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Posted by don findlay on June 22, 2006, 9:52 pm
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pete wrote:
> Yes, that's exactly what the math suggests. The velocity of a gas
> at a given temperature is related to its mass (temp=energy; mv^2..).
> As hydrogen and helium are so light, their velocities are high
> enough that they approach escape velocity, and the top end of
> the distribution curve gets stripped off the planet over a few
> million years.
Why do you think in terms of escape velocity, rather than 'winnowing'
--
> ==========================================================================
> vincent@triumf[munge].ca Pete Vincent
> Disclaimer: all I know I learned from reading Usenet.
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Posted by Perplexed in Peoria on June 23, 2006, 7:05 pm
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>
> > George wrote:
> >>
> >> It also doesn't sit well with the current configuration of the planets,
> >> which have rocky inner worlds and gaseous outer ones. If the Earth was
> >> a
> >> gaseous inner world, one might consider the possibility that the other
> >> inner worlds were also gaseous.
> >>
> > Agreed.
> >>
> >>Then you have to explain why/how those
> >> worlds also lost their hydrogen atmospheres.
> >>
> > Some planets lost their hydrogen because of their proximity to the Sun.
> > Others did not have enough gravity to hold on to hydrogen. John Curtis
>
> That doesn't explain why three of the four rocky planets were able to
> retain substantial atmospheres. If the Earth ever had a significant
> hydrogen atmosphere, it lost it very early in its history.
In this context, the following online slideshow/lecture may be interesting.
Reposted from sci.bio.evolution. Acknowlegements to Wirt Atmar, who posted
the original.
------------------
The Evolutionary Biology Lecture of the Week for June 19, 2006 is now
available at:
http://aics-research.com/lotw/
The talks center primarily around evolutionary biology, in all of its
aspects: cosmology, astronomy, planetology, geology, astrobiology,
ecology, ethology, biogeography, phylogenetics and evolutionary biology
itself, and are presented at a professional level, that of one
scientist talking to another. All of the talks were recorded live at
conferences.
This is the fifth lecture in a summer-long series on the new science of
astrobiology.
=====================================
June 19, 2006
Part V: Astrobiology
Sympathy for the Devil:
The Case for Life on Venus
David Grinspoon, Southwest Research Institute, Boulder
33 min.
Venus favors the bold.
-- Ovid (43 BC - 17 AD)
The next four lectures will ask the question: "Why did things go so
right for life on Earth?" To answer that question, we ask, "Why did
things go so badly on Earth's nearest neighbors, Venus and Mars?"
Mars is half the size of Earth, but Venus is nearly Earth's identical
twin. All three planets likely had substantial oceans, but Venus and
Mars lost their oceans some time ago. Mars' oceans apparently
evaporated almost immediately, but Venus may have kept its oceans for
600 million years, as estimated by Jim Kasting of Penn State in 1988,
or for as long as two billion years, as suggested by David Grinspoon in
this week's lecture.
For all of its history, Mars has lain outside the "habitable zone,"
where planetary temperatures make long-term surface liquid water
possible. Worse, because of Mars' small size, the internal heat engine
of Mars sputtered off sometime ago, thus the world stopped evolving
geologically, no longer replenishing its atmospheric gases.
Venus, on the other hand, lies interior to the habitable zone and has
received too much solar flux. While its oceans were evaporating, UV
radiation was dissociating its water into its constituent components,
providing sufficient energy for the escape of hydrogen into space. For
a bit of time, perhaps a billion years, Venus may had water oceans
overlain by an oxygen-rich atmosphere before any other planet. But
that's not Venus today.
Grinspoon outlines the two great transitions in the geological history
of Venus in this lecture: the initial loss of its oceans and the later
overturn of its crust, which may in fact be the consequence of a single
continuous dehydrating evolution. With the dessication of Venus'
surface, water-borne tectonic subduction ceased and the planetary
surface became in effect one large plate. The result was that about 700
million years ago, internal heat built to the point that the entire
surface of Venus may have melted in one global event.
Venus' present may well be Earth's future. As the Sun continues to
brighten, it is expected that the Earth will also lose its oceans in
500 million to one billion years, thus in appproximately the same short
period of time that it has taken life on Earth to progress from
trilobite to astronaut, life will come to an end on Earth as well -
at least on its surface.
Venus has not traditionally been considered a promising target for
astrobiological exploration, yet Grinspoon proposes that Venus should
be central to such an exploration program for several reasons. All of
our ideas about extraterrestrial biochemistry are, of necessity,
extrapolations from the single example of life which we have been able
to study. Planetary exploration, with an increasing focus on
astrobiology, has been designed to "follow the water." This is a
reasonable strategy but it is based, at best, on an educated guess
about life's universals.
If we think beyond the specifics of a particular chemical system
required to build complexity and heredity, we can ask what general
properties a planet must possess in order to be considered a possible
candidate for life. Grinspoon argues that the answers might include an
atmosphere with signs of chemical disequilibrium and active, internally
driven cycling of volatile elements between the surface, atmosphere and
interior, what he calls his "Living Worlds Hypothesis." At present, the
only two planets we know of which possess these characteristics are
Earth and Venus.
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Posted by robert casey on June 18, 2006, 5:59 pm
Please log in for more thread options John Curtis wrote:
> Comparing the cummulative mass of satellites to the mass of their
> central gas giants produces a constant ratio of 0.0001 which is a
> hundred times smaller than Moon to Earth ratio of 0.012:
> http://www.spaceref.com/news/viewpr.html?pid=20099
> This discepancy in ratios becomes smaller when viewed from
> planet-evaporation-model standpoint, where terrestrial planets
> lost their hydrogen-helium atmospheres to UV stripping:
Then those hot Jupiters like 51 Peg should have shrunk down to something
the size of Earth by now. 51 Peg is somewhat older than the Sun, and
its big close in planet should be long shrunk down by now if your idea
actually worked. Or am I missing something?
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Posted by John Curtis on June 18, 2006, 8:21 pm
Please log in for more thread options robert casey wrote:
> John Curtis wrote:
> > Comparing the cummulative mass of satellites to the mass of their
> > central gas giants produces a constant ratio of 0.0001 which is a
> > hundred times smaller than Moon to Earth ratio of 0.012:
> > http://www.spaceref.com/news/viewpr.html?pid=20099
> > This discepancy in ratios becomes smaller when viewed from
> > planet-evaporation-model standpoint, where terrestrial planets
> > lost their hydrogen-helium atmospheres to UV stripping:
>
> Then those hot Jupiters like 51 Peg should have shrunk down to something
> the size of Earth by now.
>
Not if 51 Peg started out with 30MJ. A rocky planet with 0.6MJ would be
its destination :-)
http://www.exoplaneten.de/51peg/english.html
John Curtis
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Posted by robert casey on June 18, 2006, 8:51 pm
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>
> Not if 51 Peg started out with 30MJ. A rocky planet with 0.6MJ would be
> its destination :-)
> http://www.exoplaneten.de/51peg/english.html
> John Curtis
>
Maybe, but some hot JUpiters transit across their star's faces as seen
from Earth. That tells inclination, diameter and accurate mass. The
mass and diameter indicate a gas giant like Jupiter. Some of the gases
in the planet's atmosphere have been identified. (Compare the spectra
with the planet transiting the star, against when the planet is behind
the star. Take the difference.) One such planet was found to be losing
small amounts of atmosphere, but the rate of loss was such that the
planet would last for tens of billions of years.
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