Seismic Shaking Erased Small Impact Craters On Asteroid Eros

SEISMIC SHAKING ERASED SMALL IMPACT CRATERS ON ASTEROID EROS
From Lori Stiles, UA News Services, 520-621-1877
November 26, 2004

University of Arizona scientists have discovered why Eros, the largest
near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from
February 2000 to February 2001, it revealed an asteroid covered with
regolith -- a loose layer of rocks, gravel and dust -- and embedded with
numerous large boulders. The spacecraft also found places where the regolith
apparently had slumped, or flowed downhill, exposing fresh surface
underneath.

But what NEAR didn't find were the many small craters that scientists
expected would pock Eros' landscape.

"Either the craters were being erased by something or there are fewer small
asteroids than we thought," James E. Richardson Jr. of UA's planetary
sciences department said.

Richardson concludes from modeling studies that seismic shaking has
obliterated about 90 percent of the asteroid's small impact craters, those
less than 100 meters in diameter, or roughly the length of a football field.
The seismic vibrations result when Eros collides with space debris.

---------------------------------------------------
Contact Information
James E. Richardson 520.621.6960

Related Web site
http://www.lpl.arizona.edu/~jrich/
-----------------------------------------------------

Richardson, Regents' Professor H. Jay Melosh and Professor Richard
Greenberg, all with UA's Lunar and Planetary Laboratory, report the analysis
in the Nov. 26 issue of Science.

"Eros is only about the size of Lake Tahoe -- 20 miles (33 kilometers) long
by 8 miles (13 kilometers) wide," Richardson said. "So it has a very small
volume and a very low gravity. When a one-to-two-meter or larger object hits
Eros, the impact will set off global seismic vibrations. Our analysis shows
how these vibrations easily destabilize regolith overlaying the surface."

A rock-and-dust layer creeps, rather than crashes, down shaking slopes
because of Eros' weak gravity. The regolith not only slides down
horizontally, but also is launched ballistically from the surface and 'hops'
downslope. Very slowly, over time, impact craters fill up and disappear,
Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a
200-meter crater would fill in about 30 million years. Because Eros is now
outside the asteroid belt, that process takes a thousand times longer, he
added.

Richardson's research results match the NEAR spacecraft evidence. Instead
of the expected 400 craters as small as 20 meters (about 70 feet) per square
kilometer (three-fifths mile) on Eros' surface, there are on average only
about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros'
internal structure.

"The NEAR mission showed Eros to most likely be a fractured monolith, a
body that used to be one competent piece of material," Richardson said. "But
Eros has been fractured throughout by large impacts and is held together
primarily by gravity. The evidence is seen in a series of grooves and ridges
that run across the asteroid's surface both globally and regionally."

Large impacts fracture Eros to its core, but many smaller impacts fracture
only the upper surface. This gradient of big fractures deep inside and
numerous small fractures near the surface is analogous to fractures in the
upper lunar crust, Richardson said. "And we understand the lunar crust --
we've been there. We've put seismometers on the moon. We understand how
seismic energy propagates through this kind of structure."

The UA scientists' analysis of how impact-induced seismic shaking has
modified Eros' surface has a couple of other important implications.

"If we eventually do send spacecraft to mine resources among the near-Earth
asteroids or to deflect an asteroid from a potential collision with the
Earth, knowing internal asteroid structure will help address some of the
strategies we'll need to use. In the nearer future, sample return missions
will encounter successively less porous, more cohesive regolith as they dig
farther down into asteroids like Eros, which has been compacted by seismic
shaking," Richardson noted.

"And it also tells us about the small asteroid environment that we'll
encounter when we do send a spacecraft out into the main asteroid belt,
where Eros spent most of its lifetime. We know the small asteroids -- those
between the size of a beachball and a football stadium -- are out there.
It's just that their 'signature' on asteroids such as Eros is being erased,"
Richardson said.

This finding is important because the cratering record on large asteroids
provides direct evidence for the size and population of small main-belt
asteroids. Earth-based telescopic surveys have catalogued few main-belt
asteroids that small. So scientists have to base population estimates for
these objects primarily on visible cratering records and asteroid
collisional history modeling, Richardson said.

