Jupiter family

Short-period comets these display a strong tendency for their farthest point from the sun (aphelia) to coincide with a giant planet s orbital radius, so that we can distinguish so-called comet families . The Jupiter family of comets is the largest and numbers around 70 comets. The shortest orbital period known is that of the short-period comet Encke—about 3.3 years. 

The latter group was probably responsible for the early bombardment of the protoplanets. Delsemme believes that the cometary nuclei of the members of the Jupiter family never experienced temperatures greater than 225 K. The values suggested for the others are Saturn family, 150K Uranus family, 75 K Neptune family, 50 K. During many million years, these comets got mixed together in the Oort cloud (which has a diameter of around 50,000 AU). 

With the success of the Stardust mission, tests of our models for solar nebula, and thus protoplanetary disk evolution, are no longer limited to asteroidal bodies (meteorites), but now can be applied to cometary bodies as well. Stardust returned dust grains that were ejected from the surface of comet Wild 2, a Jupiter-family cometthatis thought to have formed at distances of >20 AU from the Sun (Brownlee et al. 2006). Thus, we now have samples of materials from the outer solar nebula that can be studied in detail. 

Figure 17 The two main comet compositional groups. Plots of C2, C3, and NH relative to CN emission from the A Heam et al. (1995) study. The filled points are Jupiter family comets, the group with depleted C2 and C3 and...

Comets can be classified into two major groups short-term and long-term comets. Short-term comets are those with orbital periods of less than 200 years. They can be further subdivided into two groups the Jupiter family, with periods of less than 20 years, and... 

Short period comets their orbital periods about the Sun are less than 200 years. At aphelion (when their distance from the Sun is at maximum) they are at distances of Jupiter and beyond. The aphelion of comet Halley is about the orbit of Neptune. Comets of the Jupiter family have orbital periods of less than 20 years. The existence of such families of comets can be attributed to the gravitational influence of Jupiter. ... 

Their measurements gave a distance four times that of the Moon. Also called Jupiter family there exists also a Saturn family. 

At the end of this section, two final points merit some attention. The first one concerns the location of the parent bodies of the chondrites. The asteroid belt between Mars and Jupiter is one possibility favoured by many authors, but some other possibilities exist (such as the families of asteroids crossing the earth s orbit). Asteroids themselves are of many different types and some of them are probably extinct comets (after too many passages at the perihelion). Some chondrites could be fragments of these extinct comets, but this hypothesis is not the most probable one. People interested in this problem will find information in Refs. 2, 5 and 9. 

Tidal breakup is not, however, the major source of fragmentation of comets. Most fragmentation occurs somewhat mysteriously far from massive bodies in the absence of tidal stress. Many comets including S4 LINEAR, Hale-Bopp, Wilson, Kohoutek, West, and Ikeya-Seki have been seen to fragment far from the Sun and Jupiter. Remarkable insight into the disintegration of comets has come from the study of the Kreutz family of sungrazing comets, a family of comets that pass within a few solar radii of the Sun. 

Titan orbits Saturn, so from its surface, the sun is smaller than a dime. Not that you d be able to see it anyway, because a dark orange haze obscures everything. Titan is just a little smaller than Mars, but it is abnormally big for its family. It is 20 times the size of all the other moons of Saturn combined, and about the same size as Jupiter s four big moons. Unlike those, it has retained an atmosphere, and beneath that, it has exotic oceans as liquid as our seven seas. 

Jupiter s big family of moons holds tantalizing liquid flows. Europa and Ganymede both have liquid oceans hidden in darkness under miles of ice. Likewise for Saturn s moon Enceladus, which is warmed by underwater vents like those suspected in the origins of Earth-life. Jupiter itself has supercritical helium and hydrogen deep inside. After the disappointingly dry rocky planets near Earth, the gas-giant moons seem like veritable oceans of possibility. 

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