ISSN:0094-8276    

DOI:10.1029/GL016i011p01203

年代:1989

数据来源: WILEY

摘要:

Since 1985, planetary astronomers have been working to take advantage of a once‐per‐century apparent alignment between Pluto and its satellite, Charon, which has allowed mutual occultation and transit events to be observed. These events, which will cease in 1990, have permitted the first precise determinations of their individual radii, densities, and surface compositions. In addition, information on their surface albedo distributions can be obtai

ISSN:0094-8276    

DOI:10.1029/GL016i011p01205

年代:1989

数据来源: WILEY

摘要:

We review recent models of the internal structure of Pluto and Charon made possible by analysis of the Pluto/Charon mutual events. At a mean density of just over 2 g cm−3and a predicted rock mass fraction of roughly 0.7, the Pluto/Charon system is significantly rockier than the icy satellites of the giant planets, a contrast which may reflect its formation in a CO‐rich outer solar nebula rather than a circumplanetary nebula. Pluto and Charon may in fact besorocky that they lost volatiles early in their history (possibly during a Charon‐forming impact event), although this is still an open issue. Finally, we review the outlook for future study of the Pluto and Charon inte

ISSN:0094-8276    

DOI:10.1029/GL016i011p01209

年代:1989

数据来源: WILEY

摘要:

A recent stellar occultation has confirmed predictions that Pluto has an atmosphere which is sufficiently thick to uniformly envelope the planet and to extend far above the surface. Pluto's atmosphere consists of methane and perhaps other volatile gases at temperatures below their freezing points; it should regulate the surface temperature of its volatile ices to a globally uniform value. As Pluto approaches and passes through perihelion, a seasonal maximum in the atmospheric bulk and a corresponding minimum in the exposed volatile ice abundance is expected to occur. The lag in maximum atmospheric bulk relative to perihelion will be diagnostic of the surface thermal properties. An estimate of Pluto's atmospheric bulk may result if a global darkening (resulting from the disappearance of the seasonally deposited frosts) occurs before the time of maximum atmospheric bulk. The ice deposited shortly after perihelion may be diagnostic of the composition of Pluto's volatile reservoir.

ISSN:0094-8276    

DOI:10.1029/GL016i011p01213

年代:1989

数据来源: WILEY

摘要:

The dynamics of the Pluto‐Charon system are reviewed from a historical perspective. Although Pluto‧s orbit crosses Neptune‧s, an intricate system of nested resonances keeps these planets apart. Pluto‧s orbit is apparently chaotic as well. Pluto always keeps the same face turned toward Charon, andvice versa. Tides also damp Charon‧s orbital eccentricity and inclination. Precession of Pluto‧s orbital plane causes Pluto‧s obliquity to vary periodically from formally prograde to retrograde. Pluto is probably an original member of the Solar system, but not an escaped satellite of Neptune. The Voyager II encounter with Neptune, the final Pluto‐Charon mutual events, and the next generation of telescopes are bound to reveal

ISSN:0094-8276    

DOI:10.1029/GL016i011p01217

年代:1989

数据来源: WILEY

摘要:

Elliotet al.[1989] invoked a haze layer near the surface of Pluto to explain certain features of a stellar occultation by that planet in June, 1988. The primary requirements for this haze layer were that it achieve unity tangential optical depth at a radius of 1174 km and be essentially transparent above 1189 km. We explore here the possibility that aerosols generated through methane photolysis could be responsible for such a haze layer. A comprehensive model of aerosol production, particle growth, sedimentation and condensation is applied to the atmosphere of Pluto using pressures, temperatures and composition derived from the stellar occultation and other data. We test two atmosphere models proposed in the literature, one from Elliotet al. [1989], and one from Hubbardet al. [1989], as well as a range of optical properties for the particles. In order to produce a haze with unity tangential optical depth at 1174 km, we had to use an aerosol mass production rate equal to twice the total methane dissociation rate due to solar UV expected for Pluto and assume that the particles produced were 10 times more absorbing than those in other hazes in the outer solar system. The possibility of condensation in the lower atmosphere was considered but did not result in distinctly different haze optical depths. If a photochemical haze on Pluto was responsible for the occultation lightcurve measured by Elliotet al.,operation of a photochemical system different from those on Titan, Uranus or Neptune is indicated. Alternatives include near surface condensation processes driven by circulation, and the possibility that the occultation light curve can be explained in its entirety by temperature effects as proposed by Hubbardet al. [1989].

