Jia-Qing Zheng

NATURAL TRANSFER OF VIABLE MICROBES IN SPACE FROM PLANETS IN EXTRA-SOLAR SYSTEMS TO A PLANET IN OUR SOLAR SYSTEM AND VICE VERSA

We investigate whether it is possible that viable microbes could have been transported to the Earth from planets in extra-solar systems by means of natural vehicles such as ejecta expelled by comets or asteroid impacts on such planets. The probabilities of close encounters with other solar systems are taken into account as well as the limitations of bacterial survival times inside ejecta in space, caused by radiation and DNA decay. The conclusion is that no potentially DNA/RNA life-carrying ejecta from another solar system in the general Galactic star field landed on the Earth before life already existed on the Earth, even if the microbial survival time in space is as long as tens of millions of years. However, if the Sun formed initially as a part of a star cluster, as is commonly assumed, we cannot rule out the possibility of transfer of life from one of the sister systems to us. Likewise, there is a possibility that some extra-solar planets carry life that originated in our solar system. It will be of great interest to identify the members of the Suns birth cluster of stars and study them for evidence of planets and life on the planets. The former step may be accomplished by the GAIA mission, the latter step by the SIM and DARWIN missions. Therefore it may not be too long until we have experimental knowledge to answer the question of whether the natural transfer of life from one solar system to another has actually taken place.

Risks threatening viable transfer of microbes between bodies in our solar system

Abstract A fraction of the number of ejecta expelled from a planet by comet or asteroid impacts end up landing on another planet. If microorganisms were living in the ground before impact, they would be transported inside ejecta to the target planet. During that perilous trip, they would be subject to four main categories of threat to their survival: dynamical stress, excess temperature, radiation, chemical attack and vacuum. The effect of these, in the form of survival fractions as a function of time, as well as approximate numbers of arriving ejecta with viable flight times, have been investigated in a quantitative study we have made. The result shows that viable transfer from Mars to Earth and vice versa was highly probable during the first 0.5 Ga, and also probable, but with lower frequency, thereafter. Here we follow up with considerations about the consequences of the result regarding the question of whether the ancestor cell of all life on Earth must have originated on Earth, or whether it could have originated on Mars, its descendants thereafter moving to Earth. Some other possible consequences are also discussed.

Computer simulations of interacting galaxies in compact groups and the observed properties of triple galaxies

Triplets of galaxies offer a good opportunity to measure the amount of dark matter in systems of galaxies. We use the extensive data set on triplets by Karachentsev et al. [Isv. Spec. Astrophys. Obs., 27, 67 (1989)], and try to model it by using computer simulations of interacting galaxies. Our simulated interactions include dynamical friction and mergers in addition to the usual, softened Newtonian potential. We start with bound five galaxy systems of various kinds. When only three galaxies remain after mergers, the systems are studied at many different projection angles in order to simulate the observed triplets. In all, nearly 10 000 five galaxy systems were calculated, and each was viewed from 20 random angles in order to derive statistical properties of the simulated triplets

Apsidal corotation in mean motion resonance: the 55 Cancri system as an example

The inner two planets around 55 Cancri were found to be trapped in a 3 : 1 mean motion resonance (MMR). In this paper, we study the dynamics of this extrasolar planetary system. Our numerical investigation confirms the existence of the 3 : 1 resonance and implies a complex orbital motion. Different stable motion types, with and without apsidal corotation, are found. Owing to the high eccentricities in this system, we apply a semi-analytical method based on a new expansion of the Hamiltonian of the planar three-body problem in the discussion. We analyse the occurrence of apsidal corotation in this MMR and its influence on the stability of the system.

