John J. Matese

The galactic disk tidal field and the nonrandom distribution of observed Oort cloud comets

Abstract We consider the interaction of the galaxy with the Oort cloud of comets. If the perturbation is limited to the secularly averaged first-order disk tidal term, analytic solutions for the time evolution of orbital elements are possible. In turn, Monte Carlo techniques then become a practical means of estimating the distributions of orbital elements for those comets which become observable. We demonstrate that the detected nonrandomness in the distribution in galactic latitudes of perihelia is a real phenomenon caused by the tidal perturbation and is not the result of observational selection effects. Results on the refilling of the liss cylinder are also presented.

A model of the galactic tidal interaction with the Oort comet cloud

We consider a model of the in situ Oort cloud which is isotropic with a random distrihution of perihelia directions and angular momenta. The energy distribution adopted has a continuous range of values appropriate for long-period (>200 yr) comets. Only the tidal torque of the Galaxy is included as a perturbation of comet orbits and it is approximated to be that due to a quasi-steady state distribution of matter with disk-like symmetry. The time evolution of all orbital elements can be analytically obtained for this case. In particular, the change in the perihelion distance per orbit and its dependence on other orbital elements is readily found. We further make the assumption that a comet whose perihelion distance was beyond 15 AU during its last passage through the Solar System would have orbit parameters that are essentially unchanged by planetary perturbations. Conversely, if the prior passage was inside 15 AU we assume that planetary perturbations would have removed the comet from the in situ energy distribution accessible by the galactic tide. Comets which had their perihelia changed from beyond 15 AU to within 5 AU in a single orbit are taken to be observable. We are able to track the evolution of 106 comets as they are made observable by the galactic tidal touque. Detailed results are obtained for the predicted distribution of new (0 < 1/α < 10−4 AU−1) comets. Further, correlations between orbital elements can be studied. We present predictions of observed distributions and compare them with the random in situ results as well as with the actual observed distributions of class I comets. The predictions are in reasonable agreement with actual observations and, in many cases, are significantly different from random when perihelia directions are separated into galactic northern and southern hemispheres. However the well-known asymmetry in the north-south populations of perihelia remains to be explained. Such an asymmetry is consistent with the dominance of tidal torques today if a major stochastic event produced it in the past since tidal torques are unable to cause the migration of perihelia across the latitude barriers ±26°.6 in the disk model.

A slightly more massive young Sun as an explanation for warm temperatures on early Mars

The valley network channels on the heavily cratered ancient surface of Mars suggest the presence of liquid water approximately 3.8 Gyr ago. However, the implied warm climate is difficult to explain in the context of the standard solar model, even allowing for the maximum CO2 greenhouse heating. In this paper we investigate the astronomical and planetary implications of a nonstandard solar model in which the zero-age, main-sequence Sun had a mass of 1.05 +/- 0.02 M solar. The excess mass was subsequently lost in a solar wind during the first 1.2(-0.2, +0.4) Gyr of the Suns main sequence phase. The implied mass-loss rate of 4(+3, -2) x 10(-11) M solar yr-1, or about 10(3)x that of the current Sun, may be detectable in several nearby young solar type stars.

Tidal Imprint of Distant Galactic Matter on the Oort Comet Cloud

We report the detection of a remarkably strong nonrandom signal in the distribution of perihelia vectors of Oort cloud comets. The strongest signal is in the Galactic longitudes of perihelia, and we show that it is most likely caused by the adiabatic perturbation of the Galactic radial tide. The probability that the signal in longitude is because of chance is found to be 2 × 10-6. There is also evidence of the adiabatic perturbation of the Galactic tide component which is in a direction normal to the midplane. This is found in the distribution of Galactic latitudes of perihelia but is statistically less significant, a somewhat counterintuitive result since the Galactic z tide has a strength ≈ 16 times that of the Galactic radial tide. We find that ≈ 1/3 of observed Oort cloud comets are attributable to the Galactic radial tide. The source of the Galactic z tide is primarily local disk matter, while the source of the Galactic radial tide is the entire distribution of matter interior to the solar orbit. Consequently, we conclude that distant matter in the Galaxy, down to the Galactic core some 1.6 × 109 AU away, is an important factor in making Oort cloud comets observable.

Planet X and the origins of the shower and steady state flux of short-period comets

