[ExI] Dawn's Early Light: Ceres and Vesta

Amara Graps amara at amara.com
Mon Jul 9 13:49:41 UTC 2007

Dear Jeff,

In your description, I didn't see the emphasis on the condensation
sequence that should be there, and I think your time sequence is not
really in order. The molecules didn't form during the planet forming
part, but much before, and were incorporated into the solids that became
the planetesimals, that became the embryos, that became the planets.

After the molecular cloud collapses (triggered by a supernova or few, or
some other kind of 'shock') to form the "solar blob", which becomes then
a "protostar+ gas/dust disk (because angular moment is transferred out,
while  material transferred in), which becomes a star+gas/dust disk
(fusion has turned on in the protostar), then we have a solar nebula,
where solids form dependent on the temperature and pressure profile of
the nebula, which is time-dependent, as the nebula disk cools too. So
one sees a molecule formation pattern as a function of heliocentric
distance and time. It's very model dependent, as you can imagine. I say
that I would like to know where was snow line during our solar system
formation, which you interpreted as a large unknown. The physical
process _is_ known, clear, and well-defined, but what is not known well
is the temperature and thickness of the solar nebula at the time of the
condensation sequence. There are many solar nebula models out giving
temperature and pressure estimates.

This page talks about the condensation sequence:


In solar nebula - dust and gas condense to form grains of solid matter
Condensation: gas atoms stick together to form grains
* allows smallest grains to grow quickly
* less effective as grains gets larger
Type of matter that can condense depends on temperature of solar nebula

Condensation Sequence:
Which types of materials can condense from a gas depends on the temperature
* the lower the temperature, the lower the density of material that 
can condense

* $T < 1500degK, (1250degC) - only refractory (ie. high melting
point) materials can condense
---> high density materials
---> e.g. metals, metal oxides

* $T<1000degK, (750degC) - high & medium melting point materials condense
---> medium and high density materials
---> e.g. silicates (rocky material)

* $T<150degK (100degC) - volatile (ie. low melting point) AND
refractory materials can condense
---> low, medium and high density materials
---> e.g. ices of water, ammonia, methane

Temperature of solar nebula decreases with increasing distance from proto-Sun
---> Close to proto-Sun: only refractory materials (eg. metallic 
grains) condense
--->  Medium distance from proto-Sun: silicate (rocky) & metallic 
grains condense
---> Furthest from proto-Sun: volatile and refractory materials ie. ice,
silicate & metallic grains condense

Also: solar nebula cools with time:
Close to proto-Sun:
* First metallic grains condense
* Later metallic AND silicate grains condense


Another facet I didn't see in your description is why there are
terrestrial planets, gas giants, ice giants, in exactly that order as a
function of distance from the Sun. The solar system's structure has a
very good logic behind, that builds further on the earlier steps of the
condensation sequence.

The refractory molecules, volatile molecules, etc. form in their
particular locations based on the solar nebula temperature profile, and
they form planets and hold gases (atmospheres) based on what body's
gravity can hold that molecule at that particular temperature.

Jupiter with its solid core and its location could attract hydrogen and
helium and all of those light gases, because at that temperature further
from the Sun, the molecules do not have enough kinetic energy to escape
from proto-Jupiter's gravity. So it builds and grows into a gas giant.

Earth could not hold hold a hydrogen atmosphere.. the molecules there
have too much kinetic energy and overcome proto-Earth's gravity

And the idea hold for the other planets.



Amara Graps, PhD      www.amara.com
Associate Research Scientist, Planetary Science Institute (PSI), Tucson
INAF Istituto di Fisica dello Spazio Interplanetario (IFSI), Roma, Italia

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