The Eons of Chaos and Hades Goldblatt, C.; Zahnle, K. J.; Sleep, N. H. ...
Solid earth (Göttingen),
01/2010, Letnik:
1, Številka:
1
Journal Article
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We propose the Chaotian Eon to demarcate geologic time from the origin of the Solar System to the Moon-forming impact on Earth. This separates the solar system wide processes of planet formation from ...the subsequent divergent evolution of the inner planets. We further propose the division of the Hadean Eon into eras and periods and naming the proto-Earth Tellus.
Cratering rates in the outer Solar System Zahnle, Kevin; Schenk, Paul; Levison, Harold ...
Icarus (New York, N.Y. 1962),
06/2003, Letnik:
163, Številka:
2
Journal Article
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This paper is a compilation by table, graph, and equation of impact cratering rates from Jupiter to Pluto. We use several independent constraints on the number of ecliptic comets. Together they imply ...that the impact rate on Jupiter by 1.5-km-diameter comets is currently
Ṅ(
d > 1.5 km) = 0.005−0.003+0.006 per annum. Other kinds of impactors are currently unimportant on most worlds at most sizes. The size–number distribution of impactors smaller than 20 km is inferred from size–number distributions of impact craters on Europa, Ganymede, and Triton; while the size–number distribution of impacting bodies larger than 50 km is equated to the size–number distribution of Kuiper Belt objects. The gap is bridged by interpolation. It is notable that small craters on Jupiter’s moons indicate a pronounced paucity of small impactors, while small craters on Triton imply a collisional population rich in small bodies. However it is unclear whether the craters on Triton are of heliocentric or planetocentric origin. We therefore consider two cases for Saturn and beyond: a Case A in which the size–number distribution is like that inferred at Jupiter, and a Case B in which small objects obey a more nearly collisional distribution. Known craters on saturnian and uranian satellites are consistent with either case, although surface ages are much younger in Case B, especially at Saturn and Uranus. At Neptune and especially at Saturn our cratering rates are much higher than rates estimated by Shoemaker and colleagues, presumably because Shoemaker’s estimates mostly predate discovery of the Kuiper Belt. We also estimate collisional disruption rates of moons and compare these to estimates in the literature.
Large asteroid impacts produced globally lethal conditions by evaporating large volumes of ocean water on the early Earth. The Earth may have been continuously habitable by ecosystems that did not ...depend on photosynthesis as early as 4.44 Gyr BP (before present). Only a brief interval after 3.8 Gyr exists between the time when obligate photosynthetic organisms could continuously evolve and the time when the palaeontological record indicates highly evolved photosynthetic ecosystems.
The explosion over Tunguska, Central Siberia, in 1908 released 10 to 20 megatons (high explosive equivalent) of energy at an altitude of about 10 km. This event represents a typical fate for stony ...asteroids tens of meters in radius entering the Earth's atmosphere at common hypersonic velocities. Comets and carbonaceous asteroids of the appropriate energy disrupt too high, whereas typical iron objects reach and crater the terrestrial surface.
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.
"Are they worlds, or are they mere masses of matter? Are physical forces alone at work there or has evolution begotten something more complex, something not unakin to what we know on Earth as life? ...It is in this that lies the peculiar interest of Mars." Percival Lowell (in ref. 1, p. 3).
The Magellan images showed presence of radar-dark shadowlike disks surrounding small impact craters on the Venusian surface, which are also found without craters. This paper examines the hypothesis ...of Phillips et al. (1991) that the dark shadows formed where shock waves from airbursting impactors met the ground, pulverizing decimeter-scale surface features. A simple analytic model is used successfully to simulate the catastrophic disruption and the deceleration of impactors in a thick atmosphere to reproduce observed Venusian cratering statistics and generate radar-dark disks by the impact of atmospheric shock waves on the surface.
The recent discovery of an apparently global soot layer at the Cretaceous/Tertiary boundary indicates that global wildfires were somehow ignited by the impact of a comet or asteroid. It is shown here ...that the thermal radiation produced by the ballistic reentry of ejecta condensed from the vapor plume of the impact could have increased the global radiation flux by factors of 50 to 150 times the solar input for periods ranging from one to several hours. This great increase in thermal radiation may have been responsible for the ignition of global wildfires, as well as having deleterious effects on unprotected animal life.
A simple description of the disruption and deceleration of 100-m- to 5-km-diameter comets striking Jupiter is combined with numerical simulations of the subsequent explosions to predict the fate of ...Comet Shoemaker-Levy 9. Kilometer-size objects of density 1 g/cm3 explode at about the 10-bar level; a fragment of the same diameter but of density 0.3 g/cm3 explodes at about the 2-bar level. Detailed numerical simulations of the first 3 min of the explosion were performed using the astrophysical hydrodynamics program ZEUS-3D. Our numerical simulations begin either with hot cylinders with dimensions suggested by the disruption and deceleration model or with an initial wake constructed from a moving line charge. In all cases, extensive plumes of hot gas are expelled from the atmosphere. The models with wakes evolve about twice as fast as the initially confined models. Models of both types generate similar pressure waves into the planet. Temperatures and negative hydrogen ion opacities were computed by solving a battery of Saha equations. For atmospheric entry, light curves were computed assuming thermal radiation by clean jovian air with a surface area consistent with the (changing) cross-sectional area of the impactor. On entry the largest bolides could be very bright, possibly as bright as Jupiter for observers placed to see them, although for kilometer-size impactors the luminosity peak is obscured by clouds. The timescale is about 10 sec. For the fireball, light curves were computed from the numerical simulations assuming a grey atmosphere. Metals from the vaporized comet provide electrons that dramatically increase the opacity of Jovian air at low temperature; the resulting effective radiating temperature of the fireball is of order 3000 K. The fireball rises through and above the atmosphere, brightening at first as its surface area increases, but later fading to invisibility as its temperature drops and its opacity plummets. The timescale is about 100 sec.
New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton's orbital motion (i.e., all are on the leading ...hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker–Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton's small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of ∼50 Myr on the age of the more heavily cratered terrains, and of ∼6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton's is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration.
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