[meteorite-list] Mars Rover Nears 'Bathtub Ring of Blueberries'

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 15 Dec 2006 09:28:30 -0800 (PST)
Message-ID: <200612151728.JAA20713_at_zagami.jpl.nasa.gov>

http://space.newscientist.com/article/dn10802-mars-rover-nears-bathtub-ring-of-blueberries.html

Mars rover nears 'bathtub ring of blueberries'
David L Chandler
New Scientist
14 December 2006

A bumper crop of "blueberries" has been found around the rim of Mars'
Victoria Crater by NASA's Opportunity rover. The discovery suggests that
when the rover descends into the crater in a few months, it may find a
bathtub ring of the tiny concretions - an ancient high-water mark
suggesting the area was altered by ground, not surface, water.

Blueberries are thought to have formed by the precipitation of
iron-bearing minerals when groundwater rose up through layers of
sediment. The spheres have been seen scattered across the surface almost
everywhere Opportunity has crossed on its 7-kilometre voyage from its
landing site to the nearly kilometre-wide Victoria Crater.

But they have become much smaller and sparser along the way, dropping
from 5 millimetres wide to less than 1 mm across. Since the rover neared
the crater's rim, however, that trend has reversed.

There, the terrain was "full of great big juicy blueberries again," said
rover chief scientist Steven Squyres on Wednesday at the American
Geophysical Union's fall meeting in San Francisco, California, US. "That
was a surprise to us."

Slight tilt

Since the spheres form underground as groundwater rises through the
soil, the find suggests the water rose only so far and then stopped -
without reaching the surface. This left a layer filled with large
blueberries, with very few in the overlying layers.

In fact, the Meridiani Planum plain Opportunity has been crossing is
tilted slightly and the rover has been moving gradually upwards, into a
higher layer with few blueberries. The great impact that produced
Victoria Crater, however, dug into the blueberry-rich layer below and
tossed thousands of the spheres all around its rim.

The finding was unexpected because Opportunity had earlier found ripples
in Endurance Crater a few kilometres away that indicated water at least
occasionally flowed on the surface there. Now, it seems such events were
rare around Victoria Crater, and that most of the time the water only
rose to a level well below the surface.

Just how far down that level is will not be known until the rover
descends into the crater in a few more months. "As we go down, we'll
cross a bathtub ring" that marks the highest level the water typically
reached, says Squyres.
          
Dry dunes

Opportunity has been moving slowly around the crater, taking detailed
pictures of the sides of each promontory it passes. Already, it has
clearly shown dramatic variations in the thickness of the layers along
the cliffsides.

"We're seeing lateral stratigraphy, something we could never do before"
because of the wide expanse of layers exposed in the large crater," says
Squyres. So far, he says, these extended layers reveal dune-like
profiles, suggesting a dry surface environment that was mostly shaped by
prevailing winds. Rising groundwater would have formed blueberries
within these layers after the dunes had been buried and compacted.

As they approach their third anniversary on Mars, the twin rovers remain
in excellent health, except for one stuck wheel on Spirit, on the
opposite side of Mars. That rover has stayed close to one spot for the
last nine months, where among other observations, it spotted the first
water-ice clouds identified from the surface of Mars.

It will soon be moving again, towards rocky areas whose compositions
show clear signs of having been affected by water.
Received on Fri 15 Dec 2006 12:28:30 PM PST