(meteorobs) Excerpts from "CCNet, 21/2000 - 15 February 2000"

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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 15 February 2000
Date: Tue, 15 Feb 2000 10:08:12 -0500 (EST)

CCNet, 21/2000 - 15 February 2000


     "The images of Eros recently sent back to Earth by the Near=20
     spacecraft show a surface covered with craters, some of which=20
     are quite large and degraded relative to Eros's size. This might=20
     seem surprising, given the general belief that Near-Earth=20
     Asteroids are fragments of main belt bodies that reached the=20
     terrestrial planet region via injection into powerful resonances=20
     like the 3:1 mean-motion resonance or the nu6 secular resonance.=20
     Since the median lifetime of NEAs is of order 10~My, NEAs are=20
     generally expected to have sparsely cratered surfaces. However,=20
     it appears that Eros may have spent a long time collisionally=20
     coupled to the main belt after its collisional birth. [...]
     We favor a scenario where many main belt asteroids slowly drift=20
     into resonance from neighbouring regions; this radial drift is=20
     probably caused by "Yarkovsky" thermal drag forces which act=20
     over the collisonal lifetime of the bodies [...]. In light of=20
     these new model results, we believe that many or perhaps most=20
     NEAs should have an age comparable to their collisional=20
     lifetime, presumably explaining the densely cratered surface of=20
         -- Alessandro Morbidelli and Bill Bottke, 15 February 2000

    Alessandro Morbidelli and Bill Bottke

    http://near.jhuapldot edu/iod/20000214g/index.html

    Space Science News <express@spacescience.com>=20

    Andrew Yee <ayee@nova.astro.utorontodot ca>


CCNet-NEWS, 15 February 2000


By Alessandro Morbidelli and Bill Bottke

The images of Eros recently sent back to Earth by the Near spacecraft=20
show a surface covered with craters, some of which are quite large=20
and degraded relative to Eros's size. This might seem surprising,=20
given the general belief that Near-Earth Asteroids are fragments of=20
main belt bodies that reached the terrestrial planet region via=20
injection into powerful resonances like the 3:1 mean-motion resonance=20
or the nu6 secular resonance. Since the median lifetime of NEAs is of=20
order 10~My (Gladman et al., Science, 277, 197-201, 1997) NEAs are=20
generally expected to have sparsely cratered surfaces. However, it=20
appears that Eros may have spent a long time collisionally coupled to=20
the main belt after its collisional birth. =20

Using a model presented at the last DPS in Abano which can=20
quantitatively reproduce the observed orbital and size distribution=20
of NEAs, we believe we can explain this inconsistency.  Our=20
computations show that 25-50% of the largest NEAs arrive on=20
terrestial planet-crossing orbits only after a slow increase of their=20
orbital eccentricity (discussed in Migliorini et al., Science, 281,=20
2022--2024, 1998;  Morbidelli and D. Nesvorny, Icarus, 139, 295-308,=20
1999).  The rest of the NEA population should reach planet-crossing=20
orbits through a "classical" fast track resonance (e.g., 3:1 or nu6=20
resonance). The number of bodies that are required to pass through 
the fast- and slow-track resonances per million year, however, is=20
inconsistent with a dominant role of collisions in the=20
resonance-feeding process. For this reason, we favor a scenario where=20
many main belt asteroids slowly drift into resonance from=20
neighbouring regions; this radial drift is probably caused by=20
"Yarkovsky" thermal drag forces which act over the collisonal=20
lifetime of the bodies (Farinella and Vokrouhlicky, Science, 283,=20
1507-1510, 1998; Bottke et al. 2000, Icarus, in press), though=20
alternative mechanisms are under study. In light of these new model=20
results, we believe that many or perhaps most NEAs should have an age=20
comparable to their collisional lifetime, presumably explaining the=20
densely cratered surface of Eros.

P.S. A paper on this topic by Bottke, Jedicke, Morbidelli, Gladman=20
and Petit is under revision for publication in Science

For further information, please contact

Alessandro Morbidelli <morby@obs-nice.fr>
Bill Bottke < bottke@astrosun.tn.cornelldot edu>


NEAR image of the day for 2000 Feb 14=20
http://near.jhuapldot edu/iod/20000214g/index.html

Today at 10:33 AM EST the NEAR spacecraft was successfully inserted=20
into orbit around 433 Eros, becoming the first artificial satellite=20
of an asteroid. Just over an hour later, NEAR pointed its camera at=20
the asteroid and took this picture from a range of 210 miles (330 km)=20
above the surface. Mission navigators and operators will use this=20
image and others to be taken later to traingulate on landmarks on the=20
asteroid's surface, precisely measuring position of the spacecraft to=20
plot NEAR's course.

Features as small as a 100 feet (30 meters) across can be seen. This=20
view shows the 3-mile (5-kilometer) impact crater which the spacecraft=20
has spied for over a week during its approach. The two smaller craters=20
superimposed on its rim are each about 1.2 miles (2 kilometers) across. =

An enormous boulder a full 170 feet (50 meters) in size sits on the=20
large crater's floor. Other key features of the surface are shallow=20
subsurface layering exposed near the tops of crater walls, and
shallow grooves crossing the surface and cutting the crater's rim.

