[meteorite-list] Interstellar Meteorites(?)
From: Kelly Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:32 2004
The May 19, 2001, issue of SCIENCE NEWS, has a nice article on the
question of whether comets could have brought organic "pre-life"
molecules to the Earth without the impact of the comet destroying the
complex organic molecules. At the U. of Chicago, they've build a "comet
cannon" which fires shells with biotic molecules in them at near-impact
speeds. They have discovered that the pressures of impact polymerized
the molecules into more complex ones, rather then destroying them.
There's a nice photo of the cannon (how do you explain that to the
building inspector?) and a photo of the contrail of the Tagish Lake
Then, the August, 2001, issue of DISCOVER contains an article
entitled "Did Life on Earth Come From Mars?" It details various lines of
research on SNC's and the chance of survival of microbes, particularly
halophiles and bacterial mats, on their unplanned excursion from Mars to
One subject that gets a lot of discussion in this article is the
question of whether it is possible for life to be transferred from one
solar system to another (as apparently it can be between planets of the
same solar system). The article quotes Curt Mileikowsky and Jay Melosh
on this question. (Ron Baalke posted a news note about Melosh's work on
this subject last month.) Both have used computer modeling of the
possibility and they say that the interstellar transfer of meteorites is
virtually impossible. One quote: "Not one rock in a billion years."
I find it fascinating that someone who claims that 55 billion
(that's 55,000,000,000) Mars rocks have landed on Earth since the
beginning of the solar system can so blithely pooh-pooh interstellar
rocks! (You know what that 55 billion Mars rocks claim would have done
to your career 30 years ago?)
So, at the risk of proving that I never learned not to quarrel with
research reported in DISCOVER, I thought about checking the likelihood
of that not one rock in a billion years claim. Their result is derived
from computer modeling, always a powerful and sensitive amplifier of
error, omission and bias.
What do we know about real interstellar rocks? What, you say, are
there real interstellar rocks? Yes. In New Zealand there is a facility
called the Advanced Meteor Orbit Radar (or AMOR for short). It tracks
and determines the velocities and orbital properties of approximately
1,000 meteors per day. Of those 1000 meteors per day, about 2 are
interstellar in origin --- they have velocities in excess of the 72.43
km/sec which is the upper limit for bodies gravitationally bound to the
Sun at the distance of the Earth's orbit.
Especially fascinating is the fact that when interstellar meteors
are plotted on the celestial chart and false colored for frequency, a
big "hot spot" develops smack dab on the location of beta Pictoris, a
star 51 light years away which is believed to be a solar system in
formation, with a big dusty disc which has been photographed.
The AMOR radars cover only 3% to 4% of the Earth's surface, of
course, so the 2 interstellar meteors per day that it observes means
that there must be 50 interstellar meteors per day falling to Earth in
total. And since the interstellar nature of these meteors is revealed
solely by their speed in excess of the perfect retrograde encounter,
there must also be detections of prograde interstellar meteors whose
speeds are less than the critical value and so "escape the net." That
means there are really more like a total of 100 interstellar meteors
reaching the Earth per day.
If you draw a sphere centered on the Sun with a radius equal to the
Earth's distance from the Sun, you have created a surface that an
interstellar meteor has to cross to hit the Earth. (Actually, it has to
cross it twice, once inbound and once outbound.) If you cover the sphere
of the Earth's orbit with patches the size of the collisional cross
section of the Earth, it takes about 2 billion patches to cover the
sphere, which means that the Earth has about a one in a billion chance
of of being hit by any object that crosses the sphere (twice, remember).
If 100 interstellar meteors hit the Earth per day, then that means
that 100 billion interstellar meteors cross the sphere of the Earth's
orbit every day, or 18 trillion interstellar meteors per year. You know,
that's a hell of a lot of interstellar meteors!
Of course, they're little meteors, about 40 microns in size. It
would take 1,500,000 of them to weigh a gram. A gram is about what a one
centimeter interstellar meteor would weigh.
We can compute the likelihood of a bigger (or smaller) object that
the ones we observe by using the power law. The power law says that in a
randomly produced assembly of different sizes of objects each size class
possesses equal mass. For example, in a population of asteroids with
1000 one kilometer ones, there will be only 100 two kilometer ones and
only 10 four kilometer ones. The numbers are declining but the mass in
each class is the same. This techniques is widely used and accepted in a
variety of applications.
So, if the sphere of the Earth's orbit is crossed by 36 trillion
(36,000,000,000,000) 40 micron interstellar meteors per year, there will
be about 36,000 one centimeter ones crossing it per year. Since the
Earth has a one in a billion chance of being hit by such objects, that
would imply the Earth is struck by a 1-cm interstellar meteor every
27,778 years. During that same time, it would have a one in ten chance
of being hit by a 2-cm object, a one in a hundred chance of being hit by
a 4-cm object, and so on. The total can be adjusted by a factor of
1.11111... (ain't decimals grand?)
That brings the mean time between hits of a 1-cm or larger
interstellar meteor to 25,000 years. 25,000 years is a long time... but
it sure as hell ain't as long as a billion years.
One objection might be that dust-sized particles may be
over-represented because collisional efficiency increases as the
particle sizes get smaller. The frequency of 1-cm interstellar meteors
calculated by the power law is less that 1/300 of 1%, considerably
smaller than the incidence of 1-cm and larger objects in the infall of
asteroidal dust and conventional meteorites to the Earth.
Of course, the other real problem is that interstellar rocks are
moving fast. The landing is going to be tricky.
I always like to check the results of computer modeling with the
real world from time to time. Me, I'm putting out the welcome mat for
Sterling K. Webb
Received on Wed 18 Jul 2001 12:33:18 AM PDT