[extropy-chat] volcanic gas- origins of life?

[Amara Graps]

hal at finney.org:

>Thanks, Amara, that was very interesting information.  The nature of
>the early Earth is apparently far less understood than I had realized.

>NASA announced yesterday that recent observations imply that the
>planet formation stage was somewhat different than previously thought,
>http://www.spitzer.caltech.edu/Media/releases/ssc2004-17/release.shtml.
>The implication is that as planetisimals joined to form planets, there
>was a longer phase of destruction and rejoining, rather than a relatively
>smooth process of accretion.  I suppose this might provide more thorough
>mixing of the initial components.

The press release says:

"According to the most popular theory, rocky planets form somewhat like
snowmen. They start out around young stars as tiny balls in a
disc-shaped field of thick dust. Then, through sticky interactions with
other dust grains, they gradually accumulate more mass. Eventually,
mountain-sized bodies take shape, which further collide to make planets."

Yes, but there is a "Late Phase" that is more-or-less accepted by planet
formation modelers that the press release doesn't mention. I think the
Spitzer press release should be saying that there is  'more' collisions
in the Late Phase in terrestrial planet formation than astronomers 
previously thought.

I was thinking of this topic last summer when a "Supper-Earth" planet
was found with a mass of about 14 Earth masses. I tried to find in the
literature some upper masses for Earthlike planets, and I found none.
(This is not my specialty area either, but I try to follow the literature.)
Too many variables, and the last phase of planet formation is the
largest variable. The following is what I collected about terrestrial
planet formation.


The article that I've seen that best describes the process of
terrestrial planet formation was published last April in Physics
Today:

[1] "Origin of Terrestrial Planets and the Earth-Moon System"
by Robin Canup, pgs. 56-62. 

More details on terrestrial planet formation can be found online
in the KITP seminars. These are particularly relevant:

[2] Terrestrial Planet Planet Formation in BInary Star Systems
by Jack Lissauer
http://online.itp.ucsb.edu/online/planetf_c04/lissauer/oh/01.html

[3] Implications of Planet Formation Models for the Initial State
of the Earth by David Stevenson
http://online.itp.ucsb.edu/online/planetf_c04/stevenson/oh/01.html

[4] Two Fluid Flights of Fancy: From Dust to Planetesimals 
by Andrew Youdin
http://online.itp.ucsb.edu/online/planetf_c04/youdin/oh/01.html

see also:
J.E. Chambers (2001), "Accretion in the Solar System" Icarus 152 205.

Terrestrial planet accretion in our solar system is typically
described in three stages [1, 2]:

I. Early Stage: 

Growth of dust grains (about micron)  --> planetesimals (1-10 km)
Timescale: Fast [3]

This early stage is the least well-understood process [1],
coagulation (sticking) is difficult, gravitational instability is
hard [4].

II. Middle Stage: 

Growth by accretion of planetesimals --> planetary embryos (1000s km)
Timescale: ~ 10^7 years  [1, 3]

Canup's paper describes this part well. She says:

This next stage is much better understood due to extensive modeling
work by the theoreticians. The rate of accretions (and hence the
growth of the planetesimals) is controlled by the rate of collisions
among the orbiting planetesimals  in the solar nebula.

The rate of collisions depends on the local orbital velocity (which
increases with decreasing distance from the Sun, so that regions
closer to the Sun generally accrete more rapidly), the number
density of the planetesimals and their sizes and relative
velocities. A source of velocity damping for the smaller objects is
the gaseous nebula.

There is a possible 'runaway growth' that can happen, due to the
largest objects growing the fastest, with a single object running
away with most of the available mass in its annular region in the
solar nebula disk. In this case, then an object of roughly 1% of
Earth mass can grow in as little as 10^5 years.

Eventually we run out of solar nebula material.  so that becomes is
our final limiting factor for growth at this stage.

III. Late Stage:

Collision of tens to hundreds of embryos to yield --> planets

Occurs in the absence of the solar nebula, but not always.

Current modeling work suggests that solid planets are 'sculpted' by
a violent, stochastic final phase of giant impacts. She says in [1]
that a seemingly inherent feature of the late stage is giant impacts,
in which lunar-to-Mars-sized objects mutually collide to yield the
final few terrestrial planets. (Canup has worked extensively on the 
Earth-Moon formation process, that occured at this stage.)
 
The bodies come from outside of the original accretion zone. Note 
that in our own solar system, these events were strongly influenced 
by Jupiter's gravity. The implication of this last stage is that our
terrestrial planets (and Moon) may only represent one possible
outcome in a wide array of potential solar-system architectures.[1]


Amara

-- 
Amara Graps, PhD     www.amara.com
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF), 
Adjunct Assistant Professor Astronomy, AUR, 
Roma, ITALIA     Amara.Graps at ifsi.rm.cnr.it