[meteorite-list] Dawn Journal - January 30, 2013

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 31 Jan 2013 17:18:15 -0800 (PST)
Message-ID: <201302010118.r111IF1g006252_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_01_30_13.asp

Dawn Journal
Dr. Marc Rayman
January 30, 2013

Dear Dawnt Look Backs,

Its long and daring interplanetary journey continuing smoothly, Dawn is
making good progress in gradually reshaping its orbit around the sun.
Its uniquely efficient ion propulsion system is gently bringing it
closer to its next destination, dwarf planet Ceres, and ever farther
from its previous one, Vesta. Although the robotic explorer's sights are
set firmly ahead, let's take one last look back at the fascinating alien
world it unveiled during its 14 months in orbit there.

Vesta, the second most massive resident of the main asteroid belt
between Mars and Jupiter, was discovered in 1807. For more than two
centuries thereafter, the mysterious object appeared as little more than
a fuzzy patch of light among the stars. The only one of the millions of
main belt asteroids to be bright enough to be visible to the naked eye,
Vesta beckoned, but its invitation was not answered until Dawn arrived
in July 2011, nearly four years after it left Earth. The cosmic ambassador
is the only spacecraft ever to have orbited an object in the main asteroid
belt, and its ambitious mission would have been impossible without ion
propulsion.

Dawn found a complex and exotic place, and it returned a fabulously rich
collection of pictures and other measurements that will continue to be
analyzed for many, many years. For now, we will simply touch on a very few
of the many insights that already have been illuminated by the light of Dawn.

Scientists recognize Vesta as being more like a mini-planet than like
the chips of rock most people think of as asteroids. The behemoth is 565
kilometers (351 miles) wide at the equator and has a surface area more
than twice that of California (although it is populated by far fewer
eccentrics, billionaires, and other colorful characters found in that
state). Dawn's measurements of the gravity field provide good evidence that Vesta
separated into layers, much like Earth did as the planet was forming.
Vesta's dense core, composed principally of iron and nickel, may be 200
to 250 kilometers (125 to 150 miles) across. Surrounding that is the
mantle, which in turn is covered by the veneer of the crust, about 20
kilometers (12 miles) thick. The once-molten core is now solid (in
contrast to Earth's, which remains hot enough to be liquid), but the
differentiation into layers gives Vesta a key distinction from most
asteroids. Because it was likely still in the process of accumulating
material to become a full-sized planet when Jupiter's immense gravity
terminated its growth, scientists often refer to Vesta as a protoplanet.

Among the most prominent features of the alien landscape is a huge gouge
out of the southern hemisphere so large that its presence was inferred
from observations with the Hubble Space Telescope. Dawn found this
gigantic crater to be even deeper and wider than expected, penetrating
about 19 kilometers (12 miles) and spanning more than 500 kilometers
(310 miles), or nearly 90 percent of the protoplanet's equatorial diameter.

The yawning hole is now known as Rheasilvia, after the Vestal Virgin who
not only was the mythical mother of Romulus and Remus, but also surely
would have been astounded by the spectacular sights on Vesta as well as
the spacecraft's capability to point any user-defined body vector in a
time-varying inertial direction defined by Chebyshev polynomials. As
Dawn has brought Vesta into focus, cartographers have needed labels for
the myriad features it has discovered. The International Astronomical
Union names Vestan craters for Vestal Virgins and other famous Roman
women; mountains, canyons, and other structures are named for towns and
festivals associated with the Vestal Virgins.

Vesta dates to the dawn of the solar system, more than 4.5 billion years
ago, and its age shows. Myriad craters tell the story of a timeworn
surface that has been subjected to the rough and tumble conditions of
life in the asteroid belt ever since. A virtual rain of space rocks has
fallen upon it. While Rheasilvia records the most powerful punch, from
an object as much as 50 kilometers (30 miles) across, there are at least
seven craters, some quite ancient indeed, more than 150 kilometers
(nearly 100 miles) in diameter. As the eons pass, craters degrade and
become more difficult to discern, their crisp shapes eroded by
subsequent impacts large and small.

