[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Nemesis

jjswaim schrieb/wrote/a écrit:

> ... what the criteria were for excluding the possiblity
> of life on the planets in a multiple star system?


Hello Julia and List,


Zones of Habitability (Sky & Telescope, December 1998, p. 54):

A star’s 'habitable zone' is the region where a habitable, Earth-like
planet or moon can exist. Such a world is usually defined to be one that
has surface temperatures allowing for the existence of liquid water over
a substantial portion of its surface. While Mars or the Jovian moon
Europa might have biocompatible environments where life may exist, they
are not promising sites for the development of advanced forms of life
and are not generally considered to be habitable.
The size and location of a star's habitable zone depend on the amount of
solar energy, or insulation, a world receives. A further constraint is
the amount of green-house gases, such as carbon dioxide, in its
atmosphere.
In the past decade planetary climate researchers have come to realize
that the amount of carbon dioxide in the atmosphere of a geologically
active world like our own is primarily controlled by the
carbonate-silicate cycle. On a terrestrial body this process starts when
rain containing dissolved carbon dioxide weathers silicate minerals. The
products of these reactions are carried to the ocean, where they
eventually precipitate to form climatically inert carbonate deposits. On
a geologically active world these deposits will eventually reach the
mantle by the subduction of crustal plates (as happens on Earth) or by
some other tectonic process. Once in the mantle, the carbonate deposits
are heated and transformed back into carbon dioxide that can be released
into the atmosphere through volcanism.
Because of the way this cycle works, excess carbon dioxide is removed
from the atmosphere when it is too hot because of enhanced amounts of
precipitation and rates of weathering. When it becomes too cold,
precipitation and weathering slows, thus allowing volcanically derived
carbon dioxide to build up in the atmosphere. In effect, the
carbonate-silicate cycle is a slow-acting, global thermostat.
James Kasting (Pennsylvania State University) and his colleagues have
performed calculations that incorporate the climate-stabilizing effects
of the carbonate-silicate cycle. Their results suggest that the inner
edge of a star's habitable zone lies where the carbonate-silicate cycle
has already removed most of the carbon dioxide from a world's
atmosphere. Closer to a star, the temperature starts to rise out of
control with increasing insolation. Because of increased evaporation,
water vapor builds up in the atmosphere to produce a moist greenhouse
effect. Eventually the heat allows water vapor to rise high in the upper
atmosphere, where it is broken down by the star's ultraviolet light and
the world's hydrogen atoms are permanently lost.
This water-loss limit, which in our solar system occurs 0.95
astronomical unit from the Sun, is the first inner boundary of the
habitable zone. Assuming that a planet retains its water despite this
mechanism, a runaway greenhouse effect that would sterilize the planet
finally sets in closer than 0.84 a.u.
As we move out from the inner edge of the habitable zone, the
carbonate-silicate cycle slowly increases the amount of atmospheric
carbon dioxide to maintain above-freezing temperatures. While these high
levels of carbon dioxide would be fatal to humans, native life forms
that evolved in such an environment would thrive.
According to Kasting's work, when an Earth-like planet is 1.37 a.u. from
the Sun, the top of its carbon-dioxide-rich atmosphere starts to freeze
and form carbon-dioxide ice clouds. If the clouds are thick and their
particles are small, they will reflect light efficiently and cool the
planet. If this is true, then 1.37 a.u. is the outer limit of our Sun's
current habitable zone.
However, recent studies by François Forget (Laboratoire de Météorologie
Dynamique, Paris) and Raymond Pierrehumbert (University of Chicago)
indicate that these clouds could contain much larger particles, which
are excellent reflectors of infrared radiation. Such clouds would
actually help a world retain heat like a blanket and augment the
greenhouse effect. The outer limit of the habitable zone could then
extend out to 2.4 a.u., where the required amount of atmospheric carbon
dioxide reaches 5 to 10 bars. At this point the atmosphere is so thick
that the addition of more carbon dioxide will not increase surface
temperatures. Beyond this limit a world's oceans freeze and it becomes a
giant ice cube.


Best regards,

Bernd

----------
Archives located at:
http://www.meteoritecentral.com/list_best.html

For help, FAQ's and sub. info. visit:
http://www.meteoritecentral.com/mailing_list.html
----------
References: