Читать онлайн "Broca's Brain: The Romance of Science" автора Саган Карл Эдуард - RuLit - Страница 89

 
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The conclusion would not be altered materially were there to have been several close passes, rather than just one, through the solar photosphere. The source of the high temperature of Venus cannot be one or a few heating events, no matter how dramatic. The hot surface requires a continuous source of heat-which could be either endogenous (radioactive heating from the planetary interior) or exogenous (sunlight). It is now evident, as suggested many years ago (see Wildt, 1940; Sagan, 1960), that the latter is the case: it is the present radiation of the Sun, continuously falling on Venus, which is responsible for its high surface temperature.

APPENDIX 4

Magnetic Field Strengths Necessary to Circularize an Eccentric Cometary Orbit

ALTHOUGH VELIKOVSKY has not, we can calculate approximately the order of magnitude of the magnetic field strength necessary to make a significant perturbation on the motion of a comet. The perturbing field might be from a planet, such as Earth or Mars, to which the comet is about to make a close approach, or from the interplanetary magnetic field. For this field to play an important role, its energy density must be comparable to the kinetic energy density of the comet. (We do not even worry about whether the comet has a distribution of charges and fields which will permit it to respond to the imposed field.) Thus, the condition is

where B is the magnetic field strength in gauss, R is the radius of the comet, m its mass, v its velocity and ρ its density. We note that the condition is independent of the mass of the comet. Taking a typical cometary velocity in the inner solar system of about 25 km/sec, and ρ as the density of Venus, about 5 gm/cm3, we find that a magnetic field strength of over 10 million gauss is required. (A similar value in electrostatic units would apply if the circularization is electrical rather than magnetic.) Earth’s equatorial surface field is about 0.5 gauss. The fields of Mars and Venus are less than 0.01 gauss. The Sun’s field is several gauss, ranging up to several hundred gauss in sunspots. Jupiter’s field as measured by Pioneer 10 is less than 10 gauss. Typical interplanetary fields are 10−5 gauss. There is no way to generate anything approaching a 10 megagauss field on a large scale in the solar system. And there is no sign that such a field was ever experienced in the vicinity of Earth. We recall that the magnetic domains of molten rock in the course of refreezing are oriented by the prevailing field. Had Earth experienced, even fairly briefly, a 10 Mg field 3,500 years ago, rock magnetization evidence would show it clearly. It does not.

Carl Sagan

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