Jupiter in Early 2024: How Hubble's OPAL Portraits Are Put Together

On January 5 and 6, 2024, Hubble spent 13 of its orbits on Jupiter. The observations belonged to the Outer Planet Atmospheres Legacy program, OPAL, which in 2024 reached its tenth year. The two portraits released on March 14, one for each side of the planet, are the visible end of a large and carefully planned set of data, and the way they were put together shows how much work hides behind a picture of a planet that never holds still.

Ten years of OPAL

OPAL exists to give astronomers a long, regular record of the four giant planets, Jupiter, Saturn, Uranus and Neptune, observed once a year in a range of wavelengths. Over a decade the archive becomes a weather history: storms forming, changing color and fading, bands shifting, haze coming and going. The same data also help scientists interpret observations of planets around other stars, which can only be seen as single points of light.

The 2024 Jupiter images show both hemispheres crowded with storms. On one side the Great Red Spot stands out, with the smaller anticyclone nicknamed Red Spot Jr. to its lower right, which had turned somewhat redder again. On the other side a deep red cyclone and a reddish anticyclone sit next to each other, rotating in opposite directions. The released pair can be viewed on the ESA/Hubble image page.

What one orbit of data looks like

Hubble data come in units of orbits: one pass of the telescope around Earth, during which it can observe a target before Earth gets in the way. In a single orbit of the January program, Hubble cycled through nine filters in a little under 33 minutes, and laid side by side those frames show the Great Red Spot visibly turning toward the upper right from one to the next.

The full program ran from 20:26:34 UT on January 5 to 15:54:02 UT on January 6. Spread over that stretch, the 13 orbits caught the planet through two complete rotations, enough to build a map of the whole globe as well as the two portraits. Because Jupiter is so bright, exposures were very short: just 1.2 seconds through the F467M filter and 45 seconds at the longest, through the narrow FQ889N filter.

From gray frames to natural color

Every Hubble color picture starts as a set of black-and-white exposures through separate filters, and planets are no exception. The complication is motion. Jupiter turns once in about ten hours and moves along its orbit at the same time, so the cloud pattern shifts even between exposures taken minutes apart. Combined without correction, each storm would appear three times in three offset colors.

To approximate natural color, three of the nine filters are used: F395N for blue, F502N for green and F658N for red. Before they are combined, the planet has to be derotated, each frame shifted on the globe so that its features line up with the others at one chosen moment. There are several ways to do this. For OPAL Jupiter images the work is done in WinJUPOS, a program written for exactly this task, which uses the precise time of every frame to calculate how far the planet has turned. The general method is explained in What's Up with Jupiter and Saturn?, and a 3D-model approach to the same problem in Rebuilding Hubble's Comet Impact in Color.

For the tenth anniversary a color image was made from every orbit of data, not just from the best two. One of the 13 observations had not succeeded, so the compilation holds 12 portraits, a near-continuous record of Jupiter turning through two of its days.

Io walks across the planet

Long planetary programs almost always catch moons, and in January 2024 the moon Io crossed in front of Jupiter. Over a series of observations spanning four orbits, Io moved across the disk and cast its dark shadow onto the cloud tops. One of those moments appears in the lead image of the release, where Io is seen near the left edge. Io is the innermost of the four large Galilean moons and the most volcanically active body in the solar system, and Hubble resolves volcanic outflow deposits on its surface.

The crossing was also turned into a short animation, with three choices that shape how it looks:

• It uses single grayscale frames, not color composites, so that the time between frames stays short.
• The frames come from very different wavelengths, each emphasizing different layers of the atmosphere, so the planet seems to change shape and blink. Io's motion is the part to follow.
• Between orbits there is a gap of about an hour, which shows as distinct jumps. The frames were deliberately not interpolated, so the animation shows only the data as recorded.

Hubble had been observing for nearly 34 years when these images were taken, and the OPAL visits had become a fixed part of its calendar. Hubble's yearly record of Saturn's seasons is in A Saturn Yearbook.

Questions readers ask

What is the OPAL program?

OPAL, the Outer Planet Atmospheres Legacy program, uses Hubble to observe Jupiter, Saturn, Uranus and Neptune every year. The long, regular record lets astronomers follow changes in the planets' atmospheres over time. In 2024 the program marked its tenth year of observations.

How long did Hubble observe Jupiter in January 2024?

The observations covered 13 Hubble orbits, from 20:26:34 UT on January 5 to 15:54:02 UT on January 6, through nine filters. That span included two full rotations of Jupiter, enough to build a global map of the planet as well as portraits of both sides.

Which filters give Jupiter its natural-looking color?

The natural-color versions use three of the nine filters: F395N for blue, F502N for green and F658N for red. The frames are derotated in WinJUPOS so that the clouds line up before the three colors are combined.

Why does Jupiter seem to blink in the Io animation?

The animation is made from single frames taken through very different filters. Each wavelength shows different layers of Jupiter's atmosphere, so the planet's appearance changes from frame to frame. The gaps of about an hour between orbits also make Io's motion jump in steps.