> Alain <kua### [at] videotronca> wrote:
>> A planetary magnetic field is not needed to protect life on the planet's
>> surface, you only need a thick enough athmosphere.
>
> However, a magnetic field may be needed to protect the /atmosphere/ itself.
> Mars once had a much thicker atmosphere, and the going hypothesis is that it was
> blown away by the solar wind after Mars' core solidified.
>
>
>

Mars is also about half the Earth's radius, making it 1/8th the mass and 
a much lower surface gravity. Been farther from the sun makes it cooler 
and reduce the athmospheric evaporation that was greatly increased by 
the lower mass. Mars DOES slowly loose some of it's athmosphere, mostly 
light gases like oxygen and nitrogen, at a greater rate, leaving about 
only carbon dioxide.

Now, take a planet with an average density of about 3, compared to 
around 5 for the Earth, and increase it's radius to get a surface 
gravity of 0.9g. Take it slightly farther from the Sun.
That planet could have a surface athmosphere similat to Earth, but it 
would extent much higher, making it thicker. The planet been farther 
from the sun receive less energy, compensating the higher thermal retention.
The Solar wind effect on it would be rather negligeable even without any 
magnetic field. At the same time, that planet can actualy collect stray 
gases, and some mass from the solar wind, to replentish itself.
On such a planet, nights would be warmer than on Earth and, for the same 
axis tilt, you'd expect less contrasted seasons as the athmosphere would 
have more thermal inertia.



Alain

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