[meteorite-list] Re: Mercury Meteorite Puzzle

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:04:49 2004
Message-ID: <200205171858.LAA15671_at_zagami.jpl.nasa.gov>

BBC News article:
>>"NWA 011 has an oxygen isotope ratio that indicates it
>>came from a body larger than a big asteroid."

>Okay. This article explains to me why this meteorite
>came from a body larger than an asteroid. But it
>doesn't explain how the other "planet-sized bodies" in
>our solar system are being ruled-out in favor of
>Mercury.

Actually, it looks like the BBC didn't quite report this
correctly. Yamaguchi reports that the meteorite came
from an "unknown, differentiated planetary object". And
Venus hasn't been ruled out.

In the April Science article by Herbert Palme,
he says the low Fe in the basaltic rock indicates it came
from a parent body with a metal core. So, that would
narrow the candidates to Mercury, Venus, Earth, Moon,
Vesta or another large asteroid. Oxygen isotopic differences
rules out Vesta. A high FeO contest indicates the core
is on the small size, so that rules out the Earth. We have
samples of the Moon, and Mars meteorites, and their differences
would rule out these two bodies. So, the candidate
list shrinks to Mercury, Venus or another large asteroid.
Herbert Palme in his Science article says that "perhaps NWA011 is a
basalt from Mercury". It seems everyone has latched onto
that statement. In the same paragraph, Palme also says that
"Alternatively, a much smaller asteroid may have produced basalts
of compositions that are so far only known from the larger
bodies of the solar system."

Anyway, at this point, we have 3 candidates for NWA 011:
Mercury, Venus or a large asteroid.

Now, let's talk a little about the E chondrites, the other meteorite
group that is speculated to come from either Mercury or Venus.
These meteorites have a very low oxygen content, which indicates they
formed inside the orbit of Earth. Since Mercury and Venus are inside
of Earth's orbit, then by the process of elimination, that would
indicate the parent body for E chondrites are either Mercury or Venus.
OK, fair enough.

Now, consider the following. In 1998, astronomers from the University
of Hawaii announced the discovery of asteroid 1998 DK36. I've attached
their press release at the end. What is interesting about 1998 DK36 is
its orbit. This is the first asteroid discovered whose orbit lies
entirely inside of Earth's orbit. Though the asteroid is rather small
at only about 40 meters in diameter, its discovery indicates that they
may be other asteroids that exist, yet to be discovered, with similar
orbits that are also entirely inside of Earth's orbit. In other words,
the parent body of the E chondrites and NWA 011 may be an asteroid.
Such an asteroid would have to be large enough to form a metal core,
and orbit closer to the Sun than the Earth. Asteroids in these
types of orbits will be hard to observe from Earth because of their close
proximity to the Sun. While it is still possible the
parent body for the E chondrites and NWA 011 may be Mercury or Venus,
you can't rule out asteroids just yet either.

Ron Baalke

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

Institute for Astronomy
University of Hawaii

Contacts: Dr. David Tholen 808-956-6930 tholen_at_ifa.hawaii.edu
          Mr. Robert Whiteley 808-956-6700 robw_at_ifa.hawaii.edu

July 1, 1998

Astronomers Find New Class of Asteroid

University of Hawaii astronomers have discovered a new type of asteroid,
whose orbits lie completely within the orbit of the Earth. Previously, all
known asteroids traveled in an orbit farther from the Sun than the Earth,
over at least a portion of their journey.

"All other efforts to discover asteroids on a collision course with the
Earth are being directed at a region of the sky almost opposite the Sun,"
said David Tholen, planetary astronomer at the Institute for Astronomy.

"The significance of this discovery is that we would have otherwise never
found this new asteroid because it apparently doesn't travel to that region
of the sky being scanned by other search efforts."

If such an asteroid's orbit around the Sun intersects with the Earth's
orbit, it could hit the Earth and we would have never seen it coming, said
Tholen. We would have been caught unaware by an asteroid approaching us from
the daytime side of the sky, he said.

Tholen and graduate student Robert Whiteley made the observation using a
specialized camera fitted on the University of Hawaii's 2.24-meter telescope
atop Mauna Kea last February.

While scanning the dusk and dawn skies to assess the size and number of
asteroids within the Earth's orbit, Whiteley spotted the object, since
designated 1998 DK36, on his computer screen, shortly after Tholen had
recorded the images at Mauna Kea Observatory and sent them to Whiteley's
computer via the Internet.

Additional observations made the following night made it possible to
compute a preliminary orbit of the object around the Sun. Tholen said the
exact size and shape of the asteroid orbit remain uncertain. However, the
orbit's farthest point from the Sun could be determined relatively
accurately, and it appears to be very close to, but slightly inside the
orbit of the Earth.

The asteroid is thought to be about 40 meters in diameter, similar in size
to the one that flattened the Tunguska region of Siberia on June 30, 1908,
as well as the iron object that produced Meteor Crater in Arizona 50,000
years ago.

Could it collide with the Earth?

"We were unable to obtain enough observations to perform a formal
probability calculation, though the best-fitting orbit has the object
passing an apparently safe 750,000 miles from the Earth's orbit," said
Tholen. "To do a better job with such discoveries, we really need to have a
telescope that we can dedicate to such difficult observations."

"1998 DK36 is nothing to lose sleep over," said Tholen. "It's the ones we
haven't found yet that are of concern."
Received on Fri 17 May 2002 02:58:12 PM PDT