This dissertation is an exploration of the effects of the collision of large asteroids and comets on the atmosphere of the Earth and Earthlike planets. The first task undertaken is the characterization of the impact rates in the inner solar system during the present time, and during the first billion years of Solar System history when the flux was changing rapidly. Once defined, these fluxes are used to model the long term cumulative effect of multiple impacts on planetary atmospheres. The implications of cometary impacts on evolution of the water and deuterium abundances on Venus are examined. The short lifetime of water on Venus suggests that the water abundance is in a quasi-steady-state balance between loss by escape and replenishment by infall. In addition, the observed deuterium-to-hydrogen ratio on Venus is consistent with a steady state and does not necessarily imply a past water excess. Results are presented of a model incorporating a stochastic cometary source and nonthermal escape of hydrogen that produces the observed water abundance and D/H ratio. The stochastic variability of each of these quantities is shown to be large. Water on Venus is likely to be in a near steady state mediated by large comet impacts. The early history of water on the planet has been obscured by a history of random impacts. A study of the effects of impact-generated dust clouds on the primitive Earth leads to the conclusion that such clouds were significant perturbers of the early climate. The Earth was shrouded by an optically-thick dust cloud for ≈ 150-250 m.y. During this time the surface temperature was equal to the planetary equilibrium temperature unless significant heating by impacts or surface heat flow existed beneath the dust cloud. An admixture of a few per cent of organic materials in the cloud may have significantly lowered the planetary bond albedo, thereby raising the equilibrium temperature. The epoch of continuous dust shrouding was followed by a period of stochastically intermittent dust clouds occuring at greater intervals as the early intense bombardment subsided towards the present day flux.

The noble gas abundances and isotopic ratios of the terrestrial planets differ from each other and from the average of chondritic meteorites. These different abundance patterns result from primordial heterogeneities or different degassing histories. Magmatic transport is the only degassing mechanism that can be demonstrated to occur on Venus, Earth, and Mars, and is presently the dominant form of volatile transport to a planet's free surface. An alternative technique was developed to determine the partitioning and solubility of noble gases in mineral/melt systems. Natural end member minerals and synthetic melts known to be in equilibrium were held in separate crucibles in a one bar flowing noble gas atmosphere. Experiments were run 7-18 days at 1300 or 1332°C, in 99.95% Ar or a Ne-Ar-Kr-Xe mix. Gas concentrations were measured by mass spectrometry. The solubility of noble gases in minerals was surprisingly high, and individual samples of a particular mineral composition are distinct in their behavior. The data is consistent with lattice vacancy defect siting. Noble gas solubility in the minerals increases with increasing atomic number; this may be related to polarizability. Noble gas solubilities in melts decrease with increasing atomic number. Solubility is directly proportional to melt molar volume; values overlap the lower end of the range defined for natural basalts. The lower solubilities are related to the higher MgO and CaO concentrations and lower degree of polymerization and Fe³⁺ concentration in synthetic vs. natural melts. Partition coefficient patterns show a clear trend of increasing compatibility with increasing noble gas atomic number, but many individual values are > 1. Calculations show that the terrestrial planet atmospheres cannot have formed from partial melting of a common chondritic source. When results are examined with isotopic constraints and MOR and hot-spot activities, there is no compelling evidence that the Earth is substantially outgassed of its primordial or even its radiogenic volatiles. If volcanic degassing was mostly responsible for the atmospheres, then initial volatile abundances were Mars < Earth < Venus. Alternatively, roughly equal abundances could have been modified by catastrophic processes.

The scattered radiation fields in spherical planetary atmospheres have been considered by a new method which is called the Quasi-Spherical method. This method is applicable to planets with radii which are much larger than the height of their atmosheres. The scattering of 0.5 (mu)m radiation in a conservative and vertically inhomogeneous atmosphere has been discussed. Results comparing the emerging radiation from plane-parallel and spherical models for the earth's atmosphere have been presented for four different aerosol distributions in addition to the normal molecular composition. These results indicate measurable differences on the order of 10 to 300% as the angle of observation and/or the angle of incident sun falls within 10(DEGREES) from the horizon. Also, the obtained results in the spherical atmosphere show that additional layers of aerosols in either the stratosphere or the troposphere can be detected by satellite or aircraft radiometric measurements, while the plane-parallel atmosphere does not permit such a detection. The accuracy of the obtained results by the present method can be increased by increasing the number of spherical shells in the spherical atmosphere. The emerging radiation in homogeneous and inhomogeneous Rayleigh atmospheres as computed by the Quasi-Spherical and the Monte Carlo methods compare quite well. By applying the divergence theorem it was shown by the present method that the total flux in the spherical atmosphere is conserved within 1.351%.

