[meteorite-list] Worried About Asteroid-Ocean Impacts? Don't Sweat The Small Stuff

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:49 2004
Message-ID: <200303171742.JAA16047_at_zagami.jpl.nasa.gov>

WORRIED ABOUT ASTEROID-OCEAN IMPACTS? DON'T SWEAT THE SMALL STUFF
>From Lori Stiles, UA News Services, 520-621-1877
March 17, 2003

The idea that even small asteroids can create hazardous tsunamis may at last
be pretty well washed up.

Small asteroids do not make great ocean waves that will devastate coastal
areas for miles inland, according to both a recently released 1968 U.S.
Naval Research report on explosion-generated tsunamis and terrestrial
evidence.

University of Arizona planetary scientist H. Jay Melosh is talking about it
today at the 34th annual Lunar and Planetary Science Conference in League
City, Texas. His talk, "Impact-Generated Tsunamis: an Over-Rated Hazard," is
part of the session, "Poking Holes: Terrestrial Impacts."

---------------------
Contact Information
H. Jay Melosh
520-621-2806
jmelosh_at_lpl.arizona.edu
----------------------

Given all life's worries, new evidence that asteroids smaller than a
kilometer in diameter wonąt generate catastrophic tsunamis is welcome news,
and not only for coast dwellers. It will save taxpayers the cost of
financing searches for small Earth-approaching asteroids, a savings of
billions of dollars, Melosh said.

(The current NASA-funded effort to search and map truly hazardous
Earth-approaching asteroids ­ those one kilometer or larger in diameter ­ is
now half done and on track to be finished by the end of the decade, Melosh
noted. NASA funds NEAT, LINEAR and the UA Spacewatch programs in this
effort.)

The idea that asteroids as small as 100 meters across pose a serious threat
to humanity because they create great, destructive ocean waves, or tsunamis,
every few hundred years was suggested in 1993 at a UA-hosted asteroids
hazards meeting in Tucson.

At that meeting, a distinguished Leiden Observatory astrophysicist named J.
Mayo Greenberg, who since has died, countered that people living below sea
level in the Netherlands for the past millennium had not experienced such
tsunamis every 250 years as the theory predicted, Melosh noted.

But scientists at the time either didnąt follow up or they didnąt listen,
Melosh added.

While on sabbatical in Amsterdam in 1996, Melosh checked with Dutch
geologists who had drilled to basement rock in the Rhine River delta, a
geologic record of the past 10,000 years. That record shows only one large
tsunami at 7,000 years ago, the Dutch scientists said, but it coincides
perfectly in time to a giant landslide off the coast of Norway and is not
the result of an asteroid-ocean impact.

In addition, Melosh was highly skeptical of estimates that project small
asteroids will generate waves that grow to a thousand meters or higher in a
4,000-meter deep ocean.

Concerned that such doubtful information was ­ and is - being used to
justify proposed science projects, Melosh has argued that the hazard of
small asteroid-ocean impacts is greatly exaggerated.

Melosh mentioned it at a seminar he gave at the Scripps Institution of
Oceanography a few years ago, which is where he met tsunami expert William
Van Dorn.

Van Dorn, who lives in San Diego, had been commissioned in 1968 by the U.S.
Office of Naval Research to summarize several decades of research into the
hazard posed by waves generated by nuclear explosions. The research included
1965-66 experiments that measured wave run-up from blasts of up to 10,000
pounds of TNT in Mono Lake, Calif.

The experiments indeed proved that wave run-up from explosion waves produced
either by bombs or bolides (meteors) is much smaller relative to run-up of
tsunami waves, Van Dorn said in the report. "As most of the energy is
dissipated before the waves reach the shoreline, it is evident that no
catastrophe of damage by flooding can result from explosion waves as
initially feared," he concluded.

The discovery that explosion waves or large impact-generated waves will
break on the outer continental shelf and produce little onshore damage is a
phenomenon known in the defense community as the "Van Dorn effect."

But Van Dorn was not authorized to release his 173-page report when he and
Melosh met in 1995.

Melosh, UA planetary sciences alumnus Bill Bottke of the Southwest Research
Institute and others agreed at a science conference last September that they
needed to find the report.

Bottke found the title - "Handbook of Explosion-Generated Water Waves" - in
a Google search.

Given a title, UA science librarian Lori Critz then discovered that the
report had been published and added to the University California San Diego
library collection in March 2002. Bottke also tracked it down, and had the
report by the time Melosh requested it by interlibrary loan. Both made
several photocopies.

Melosh said, "I since found out it was actually read into the Congressional
Record as part of the MX Missile controversy."

BIOSKETCH: H. JAY MELOSH
Melosh, a professor in the UA planetary sciences department and Lunar and
Planetary Laboratory, is well known for his work in theoretical geophysics
and planetary surfaces. His principal research interests are impact
cratering, planetary tectonics, and the physics of earthquakes and
landslides. His recent research has focused on studies of the giant impact
origin of the moon, the K/T boundary impact that extinguished the dinosaurs,
the ejection of rocks from their parent bodies, and the breakup and
collision of comet Shoemaker-Levy 9 with Jupiter. Melosh also is active in
astrobiological studies that relate chiefly to the exchange of
microorganisms between the terrestrial planets.
Melosh earned his doctorate from Caltech in 1973 and joined the UA faculty
in 1982. He is on the 12-member science team for Deep Impact, a $279 million
robotic mission that will become the first to penetrate the surface of a
comet when it smashes its camera-carrying copper probe into Comet Tempel 1
on July 4, 2005.
Received on Mon 17 Mar 2003 12:42:50 PM PST