- --------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics=20
Laboratory, Laurel, Maryland, NEAR was the first spacecraft launched in =

NASA's Discovery Program of low-cost, small-scale planetary missions.=20
See the NEAR web page at http://near.jhuapldot edu for more details.


>From Space Science News <express@spacescience.com>=20

Space Science News for February 14, 2000
First Light from Eros Orbit:  NEAR's first close-up pictures from Eros  =

orbit have arrived at Earth. This story includes a beautiful image of a =

large crater on the asteroid and highlights from this afternoon's NASA=20
press briefing.  FULL STORY at



>From Andrew Yee <ayee@nova.astro.utorontodot ca>

>From NATURE, Friday, 11 February 2000

The ice microbes cometh

At the end of last year, microbes were found under thousands of metres=20
of ice in Antarctica. The discovery not only stretched the habitable=20
regions of the Earth to new extremes but also lent hope to the idea=20
that life might eke out a precarious existence on other worlds. Now new =
research shows where these microbes might come from, and how they might =
survive the rigours of a life in ice.

The plucky bacteria, encased in ice many thousands of years old, were=20
first reported last December by a team of US scientists[1]. They were=20
found in the bottom 100 metres of a core of ice drilled 3590 metres=20
into the ice sheet at East Antarctica's Vostok Station.

Why are the bacteria in the lowest part of the ice, not closer to the=20
surface, where they might have been deposited on wind-borne dust? The=20
answer is supplied by results reported in Nature[2] by Martin Siegert=20
of the University of Bristol and co-workers. Hidden beneath the ice=20
sheet on which Vostok Station stands is a vast lake, called Lake=20
Vostok, discovered in the 1970s.=20

Lake Vostok, all of which is below several kilometres of solid ice, has
been mapped out using radar signals, which bounce back from the ice at=20
the top and bottom of the lake to reveal its buried profile. At 670=20
metres deep and covering 14,000 square kilometres, it is the largest=20
known sub-ice lake.

Siegert's group has analysed radar data from airborne measurements=20
revealing that ice is being lost from the base of the sheet in the=20
north and west of the lake. To the south, on the other hand, the ice=20
over the lake is about 150 metres thicker on average, owing to freezing =
of the lake water.=20

This suggests that ice is melting over one part of the lake and being=20
reformed over another. This may induce circulation in the lake water,=20
just as the water in surface lakes circulates because of convection. It =
may also release rocky debris and other ice-bound impurities into the=20
water, which, the researchers say, could provide nutrients for any=20
organisms living in the lake.

It was precisely because of the presence of the lake that the ice core=20
was drilled. The core was taken from a region where refrozen ice had=20
been accreted from the lake onto the bottom of the ice, and the=20
drilling stopped just 120 metres short of the top of the lake. This=20
meant that it penetrated about 100 metres into the 'accretion' ice, and =
it was here that bacteria were found -- some still living after being=20
released from the ice core. This suggests that they may grow within the =
lake itself.

But how, asks physicist P. Buford Price of the University of California =
in the Proceedings of the National Academy of Sciences[3], could=20
bacteria go on living within ice several degrees below freezing point,=20
at pressures four hundred times greater than the air pressure at the=20
Earth's surface?=20

Price says that the accretion ice above Lake Vostok provides all three=20
of the ingredients essential to life: water, energy and carbon. Glacier =
ice, he points out, is laced with a network of water-filled veins=20
between solid ice grains, in which salts accumulate, lowering the=20
freezing point and preventing the veins from icing up. Price estimates=20
that these veins could be several thousandths of a millimetre across in =
the Vostok accretion ice -- wide enough to accommodate bacterial cells.

The liquid veins also concentrate dissolved acids, including organic=20
acids such as formic and acetic acid (the main component of vinegar).=20
Price argues that chemical reactions involving these acids, which have=20
been detected from the ice-core studies, could provide sufficient
energy and carbon to support the number of microbes found in the ice=20

Lake Vostok is the best terrestrial analogue of Jupiter's moon Europa.=20
Over the past few years, the Galileo spacecraft orbiting Jupiter has=20
sniffed out strong evidence that below the crust of ice covering=20
Europa's surface lurks an ocean of liquid water stretching from pole to =
pole. This is the only known world in the solar system other than Earth =
on which a large body of liquid water seem likely to exist (although it =
is possible that Callisto, another of Jupiter's moons, might also have=20
a subsurface ocean). If life can exist in the ice above Lake Vostok,=20
thousands of metres below frozen Antarctica, who is to say that it =
might not be found also below the ice fields of Europa?

[1] Jouzel, J., Petit, J.R., Souchez, R., Barkov, N.I., Lipenkov, V.Y., =

Raynaud, D., Stievenard, M., Vassiliev, N.I., Verbeke, V. & Vimeux, F.=20
More Than 200 Meters of Lake Ice Above Subglacial Lake Vostok,=20
Antarctica Science 286, 2138-2141 (1999).=20

[2] Siegert, M.J., Kwok, R., Mayer, C. & Hubbard, B. Water exchange=20
between the subglacial Lake Vostok and the overlying ice sheet Nature=20
403, 643 (2000).=20

[3] Price, P.B. A habitat for psychrophiles in deep Antarctic ice. PNAS =

97, 1247-1251 (2000).=20

=A9 Macmillan Magazines Ltd 2000 - NATURE NEWS SERVICE

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