The long history of cratering is particularly evident in the startling
difference between the northern and southern hemispheres. The north is
very densely cratered, but the south is not. Why? The titanic blow that
carved out Rheasilvia is estimated to have occurred over one billion
years ago. It excavated a tremendous volume of material. Much of it fell
back to the surface, wiping it clean, so the cratering record had to
start all over again. Recall that the crater itself is 500 kilometers
(310 miles) in diameter, and scientists estimate that 50 kilometers (30
miles) outside the rim, the debris may have piled about 5 kilometers (3
miles) high. Even at greater distances, preexisting features would have
been partially or completely erased by the thick accumulation. The
effect did not reach to the northern hemisphere, however, so it retained
the craters than had formed before this enormous impact.

Some of the rocks were ejected with so much energy that they broke free
of Vesta's gravitational grip, going into orbit around the sun. They
then went their own way as they were yanked around by the gravitational
forces of Jupiter and other bodies, and many of them eventually made it
to the part of the solar system where your correspondent and some of his
readers spend most of their time: Earth. When our planet's gravity takes
hold of one of these Vesta escapees, it pulls the rock into its
atmosphere. Some lucky witness might even observe it as a meteor. Its
blazing flight to the ground is not the end of its glory, however, for
these rocks are prized by planetary geologists and other enthusiasts who
want a souvenir from that impact.

Scientists now know that about 6 percent of the meteorites that have
been found originated on Vesta. Six percent! One of every 16 meteorites!
This is an astonishingly large fraction. Apart from Mars and the moon,
Vesta is the /only/ known source of specific meteorites. Although rocks
from Vesta had to travel much farther, they far outnumber meteorites
from these other two more familiar celestial bodies.

Combining laboratory studies of the numerous samples of Vesta with
Dawn's measurements at the source provides an extraordinary opportunity
to gain insights into the nature of that remote world. Meteorites from
Vesta are so common that they are often displayed in museums
(occasionally even without the curators' awareness of their special
history) and can be obtained from many vendors. Anyone who has seen or
held one surely must be moved by contemplating its origin, so distant in
space and time, from well beyond Mars and long before animal or plant
life arose on Earth.

The impact that formed Rheasilvia partially obliterated the second
largest crater on Vesta, Veneneia. That 400-kilometer (250-mile) crater
was about 12 kilometers (7.5 miles) deep. It was formed more than two
billion years ago, around the time life on Earth grew to macroscopic
proportions and photosynthesis by cyanobacteria was still introducing
oxygen to the atmosphere.

Scientists are deciphering a possible consequence of the colossal
collisions that formed Rheasilvia and Veneneia. The events were so
forceful that they sent shock waves reverberating through the entire
world. (It is fortunate that Vesta was not destroyed, because if it had
been, we would not have such a fascinating place to explore, although it
may well be that other protoplanets in the asteroid belt did meet that
fate.) As the energy surged through Vesta's interior, the material
deformed in complex ways that just a chunk of rock could not. But Vesta
is not just a chunk of rock. As a mini-planet, its interior composition
and properties have been altered by the geological forces that formed
and shaped the world. The seismic stretching apparently caused faults
400 kilometers (250 miles) from the impact sites, and those scars are
now evident in another of the most conspicuous features: a vast network
of chasms near the equator.

Eighty-six gorges mapped at the equator appear to have been caused by
the Rheasilvia impact, and another seven may be from Veneneia.
Individual troughs extend to as much as 465 kilometers (289 miles) in
length. One is more than 39 kilometers (24 miles) wide and 4.0
kilometers (2.5 miles) deep. These dimensions rival those of the Grand
Canyon. This would be as if huge impacts on Earth in Barrow, Alaska and
in London (at similar latitudes but the opposite hemisphere as
Rheasilvia and Veneneia, respectively) had triggered the formation of
giant canyons near the equator.

