Encyclopedia of the Solar System 2nd ed

564 Encyclopedia of the Solar System

FIGURE 9 Splitting of comet LINEAR. (Top) TheHubble
Space Telescope(HST) image on 5.4 August 2000 showing the
dust tail (extending to the left) and several large remnants near
the tip of the tail; 22 July 2000 is the estimated date of
disintegration of the nucleus. (Bottom) TheVery Large Telescope
(VLT) image on 7.0 August 2000 showing fragments. Image
processing was used to suppress light from the diffuse tail. The
streaks are star trails. (Courtesy of H. Weaver, Johns Hopkins
University; C. Delahodde, O. Hainaut, R. Hook, European
Southern Observatory; Z. Levay, Space Telescope Science
Institute; and theHST/VLTobserving team; NASA/ESA, ESO.)

brightness is due to an increase in the number of dust par-

ticles that scatter sunlight. Comet Halley displayed an ex-

traordinary outburst on 12 February 1991 when it was 14.3

AU from the Sun.

Splitting exposes fresh ice surfaces and hence produces

enhanced loss of material. An impact from an interplan-

etary boulder would have much the same effect. A plau-

sible mechanism not involving splitting or impacts uses

the crystallization of amorphous ices as the energy source.

On this picture, a heat wave propagates inward, trigger-

ing the energy release from the amorphous ice and pro-

ducing pockets of gas that break through to the surface to

produce the outburst. This mechanism is plausible for the

outburst in comet Halley (mentioned earlier) and in comets

that have repeated outbursts, such as comet Schwassmann–

Wachmann 1.

A summary schematic of a comet nucleus is shown in

Fig. 11. Because our knowledge of interiors is insecure, the

figure presents processes at work rather than a specific inte-

rior. Some hints about nucleus structure have already come

from theStardustimages of comet Wild 2. The nucleus

shows a highly structured surface that can be described as

pockmarked. Some of the features, possibly impact craters,

have steep slopes, and the surface must have some cohesive

strength. How did these sharp features persist if layers were

peeled off by sublimation during every perihelion passage?

For comet Wild 2, the answer lies in its orbital history.

Comet Wild 2 was captured into its current (Jupiter-family)

orbit by a close encounter with Jupiter only 30 years ago.

With an orbital period of 6.4 years, this comet has probably

made only a handful of passes through the inner solar

system. By comparison, comet Halley has probably made

FIGURE 10 Comet Shoemaker–Levy 9 on 17 May 1994 as imaged by theHubble Space Telescope.

The fragments extended for over 1.1 million km. (Courtesy of H. A. Weaver and T. E. Smith,

Space Telescope Science Institute/NASA.)