566 Encyclopedia of the Solar System
through the coma) falls off asρ−^1 (ρis the projected dis-
tance from the nucleus). The slope on a logB(brightness)
versus logρplot would be−1. Shallower slopes indicate a
creation process, and steeper slopes indicate a destruction
process. This behavior is observed in molecules such as C 2.
These results and results from more detailed modeling lead
to an important conclusion.
The molecules measured and observed in the coma are
not necessarily the molecules coming directly from the nu-
cleus, but they are part of a chain of creation and destruc-
tion of species, presumably from complex molecules in the
nucleus to progressively simpler molecules with increas-
ing distance from the nucleus. Thus, the molecules ob-
served are simply the ones that are caught at some specific
distance from the nucleus or with the method of obser-
vation.
Calculations that include the various changes in com-
position with the goal of understanding the composition of
the original material from the nucleus are very complex and
must include gas-phase reactions and photolytic (involving
photons) reactions as well as possible interactions between
the gas and dust. While progress has been made, final reso-
lution of this problem may require measurements obtained
at a cometary surface. As discussed in Section 6, the knowl-
edge of the bulk composition of comets seems secure and
is consistent with condensation from a cloud that initially
had solar abundances.
Table 2 (in Section 6) lists chemical species observed
spectroscopically and measured by mass spectrometry in
comets. This table shows the variety of species in comets
and the similarity to interstellar material. This relationship
is discussed in Section 6.
Thehydrogen cloudaround comets is much larger
than the coma but was not observed until the 1970s. Its
existence was predicted in 1968 by L. Biermann. Observa-
tions above the Earth’s atmosphere were needed because
the hydrogen cloud is best seen in Lyman-α(121.6 nm),
the resonance line of hydrogen. Figure 13 shows the hy-
drogen cloud of comet Hale–Bopp along with a visible light
image. The huge size of the cloud is shown by the yellow
disk at the right. This disk is the angular size of the Sun at
the comet’s distance. The hydrogen cloud has the largest
size; however, smaller clouds of oxygen and carbon are also
seen.FIGURE 13 Hydrogen Lyman-a
image taken on 1 April 1997
showing the hydrogen cloud of
comet Hale–Bopp (contours in
shades of blue) along with a visible
image showing the plasma and dust
tails. The image is approximately
40 ◦on a side. The small yellow disk
shows the angular size of the Sun
and the solar direction. (Courtesy
of M. Combi, University of
Michigan; visual photograph by
Dennis di Cicco andSky and
Telescope.)