2024 marks the 10th anniversary of a prolific Hubble observing program known as the Outer Planet Atmospheres Legacy, or OPAL. The goal of OPAL is to provide a long baseline look at the outer planets of our solar system (Jupiter, Saturn, Uranus, and Neptune – sorry Pluto!) to study their atmospheric dynamics and the evolution of gas giant planets. These observations are conducted on an annual basis for each of the planets. They provide a treasure trove of data for astronomers in a variety of wavelengths, allowing for detailed analysis and extrapolation to better understand observations of planets beyond our solar system, known as exoplanets.
Here at Space Telescope, we almost always feature OPAL observations in a news release and this year is no exception. Just this past January, Hubble once again grabbed some glamor shots of the king of the solar system and I thought this would be a great opportunity to provide a little insight about how these images are put together and offer some bonus material that did not make it into our press release.
Hubble observations are usually lumped into chunks of time denoted by an orbit of the telescope around the Earth, hence we refer to one set of observations as an orbit. Figure 1 shows a collection of images taken in different filters during a single orbit of Jupiter observations. These observations span a time frame of almost 33 minutes, and you can clearly see the Great Red Spot rotating away toward the upper right in each frame. The OPAL program observed Jupiter in January for a total of 13 orbits across nine filters starting at 20:26:34 UT on January 5th and ending at 15:54:02 UT on January 6th. This number of observations and their spacing in time allows the team to capture two full rotations of the planet and create a planetary map from the data. Because Jupiter is so bright, exposure times are just a few seconds per filter with the shortest being 1.2 seconds in the F467M filter and the longest 45 seconds in the FQ889N filter.
As we have documented in detail in prior posts, the beautiful color images we see from telescopes like Hubble and Webb are not sent like that by the telescopes: Several monochromatic exposures are taken in different filters and combined by image processors to create a color image from the data. This process is no different with solar system objects, but there is one complication. Objects like planets are constantly moving, both through their orbital motion and their planetary rotation. In a past blog post, I showed how both Jupiter and Saturn complete their full day’s rotation in about 10 Earth hours. That rotation is fast enough to cause noticeable changes even within the relatively fast time frame of single exposures with Hubble!
As documented in the post linked above, to create a clean, crisp color image of Jupiter requires “derotating” the planet in different filters so that all of the planet’s atmospheric features line up in the composite image. For Jupiter, we try to approximate natural color by using three filters to create color images in the F395N filter (blue), F502N (green), and F658N (red). There are a few ways to achieve the derotation effect, and I use software specifically designed for this called WinJUPOS.
Since we are celebrating 10 years of OPAL observations of Jupiter this year, I decided to process color images from every single orbit of data that was taken. (Note that one of the 13 observations was unsuccessful, so there are 12 images in this compilation.)
Among the many amazing things to spot in these images, the moons have to be one of my favorites. Given the amount of time spent looking at the planets in an observing program, it’s usually a safe bet that some moons will show up. For this set of Jupiter observations, we were treated to a spectacular photobombing by none other than Jupiter’s moon Io, which casually saunters across the planet’s disk, casting its dark shadow onto the Jovian atmosphere in a series of observations spanning four orbits. One of those images is featured in the two-panel lead image for the press release seen at the top of this post. I also put together an animation from the raw images to see what the motion of the moon would look like. Some things to consider as you watch:
– Obviously, these are not color images. We can get a shorter time between frames by using single frames instead of combining them in color composites.
– Because we’re seeing Jupiter in several very different wavelengths, which emphasize certain features of the atmosphere, the planet appears to change shape and blink in somewhat unsettling ways. It’s best to just focus on Io and its motions, and try to tune out the changing face of the planet (although that is also fascinating in its own right).
– There is a gap in observing time of about an hour between orbits. These observations show Io in four orbits, so we see several distinct jumps in the animation. I didn’t want to interpolate between frames because this is a pure visualization of the actual data as it was taken.
With those caveats in mind, please enjoy!
Much like Hubble itself and its nearly 34 years of observing the cosmos, the OPAL program continues to be the gift that keeps on giving!
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