Capture of comets during the evolution of a star cluster and the origin of the Oort Cloud

It is generally assumed that the Solar System is surrounded by a swarm of comets, the so-called Oort Cloud, which contains approximately 1011 members. The observed comets belong to a small subsection of the Cloud, and they have very elongated orbits. The origin of the Cloud is presently unclear. Here we consider the possibility that the comets were born in a star cluster together with the Sun. We follow the evolution of the star cluster with its embedded swarm of comets and calculate the rate at which stars accumulate stable comet companions. We conclude that if the Oort Cloud of comets was born in this process, then the present day density of comets in interstellar space has to be high, and that comets make a significant contribution to the overall mass density of the Galaxy.

Orbits of Short Period Comets Captured from the Oort Cloud

Oort cloud comets occasionally obtain orbits which take them through the planetary region. The perturbations by the planets are likely to change the orbit of the comet. We model this process by using a Monte Carlo method and cross sections for orbital changes, i.e. changes in energy, inclination and perihelion distance, in a single planet-comet encounter. The influence of all major planets is considered. We study the distributions of orbital parameters of observable comets, i.e. those which have perihelion distance smaller than a given value. We find that enough comets are captured from the Oort cloud in order to explain the present populations of short period comets. The median value of cos i for the Jupiter family is 0.985 while it is 0.27 for the Halley types. The results may explain the orbital features of short period comets, assuming that the active lifetime of a comet is not much greater than 400 orbital revolutions.

Origin Of Oort Cloud Comets In The Interstellar Space

Comets must form a major part of the interstellar medium. The solar system provides a flux of comets into the interstellar space and there is no reason to suspect that many other stars and their surrounding cometary systems would not make a similar contribution. Occasionally interstellar comets must pass through the inner solar system, but Whipple (1975) considers it unlikely that such a comet is among the known cases of apparently hyperbolic comets. Even so the upper limit for the density of unobserved interstellar comets is relatively high.

Improved Models for the Evolution of the Coma Cluster of Galaxies

The analysis by Fitchett & Webster of the observations of the Coma cluster of galaxies has demonstrated that the center of the Coma Cluster consists of two subclusters. Therefore, it is important to construct realistic dynamical models of a galaxy cluster with two mass centers. Our previous N-body models for the Coma Cluster consisted of point masses or particles with simple interaction properties. In the current paper, we employ a more sophisticated N-body code, which includes dynamical friction, mass exchange, and mergers between galaxies. Our starting point is a model where the two subclusters form a binary system. The rest of the cluster galaxies are in nearly radial, bound orbits around the center of mass of the binary. The initial galaxy densities and velocities are chosen according to a particular cosmological model. At the end of the N-body simulation of 250 galaxies, we extract the projected galaxy surface density and radial velocity dispersion profiles as a function of the distance from the center of the mass of the cluster. With certain initial parameters, excellent agreement with observations is obtained. In such models, the use of the virial theorem in the standard way gives an overestimate of the cluster mass by a factor of about 3. Therefore, the true mass of the Coma Cluster should be smaller than the usually quoted value by the same factor. The mass-to-light ratio of the Coma Cluster should be about 100 in solar units, in agreement with the analysis of the X-ray data by Cowie et al.

Monte Carlo Simulations of Oort Cloud Comets: the Influence of Mantle Blow-off on the Inclination Distribution of Jupiter Family

We have developed an efficient Monte Carlo method by which we can evaluate the evolution of comets. There are many poorly known evolutional parameters, and we have investigated the influence of these parameters on the final populations and the inclination distributions of short-period comets. We compare the observed and calculated inclination distributions of different comet populations and obtain a good fit for the inclinations of the Jupiter family comets by assuming a mantle blow-off and a sudden brightening of the comet when its perihelion distance is lowered in a major jump.

Dynamics and Merging in Compact Galaxy Groups

By using a few body code with a frictional force we study the dynamics of small galaxy groups. The results have been compared with observational data on triples and binaries by Karachentsev et al. We start with four to five galaxy groups initially and take samples of the groups when the membership has dropped to two or three. It is possible to find merger evolution models which agree with observations. One interesting factor is the mass-to-light ratio of the observed systems: we find the best fit with the models when the mass-to-light ratio is about 50 in solar units.