U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Untitled Document
Astrogeology Analysis Ready Data
Toggle Dark/Light/Auto mode Toggle Dark/Light/Auto mode Toggle Dark/Light/Auto mode Back to homepage
Edit page

Photogrammetrically Controlled Galilleo Observation Sequences

Browse the data

Overview

The Galileo spacecraft deployed from NASA’s Space Shuttle Atlantis on October 18,1989 and entered Jupiter orbit on December 7, 1995. The spacecraft was equipped with a framing camera (the Solid State Imaging, or SSI) with a 1,500-mm nominal focal length, 8.1-mrad field of view, 10.16-µrad/pixel angular resolution, 800 x 800-pixel charge-coupled device detector, and 8 filter positions (mounted on a filter wheel) ( Citation: , & al., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , & (). Galileo’s First Images of Jupiter and the Galilean Satellites. Science, 274(5286). 377–385. https://doi.org/10.1126/science.274.5286.377 ) . Over the course of the Galileo mission, the spacecraft acquired more than 700 images of Jupiter’s moon Europa providing the only moderate- to high-resolution images of Europa’s surface. Galileo images of Europa were acquired as observation sequences during each orbit that targeted the moon. Each of these observation sequences consists of a small number of images (between 1 and 19) that were acquired close in time, that typically overlap, and that have consistent illumination conditions and similar pixel scale. The observations vary from low-resolution hemispherical imaging to high-resolution targeted “postage stamps.” Unfortunately, uncertainty in the position and pointing of the spacecraft, as well as in the position and orientation of Europa, resulted in significant errors in estimated image locations on the surface. The result of these errors is that images acquired during different Galileo orbits, or even at different times during the same orbit, are significantly misaligned (errors of up to 100 km on the surface). Global photogrammetric control of nearly the entire Galileo Europa image dataset (along with 221 Voyager 1 and 2 images) improved the relative and absolute location of the images and enabled the creation of mosaics of each of the individual observation sequences acquired by the Galileo spacecraft. The 92 observation mosaics provide users with nearly the entire Galileo Europa imaging dataset at its native resolution and with improved relative image locations. The dataset therefore provides a set of image mosaics that can be used for scientific analysis and mission planning activities.

Processing

Raw Galileo images and labels were downloaded from the Planetary Data System (PDS) archive and ingested into the USGS’ Integrated Software for Imagers and Spectrometers (ISIS 3.10) using the application gllssi2isis to create ISIS cubes (i.e., images in .cub format). Reconstructed SPICE kernels (i.e., the default kernels) were applied using ISIS’ spiceinit application. A standard radiometric calibration (ISIS’ gllssical application) and noise filter (ISIS’ noisefilter application) was applied, and the edge of each image was then trimmed by 2 pixels (ISIS’ trim application). In some cases, image downlink was interrupted mid-transmission and had to be completed later, with the result that lines from a single frame are split into multiple image files. These images were reconstructed using ISIS’ handmos application, which combines two ISIS cubes by line/sample.

Although not included in the mosaics, Voyager data were included in the photogrammetric control network used to create the mosaics. Voyager images ranged in scale from 1.63 km/pixel to 32 km/pixel, with the highest resolution images concentrated at 180°E. Voyager images were ingested into ISIS with the voy2isis command and reconstructed SPICE kernels were applied (we used the pck00010_msgr_v23.tpc planetary constants kernel, or PCK). In most cases, the reconstructed Voyager SPICE is so inaccurate that the data lie completely off the body and had to be manually adjusted using ISIS’ deltack application before the images could be used (see ( Citation: , & al., , , , & (). Improving the Usability of Galileo and Voyager Images of Jupiter’s Moon Europa. Earth and Space Science, 8(12). e2021EA001935. https://doi.org/https://doi.org/10.1029/2021EA001935 ) for details). Once the SPICE was corrected, we applied the standard Voyager radiometric calibration (ISIS’ voycal) and removed image reseaux (ISIS’ findrx and remrx). The highest resolution (smallest pixel scale) images redundantly cover the anti-Jovian hemisphere at pixel scales of 1.6–2 km.

To improve the locations of Galileo and Voyager images, a global network of image tie points (a control network) was developed using ISIS. The control network is the input to the photogrammetric control process, in which a least-square bundle adjustment is performed to triangulate the ground coordinates (latitude, longitude, and radius) of each tie point and minimize location residuals globally. The ISIS application jigsaw was used to perform all the bundle adjustments. In order to create a global control network for Europa images, three independent networks were first generated: a Voyager-only network, a Galileo-only network, and a bridge network that included key Voyager and Galileo images. Each of these networks was bundle adjusted separately to ensure a clean network (i.e., free from image mis-registrations). The three clean networks were then merged into a single network and bundled together to update image locations. The final bundle adjustment solved for camera angles (including twist) using a priori constraints of 1 degree on camera angles and 500 m in radius. The orientation of Europa was then adjusted (parametrized as the prime meridian offset Wo) to ensure the data are aligned with the IAU-defined coordinate system for Europa (i.e., the longitude of the center of the crater Cilix must be at 182°W / 178°E).

