S. Alan Stern

On the number of planets in the outer solar system: Evidence of a substantial population of 1000-km bodies

Where have all the Tritons gone, long time ago? n nA natural consequence of the accumulation stage of planetary formation is the scattering of planetesimals during the accretion epoch. In the outer Solar System, such scattering contributed to the formation of the Oort cloud (with an orbital semimajor axis distribution 103 < a < 5 < 104 AU) and the Kuiper disk (30 < a < 500 AU). We suggest here that in addition to comets, a potentially large number of planetary bodies in the size class near 1000 km may have also been scattered into the Kuiper disk and Oort cloud. This hypothesis implies that the present population of planets in the outer Solar System is much larger than previously recognized. To support this claim, we present calculations showing that the large tilts of Uranus and Neptune, the capture of Triton, and the formation of the Pluto-Charon binary each argue for the past presence of numerous 1000-km bodies in 20–50 AU region. We then summarize arguments that scattering dominated other loss processes for these objects, leading to the conclusion that many of these bodies should presently be located beyond the “regular” planetary system. Finally, we examine the detection prospects for such bodies, and compare the existing observational constraints to our model predictions of the number of such bodies in the region between 30 and 500 AU from the Sun.

Why is Pluto bright? Implications of the albedo and lightcurve behavior of Pluto

Abstract Methane is abundant on Pluto, however, CH 4 rapidly darkens in the Plutonian solar insolation and charged particle environment. We therefore call attention to the fact that Plutos high albedo is at odds with the observation of methane frost on Plutos surface. We examine a variety of mechanisms to resolve this dilemma, and conclude that Plutos surface is being replenished with fresh (bright) volatile frosts. We propose that this replenishment is due to orbitally driven sublimation and freezout of volatiles in the atmosphere. Thus, Plutos high albedo adds to the case for an atmosphere, and argues for annual volatile transport cycles. Orbitally driven replenishment can also account for the observed secular changes in Plutos lightcurve. We show that thermally driven sublimation is capable of replacing volatiles lost to escape and photolysis. Our model is consistent with present data on the surface composition, albedo, aldebo distribution, and surface color of Pluto, and presents an explanation of the time variability of Plutos lightcurve. Charons darker albedo is consistent with our model because Charons surface is today devoid of volatiles. We predict that the secular variation in Plutos rotational lightcurve should reverse 7–17 years after perihelion due to the thermal inertia of Plutos surface. Based on the minimum mass of CH 4 required to cover newly created dark hydrocarbons each Pluto year, we develop a lower limit of 7–70 cm-am on Plutos maximum atmospheric abundance. Based on the time-lag constraint, we develop a lower limit of between 16 and 45 cm-am on Plutos present atmospheric abundance. Finally, we discuss a number of observational tests for our model.

Collisions in the Oort Cloud

Abstract A first generation model has been constructed to evaluate the importance of physical collisions between objects in the Oort cloud. While the collision rate is strongly dependent on the population structure of the Oort cloud, it is shown that natural power-law population structures produce significant numbers of collisions between each comet and smaller objects over the age of the Solar System, and that such collisions provide a feedback mechanism for the production of small debris in the cloud. The effects of bombardment on the evolution of cometary surfaces are also explored. It is shown that impacts cause extensive surface evolution to develop on comets in the cloud, if the number of small objects orbiting in the cloud is in accordance with “standard” power-law populations.

ISM-induced erosion and gas-dynamical drag in the Oort Cloud

We examine the physical interactions between material in the Oort Cloud and the interstellar medium (ISM). The model employed here accounts for sputtering, sticking, and grain-impact erosion, as well as gas drag. The model represents the ISM as a multiphase medium with two cloud-phase regimes (atomic and molecular) and two gas-phase regimes (coronal and warm/ambient). The effects of both supernova remnants and stellar winds on the Oort Cloud are also evaluated. It is found that erosion is the dominant ISM interaction. Owing to the spatial inhomogeneities in the ISM, the erosion rate is highly time variable, with essentially all erosion taking place during brief encounters with atomic and molecular clouds. Depending on the actual mechanical and surface roughness properties of cometary surfaces, 60–600 g cm−2 of material may have been lost from each comet or smaller object. ISM drag effects were found to efficiently remove submicron particles from the Cloud. Either by direct ejection or through drag forces, eroded debris particles will be lost from the Oort Cloud to the ISM. Erosion reduces the effectiveness of the thermal and radiation damage processes acting on cometary surfaces in the Oort Cloud.

