570 PART 4^ |^ THE SOLAR SYSTEM
missions produced the surprising discovery that some comet
dust is crystalline and must have formed originally in very
warm environments close to the sun, but then was incorpo-
rated somehow into comet nuclei in the cold outer solar
system.
Th e Solar and Heliospheric Observatory (SOHO) space-
craft was put into space to observe the sun, but it has also
discovered over a thousand comets, called “sun grazers,” that
come very close to the sun, in some cases 70 times closer to
the sun’s surface than the planet Mercury (■ Figure 25-16).
As many as three comets per week plunge into the sun and
are destroyed. Most sun grazers belong to one of four groups,
and the comets in each group have very similar orbits. Like
the Hirayama families of asteroids, these comet groups
appear to be made up of fragments of larger comet nuclei.
Th e original comet may have been ripped apart by the vio-
lence of gases superheated near the sun and bursting through
the crust, or by solar tidal forces, or both.to volcanic faults or vents (■ Figure 25-14). As the rotation of a
comet nucleus carries an active region into sunlight, it begins
venting gas and dust, and as it rotates into darkness it shuts
down.
Th e nuclei of comets appear to have a crust of rocky dust left
behind as the ices vaporize. Breaks in that crust can expose ices
to sunlight, and vents can occur in those regions. It also seems
that some comets have large pockets of volatiles buried below the
crust. When one of those pockets is exposed and begins to vapor-
ize, the comet can suff er a dramatic outburst, as Comet Holmes
did in 2007 (■ Figure 25-15).
Astronomers have devised ways to study comet material
more directly. Th e Stardust spacecraft passed through the tail of
Comet Wild 2 in 2004, collected dust particles that had been
ejected from the comet’s nucleus, and returned the samples in a
sealed capsule to Earth in 2006 for analysis. In 2005, the Deep
Impact spacecraft released an instrumented impactor probe into
the path of comet Tempel 1. As planned, the nucleus of the
comet ran into the impactor at almost 10 km/s (22,000 mph).
Th e probe broke through the crust of the nucleus and blasted
vapor and dust out into space where the Deep Impact “mother
ship,” as well as the Spitzer and Hubble space telescopes and
observatories on Earth, could analyze it (page 569). Th ose
■ Figure 25-14
The crusts of comets are evidently delicate mixtures of rock, ice, and dust.
The dust is ejected along with gases as the ices in a comet vaporize in
sunlight, as shown in this artist’s conception. (NASA/NSSDC; Tom Herbst, Max-
Planck-Institut für Astronomie, Heidelberg; Doug Hamilton, Max-Planck-Institut für
Kernphysik, Heidelberg; Hermann Bo ̈ ehnhardt, Universitäts-Sternwarte, Mu ̈ nchen; and
Jose Luis Ortiz Moreno, Instituto de Astrofi sica)
■ Figure 25-15
Composite of 19 snapshots of Comet Holmes, showing its changing bright-
ness and position spanning the period from October 2007 until March 2008.
During its outburst in late October 2007, the comet brightened by a factor
of about 500,000 as a large pocket of volatile material exploded through its
crust and spread into space. (John Pane)