The final bundle solution included 694 Galileo and Voyager images, of which 481 were from Galileo and have pixel scale ranging from 5.7 m/pixel to 19,500 m/pixel. Each Galileo image was trimmed (ISIS’ photrim application) to remove regions with high emission and incidence angles. In general, images were trimmed at maximum emission and incidence angles of 90 degrees; however, 25 images were trimmed at maximum angles between 80 and 90 degrees to remove distorted data. ISIS’ cam2map application was then used to project all the images within a single Galileo observation sequence to a common projection and pixel scale (equivalent to the smallest pixel scale in the observation sequence, pixel scales typically only varied by 10s of meters, or a few percent, between images). An equirectangular projection with Europa’s mean radius of 1,560.8 km ( Citation: , & al., , , , , , , , , , , , , , , , , & (). Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015. Celestial Mechanics and Dynamical Astronomy, 130(3). 22. https://doi.org/10.1007/s10569-017-9805-5 ) was used except when images within the observation extended above or below 55°north or south, respectively. In those cases, a stereographic projection was used. In either case, an east-positive, planetocentric, 0–360° longitude coordinate system was adopted for consistency with the upcoming Europa Clipper and JUICE missions. Average mosaics were created using ISIS’ automos application. The result was a set of 92 observation sequence mosaics that together provide near-global coverage of Europa at the native pixel scale of the images. Additional details can be found in ( Citation: , & al., , , , & (). Improving the Usability of Galileo and Voyager Images of Jupiter’s Moon Europa. Earth and Space Science, 8(12). e2021EA001935. https://doi.org/https://doi.org/10.1029/2021EA001935 ) .

Available Assets

Assets available with these data are:

  • Galileo, Voyager 1, and Voyager 2 individual projected (equirectangular or polar stereographic) images
  • Galileo stereo DTMs spatially aligned with the image mosaics described here

Accuracy, Errors, and Issues

Final bundle adjustment yielded root mean square (RMS) uncertainties of 246.6 m, 307.0 m, and 70.5 m in latitude, longitude, and radius, respectively. The total RMS uncertainty (over all points) was 0.32 pixels. These values should be thought of as the uncertainty in the location of images relative to one another. The data are also tied to Europa’s geodetic reference system. We have confirmed that the center of the crater Cilix, which defines the reference system at 182°W (178°E), is located at 181.9991414°W (178.0008586°E) in the mosaic. The difference is approximately equivalent to 23 m. The highest resolution images of Cilix have a pixel scale of 63 m/pixel, so the location is “known” to a fraction of a pixel. The precision therefore exceeds that with which the center of Cilix (a 19-km-diameter crater) is known, especially given the natural irregularity of the crater rim. However, given that the data set is tied to the reference frame only at a single point, the certainty with which absolute coordinates are known degrades with distance from Cilix (generally as the sqrt(N) where N is the number of images away from Cilix).

General Usability

The observation sequence mosaics provide the user with a simple means of viewing nearly the entire Galileo image data set of Europa at its native resolution (or very nearly) in a GIS environment. The mosaics are therefore especially well-suited to morphological analysis of landforms, where it is desirable to have the highest-resolution images available, and lower-resolution images can be used for geologic context.

The mosaics names follow the same convention as the original Galileo observation sequences. The name begins with a letter and number combination. The letter indicates the primary target of the orbit (some Europa images were acquired on orbits targeting other satellites) and the number indicates the Galileo mission orbit number. For example, observations mosaics starting with G1 indicate the first Galileo orbit, which targeted Ganymede. Note that after G7 the letter and number are switched, such that 10E indicates the 10th Galileo orbit, which targeted Europa. The observation sequence name ends with 6 letters that briefly summarize the objective of the observation, and two numbers permitting designation of multiple observations of the same objective (e.g., sometimes at different image scales or even different locations). For example, 17ESAGENOR01, 17ESAGENOR02, and 17ESAGENOR03 show portions of Agenor Linea at different scales during the 17th Galileo flyby, which targeted Europa. More complete descriptions of each flyby mosaic can be found in the supporting information of ( Citation: , & al., , , , & (). Improving the Usability of Galileo and Voyager Images of Jupiter’s Moon Europa. Earth and Space Science, 8(12). e2021EA001935. https://doi.org/https://doi.org/10.1029/2021EA001935 ) .

The mosaics should not be combined and used for rigorous geologic mapping, since mapping should be performed at a consistent scale. These mosaics should also not be used, under any circumstances, for quantitative or qualitative analysis of albedo variations, except under the limited circumstances of relative albedo differences over small spatial scales within a single mosaic. The images have not been photometrically corrected and therefore quantitative albedos cannot be derived from them, nor can they be compared to other mosaics, even where overlaps occur. The mosaics are also average mosaics, which means that overlapping pixel values (calibrated but photometrically uncorrected I/F) are averaged. In many cases, the mosaics include images acquired under different color filters, and the DN values are therefore the average of the DNs from each color. The observation sequence mosaic 12ESGLOCOL01 is an excellent example. The mosaic consists primarily of four images with identical footprints (a fifth image in the observation does not overlap them) that were acquired with four different filters (green, violet, IR-7560, IR-9680) and the DN values are the average of all four colors and therefore are quantitatively meaningless. If rigorous DN values are needed, a photometric correction should be applied to individual Galileo images, such as those provided elsewhere in this STAC catalog.

The user should also be aware that no attempt was made to make the mosaics spatially consistent with existing data products, such as the 500 m/pixel, USGS Galileo-Voyager mosaic , or Dr. Paul Schenk’s mosaics . We recognize and endorse the utility of having well-aligned spatial data sets; however, our use of the entire Galileo Europa image data set required a “clean start” to the photogrammetric control.

Traditional, global mosaics are available from the USGS and Dr. Paul Schenk .

Discuss the Data

References

  • , , , , , , , , , , , , , , , , & (). Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015. Celestial Mechanics and Dynamical Astronomy, 130(3). 22. https://doi.org/10.1007/s10569-017-9805-5
  • , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , & (). Galileo’s First Images of Jupiter and the Galilean Satellites. Science, 274(5286). 377–385. https://doi.org/10.1126/science.274.5286.377
  • , , , & (). Improving the Usability of Galileo and Voyager Images of Jupiter’s Moon Europa. Earth and Space Science, 8(12). e2021EA001935. https://doi.org/https://doi.org/10.1029/2021EA001935