Astronomical Engineering: A Strategy For Modifying Planetary Orbits

The Sun's gradual brightening will seriously compromise the Earth'sbiosphere within 10⁹ years. If Earth's orbit migrates outward,however, the biosphere could remain intact over the entiremain-sequence lifetime of the Sun. In this paper, we explore thefeasibility of engineering such a migration over a long timeperiod. The basic mechanism uses gravitational assists to (in effect)transfer orbital energy from Jupiter to the Earth, and therebyenlarges the orbital radius of Earth. This transfer is accomplishedby a suitable intermediate body, either a Kuiper Belt object or a mainbelt asteroid. The object first encounters Earth during an inward passon its initial highly elliptical orbit of large ( 300 AU)semimajor axis. The encounter transfers energy from the object to theEarth in standard gravity-assist fashion by passing close to theleading limb of the planet. The resulting outbound trajectory of theobject must cross the orbit of Jupiter; with proper timing, theoutbound object encounters Jupiter and picks up the energy it lost toEarth. With small corrections to the trajectory, or additionalplanetary encounters (e.g., with Saturn), the object can repeat thisprocess over many encounters. To maintain its present flux of solarenergy, the Earth must experience roughly one encounter every 6000years (for an object mass of 10²² g). We develop the details ofthis scheme and discuss its ramifications.     

Run-up from impact tsunami

We report on calculations of the on-shore run-up of waves that might be generated by the impact of subkilometre asteroids into the deep ocean. The calculations were done with the COULWAVE code, which models the propagation and shore-interaction of non-linear moderate- to long-wavelength waves  ( kh < π)  using the extended Boussinesq approximation. We carried out run-up calculations for several different situations: (1) laboratory-scale monochromatic wave trains onto simple slopes; (2) 10–100 m monochromatic wave trains onto simple slopes; (3) 10–100 m monochromatic wave trains onto a compound slope representing a typical bathymetric profile of the Pacific coast of North America; (4) time-variable scaled trains generated by the collapse of an impact cavity in deep water onto simple slopes and (5) full-amplitude trains onto the Pacific coast profile. For the last case, we also investigated the effects of bottom friction on the run-up. For all cases, we compare our results with the so-called 'Irribaren scaling': The relative run-up   R / H ₀=ξ= s ( H ₀/ L ₀)^−1/2  , where the run-up is   R , H ₀  is the deep-water waveheight, L ₀ is the deep-water wavelength, s is the slope and ξ is a dimensionless quantity known as the Irribaren number. Our results suggest that Irribaren scaling breaks down for shallow slopes   s ≤ 0.01  when  ξ < 0.1 − 0.2  , below which   R / H ₀  is approximately constant. This regime corresponds to steep waves and very shallow slopes, which are the most relevant for impact tsunami, but also the most difficult to access experimentally.     

Astrophysical applications of a boundary integral method for fluid dynamics

A boundary integral formulation for the dynamics of incompressible, inviscid, self-gravitating bodies is described. The method is applied to several problems of astrophysical interest: spheroidal equilibria (Maclaurin and Jeans), oscillations, and a simple version of tidal encounter and breakup.     

Impact evolution of asteroid shapes: 1. random mass redistribution

We explore whether the cumulative effect of small-scale meteoroid bombardment can drive asteroids into nonaxisymmetric shapes comparable to those of known objects (elongated prolate forms, twin-lobed binaries, etc). We simulate impact cratering as an excavation followed by the launch, orbit, and reimpact of ejecta. Orbits are determined by the gravity and rotation of the evolving asteroid, whose shape and spin change as cratering occurs repeatedly. For simplicity we consider an end-member evolution where impactors are all much smaller than the asteroid and where all ejecta remain bound. Given those assumptions, we find that cumulative small impacts on rotating asteroids lead to oblate shapes, irrespective of the chosen value for angle of repose or for initial angular momentum. The more rapidly a body is spinning, the more flattened the outcome, but oblateness prevails. Most actual asteroids, by contrast, appear spherical to prolate. We also evaluate the timescale for reshaping by small impacts and compare it to the timescale for catastrophic disruption. For all but the steepest size distributions of impactors, reshaping from small impacts takes more than an order of magnitude longer than catastrophic disruption. We conclude that small-scale cratering is probably not dominant in shaping asteroids, unless our assumptions are naive. We believe we have ruled out the end-member scenario; future modeling shall include angular momentum evolution from impacts, mass loss in the strength regime, and craters with diameters up to the disruption threshold. The ultimate goal is to find out how asteroids get their shapes and spins and whether tidal encounters in fact play a dominant role.     

High-resolution simulations of the impacts of asteroids into the venusian atmosphere III: further 3D models

We report on high-resolution three-dimensional calculations of oblique impacts into planetary atmospheres, specifically the atmosphere of Venus, extending the results of Korycansky et al. (2000, Icarus 146, 387-403; 2002, Icarus 157, 1-23). We have made calculations for impacts at 0°, 45°, and 60° from the vertical, different impactor velocities (10, 20, and 40 km s⁻¹), and different impactor masses and orientations. We present results for porous impactors using a simple model of porosity. We have investigated the sensitivity to initial conditions of the calculations [as a follow-up to the results found in Korycansky et al. (2002)] and resolution effects. For use in cratering calculations, we fit simple functions to the numerical results for mass and momentum that penetrate to a given altitude (column mass) and investigate the behavior of the fit coefficients as functions of impactor parameters such as mass, velocity, and impact angle. Generally speaking, the mass and momentum (and hence resulting crater diameters) depend primarily on impactor mass and mass of atmosphere encountered and weakly or not at all on other parameters such as impactor velocity, impact angle, or porosity. The column mass to which the last portion of the impactor penetrates is approximately equal to the mass of impactor at the top of the atmosphere before the impact takes place. Finally, we present the beginnings of a simplified but physically based model for the impactor and its fragments to reproduce the mass and momentum fluxes as a function of height during the impact.     

