Re: "Sagan and Velikovsky" to appear in local book stores

Dallas Kennedy (
25 Jan 1995 05:59:20 GMT

The main conventional book on Venus I was thinking of, was _Venus_, published
by U. Arizona Press in 1983 [?]. It uses the Pioneer and Venera data, but
clearly not the Magellan data.

One item that is clear from that book is that Venus is not in thermal balance;
that is, its heat out flux (157 W/m^2) is larger than its light in flux
(106 W/m^2), after the albedo (0.84) is taken into account. The chapter
in the book that deals with this concludes that there "cannot" be an internal
heat source, so it just ad hoc changes the albedo to adjust the heat budget
into balance.

The runaway greenhouse model of Sagan to explain Venus' high surface tempera-
tures has a number of fatal problems. The conventional version assumes thermal
balance. But even if there were thermal balance, the greenhouse suffers from
these flaws: (1) no adverse thermal gradient below the cloud tops; and (2)
almost no water vapor. (1) For the trapping of heat to occur, thermodynamics
requires the presence of a temperature inversion to prevent the heat at the
surface from flowing out (by diffusion or conduction). There is evidently no
significant convection in the lower atmosphere (below the cloud tops). The
Earth's "atmosphere" effect is based on the temperature inversion in the lower
atmosphere. This inversion is absent in the case of Venus. (2) The trapping
of IR radiation requires, for full coverage of the IR spectrum, water vapor
in addition to CO2. But Venus' atmosphere has much less water vapor than
necessary to make the runaway greenhouse work. Substitute gases with the
right absorption properties are not evident. The upshot of these two
problems is that Venus' surface should be quite a bit cooler than it is --
perhaps 350 K, not 750 K. *The surface of Venus receives less sunlight than
does the Earth's surface*, in spite of its closer distance to the Sun, because
of its high albedo.

It won't do to claim that Venus may have had much more water vapor in the past,
enough to establish a runaway greenhouse then. The model needs the water
vapor (or a suitable substitute) to be there *now*, to keep the high tempera-
ture in a steady state. The same goes for the inverse temperature gradient.

I think that the greenhouse model should have been junked after the Pioneer
data were available (after 1979). I do not argue that an ordinary or runaway
greenhouse effect is impossible under the right conditions, but only that
those conditions are not present on Venus today, given the data I have seen.
The greenhouse model is a good example of people committing to a flawed idea
before there was much information to judge the idea and then clinging to that
idea well after more and better information had become available that refuted
the idea.

There are other strange features of Venus, not related to the greenhouse, but
worth noting. (1) The atmosphere does not have much argon (from the radio-
active decay of potassium, I think). An old atmosphere, like the Earth's,
should have more argon. The beta decay half-life of potassium-40 is about
10^9 yr. (2) If solar heating were the dominant heating at Venus' surface,
one would expect the equatorial region to be hotter than the poles (like the
Earth). However, Venus' poles have measured to have temperatures close to or
even about equal to the equatorial temperatures. From this, I infer that
the dominant heating at Venus' surface is not solar, but internal. (3) Venus
has a youthful-looking surface, not gnarled and wrinkly like the Earth's :)

How the planet science community could have kept up the greenhouse model,
especially with no inverted temperature gradient, is bizarre. I assume it
is by inertia or politics.