Titan casts revealing shadow (Forwarded)

Steve Roy
Media Relations Dept.
Marshall Space Flight Center, Huntsville, AL

(256) 544-0034

Megan Watzke
Chandra X-ray Observatory Center, CFA, Cambridge, MA

(617) 496-7998

Science Contacts:

Koji Mori, , 814-863-0182
Hiroshi Tsunemi,
David Burrows, , 814-863-2466

April 5, 2004

RELEASE: 04-03

Titan casts revealing shadow

A rare celestial event was captured by NASA's Chandra X-ray Observatory as Titan
-- Saturn's largest moon and the only moon in the Solar System with a thick
atmosphere -- crossed in front of the X-ray bright Crab Nebula. The X-ray shadow
cast by Titan allowed astronomers to make the first X-ray measurement of the
extent of its atmosphere.

On January 5, 2003, Titan transited the Crab Nebula, the remnant of a supernova
explosion that was observed to occur in the year 1054. Although Saturn and Titan
pass within a few degrees of the Crab Nebula every 30 years, they rarely pass
directly in front of it.

"This may have been the first transit of the Crab Nebula by Titan since the
birth of the Crab Nebula," said Koji Mori of Pennsylvania State University in
University Park, and lead author on an Astrophysical Journal paper describing
these results. "The next similar conjunction will take place in the year 2267,
so this was truly a once in a lifetime event."

Chandra's observation revealed that the diameter of the X-ray shadow cast by
Titan was larger than the diameter of its solid surface. The difference in
diameters gives a measurement of about 550 miles (880 kilometers) for the height
of the X-ray absorbing region of Titan's atmosphere. The extent of the upper
atmosphere is consistent with, or slightly (10-15%) larger, than that implied by
Voyager I observations made at radio, infrared, and ultraviolet wavelengths in 1980.

"Saturn was about 5% closer to the Sun in 2003, so increased solar heating of
Titan may account for some of this atmospheric expansion," said Hiroshi Tsunemi
of Osaka University in Japan, one of the coauthors on the paper.

The X-ray brightness and extent of the Crab Nebula made it possible to study the
tiny X-ray shadow cast by Titan during its transit. By using Chandra to
precisely track Titan's position, astronomers were able to measure a shadow one
arcsecond in diameter, which corresponds to the size of a dime as viewed from
about two and a half miles.

Unlike almost all of Chandra's images which are made by focusing X-ray emission
from cosmic sources, Titan's X-ray shadow image was produced in a manner similar
to a medical X-ray. That is, an X-ray source (the Crab Nebula) is used to make a
shadow image (Titan and its atmosphere) that is recorded on film (Chandra's ACIS
detector).

Titan's atmosphere, which is about 95% nitrogen and 5% methane, has a pressure
near the surface that is one and a half times the Earth's sea level pressure.
Voyager I spacecraft measured the structure of Titan's atmosphere at heights
below about 300 miles (500 kilometers), and above 600 miles (1000 kilometers).
Until the Chandra observations, however, no measurements existed at heights in
the range between 300 and 600 miles.

Understanding the extent of Titan's atmosphere is important for the planners of
the Cassini-Huygens mission. The Cassini-Huygens spacecraft will reach Saturn in
July of this year to begin a four-year tour of Saturn, its rings and its moons.
The tour will include close flybys of Titan that will take Cassini as close as
600 miles, and the launching of the Huygens probe that will land on Titan's surface.

"If Titan's atmosphere has really expanded, the trajectory may have to be
changed," said Tsunemi.

The paper on these results has been accepted and is expected to appear in a June
2004 issue of The Astrophysical Journal. Other members of the research team were
Haroyoski Katayama (Osaka University), David Burrows and Gordon Garmine (Penn
State University), and Albert Metzger (JPL). Chandra observed Titan from 9:04 to
18:46 UT on January 5, 2003, using its Advanced CCD Imaging Spectrometer instrument.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra
program for the Office of Space Science, NASA Headquarters, Washington. Northrop
Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development
contractor for the observatory. The Smithsonian Astrophysical Observatory
controls science and flight operations from the Chandra X-ray Center in
Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2004/titan/
and
http://www.msfc.nasa.gov/news/news/p...tos04-087.html