Modeling crater populations on Venus and Titan

We describe a model for crater populations on planets and satellites with dense atmospheres, like those of Venus and Titan. The model takes into account ablation (or mass shedding), pancaking, and fragmentation. Fragmentation is assumed to occur due to the hydrodynamic instabilities promoted by the impactors’ deceleration in the atmosphere. Fragments that survive to hit the ground make craters or groups thereof. Crater sizes are estimated using standard laws in the gravity regime, modified to take into account impactor disruption. We use Monte Carlo methods to pick parameters from appropriate distributions of impactor mass, zenith angle, and velocity. Good fits to the Venus crater populations (including multiple crater fields) can be found with reasonable values of model parameters. An important aspect of the model is that it reproduces the dearth of small craters on Venus: this is due to a cutoff on crater formation we impose, when the expected crater would be smaller than the (dispersed) object that would make it. Hydrodynamic effects alone (ablation, pancaking, fragmentation) due to the passage of impactors through the atmosphere are insufficient to explain the lack of small craters. In our favored model, the observed number of craters (940) is produced by ∼5500 impactors with masses , yielding an age of (1-σ uncertainty) for the venusian surface. This figure does not take into account any uncertainties in crater scaling and impactor population characteristics, which probably increase the uncertainty to a factor of two in age.We apply the model with the same parameter values to Titan to predict crater populations under differing assumptions of impactor populations that reflect present conditions. We assume that the impactors (comets) are made of 50% porous ice. Predicted crater production rates are ≈190 craters . The smallest craters on Titan are predicted to be in diameter, and ≈5 crater fields are expected. If the impactors are composed of solid ice (density ), crater production rates increase by ≈70% and the smallest crater is predicted to be in diameter. We give cratering rates for denser comets and atmospheres 0.1 and 10 times as thick as Titan's current atmosphere. We also explicitly address leading-trailing hemisphere asymmetries that might be seen if Titan's rotation rate were strictly synchronous over astronomical timescales: if that is the case, the ratio of crater production on the leading hemisphere to that on the trailing hemisphere is ≈4:1.     

Atmospheric impacts, fragmentation, and small craters on Venus

We use high-resolution three-dimensional numerical models of aerodynamically disrupted asteroids to predict the characteristic properties of small impact craters on Venus. We map the mass and kinetic energy of the impactor passing though a plane near the surface for each simulation, and find that the typical result is that mass and energy sort themselves into one to several strongly peaked regions, which we interpret as more-or-less discrete fragments. The fragments are sufficiently well separated as to imply the formation of irregular or multiple craters that are quite similar to those found on Venus. We estimate the diameters of the resulting craters using a scaling law derived from the experiments of Schultz and Gault (1985, J. Geophys. Res. 90 (B5), 3701-3732) of dispersed impactors into targets. We compare the spacings and sizes of our estimated craters with measured diameters tabulated in a Venus crater database (Herrick and Phillips, 1994a, Icarus 111, 387-416; Herrick et al., 1997, in: Venus II, Univ. of Arizona Press, Tucson, AZ, pp. 1015-1046; Herrick, 2003, http://www.lpi.usra.edu/research/vc/vchome.html) and find quite satisfactory agreement, despite the uncertainty in our crater diameter estimates. The comparison of the observed crater characteristics with the numerical results is an after-the-fact test of our model, namely the fluid-dynamical treatment of large impacts, which the model appears to pass successfully.     

北大别黄土岭麻粒岩锆石U—Pb离子探针定年

阴极发光显微结构分析表明 ,北大别黄土岭麻粒岩中存在三种类型的锆石 :原岩锆石、麻粒岩相锆石和残留锆石。对它们分别进行离子探针定年 ,得到黄土岭麻粒岩的原岩年龄约为 2 70 0 Ma、麻粒岩相变质事件的年龄为 2 0 5 2± 10 0 Ma、残留锆石的年龄为约 3.4 Ga。以上研究表明黄土岭麻粒岩为残存的扬子板块的结晶基底 ,而约 3.4 Ga残留锆石的发现 ,则首次证明大别山地区存在古太古代的陆壳物质     

Ocean robotics in support of fisheries research and management

South Africa’s small-pelagic fishery is a socio-economically important component of the country’s commercial fisheries sector, second in value only to the demersal trawl fishery. Management of this sector relies on infrequent hydro-acoustic surveys, which provide measures of anchovy Engraulis encrasicolus and sardine Sardinops sagax biomass used in the assessments of stock status and in the development of management plans for the sustainable utilisation of these resources. We demonstrate how technological capabilities in ocean robotics at the Council for Scientific and Industrial Research (CSIR) could augment the current resource-intensive hydro-acoustic ship-based survey programme and create opportunities for expanding its spatial and temporal resolution. We successfully implement and demonstrate an autonomous wave glider, fitted with a hydro-acoustic sensor and compare the data to a collocated ‘traditional’ ship-based acoustics survey. In the future these autonomous systems approaches could be seen as a means to lessen the cost burden of the ship-based survey, while at the same time with the added advantage of continuous collection over much wider spatial and temporal domains. This could enable a more reflexive stock management approach taking into account the seasonal characteristics of the fishery and its ecosystem. Gliders thus have potential to increase dramatically the quantity of information available to fisheries managers, thereby reducing uncertainty and contributing to improved management of valuable fish resources. They are likely to contribute to improved knowledge of the ecology of small pelagic fish species off the coast of South Africa in a changing climate and should potentially also permit the collection of biomass data for other marine resources currently not routinely monitored.