67P/Churyumov-Gerasimenko, which is currently being orbited by the Rosetta spacecraft. As some have proposed for 67P, it is possible that Hydra is the result of a low-speed collision of two older moons. We haven’t yet had a chance to download the LORRI images of Styx and Kerberos, but they are coming soon, and will be of similar resolution to this image of Hydra.
Finally, I want to tell you how I processed these images of Nix and Hydra. I created them in Python with AstroPy to read the images and translate from pixels to on-sky coordinates, and Scikit-Image to process and sharpen the images. LORRI’s pixels are smaller than the resolution limit of the imager, which makes the images appear a little bit blurry. But since the optics of LORRI are very stable, we can use a process called “deconvolution” to back out what the image would have been if the camera were perfectly sampled. This makes the images sharper, but also adds “noise.” We can minimize the noise by taking several deconvolved images and finding the median value at each pixel location; this keeps the real detail while throwing out much of the deconvolution noise. Both Astropy and Scikit-Image are free and open sources, and make astronomical image processing fun (and a bit addicting) for anyone with a basic knowledge of Python.