Vesta has another feature that exceeds the dimensions of anything found
on Earth. At the center of Rheasilvia is a mountain of staggering
proportions. The summit soars to a fantastic 20 to 25 kilometers (12.4
to 15.5 miles) above the variable elevation of the terrain around it
(even ignoring the smaller craters that go deep into the floor of
Rheasilvia) and 180 kilometers (110 miles) across at its base. This
colossus is well over twice the height of puny Mt. Everest, which rises
to less than 9 kilometers (5.5 miles) above the distant seas. Vesta's
peak is the second tallest known in the solar system. Olympus Mons on
Mars is (slightly) higher.

Dawn's extensive stereo measurements
showed that Vesta's extreme topography is not limited to Rheasilvia.
This craggy world has many steep slopes. When a space rock smashes into
such a slope, the crater it forms may be unstable. The uphill portion
succumbs to the pull of Vesta's gravity and the resultant landslide
leaves a very deformed crater, as Dawn observed in many locations.

Vesta's surface displays more variety than dramatic craters, towering
mountains, expansive chasms, and other impressive topographic features.
Among the surprises are strong variations in the brightness of the
material itself. Dark splotches here and there are likely deposits from
dark rocks that formed in a different location in the solar system and
eventually crashed into Vesta. Bright areas are interpreted to be
material that originated on Vesta and which has hardly changed at all
since the giant protoplanet formed. Gradually it was covered by debris
settling to the ground from impacts elsewhere, but occasionally an
impact exposes it.

The sophisticated probe from Earth has allowed scientists to make many
more wonderful discoveries, far too many to be presented here (or, at
least, far too many for your correspondent to describe without exceeding
his self-imposed limit of 2,173 words). Some of them have been reported
in JPL/NASA news releases as well as in other
websites, printed publications, news broadcasts, at dinner tables, and
perhaps even on playgrounds (the kind this writer would have enjoyed
anyway). And if you haven't been to Vesta to behold the marvelous sights
yourself, then either go there or go here
<http://dawn.jpl.nasa.gov/multimedia/imageoftheday/archives.asp?month=2012-September>
to see a few of the best views.

As half a second in a person's lifetime, for one of its nearly 4.6
billions years, Vesta had a companion from Earth, but now it is alone
again. Dawn has given us stunning views of this survivor from the dawn
of the solar system. Even as dazzling knowledge is gained about Vesta,
that distant orb grows only fainter for the probe itself, their
separation already being more than 10 times the distance between Earth
and the moon. The world Dawn orbited not so long ago now appears only as
a pinpoint. It glows among the stars about as bright as Canopus, the
second brightest star (apart from the sun) visible from our solar
system. Only Sirius is brighter, about twice as luminous. (For Dawn, as
well as for terrestrial observers, the planet Jupiter also shines
brighter now.)

The discoveries so far are only the beginning. Dawn's sensors returned
so much data that it will take decades to mine their riches. Surprising
new understandings will be gained by further studies, combined with more
investigations of the meteorites from Vesta, comparisons with other
worlds, and additional new information about the cosmos. One of the
beauties of science is that it allows us not only to comprehend more and
more about the universe but also to ask -- and ultimately answer -- more
and more insightful questions. Dawn has already made great contributions
to this splendid enterprise. The explorer promises still more rewards as
it travels ever farther on its exhilarating quest for knowledge, taking
us all on a bold and exciting interplanetary adventure to uncharted worlds.

Dawn is 4.2 million kilometers (2.6 million miles) from Vesta and 57
million kilometers (35 million miles) from Ceres. It is also 1.92 AU
(288 million kilometers or 179 million miles) from Earth, or 750 times
as far as the moon and 1.95 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 32
minutes to make the round trip.
Received on Thu 31 Jan 2013 08:18:15 PM PST


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