A little while ago, I wrote the following, responding to a question that
Larry Barnes asked. Although written in the fashion of Old Testament poetry, it's
a reasonably accurate statement of our current understanding of the evolution
of the elements, galaxies, stars, planets and thus ultimately life:

> Larry asks:
>
>  > Interesting article.  It does require asking, where did the clay come
from?
>
> In the beginning was the Bang, and the Bang was good, but it was without
> form. As time progressed, small ripples in the fabric of the Bang
> precipitated mass and energy out of the stuff of the Bang, but the stuff of
> the Bang was without energy. In the beginning was the Void, and the Void
was
> dark, but from the precipitation of the Bang, hydrogen -- and a very little
> helium and a miniscule amount of lithium -- was formed. This was the
periodic
> table for the first several hundred million years.
>
> Nature said, "Let there be light," and there was light. The almost
> imperceptible ripples of the Bang grew and agglomerated to the point of
> accreting large spinning things, composed of almost nothing but hydrogen,
and
> from these large spinning things the first galaxies of stars formed. The
> force of gravity collapsed pockets of this hydrogen gas, and as those
pockets
> collapsed, they self-ignited into the fire of thermonuclear fusion. As
these
> first stars were born, grew and died, they consumed their primordial
hydrogen,
> elevating it into the upper elements of the periodic table through proton-
> proton fusion, but the energies of these first stars were weak too. Iron
was
> the most complex element that was formed in these early stars.
>
> This was the time of giant stars, and when their fuel was exhausted and
they
> could exist no longer, many of the stars died the cataclysmic deaths of
> supernovae, and from this cataclysm, the remaining higher elements of the
> naturally occurring elements were born.
>
> The first generation stars begat the second generation stars, and in turn,
> the second generation stars begat the third, and with each generation, "
> metallicity" increased, and Nature said that "metallicity" was good. The
new
> generation of stars were born with protoplanetary accretion discs. From
these
> accretion discs, planets, with all of their elemental complexities and high-
> volume volatiles, were formed from the void. Covalently bound molecules
> formed in this abundance, centered on the bottom rungs of the periodic
table
> in the fourth group, and from these covalent molecules, hydrocarbons and
> carbohydrates bountifully flowed. The precusor molecules of life were now
> everywhere, and that abundance too was good, and Nature said, "Let there be
> life," and there was life.

In that same regard, I received the following press release from JPL
yesterday that speaks to at least a fragment of that evolutionary sweep: the formation
of protoplanetary discs. Right at the moment, we're seeing two kinds of
circumstellar discs, very thin discs (called "debris discs") and thick
circumstellar discs ("protoplanetary discs"), but the middle ground discs are missing. The
last of NASA's "Great Observatories," the Space Infrared Telescope Facility
(SIRTF), which was just recently launched, was designed in part to seek out
these intermediate discs in order to better understand the evolution of planetary
systems -- and thus ultimately life itself.

I've included the press release below, although it apparently took two weeks
to reach me. It's dated October 20, 2003:

========================================

<A HREF="http://www.jpl.nasa.gov/news/features/planets-102003.cfm">
http://www.jpl.nasa.gov/news/features/planets-102003.cfm</A>

Spotlight: Missing Link Sought in Planetary Evolution
Jet Propsulsion Laboratory
October 20, 2003

Just as anthropologists sought "the missing link" between apes and
humans, astronomers are embarking on a quest for a missing link in
planetary evolution. Only instead of dusty fields and worn shovels,
their laboratory is the universe, and their tool of choice is NASA's
new Space Infrared Telescope Facility.

Launched on Aug.25, NASA's fourth and final Great Observatory will
soon set its high-tech infrared eyes on, among other celestial
objects, the dusty discs surrounding stars where planets are born.

While other ground- and space-based telescopes have spied these
swirling "circumstellar" discs, both young and old, they have missed
middle-aged discs for various reasons. The Space Infrared Telescope
Facility's unprecedented sensitivity and resolution will allow it to
fill in this gap and in the process answer fundamental questions
regarding how planets, including those resembling Earth, may form.

"With the Space Infrared Telescope Facility, we anticipate seeing many
planetary discs at all stages of development," says Dr. Karl
Stapelfeldt of JPL, a scientist with the mission. "By studying how
they change over time, we may be able to determine what conditions
favor planet formation."

Circumstellar discs are a natural step in the evolution of stars.
Stars begin life as dense cocoons of gas and dust, then as pressure
and gravity kick in, they begin to coalesce, and a flat ring of gas
and dust takes shape around them. As stars continue to age, they suck
material from this disc into their core. Eventually, a state of
equilibrium is reached, leaving a more mature star encircled by a
stable disc of debris.

It is around this time, about 10 million years into the lifetime of
the star, that astronomers believe planets arise. Dust particles in
the discs are thought to collide to form larger bodies, which
ultimately sweep out gaps in the discs, much like those lying between
the rings of Saturn.

"You can think of planets as wrecking balls that either clear away
debris or gather it up as if it were mud," says Dr. George Rieke,
principal investigator on one of the three science instruments onboard
the observatory.

Infrared telescopes can sense the glow of the cosmic dust that makes
up these discs; however, they cannot detect planets directly. Planets
have less surface area than their equivalent in dust grains and thus
give off less infrared light. This is the same reason coffee is ground
up before brewing: the larger combined surface area of the coffee
grains results in a more robust pot of coffee.

Past observations of circumstellar discs generally fall into two
categories: young, opaque discs (called protoplanetary discs) with
more than enough mass to match our own solar system's planetary
bodies; or older, transparent discs (called debris discs) with masses
equal to a few moons, and doughnut-like holes at their center.
Middle-aged discs linking these two developmental stages have gone
undetected.

One of the questions astronomers hope to address with the Space
Infrared Telescope Facility is: What happened to all the mass observed
in the younger discs? Somewhere in their evolution, mass is either
eaten up by the star, ejected by the star or transformed into planets
that lie in the doughnut holes of the discs. By analyzing the
composition and structure of the "missing link" discs, astronomers
hope to solve this riddle, and better understand how planetary systems
like our own evolved.

========================================

SIRTF was designed to help answer at least a small part of Larry's question:
"Where did the clay come from?"

Wirt Atmar

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