On July 11, 2005, the Cassini ultraviolet imaging spectrograph observed
the star Bellatrix as it passed behind Enceladus, as seen from the
spacecraft. The starlight was observed to dim when it got close to
Enceladus, indicating the presence of an atmosphere, as illustrated in
figure A. The ultraviolet imaging spectrograph team was able to identify
water vapor as the composition of the atmosphere from absorption features
in the spectrum of the star. From the depth of the absorption features, it
was also possible to estimate the quantity of water vapor the starlight
passed through. The colors show the undimmed star signal (blue) versus the
dimmed star signal (pinkish).
Enceladus' atmosphere is localized, not global in extent. As Bellatrix
re-emerged from behind Enceladus, there was no dimming of the starlight
observed. An occultation of the star Lambda Scorpius in February also
showed no sign of an atmosphere, as illustrated in figure B. In figure A
and B, the arrow marks the path of the star as it was blocked from view by
Enceladus. In figure A, the dimming of the starlight shows as a gradual
decrease in brightness, while in figure B the starlight drops abruptly
just at the point in time that the star goes behind Enceladus.
The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory,
a division of the California Institute of Technology in Pasadena, manages
the mission for NASA's Science Mission Directorate, Washington, D.C. The
Cassini orbiter and its two onboard cameras were designed, developed and
assembled at JPL. The ultraviolet imaging spectrograph was built at, and
the team is based at the University of Colorado, Boulder. The imaging
operations center is based at the Space Science Institute in Boulder,
Colo.
For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov.
The ultraviolet imaging spectrograph team home page is at http://lasp.colorado.edu/cassini.
The Cassini imaging team homepage is at http://ciclops.org .