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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1980. / Microfiche copy available in Archives and Science. / Bibliography: leaves 77-80. / by Dushan Stephen Bukvic. / M.S.

Since the discovery of 51 Pegasi b in 1995, continued observations have indirectly identified over 110 planetary objects. These Jupiter-mass objects cause their host star to "wobble" slightly about the common centre-of-mass of the system, which is detectable as radial motion in high-precision Doppler spectroscopy. Of the known planets, approximately 20% are found to orbit within 0.1 AU of the star, whilst the transit of HD209458 has inferred the gas-giant nature of these close-in extrasolar giant planets (CEGPs). The discovery of CEGPs has produced a wave of speculative theory as to the exact nature of these objects, and how they came to exist so close to their parent star. Our spectroscopic technique provides a method of achieving the direct detection of a CEGP atmosphere, the results of which will allow us to test emerging models that aim to predict the atmospheric nature of CEGPs and may provide additional information on the orbital inclination and mass of the planet. We start with a historical review of the field of extrasolar planets, followed by an introduction to the fundamental concepts which underpin the reflection of starlight from a planet's surface. We then investigate the prospects of detecting such a reflection, before detailing the technique we have devised and applied here to two known CEGP hosts. In the first instance, r Bootis, we combined observations at the 4.2-m William Herschel telescope in 1998, 1999 and 2000. The dataset comprised 893 high-resolution échelle spectra with a total integration time of 75hr 32min spanning 17 nights. We establish an upper limit on the planet's geometric albedo p < 0.39 (at the 99.9% significance level) at the most probable orbital inclination i ~ 36 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp - 1.2RJup. Although a weak candidate signal appears near to the most probable radial velocity amplitude, its statistical significance is insufficient for us to claim a detection with any confidence. In the second instance, HD75289, 4 nights of VLT(UT2)/UVES observations were secured in 2003 Jan, yielding 684 high-resolution spectra with a total integration time of 26 hours. We establish an upper limit on the planet's geometric albedo p < 0.12 (to the 99.9% significance level) at the most probable orbital inclination i ~/= 60 deg, assuming a grey albedo, a Venus-like phase function and a planetary radius Rp = 1.6RJup. In both cases, we are able to rule out some combinations of the predicted planetary radius and atmospheric albedo models with high, reflective cloud decks.

This thesis presents algorithms for performing the next two reduction steps, namely orthogonalization and extraction. More specifically, this thesis addresses the following research question: What are proper methods of orthogonalizing spectral images in preparation for extraction?

Over two hundred extrasolar planets have been discovered to date with various methods. This thesis reports on searching for extrasolar planets and characterising them by simulating their atmospheres. We used open clusters as targets for deep transit searches, with specific emphasis on the University of St. Andrews Planet Search at the Isaac Newton Telescope. We reduced CCD photometry and described the algorithm we used to search for transits. We estimated the number of transits we expect from our data. We then reduced photometry for the open cluster NGC 6940. From that data we found 18 low-amplitude, short-duration events, though none are transiting planets. They are all eclipsing binary stars. However, our null result constrains the number of planets around M dwarfs, the most numerous stars in our sample. In order to characterise reflected light from extrasolar planets, we built a three-dimensional Monte Carlo based radiation transfer model of extrasolar planetary atmospheres. We detailed the input parameters of the model, and show results of various models, focusing especially on the fractal nature of the clouds of our models, because these are the first three dimensional radiation transfer models of extrasolar planet atmospheres. We found very low geometric albedos in our simulations. Using data specific to the transiting planet HD 209458b, we built a model atmosphere with Rayleigh-scattering hydrogen gas and clouds of enstatite and iron. We show in several models the rarity of a bright HD 209458b, and conclude with some explanations on why extrasolar planets are likely dark and not detected with reflected light.