ISSN:0094-8276    

DOI:10.1029/GL016i011p01221

年代:1989

数据来源: WILEY

摘要:

Best guesses as to the thermal structure and composition of Pluto‧s atmosphere have led to speculations of substantial loss rates (∼1028s−1) of methane from the planet over cosmogonic time scales. Results from recent stellar occultation measurements, and using a Parker‐type hydrodynamic calculation, show that the loss rates may actually be lower by as much as a factor ∼5, depending upon the efficiency of heating of the atmosphere via the absorption of solar EUV and upon the true atmospheric composition, if the thermal structure of the upper atmosphere is properly taken into account. The loss rate may even be less (by another factor ∼10) if there is minimal heating of the upper

ISSN:0094-8276    

DOI:10.1029/GL016i011p01225

年代:1989

数据来源: WILEY

摘要:

If Pluto‧s atmospheric escape rate is significantly greater than 1.5 × 1027molecules s−1then the interaction with the tenuous solar wind at 30 A. U. will be like that of a comet: there will be extensive ion pick‐up upstream and the size of the interaction region will vary directly with variations in the solar wind flux. If the escape flux is much less, then one expects that the solar wind will be deflected around Pluto‧s ionosphere in a Venus‐like interaction. In either case, the weak interplanetary magnetic field at 30 A. U. results in very large gyroradii for the picked up ions and a thick bow shock, necessitating a kinetic treatment of the interaction. Strong variations in the size of the interaction region are expected on time scales of days due to changes in the

ISSN:0094-8276    

DOI:10.1029/GL016i011p01229

年代:1989

数据来源: WILEY

摘要:

The surface of Pluto is exposed to cosmic ray particles which slowly alter the reflectance of the condensed methane and the UV absorbed in the atmosphere may produce precipitates. Depending on the rates of the competing regolith processes and the rate of replenishment of the methane the surface can appear bright, ‘red‧, or ‘dark‧. Here the relevant laboratory data show that, in the absence of any local particle precipitation, the amount of darkening occurring in one orbit i

ISSN:0094-8276    

DOI:10.1029/GL016i011p01233

年代:1989

数据来源: WILEY

摘要:

Jetting can occur during oblique impacts of water ice bodies at relative velocities as low as ∼500 m s−1, because of the low Hugoniot elastic limit and high compressibility of ice compared to rock. In jetted ice, incipient melting, complete melting, and incipient vaporization occur, upon release to low pressure, at impact velocities of 1.3, 2.0, and 2.7 km s−1, respectively, much less than the 3.4, 4.4, and 5.3 km s−1, required in head‐on collisions. Uncertainties in the shock equation‐of‐state may allow complete melting during jetting at relative velocities as low as 1.2 km−1. Because jet speeds exceed impact speeds, often by a factor of several, during the accretion of icy bodies greater than a few 100 km in radius there may be a significant loss of icy material. This is more true if the accreting body is large enough to differentiate so that its surface layers are closer to pure ice in composition, and especially true if bodies of comparable size are involved, which emphasizes the obliqueness of the collision. I suggest that it is jetting during a Charon‐forming collision (and not vaporization) that may account for Pluto‐Charon's relatively large rock/ice ratio, should the C/O ratio of the solar nebula turn out to be too low to sufficiently raise the rock/ice ratio of outer solar nebula condensates by formation of

ISSN:0094-8276    

DOI:10.1029/GL016i011p01237

年代:1989

数据来源: WILEY