In message , Ron
writes
SEISMIC SHAKING ERASED SMALL IMPACT CRATERS ON ASTEROID EROS
From Lori Stiles, UA News Services, 520-621-1877
November 26, 2004

"If we eventually do send spacecraft to mine resources among the near-Earth
asteroids or to deflect an asteroid from a potential collision with the
Earth, knowing internal asteroid structure will help address some of the
strategies we'll need to use. In the nearer future, sample return missions
will encounter successively less porous, more cohesive regolith as they dig
farther down into asteroids like Eros, which has been compacted by seismic
shaking," Richardson noted.

"And it also tells us about the small asteroid environment that we'll
encounter when we do send a spacecraft out into the main asteroid belt,
where Eros spent most of its lifetime. We know the small asteroids -- those
between the size of a beachball and a football stadium -- are out there.
It's just that their 'signature' on asteroids such as Eros is being erased,"
Richardson said.

This finding is important because the cratering record on large asteroids
provides direct evidence for the size and population of small main-belt
asteroids. Earth-based telescopic surveys have catalogued few main-belt
asteroids that small. So scientists have to base population estimates for
these objects primarily on visible cratering records and asteroid
collisional history modeling, Richardson said.

There's been a recent paper ("Extreme albedo comets and the impact
hazard" W. M. Napier J. T. Wickramasinghe and N. C. Wickramasinghe,
Monthly Notices of the Royal Astronomical Society Volume 355 Issue 1
Page 191 - November 2004
doi:10.1111/j.1365-2966.2004.08309.x) suggesting that there may be many
more comets than previously supposed. Does this result from Eros tell us
anything about whether that theory is plausible?
--
What have they got to hide? Release the ESA Beagle 2 report.
Remove spam and invalid from address to reply.

Ron wrote:

SEISMIC SHAKING ERASED SMALL IMPACT CRATERS ON ASTEROID EROS
From Lori Stiles, UA News Services, 520-621-1877
November 26, 2004

University of Arizona scientists have discovered why Eros, the largest
near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from
February 2000 to February 2001, it revealed an asteroid covered with
regolith -- a loose layer of rocks, gravel and dust -- and embedded with
numerous large boulders. The spacecraft also found places where the regolith
apparently had slumped, or flowed downhill, exposing fresh surface
underneath.

But what NEAR didn't find were the many small craters that scientists
expected would pock Eros' landscape.

"Either the craters were being erased by something or there are fewer small
asteroids than we thought," James E. Richardson Jr. of UA's planetary
sciences department said.

Richardson concludes from modeling studies that seismic shaking has
obliterated about 90 percent of the asteroid's small impact craters, those
less than 100 meters in diameter, or roughly the length of a football field.
The seismic vibrations result when Eros collides with space debris.

---------------------------------------------------
Contact Information
James E. Richardson 520.621.6960

Related Web site
http://www.lpl.arizona.edu/~jrich/
-----------------------------------------------------

Richardson, Regents' Professor H. Jay Melosh and Professor Richard
Greenberg, all with UA's Lunar and Planetary Laboratory, report the analysis
in the Nov. 26 issue of Science.

"Eros is only about the size of Lake Tahoe -- 20 miles (33 kilometers) long
by 8 miles (13 kilometers) wide," Richardson said. "So it has a very small
volume and a very low gravity. When a one-to-two-meter or larger object hits
Eros, the impact will set off global seismic vibrations. Our analysis shows
how these vibrations easily destabilize regolith overlaying the surface."

A rock-and-dust layer creeps, rather than crashes, down shaking slopes
because of Eros' weak gravity. The regolith not only slides down
horizontally, but also is launched ballistically from the surface and 'hops'
downslope. Very slowly, over time, impact craters fill up and disappear,
Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a
200-meter crater would fill in about 30 million years. Because Eros is now
outside the asteroid belt, that process takes a thousand times longer, he
added.

Richardson's research results match the NEAR spacecraft evidence. Instead
of the expected 400 craters as small as 20 meters (about 70 feet) per square
kilometer (three-fifths mile) on Eros' surface, there are on average only
about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros'
internal structure.

"The NEAR mission showed Eros to most likely be a fractured monolith, a
body that used to be one competent piece of material," Richardson said. "But
Eros has been fractured throughout by large impacts and is held together
primarily by gravity. The evidence is seen in a series of grooves and ridges
that run across the asteroid's surface both globally and regionally."

Large impacts fracture Eros to its core, but many smaller impacts fracture
only the upper surface. This gradient of big fractures deep inside and
numerous small fractures near the surface is analogous to fractures in the
upper lunar crust, Richardson said. "And we understand the lunar crust --
we've been there. We've put seismometers on the moon. We understand how
seismic energy propagates through this kind of structure."