Jupiter's nightside airglow and aurora

Observations of Jupiters nightside airglow (nightglow) and aurora obtained during the flyby of the New Horizons spacecraft show an unexpected lack of ultraviolet nightglow emissions, in contrast to the case during the Voyager flybys in 1979. The flux and average energy of precipitating electrons generally decrease with increasing local time across the nightside, consistent with a possible source region along the dusk flank of Jupiters magnetosphere. Visible emissions associated with the interaction of Jupiter and its satellite Io extend to a surprisingly high altitude, indicating localized low-energy electron precipitation. These results indicate that the interaction between Jupiters upper atmosphere and near-space environment is variable and poorly understood; extensive observations of the day side are no guide to what goes on at night.

Pluto: Comments on crustal composition, evidence for global differentiation

Abstract The implications of rapid atmospheric escape and the established presence of methane on Plutos surface leads to the conclusion that unless Plutos crust is young, it cannot contain more than a few percent by volume nonvolatile material (e.g., water ice, rock). Were this not the case, the escape of volatiles would cause a deepening lag deposit to form and eventually halt the resupply of CH 4 to Plutos surface. Unless Plutos surface methane has been recently deposited, it is therefore possible to conclude that Pluto has likely differentiated. The consequences of differentiation are explored as they relate to Plutos bulk composition, surface topography, and atmospheric composition. Several relevant observational tests of the differentiation hypothesis are suggested.

Constraints on Pluto's density and composition

By combining the bulk-parameter constraints placed on the Pluto-Charon system by the 1987 Pluto-Charon eclipse observations, IRAS data, and an occultation of Charon, it is possible to isolate Plutos density from the bulk density of Pluto-Charon. In particular, it can now be demonstrated without ad hoc assumptions that Plutos density must lie between 1.07 and 2.42 g/cm3. More severe constraints may be obtained by assuming that most of the Pluto-Charon system mass is in Pluto. The compositional and evolutionary implications for Pluto which derive from these density constraints are discussed. It is also demonstrated that eclipse observations alone can be used to calculate the individual masses of Pluto and Charon.

The effects of mechanical interaction between the interstellar medium and comets

Abstract Oort Cloud comets mechanically interact with the interstellar medium (ISM). In this report we discuss and evaluate the importance of accretion of interstellar material onto comets, the erosion of cometary surfaces by impacting interstellar grains, and the consequences of these interactions. Based upon scaling analyses, we find that collisions with interstellar grains can provide a significant evolutionary mechanism for the modification of cometary surfaces and, in doing so, can also contribute an appreciable number of low-mass particulates to the ISM.

Helium and argon abundance constraints and the thermal evolution of Comet Austin (1989c1)

Abstract We report results from the flight of a rocket-borne, far ultraviolet (FUV) spectrometer observation to establish upper limits on He and Ar abundances in the bright, dynamically “new,” comet Austin (1989c1). Previous to comet Austing, no comet had been spectroscopically observed to set helium and argon abundance constraints. Relative to solar abundance, our upper limits imply Comet Austin is at least 1.5 × 10 4 depleted in He/O, and no more than 30 times enriched in Ar/O. These upper limits allow us to discuss the thermal conditions experienced by this comet during its formation and during its subsequent evolution. The upper limits achieved by this 258-sec subortial rocket observation indicate the usefulness of future, longer-integrated or otherwise more sensitive observations of comets below 1200 A, in the FUV.

The spectrum of Comet Austin from 910 to 1180 A

A spectrum of comet Austin (1988 c1) has been obtained from 910 to 1180 �. Three bright emission lines were detected, including a forbidden oxygen line (1128 �), which are attributable to radiative pumping of neutral oxygen by solar Lyman β. The relative strengths of the observed features should prove to be a useful diagnostic of the physical conditions and radiation fields in cometary comae. In addition, the absence of strong spectral features from highly volatile species such as He, Ar, or N2 can be used to place constraints on the thermal environment under which the comet was formed and has been processed.