Sky & Telescope October 1991 [antikvár]

FOCAL POINTWorlds ApartThanks to hard-won observations from Earth and Voyager 2, astronomers now realize that Pluto and Neptune's satellite Triton are truly unusual worlds. Pluto's orbit is inclined, eccentric, and overlaps Neptune's. Pluto also has a moon, Charon, with a diameter half as big as its own. Triton's motion around Neptune is retrograde, and this moon will eventually be tidally dragged into the planet. Yet Pluto and Triton have much in common maybe too much to be mere coincidence. They are comparable in size, mass, and overall density. Both have tenuous methane atmospheres and surface veneers of methane frost.So what formed these oddballs of the outer solar system? Two planetary specialists offer contrasting hypotheses.Triton has a retrograde orbit, like those of Jupiter's outermost four satellites and Saturn's distant moon Phoebe. Planetary scientists have long considered capture from solar orbit to be the most likely origin for these other satellites, and such an origin for Triton has always been thought plausible.Capture would have placed Triton in a very elliptical orbit about Neptune. Tides raised in the satellite by the planet would then have caused this orbit lo circularize over several hundred million years. During that time Triton would have passed through the entire region occupied by any original satellite system, coming inward to as little as 5 Neptune radii, where the outermost of Neptune's inner satellites now lies. Since Triton is relatively massive, it would have gravitationally ejected any smaller satellites it passed near, or even destroyed (or accreted) them outright through collision. Thus Neptune's lack of an extended satellite system today is consistent with the orbital evolution of a captured Triton.Arguments based on composition are also compelling. The chemistry of the outer solar system is quite complex. Many compounds that might have been stable in the outer solar nebula (the disk of gas and dust that gave rise to the planets) probably never formed due to the extremely cold conditions that prevailed so far from the young Sun. Notable among these are methane (CH4) and ammonia (NHj), which derive from carbon monoxide (CO) and molecular nitrogen (N2). Water ice, however, should have been common.One consequence of having abundant, unreactive CO in the outer solar nebula is that its oxygen is unavailable to form even more water. The net effect is to change the ratio of rock to water ice accreted by a solid body forming in the outer solar system. For example, these components exist in a roughly 50:50 mix (by mass) in icy satellites of the giant, outer planets. But these objects formed in planet-enveloping "subnebulae," regions warm enough to allow at least partial conversion of CO to methane and water ice.In the outer solar nebula, however, with CO hogging so much oxygen, the rock-water ratio could have been as great as 70:30 and the resulting mean density near 2 grams per cubic centimeter. Thus, it came as no surprise that the Pluto-Charon system has an overall density very near 2. This argues strongly that Pluto and Charon formed in the solar nebula, as opposed to being a pair of escaped outer-planet satellites.Before Voyager 2's 1989 flyby we suspected that Triton was similar to Pluto in size. Furthermore, if Triton formed separately in the outer solar system (as Pluto did) and was subsequently captured by Neptune, it should also mimic Pluto's bulk density and thus its rock-ice ratio. And Voyager did show that Triton's density is near 2 g/cm^ significantly less icy than are typical outer-planet satellites.For the capture hypothesis to be accepted, a dynamically plausible series of events must have occurred. Specifically, to evolve from solar orbit into an orbit permanently bound to Neptune, Triton must have been slowed significantly while sweeping past Neptune. One way toachieve this is for Triton to have passed through a dense gas-dust cloud around Neptune, one which if left alone would have produced a regular satellite system. Although Triton is fairly massive, the drag experienced during a single pass through such a subnebula would have been sufficient to capture it.Alternatively, Triton could have ventured close to Neptune after a regular satellite system had formed, in which case capture was triggered by a chance collision with one of these satellites. The putative gas-dust nebula around Neptune was probably so short-lived that capture by collision is arguably the best hypothesis. In fact, it may be one more example of how impact processes have profoundly shaped solar system history.Admittedly, the odds of Triton colliding with one of Neptune's primordial satellites is not great. Then again, no current theory for the origin of Triton can be considered truly likely. That is, if any mechanism were innately probable, then all the giant planets would have Tritons. What is important in weighing various origin scenarios is relative probability. From this perspective, Triton's capture is an uncommon but possible event given the large number of planetesimals likely in the outer solar system at that time and four giant planets with which to interact. Scenarios that require Neptune's original satellites (including Triton) to interact with hypothetical rogue planets of at least one Earth mass are much less probable.WILLIAM B. McKINNONMcKinnon is a professor of Earth and planetary sciences at Washington University in Sl. Louis, Missouri.340 Sky & Telescope, October, 1991