Cassini Observations of Io's Visible Aurorae

As the Cassini spacecraft passed through the Jovian system on its way to Saturn, it captured more than 500 images of Jupiter's moon Io in eclipse. Cassini's near-ultraviolet filters detected the moon's bright equatorial glows, supporting the interpretation that the visible emissions are predominantly due to molecular sulfer dioxide (SO₂). Detailed comparisons of laboratory SO₂ spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions:

Potassium (K) is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm.
Neutral atomic oxygen (O) and sodium (Na) are required to explain the brightness of the glows at visible wavelengths.
The sulfer molecule, S₂, is postulated to emit most of the glow intensity in the ultraviolet to blue wavelength interval from 390 to 500 nm.

The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the Jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: SO₂ emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse.

Cassini Images of Io's Aurorae

Overview of Eclipses
3303 x 2202, 2.6 MB Figure 1: Comparison of Cassini images from the three successfully recorded eclipses. These clear-filter, full resolution images were acquired near the midpoint of the eclipses and have been cropped to show just the area around the satellite. Galileo images reprojected to similar viewing geometries are shown along the bottom, with latitudes and longitudes marked at 30 degree intervals. The resolution of the Cassini images decreased from 60.5 km/pixel through 61.3 km/pixel to 68.2 km/pixel during the period of observation. The brightest spot shows thermal emission from the volcano Pele.

Eclipse C
725 x 725, 95 KB Figure 4: Time-lapse sequence of clear-filter images of Io during the eclipse of January 1, 2001. Shown for comparison are the approximate locations of the tangent points of the jovian magnetic field lines at the sub-Jupiter and anti-Jupiter points.

Eclipse D
1280 x 768, 565 KB Figure 5: Montage of spectral images from Eclipse D (January 5, 2001). Equatorial glows were detected at wavelengths from at least 330 nm to 880 nm and in all filters except UV1, HAL and IR4. Limb glows were detected in GRN, RED+GRN, CB1, RED and IR1 filter images.

Eclipse C
800 x 800, 100 KB Plate 1. Multispectral images of Io during the eclipse of January 1, 2001. (a) False color composite made up of IR4, CB1 and UV3 images portrayed as red, green and blue, respectively. (b) CB1 (red) and UV3 (blue) images superposed on clear filter image. (c) Location reference map with grid lines at 30 degree intervals. (d) Annotated clear-filter image showing locations of volcanoes and plume glows discussed in text.

Animations
403x403, 1.8 MB animated GIF Eclipse B	256 x 256, 2.8 MB animated GIF Eclipse C	256 x 256, 2.5 MB animated GIF Eclipse C in color

Additional Information

The full paper, Cassini Observations of Io's Visible Aurorae, was written by Paul Geissler (USGS), Alfred McEwen (LPI/UA), Carolyn Porco (SSI), Darrell Strobel (JHU), Joachim Saur (JHU), Joseph Ajello(JPL), and Robert West(JPL). The paper will appear in the Icarus planetary science journal. For additional information, please contact:

Paul Geissler
U.S. Geological Survey
2255 N. Gemini Dr.
Flagstaff, AZ 86001 USA
(928) 556-7257
pgeissler@usgs.gov

Related Resources

USGS Astrogeology: Cassini Mission

USGS Astrogeology: Jupiter's Moon Io