Daniel P. Whitmire

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.

Nuclear waste spectrum as evidence of technological extraterrestrial civilizations

Abstract We consider the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a relatively small fraction of the nuclear resources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geological time periods provided the star has a sufficiently shallow outer convective zone. Consideration of surface convective mixing and stellar lifetimes restricts the possible candidate stars to the approximate spectra range A5-F2. The abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 are presented and it is argued that these anomalous distributions are unlikely to be duplicated by natural nucleosynthesis processes. Relative to solar system abundances, the elements praseodymium and neodymium are found to be the most overabundant. These elements, along with the radioactive elements technetium and plutonium, could be used to identify A5-F2 candidate stars in a preliminary spectral survey.

Relativistic spaceflight and the catalytic nuclear ramjet

Abstract The interstellar ramjet concept allows us, at least hypothetically, to get around the mass-ratio limit inherent with relativistic fuel-carrying rockets. The required ramjet energy must be extracted from the nuclear reserves of the interstellar hydrogen (interstellar deuterium is too scarce). Unfortunately, the usual PPI chain for converting hydrogen to helium is hopelessly slow due to the minute weak interaction cross section of the reaction p + p → 2 D + e + + v at essentially any energy. It is shown here that this problem can be avoided, in principle, by exploiting a proton burning catalytic cycle. The best known catalytic cycle is the CNO Bi-Cycle occurring in sufficiently hot main sequence stars. The catalyst “fuel” may be carried on board the ship since it is not depleted, and the ultimate source of energy is the interstellar hydrogen. It is shown that with potentially realistic parameters the energy requirements for a ramjet accelerating at 1 g to near luminal velocities can be met with either the “hot” CNO Bi-Cycle or the Ne-Na cycle. Some of the formidable technological problems associated with a conventional heavy ion fusion reactor are briefly discussed. It is not absolutely necessary that a conventional plasma reactor model be used; for example a laser fusion reactor or a large scale Migma-type reactor (if technologically possible) might be employed. The problem of interstellar drag is considered and a model is suggested in which the incident protons kinetic energy is stored in the electric field between charged grids and returned to the exhaust particles after the relevant nuclear reactions are completed. Such a combination of electric and magnetic fields would make less severe other suggested limitations to the ramjets performance. The problem of ionizing the interstellar medium directly in front of the ramjet when operating in non-HII regions of space is also discussed. It is shown that laser ionization is energetically possible for interstellar number densities ≳10 cm −3 ; however, since the laser also tends to sweep the interstellar matter out of the ramjets path, more quantitative study is necessary before the method can be considered feasible at any density. The possibility of using a thin stripper foil, placed over the intake cross section, to ionize the interstellar medium is suggested. This method has the advantage of efficiently ionizing the interstellar matter independent of density.

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.

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

Habitable zones for earth-like planets around main sequence stars

As stars evolve and brighten, the radial zone within which liquid water can exist at the surface of an Earth-like planet expands outward. Using a new planetary climate model we have calculated the evolution of these habitable zones around several main sequence stars of masses between 0.50 and 1.25 solar masses. This evolution is presented in the form of a habitability continuum diagram for each star. We also give results for post main sequence evolution of the habitable zone around a 1.0 M⋅ star. The inner radius of the habitable zone is determined by the moist greenhouse effect and the outer radius is taken to be the distance at which a dense CO2 atmosphere begins to condense. Preliminary results indicate that the range of planetary radii for which liquid water can exist for > 4.5 Byr is considerably broader than previously calculated.

The distribution of stars most likely to harbor intelligent life.

Simple heuristic models and recent numerical simulations show that the probability of habitable planet formation increases with stellar mass. We combine those results with the distribution of main-sequence stellar masses to obtain the distribution of stars most likely to possess habitable planets as a function of stellar lifetime. We then impose the self-selection condition that intelligent observers can only find themselves around a star with a lifetime greater than the time required for that observer to have evolved, T(i). This allows us to obtain the stellar timescale number distribution for a given value of T(i). Our results show that for habitable planets with a civilization that evolved at time T(i) = 4.5 Gyr the median stellar lifetime is 13 Gyr, corresponding approximately to a stellar type of G5, with two-thirds of the stars having lifetimes between 7 and 30 Gyr, corresponding approximately to spectral types G0-K5. For other values of T(i) the median stellar lifetime changes by less than 50%.