Abstract In the planet X model periodic comet showers are associated with passages of the planets perihelion and aphelion points through a primordial disk of comets believed to lie beyond the orbit of Neptune. A strong feature of this model is that the required orbital elements and mass of planet X are consistent with independently predicted values based on the residuals in the motions of Uranus and Neptune. Here we present a more extensive analysis of the model taking into account the fact that only those comets scattered directly into the zones of influence of Saturn and Jupiter can contribute to a shower whose duration is consistent with observation (≲ 15 myr). These requirements impose a minimum planetary inclination of ≈25°, which in turn restricts the semimajor axis to be ≲100 AU. A fraction of the comets scattered directly into the zones of influence of Uranus and Neptune will evolve on time scales of ∼108 years into the steady state flux of short-period comets. We find that the absolute numbers of shower and steady state are comparable and compatible with the known terrestrial cratering rate, assuming the existence of long-lived extinct comet cores. Canonical planet X model parameters, deduced in part from the scattering dynamics analysis, are: semimajor axis ≈80 AU, eccentricity ≈0.3, inclination ≈45°, and mass ≈5m⊕. An analysis is given which suggests that planet X, in its present orbit, can create the requisite density gradient of comets near perihelion and aphelion during the lifetime of the Solar System. The required inclination of planet Xs orbit (≳25°) may explain the failure of previous surveys to discover the planet as its present latitude is not likely to be near the ecliptic. It it exists, the best immediate hope of finding planet X is the ongoing IRAS search in the 100-μm band and the full sky optical survey by Shoemaker and Shoemaker. Independent of the question of periodic comet showers, the existence of planet X and the comet disk can readily explain the origin of the steady state flux of short-period comets over a wide range of parameters.

The Pioneer 10 anomalous acceleration and Oort cloud comets

Anderson et al. [Phys. Rev. D 65 (2002) 082004] recently reported new evidence that both Pioneer 10 and 11 are experiencing nearly the same unmodeled anomalous acceleration directed toward the Sun. Numerous mechanisms, both internal and external to the spacecraft, have been proposed to explain this unmodeled acceleration. If we assume that the cause of the anomalous acceleration is (1) external to the spacecraft, (2) isotropic, and (3) acts on bodies of cometary mass, then it would imply that new comets are more tightly bound to the Sun than previously believed. Here we show that the implied higher binding energy is incompatible with the established evidence that the galactic tide is dominant in making Oort cloud comets observable. We conclude that one or more of these assumptions must be invalid.

OORT CLOUD COMET PERIHELION ASYMMETRIES: GALACTIC TIDE, SHOWER OR OBSERVATIONAL BIAS?

We investigate the distribution of Oort cloud comet perihelia. The data considered includes comets having orbital elements of the two highest quality classes with original energies designated as new or young. Perihelion directions are determined in galactic, ecliptic and geocentric equatorial coordinates. Asymmetries are detected in the scatter and are studied statistically for evidence of adiabatic galactic tidal dynamics, an impulse-induced shower and observational bias. The only bias detected is the well-known deficiency of observations with perihelion distances q > 2.5 AU. There is no significant evidence of a seasonal dependence. Nor is there a substantive hemispherical bias in either ecliptic or equatorial coordinates. There is evidence for a weak stellar shower previously detected by Biermann which accounts for ≈ 10% of the total observations. Both the q bias and the Biermann star track serve to weaken the evidence for a galactic tidal imprint. Nevertheless, statistically significant asymmetries in galactic latitude and longitude of perihelia remain. A latitude asymmetry is produced by a dominant tidal component perpendicular to the galactic disk. The longitude signal implies that ≈ 20% of new comets need an additional dynamical mechanism. Known disk non-uniformities and an hypothetical bound perturber are discussed as potential explanations. We conclude that the detected dynamical signature of the galactic tide is real and is not an artifact of observational bias, impulsive showers or poor data.

Near-Earth populations of bodies coming from the Oort cloud and their impacts with planets

When Oort cloud comets enter the planetary region their orbital evolution is dominated by encounters with the planets. Some of them become short period comets and enter the terrestrial planetary region to form a potential source of cratering. We have computed the orbital evolution of comets by encounters with the seven major planets. We find 0.5–2 impacts/Myr on the Earth, somewhat lower than the observed rate of about 2.8 impacts per Myr causing craters ≥ 20 km in diameter. Thus as far as numbers go, it is quite possible that dead comets are a major, even if not the dominant source of cratering on the Earth. We have also tested how well the expected variations in the Oort cloud comet flux show up in the rate of impacts. We find that the periodicity is reflected also in the cratering rate, though with a time delay and with added noise.

Velocity streaming of IRAS main-sequence disk stars and the episodic enhancement of particulate disks by interstellar clouds

We have discovered evidence of velocity streaming in a set of nearby main-sequence A-type IRAS disk stars. This conservatively chosen set of the five most significant particulate-disk stars consists of β Pic, α Lyr, α Psa, β Leo, and ζ Lep. Monte Carlo simulations were used to compare the velocity dispersion of this set to the dispersions of sets chosen at random from the remaining 17 main-sequence A stars listed in the Gliese catalog (<22 pc). It was found that dispersion velocities less than those of the particulate-disk star set occurred by chance in only 2% of the cases. Small dispersion velocities are normally indicative of streaming clusters or young field stars. However, the velocity dispersion of the set of particulate-disk stars is inconsistent with either interpretation

Variable Oort Cloud Flux due to the Galactic Tide

We review the subject of the time dependence of the component of Oort cloud comet flux due to the adiabatic Galactic tide, including the possibility of detecting such a signal in the terrestrial cratering record.