The UA scientists' analysis of how impact-induced seismic shaking has
modified Eros' surface has a couple of other important implications.

"If we eventually do send spacecraft to mine resources among the near-Earth
asteroids or to deflect an asteroid from a potential collision with the
Earth, knowing internal asteroid structure will help address some of the
strategies we'll need to use. In the nearer future, sample return missions
will encounter successively less porous, more cohesive regolith as they dig
farther down into asteroids like Eros, which has been compacted by seismic
shaking," Richardson noted.

"And it also tells us about the small asteroid environment that we'll
encounter when we do send a spacecraft out into the main asteroid belt,
where Eros spent most of its lifetime. We know the small asteroids -- those
between the size of a beachball and a football stadium -- are out there.
It's just that their 'signature' on asteroids such as Eros is being erased,"
Richardson said.

This finding is important because the cratering record on large asteroids
provides direct evidence for the size and population of small main-belt
asteroids. Earth-based telescopic surveys have catalogued few main-belt
asteroids that small. So scientists have to base population estimates for
these objects primarily on visible cratering records and asteroid
collisional history modeling, Richardson said.

There is still the question of the origin of all boulders (sizes of a house
to the size of a pebble). I find it very hard to accept the theory that the
boulders were produced by the impact that created the largest crater.
(With its low gravity why were not all boulders ejected into space?)
Also is there evidence of boulders in other small bodies like Phobos?

There is still the question of the origin of all boulders (sizes of a house
to the size of a pebble). I find it very hard to accept the theory that the
boulders were produced by the impact that created the largest crater.
(With its low gravity why were not all boulders ejected into space?)
Also is there evidence of boulders in other small bodies like Phobos?


Most did escape. A few didn't.

Zdenek Jizba wrote

There is still the question of the origin of all boulders (sizes of a house
to the size of a pebble). I find it very hard to accept the theory that the
boulders were produced by the impact that created the largest crater.
(With its low gravity why were not all boulders ejected into space?)
Also is there evidence of boulders in other small bodies like Phobos?


Exploded Planet Hypothesis predicted that the debris would be
orbiting Eros. Perhaps, the boulders, craters and fractures were
produced during a planetary fragmentation. John Curtis

John Curtis wrote:

Zdenek Jizba wrote

There is still the question of the origin of all boulders (sizes of a house
to the size of a pebble). I find it very hard to accept the theory that the
boulders were produced by the impact that created the largest crater.
(With its low gravity why were not all boulders ejected into space?)
Also is there evidence of boulders in other small bodies like Phobos?


Exploded Planet Hypothesis predicted that the debris would be
orbiting Eros. Perhaps, the boulders, craters and fractures were
produced during a planetary fragmentation. John Curtis

This is a reasonable hypothesis. However, I do have an objection to
the theory of fractures. Since a study concluded that Eros' rotation (from
the time it was tumbling) was produced by a gradual increase of its angular
momentum, any pre-existing fractures would have resulted in a breakup of
the asteroid.

Zdenek Jizba writes:

John Curtis wrote:

Exploded Planet Hypothesis predicted that the debris would be
orbiting Eros. Perhaps, the boulders, craters and fractures were
produced during a planetary fragmentation.

This is a reasonable hypothesis.

The Exploded Planet Hypothesis is not a reasonable hypothesis.
Planetary fragmentation can occur via mechanisms other than
explosion, however.

The Exploded Planet Hypothesis is not a reasonable hypothesis.
Planetary fragmentation can occur via mechanisms other than
explosion, however.


When that Mars sized planet hit the old Earth, some of the
fragments may have escaped into solar orbit. But that's
not really an explosion though. Planets
don't explode like Superman's home planet Krypton...

When that Mars sized planet hit the old Earth, ...

QED - that's where earth's intellignet life came from. Mars.

Happy now EC?

Jack

_____Evaporating exoplanet hints at the origin of pressure
inside a rocky planet:
http://hubblesite.org/newscenter/new.../2003/08/text/
Evaporation of Jupiter's atmosphere would produce a pressure
difference of 4.5 million bars between the trapped gases
inside Jupiter's rocky interior and its now thinner atmosphere.
Could a smaller pressure differential be responsible for the
breakup of Earth's crust into plates? Primordial gases are
still escaping Earth's interior through these breaks in the
crust:
http://www.soes.soton.ac.uk/staff/wjj/geol/geol.html John Curtis