Digest Number 735

Hi,

>Why should cosmic radiation bother the inhabitants?

Because it will kill them or at least produce mutant offspring.

>D2O (Heavy water, where the 2 should be a subscript in this email) doesn't
>chemically work /exactly/ like H2O. You might be building in trouble down
>the line for your colonists.

Heavy water is chemically identical to H2O. They differ only in kinetics.
It would be a nice little experiment to try. If it fails then we know not
to try it again and the equations can be adjusted.

>The engineering involved ("remove all but one Earth-atmosphere", "remove
>mass from the planet") assumes a very great deal indeed.

It assumes the logistical ability. It assumes nothing about the
engineering. It's entirely possible with current technology, the
engineering is sound. Considering we are not attempting to try it right now
the point is mute :]

>Why not add "add neutron star material" to increase the planet mass?

OMG...because it would reduce the entire planet to a neutron star or
expoded in the largest neutron bomb you'd ever see, assuming you survive.

>Bad maths:

Bloody good math!

>Action 5: orbiting mirrors /will/ act like light sails, but it doesn't
>necessarily follow that these will "impact the planet". If each is in a
>reasonable orbit over the world (and therefore only working part of the
>time) the net effect will be zero.

No, it will not. The sails will have a large surface area and small mass.
The force created from reflected light will push the sails into an
eccentric orbit until they burn up in the planets atmosphere. Reflected
light creates a force, force applied to a mass creates acceleration,
acceleration changes velocity, a change in velocity changes position. That
is why solar panels are used, they absorb light, so there is no reflection,
there is no force (or at least it is negligable) and the panels stay in
orbit.

>Action 7: reducing the atmosphere is very unlikey to have an effect on the
>mantle. 1 atmosphere is about 10 tonnes per square metre, for Earth. That's
>nothing for bedrock. A decent ice sheet (which would have some effect in
>terms of land movement, perhaps a few metres per century) is of the order
>of 100 times more mass than this...but no fissures spurting lava...

I did say the effect will be slight, not dramatic.
Almost all mantles are made from silicates and the core's made from
iron-nickel (or at least from metals). The insulating silicate mantle will
trap heat in the core. When the atmospheric pressure is reduced the mantle
will cool faster than the core, and the hot pressurised core will push
upwards. As the mantle is cool (and not a flowing lquid) it will crack as
the core expands. Also, you cannot compare the slow effects of an eroding
icesheet to the sudden removal of 90 atmospheres at 700 K.

>ultra-violet, but since everyone in a planetary colony will be indoors
>or in a space suit, UV is not a problem.

No, the point is to have them moving about freely on the surface. That's
why the air must be breathable before colonisation.

>Terraform the planet as little as possible, perhaps not at all.
>Modifie your colonists so that this is not a problem.

First, you would have to find an entirely new form of life to duplicate,
because life as we know it cannot reproduce at 700 K. Then, you would have
to modify an entire ecosystem to go with it. This is ignoring the basic
point of terraforming: to create a planet for life AS WE KNOW IT.

Michael
:]