WWW.ASTRONOMY.COM 47“head”) by a narrow “neck.” The body measures 2.5 by
2.1 by 1.1 miles (4.1 by 3.3 by 1.8km) and the head
spans 1.6 by 1.4 by 1.1 miles (2.6 by 2.3 by 1.8km).
More importantly, the mass and volume of 67P
reveal that the comet has a density less than half that
of water and a porosity — a measure of how much
empty space lurks inside — of more than 60 percent.
Dust in the nucleus outweighs ice by roughly four to
one. And the surface is exceedingly dark, ref lecting
only about 6 percent of the sunlight hitting it, or half
that of the Moon’s surface.
Rosetta scientists argue that these stats suggest the
nucleus is a mix of dust grains, organic compounds,
and ice cemented together. Furthermore, they contend
that the substances likely mimic the organic-rich soup
created during the collapse of the solar nebula that
gave birth to our solar system. If true, astronomers
would be approaching a better understanding of the
solar system’s origin and evolution — one of the key
reasons scientists wanted to send a mission to rendez-
vous with a comet. The Rosetta team bases these find-
ings on orbiter observations of the gases released from
the nucleus and stress that a Philae analysis of surface
materials could change their thinking.
A magnetic non-attraction
Rosetta has proven to be a trailblazer. For example,
no previous spacecraft encounter yielded much infor-
mation about cometary magnetic fields. From studies
of meteorites, astronomers know that iron, some of
it in the form of magnetite, existed in the solar sys-
tem’s earliest days. Some thought that magnetic fields
might help matter clump together and build comets,
asteroids, and other protoplanetary bodies.
But Philae’s ROMAP instrument quashed this
idea. The lander’s bouncing trek actually improved
the data. “If the surface was magnetized, we would
have expected to see a clear increase in the magnetic
field readings as we got closer to the surface,” said
Auster when reporting on the team’s findings at an
April meeting of the European Geosciences Union in
Vienna, Austria. “But this was not the case at any of
the locations we visited, so we conclude that Comet
67P/Churyumov-Gerasimenko is a remarkably non-
magnetic object.” The comet’s magnetic signature
instead derives from the solar wind, which carries the
Sun’s magnetic field throughout interplanetary space.
Close-up views of 67P also allow Rosetta to
resolve the behavior of individual regions, something
impossible to do from Earth. Since last September,
the Microwave Instrument for the Rosetta Orbiter
(MIRO) has studied the distribution of water mol-
ecules in the comet’s coma. The team finds the high-
est concentration of water above the neck, where the
density is up to 100 times larger than elsewhere.
Significant amounts of water also exist across the
comet’s day side while little is on the night side.Scientists: Rewrite the textbooks
For decades, astronomers have watched comet ices
— frozen water and frozen carbon dioxide (dry ice),
for example — sublimate. The gases created quickly
break up into their constituent atoms. Most research-
ers thought that energetic solar photons did the job,
but Rosetta data have made them change their tune.
Observations made with the Alice spectrograph,
an instrument that splits the comet’s light into its
component colors and thus allows scientists to iden-
tify the unique spectral fingerprints of coma gases,
show the Sun needs a lot of help. Alice operates at
far-ultraviolet wavelengths, where it can detect the
hydrogen and oxygen atoms released from the break-
up of water molecules and the carbon atoms freed
from carbon dioxide molecules. (A similar Alice
spectrograph f lies on the New Horizons spacecraft
that flew past Pluto in July.)
The Alice team found that the molecules split in
a two-step process. First, a high-energy solar photon
strikes a water molecule in the coma and strips an
electron from it. The liberated electron then hits
another water or carbon dioxide molecule and
breaks it apart.
“The discovery we’re reporting is quite unex-
pected,” said S. Alan Stern, a planetary scientist at theROSETTA CAST ITS
SHADOW (bottom)
onto the comet’s sur-
face from 4 miles
(6 kilometers) up.
ESA/ROSETTA/MPS/OSIRIS TEAMBOULDERS
ABOUND near the
edge of the comet’s
broad and largely
smooth Imhotep
region. ESA/ROSETTA/MPS/
OSIRIS TEAMThe lander Philae set down on the comet’s nucleus in
November, where it operated for approximately 57 hours
before entering hibernation. The two images contained in
this mosaic, the first taken by Philae, recorded one of the
lander’s three feet (bottom left). ESA/ROSETTA/PHILAE/CIVA