Message-Id: <v01530501af666d4c7e6a@[207.90.162.211]>
Date: Tue, 1 Apr 1997 02:27:40 -0800
To: pinhole@exploratorium.edu
From: ronwong@unleashed.net
Subject: Re: Oort cloud and comet composition
Dan Gray asked a few more questions:
>You said that the existance of the Oort cloud is more or less taken for
>granted now. Why is that?
First, a disclaimer:
I have made a number of comments regarding questions in the area of
astronomy that have been posted here lately, but I am not an astronomer. My
remarks have been based on whatever I have happened to have come across
over the past 40 years. In print, they convey a certain amount of authority
that is not at all warrented. My background is in physics and, although
astronomy finds its way into my work, it's not my primary interest. I fully
expect others to post corrections to remarks that I have been making here.
Paul Dougherty's explanation of how the Oort cloud came about sounds fine
to me - at least for the inner clouds from which come many of the short
period comets. The fact that others haven't chimed in probably means
they're too busy doing something else. So, caveat emptor.
Regarding the Oort cloud:
Right now the feeling in astronomy about the Oort cloud is not unlike that
which existed in nuclear physics regarding the neutrino. There wasn't any
experimental evidence of the neutrino's existance until physicists demanded
that there should be experimental evidence for it's existance. Their
explanation for a particular type of nuclear change required the existance
of such a particle despite it's outrageous properties. Its discovery simply
confirmed what everyone knew had to be.
The Oort cloud is the astronomer's way of explaining the large number of
long period comets that are actually observed in the heavens. Without it,
the long period comets would have been perturbed in a way that made them
short period comets at a rate that would have left us with very few long
period comets. Like the neutrino, the cloud is a theoreticians way of
explaining what is known. The model has been fine tuned to where it's
predictions regarding long period comets and the rate of conversion from
long period comets to short period comets seem consistent with what is
observed (so far). There still is a problem with short period comets and
the resolution of that problem involves regions substantially closer to the
sun than the Oort cloud. That some of these explanations may require later
modifications to the model of the Oort cloud remains to be seen but
astronomers seem comfortable with the present situation (at least as of
about 6 years ago - the last time I came across anything regarding this
aspect of comets).
>I do have another question about comets:
>Why are they primarily ice?
>How do water molecules even manage to get together out there?
The prevailing theory is that proposed by Kuiper at about the same time
that Oort proposed the Oort cloud: Matter was distributed about what was
later to become our sun in a fashion approximating the orbits of the
present planets. As things went around collisions occurred and gravity
acted in such a way as to bring together our sun and it's surrounding
proto-planets. In time, much of the lighter, gaseous, outer material of the
inner planets (known as the terrestial planets) was driven off due to their
proximity to the sun. They condensed into ice particles of water, methane,
carbon dioxide, etc, beyond the region of Mars. Like the inner planets,
these particles formed the outer, gaseous planets as a result of collisions
and gravitational attraction. These planets swept the region clear of the
remaining ice particles after their formation. What was left was a
suspicious object called Pluto and it's satellite Charon and the ice
particles that avoided assimulation -the Kuiper belt - which lies beyond
Neptune and extends out to the inner Oort cloud. From the Kuiper belt comes
the comets. Those that fall inward produce the short period comets, and
those flung outward produce the Oort cloud from which come eventually the
long period comets (of which some become short period comets and others
projectiles hurtling out to the remote regions of our universe).
Ron Wong
Lowell High School
San Francisco, CA