Astronomical Hypothesis for the YDB Impact Event
Regarding the probability of a swarm of cometary fragments hitting the Earth, Boslough et al. (2013) claimed that the YDB event is “statistically and physically impossible,” whereas Napier et al. (2013) argued that such an encounter in the late Quaternary is a “reasonably probable event.” We outline the latter hypothesis below; details and prime references are given in Napier (2015).
With currently accepted impact rates, there is an expectation of one extraterrestrial impact of energy 100–200 megatons over the past 20,000 y, which is inadequate to produce the observed global trauma (Bland and Artemieva 2006). However, near-Earth surveys of hazardous interplanetary objects are limited to the past ∼30 y, and extrapolation of con- temporary impact rates to timescales beyond 104 y cannot be justified without further investigation, especially for comet populations.
Comets entering Earth-crossing orbits, which are thus potential collision hazards, may be long-period (LP) objects (200 ky ≲ P ≲4 My), Halley-type (HT; 20y≲P≲200y),Jupiterfamily(JF)–type(4y≲P≲ 20 y), or Encke-type (P ≲ 4 y), the latter currently with a population of 1. The LP system is spherical, containing as many comets in retrograde orbits as in prograde, and derives from the Oort cloud. The HT system is spheroidal, with a preponderance of comets in direct orbits, while the JF and Encke comets are in direct orbits close to the plane of the ecliptic. There may be ∼100 active comets with diameters over 2.3 km in the HT population and ∼450 in the JF system, although these numbers are very uncertain. Both these populations are evanescent, with a typical JF comet surviving for only 200–300 revolutions (∼3000 y) before disintegration is complete. To maintain a steady state, new comets must enter the JF system about once a decade and the HT system about once every century. The likely replacement reservoirs are the Oort cloud and a trans-Neptunian population of icy bodies on the fringes of the planetary system, the latter of which was largely unknown 25 y ago. Its properties are still being explored, but it is estimated to contain 8 billion comets more than 1 km in diameter; the Oort cloud may contain a trillion comets.
Dormant comets, called “centaurs,” have been detected in transition from these reservoirs to the JF population. They are in unstable orbits crossing those of Jupiter, Saturn, and Uranus, becoming more unstable as they move inward and becoming active when they cross the water-snow line at ∼2.9 astronomical units (au) from the Sun (1 au p the mean Earth-Sun distance). The archetypal centaur, Chiron, currently orbits between Saturn and Ura- nus. Its half-life for ejection from the solar system is about 1 My, and that for evolution into a Jupiter- crossing orbit is 0.1–0.2 My (Hahn and Bailey 1990). Population-balance arguments indicate that at any given time there may be four to seven centaurs larger than 240-km-wide Chiron inside 18 au and about 30 that are 1100 km in diameter. Their orbits are chaotic and can be followed only statistically, because small changes in initial conditions induce large subsequent variations (the butterfly effect). Eventually, about half the centaurs in Chiron-like orbits become Jupiter crossers at some point, and a tenth become Earth crossers, moving in and out of Earth-crossing epochs repeatedly (fig. 10).
The mass distribution of centaurs is top-heavy, and the replenishment of the JF and comets in Encke- like orbits is erratic, with occasional large injections of mass into the inner planetary system. A 250-km comet with typical density 0.5 g/cm3 has 2000 times the current mass of the JF and 1000 times that of the entire current near-Earth asteroid system. In terms of terrestrial interactions, its disintegration products, during its active lifetime, will thus greatly dominate over those of the near-Earth asteroids. For a 100-km comet, the factors are 128 and 64, respectively. The timescale for an enhancement in mass of the near-Earth environment is ∼0.5 My for a factor 1000 and ∼0.03–0.1 My for a factor 100.
The main modes of disintegration of a comet are sublimation and fragmentation. The latter has two prime modes: tidal splitting, in which the comet breaks into fragments after a close planetary or solar encounter, and spontaneous splitting, in which the main nucleus stays intact while a number of short-lived cometary fragments split off, often disappearing from sight after a few weeks or days.
Fragmentation is the major mode of comet disintegration: the total mass lost by small fragments repeatedly spalling from the nucleus may be comparable to the mass of the nucleus itself (Boehn- hardt 2004). A 100-km comet of mass 2.5 # 1020 g, with perihelion of ∼0.34 au, loses typically ∼1016 g of material through sublimation during each peri- helion passage, but ∼1017–1018 g during a splitting event, equivalent in mass to ∼107–108 Tunguska bolides. Such an event may happen anywhere along its orbit. For a comet in an Encke-like orbit, such fragmentations are expected every third or fourth orbit (di Sisto et al. 2009). These splittings may occur anywhere but have a tendency to occur near perihelion. Debris from disintegrating comets readily spreads out to cross sections much greater than Earth’s diameter and so is encountered more frequently, as can be seen by the prevalence of meteor showers in the night sky, the products of cometary decay. During dormant phases, in which sublimation decreases, an active comet becomes more asteroidal in appearance through acquiring a mantle of dust and heavy organics. These phases may persist for up to 40% of the lifetime of the comet.
The structure of the meteoroid population in the inner planetary system has been determined both through numerous individual studies of meteors, going back to the 1950s, and from recent large-scale radar and optical surveys. As many as 100 meteor showers are accepted by the International Astronomical Union, but a total of 230 showers and shower components have now been identified from video- based meteoroid orbit surveys (Wiegert et al. 2009; Jenniskens et al. 2016). Some of these streams are multicomponent, indicating that they result from cascades of disruption of a parent body into subcomponents.
A prominent feature of this orbiting material is an interrelated system of meteoroidal material called the Taurid Complex. At least 20 observed streams are embedded within it, with the meteoroids moving in low-inclination, short-period, Earth-crossing orbits. Many of these streams contain bodies of killometer and subkilometer dimensions, including the 4.8-km-wide Comet Encke. A limitation of meteor surveys is that they detect only material that hits Earth. It is therefore possible that the number of meteoroid streams associated with the progenitor of the Taurid Complex is greater than the ∼20 that have been observed. A dust trail along the orbit of Encke has also been detected, with a lower mass limit of 7 million tons inferred from the Spitzer infrared space telescope (Reach et al. 2007). This trail, which is well away from Earth intersection, extends around the entire orbit and would disperse in some revolutions. Such trails, distinct from comet tails, are a generic feature of short-period comets.
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