Re: How low-density can a terrestrial body be?, or, help

On Nov 3, 12:20 pm, "carey...@xxxxxxxxx" <carey...@xxxxxxxxx> wrote:

Someone other than Carey wrote:

Calcium-aluminum-silicate comes in at a specific gravity of 2.6-2.8,
depending on composition.

And could actually be significantly denser, due to self-compression.
In short, this large a body should not be this low density.

You would have to do without a nickel-iron
core, and the attendant magnetic field.

Which should be too much of a problem, depending on the age of the
planet (solar wind atmospheric stripping). Radiation should be handled
just fine by that big fluffy atmosphere (higher column density than
Earth's).

 A low concentration of heavy radioactive elements would mean
minimal vulcanism...

First, if we're positing a very-low-density world in the habitable
zone, the assumption of cosmogenic abundances is already out the
window, so you might be able to fudge the radioactives issue (tough,
but perhaps possible).

As Carey mentions, this isn't inevitable. If the planet is young,
there's accretion, core formation, and short-lived isotopes around for
heating. If it has a moon or two, there's tidal heating. Note that a
complete lack of internal heat is going to cause issues with a lot
more than a flattened well-eroded world - lack of plate tectonics and
vulcanism would mean the carbonate-silicate cycle is shutting down,
possibly leading to a run-away climate condition.

Up to 50% of the heat outflow from Earth right now may be due to
simple cooling of the interior (that is, it is ancient heat left from
the early formation of the planet).

That I find surprising. In the lecture you cite, they mention that the
current steady-state heat flux from the surface can mostly be
accounted for by radioactive heating, and then go on to say as much as
half the remaining could be due to "fossil" heat... while not
mentioning tidal dissipation at all (significant even now for the
Earth), nor showing (in a 400-level class) calculating the time-
temperature history based on a conductivity model or accounting for
convective transport. Odd.

--
Brian Davis
.

Relevant Pages

  • Re: Sirius B
    ... left the core of the galaxy empty. ... On top of it the magnitude of the flare not weakens as it exist the ... Imagine 500 years of accelerating heat. ...
    (sci.astro)
  • Re: AKICIF: Origin of film "Sunshine"
    ... somehow had a way to tap large amounts of it (trillions of heat pipes ... time-traveled into this far age dropped an A- or maybe H-bomb down ... one of the shafts and rekindled the core. ... The sun is very gradually heating up, ...
    (rec.arts.sf.fandom)
  • Re: fission question
    ... > A core such as this, in five seconds, would be fairly accurately ... Just not time for the heat to move around ... > (thermally as well as by induced radiation) fairly quickly. ... Figure 2.5 neutrons per fission and you get 5 MeV in neutrons ...
    (sci.physics)
  • Re: AKICIF: Origin of film "Sunshine"
    ... somehow had a way to tap large amounts of it (trillions of heat pipes ... an end to their supply of geothermal power. ... time-traveled into this far age dropped an A- or maybe H-bomb down ... one of the shafts and rekindled the core. ...
    (rec.arts.sf.fandom)
  • Re: Owens Two-Phase Model of Earth Expansion
    ... >> we're in the same boat when it comes to the Earth's heat engine. ... > mantle and core, latent heat of fusion release in the core, ... of the Earth into a core, mantle and a crust (by the dissipation of its ...
    (sci.geo.geology)