[meteorite-list] Meteorites from Outside the Solar System

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:28 2004
Message-ID: <3D8AB379.EB079A51_at_bhil.com>

Hi, Rafael, List,

    Yes, there are interstellar meteorites. Not just incorporated interstellar
grains in 4.5 billion year old meteorites, but real honest to god interstellar
meteorites hitting the earth every day. Actually about one every 15 minutes...
     We had a thread about this question back in mid-July, 2001. Ron Baalke had
posted a news note about Jay Melosh's work on this subject in June, 2001. That
work was computer modeling of the possibility of interstellar meteorites and he
(Melosh) said that the interstellar transfer of meteorites is virtually
impossible: "Not one rock in a billion years."
    I really disagreed with that conclusion, so here's (some) of what I said
about it in my July 17, 2001, post to the Meteorite List, sort of my two
interstellar grains' worth:

QUOTE:
    I find it fascinating that someone (Melosh) 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? Jeesh!)
    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 an equal number of prograde interstellar
meteors whose speeds are less than critical 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 one 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. A landing on Earth is going to be tricky. But NOT impossible...
UNQUOTE

    And, of course, these odds are derived from the very, very quiet
interstellar neighborhood we are in right now. At many times in the past, the
Sun has spent tens of millions of years travelling through much more crowded
stellar environments where the odds of an interstellar rock arriving would be
hundreds or even thousands of times greater than they are now.


Sterling K. Webb
----------------------------------------------

For those that love references...

    The information on the AMOR radars, interstellar meteors, and beta
Pictoris was from an article on interplanetary (and interstellar) dust in
the January, 2001 issue of SKY & TELESCOPE magazine. Their website is
<http://www.skypub.com/>.

    The work by Mileikowsky and Melosh on the transfer of Martian
meteorites and/or interstellar meteorites was:

Mileikowsky C., Cucinotta F.A., Wilson J.W., Gladman B., Horneck G.,
Lindegren L., Melosh H.J., Rickman H., Valtonen M. and Zheng J.Q. (200)
'Risks threatening viable transfer of microbes between bodies in our solar
system', Planetary and Space Science 48 (2000) 1107-1115.

Melosh H.J. (1994) 'Swapping Rocks: Exchange of Surface Material Among the
Planets', The Planetary Report, The Planetary Society, July 1994.

Melosh H.J. (2001) 'Exchange of Meteoritic Material Between Stellar
Systems', 32nd Annual Lunar and Planetary Science Conference, Abstract
no.2022.

    For a popular summary of the issue, look at the following:
<http://www.spacedaily.com/news/life-01r.html>.

--------------------------------------------------------------------------

"Rafael B. Torres" wrote:

> Hello List Good Morning to all and happy dayˇˇˇ. I was reading Rocks from
> Space and looking at my collection, thinking about meteorites. In one part
> of the book Norton says that asteroids in early solar system were perturbed
> by Jupiter's gravity, sending them aoutside the solar systemˇ...I thought a
> bout that, there must be several asteroids from Earth that are wandering and
> traveling somewhere there aoutside our solar system within out Milky
> Way....If thats possible, maybe the other way is tooˇˇˇˇ.
>
> This is what I mean, there are millions of stars in our Milky WAy, which
> thousands (and moreˇˇ) may have planets forming or formed, there are also
> millions of dark bodies, and gas clouds, where asteroids or planetesimals
> may be forming, well if a star passes near one of them or just another big
> planet, they might be millions of pieces of rock wandering our Milky WAy,
> what are the chances of one entering the solar system, making its way
> through Earth and more important what chracteristics should it
> show?????...Is there any expert on this topic that can answer this, maybe
> they have an older isotopic record, but the isotopes and daughter maybe are
> destroyed after several million years, do we have any chances to idenify an
> intruder from outside the Solar System??????
>
> Maybe we have a meteorite from Outside the Solar System in our collections,
> but we don't know, maybe we should look closer and remenber that next time
> we hold a NWA meteorite. Thats what I like of NWA meteorites, we never know
> what have they seenˇˇˇˇ
>
> Rafael B. Torres
> 2001 Space Collection
> http://www.geocities.com/rafael_blando
Received on Fri 20 Sep 2002 01:34:50 AM PDT