Abstract DGP2026-74

Synoptical examples of work with JunoCam data from geometrical in-flight calibration to attempts of fluid dynamical modeling of Jupiter’s polar regions

Gerald Eichstädt (1) Candice Hansen-Koharcheck (2) Glenn Orton (3) John Rogers (4) Scott Bolton (5)

(1) Independent Scholar, Stuttgart, Germany (2) Planetary Science Institue, Tucson, AZ, USA (3) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA (4) British Astronomical Association, London, UK (5) Southwest Research Institute, San Antonio, Texas, USA

NASA’s Juno spacecraft flew by Earth in October 2013. This was the first opportunity to test its instruments next to a planetary target. Juno’s visible light imager JunoCam took a sequence of images of Earth. Those images gave a first opportunity to develop, test, and refine an image processing pipeline on the basis of real-world planetary image data.

It turned out that the preliminary geometrical calibration parameters published on the basis of lab tests required a substantial in-flight refinement. Cruise images of stars, as well as distant images of Jupiter, informally known as Marble Movie images, that were taken briefly before Jupiter orbit insertion in 2016, and during several of the early polar elliptical orbits of the spacecaft around Jupiter served as a basis for geometrical camera calibration.

The Juno spacecraft is usually rotating with about 2 rpm to get spin-stabilized. The camera’s optical axis points close to perpendicular to this spin axis. The camera takes an image each about 370 to 380 ms. Its 1648 x 1200 pixels CCD detector is sensitive to visible light with a flank extending to the 889 nm methane band. Four rectangular color filters are attached to the CCD in non-overlapping way. Three of the filters are broad-band for red, green, and blue visible color bands. The fourth filter is narrow-band around 890 nm. Only rectangular regions of 1648 x 128 pixels are read out for each filter, and transmitted to Earth. The camera applies time-delayed integration (TDI) in order to avoid motion blur induced by the spacecraft rotation. The camera has a horizontal field of view of about 58°. These properties taken together result in an overlap of each of the three RGB color components, such that a full RGB image can be reconstructed. However, the viewing geometry changes with each exposure. An accurate reconstruction of an RGB color image from a swath of exposures taken during one spacecraft rotation requires the three-dimensional trajectory data provided by SPICE kernels, together with a surface model of the target. In the case of Jupiter, assuming a properly parameterized rotating spheroid is sufficient for most applications.

With all these relevant geometrical parameters known, reprojected image products as well as cylindrical and polar maps of Jupiter’s cloud tops can be reconstructed.

Swathes can be combined into a larger surface coverage. They can also be used to derive short sequences of cloud motion animations, if a cloud-top area is covered by several swathes during the same perijove flyby.

This kind of animations revealed that some of Jupiters rather stable circumpolar cyclones show counter-rotation of their cores. Finding fluid dynamical models that duplicate the observed dynamics of those crystals of cyclonic vortices are among the ongoing research efforts.

JunoCam used its opportunities to take close looks at the Galilean moons Ganymede, Europa, and Io, including the observation of effects of Io’s volcanic activity.