Joseph Lazio <jlazio@xxxxxxxxxxxxxxxxx> wrote:
> Steve Pope writes,
>SP> I found the statement quoted in my first posting that a GMC passes
>SP> by every 500 million years or so interesting. This raises some
>SP> immediate questions, such as do we know when in geologic history
>SP> these things passed by, and did they leave a record -- was there
>SP> altered climate, orbital changes, more comets? It seems they are
>SP> quite massive so likely there would be an effect.
>Actually, it is probably not the mass but the density that would be
>most important. The heliosphere or the region of the Sun's influence
>is determined by pressure balance between the outflowing solar wind
>and the surrounding medium. If the density of the surrounding medium
>increases, its pressure increases, and the heliosphere shrinks. That
>could result in more cosmic rays getting to the Earth, which could
>result in more clouds forming, in turn affecting the Earth's albedo.
>Priscilla Frisch has done a fair amount of work in this area, e.g.,
Thanks. I'll take a look. I'm particularly interested in
whether there specific evidence that a GMC has passed by on
average every 500 million years, or if this is an extrapolation
of some sort.
Also, what "molecules" would be in a "molecular cloud" other