New Horizons is the gift that keeps on giving. Last week we learned that Pluto has blue sunrises and sunsets, much like the one Curiosity captured on Mars. We also confirmed what scientists have long suspected – surface water ice. In the latest round of images released, we also see new images of Pluto’s small moons, including our first look at the tiny moon Styx.
Styx is Pluto’s smallest and faintest moon, and was first discovered by the Hubble Space Telescope in 2012 when New Horizons was already en route to Pluto. The image we see may not look like much more than a fuzzy blob, but scientists can discern a great deal about Styx by studying the image. For instance, we now know the moon's approximate dimensions.
“Although it may not look like much, the new composite image of Styx reveals a highly elongated satellite, roughly 4.5 miles [7 kilometers] across in its longest dimension and 3 miles [5 kilometers] in its shortest dimension,” New Horizons project scientist Hal Weaver, of the Johns Hopkins University Applied Physics Laboratory, said in a statement.
Scientists can also measure Styx’s albedo (how much light is reflected off the surface) and determined that Styx has a highly reflective, icy surface much like Nix and Hydra. The new images were taken by New Horizons’ Long Range Reconnaissance Imager (LORRI) on July 13, approximately 12.5 hours before closest approach to Pluto and were downlinked on October 5. At the time these images were taken, New Horizons was 631,000 kilometers (391,000 miles) from Styx. Due to the distance and Styx’s small size, we may not get to see many detailed views of the tiny moon.
Nix is the second largest of Pluto’s small moons and also was the closest to New Horizons during the flyby, allowing for better images. We’ve seen three different views of Nix taken by LORRI, but the team says the best is yet to come. The "potato-shaped" moon has a surprising crater on its surface. Since the moon is so small, there’s a fine line between an impact that will break the moon apart and one that will simply leave a crater. So Nix could be incredibly lucky, or could be part of a once larger moon.
The orbits of Pluto and its moons Charon, Styx, Nix, Kerberos and Hydra are illustrated around their common center of mass. Credit: SwRI/S. Porter
We do know that the crater is a different color than the rest of Nix thanks to a color image taken by New Horizons’ Ralph/MVIC camera (below). MVIC doesn’t have the same great resolution that LORRI does, but it can see in four “colors”: red, blue, near-infrared and methane. With the MVIC we see something very interesting – the crater and ejected material is reddish in color compared to the white surface of Nix. This tells us that the moon has a darker subsurface. We also get a peek at Nix’s crescent backlit by the Sun. Since Nix doesn’t have an atmosphere, it’s not as breathtaking as Pluto’s crescent, but scientists can measure the brightness and learn more about Nix’s surface composition and topography.
Pluto’s moon Nix in high-resolution (black-and-white) and lower resolution (color). NASA/JHUAPL/SwRI
In the image of Hydra (below), we see a much more complex shape. Unfortunately Hydra was on the opposite side of Pluto to New Horizons, so the images we see will have a lower resolution. Hydra’s shape looks somewhat like a larger version of Rosetta’s comet 67P/Churyumov-Gerasimenko. Recently we learned that comet 67P’s shape is possibly the result of a low-speed collision, and scientists speculate the same could be true with Hydra.
Comet 67P-like Hydra, as seen from New Horizons. NASA/JHUAPL/SwRI
Pluto’s fifth moon, Kerberos, has a much darker surface than the rest and as such could be more difficult to image. We have yet to see any images of Kerberos from New Horizons, but stay tuned because they are coming. NASA plans to combine the images captured by New Horizons with measurements taken throughout the spacecraft’s journey, to learn more about Pluto’s small moons and their rotational properties.
“Ultimately, we hope to learn more about all four of Pluto’s small moons, to understand their similarities and differences, how they formed, and how they evolved,” said New Horizons Principal Investigator Alan Stern in a statement.