October 8, 1997 Press Conference - Dr. Henry Moore
"Cabbage Patch"
This
Sojourner rover image of the "Cabbage Patch" shows small rounded objects on the
surface that are about 3-4 cm across. Some of these are within excavations, which are
about 0.5 cm wide. Several questions arise about the pebbles: 1. Why are they rounded? 2.
Where did they come from? 3. What do they mean? Geologists use MULTIPLE WORKING HYPOTHESES
when attempting to explain observations. Some hypotheses that could account for the
pebbles are:
They were rounded during transport by waters of catastrophic floods and deposited on the Ares Vallis flood plain.
They were rounded by wave action on an ancient Martian beach.
They were rounded during glacial transport.
They are glasses that were produced by melting during impact cratering. The glass was first ejected from the crater, then molded into spherical shapes or drops as it traveled through the atmosphere, and finally was deposited at the sites.
They are spatter from lava flows.
They are nodules brought up from the deep Martian interior by lava flows or pyroclastic eruptions.
They are concretions formed in sedimentary rocks.
They came from ancient conglomerate rocks. The pebbles were rounded by water action and subsequently lithified into conglomerate rocks. Later, the waters of catastrophic floods transported the conglomerates and deposited them on the Ares Vallis flood plain. The pebbles were then freed from the rocks by weathering.
A combination of the above.
Pebbles are also seen in lander images, along with cobbles. For
example, in this picture, we see the same pebbles that were visible in the Sojourner rover
image of the "Cabbage Patch" (Figure 1). In addition, a cobble within the rock
"Lamb" (upper left) is apparent. This indicates that Lamb may be a conglomerate
(Lamb is 0.32 m x 0.15 m).
"Shark", "Half Dome" Rock
This
color composite image from the Pathfinder lander shows the rock "Shark" at upper
right (Shark is about 0.69 m wide, 0.40 m high, and 6.4 m from the lander). The rock looks
like a conglomerate in Sojourner rover images, but only the large elements of its surface
textures can be seen here. This demonstrates the usefulness of having a robot rover
"geologist" able to examine rocks up close.
This color composite image of the
"Rock Garden" shows the rocks "Shark" and "Half Dome" at
upper left and middle, respectively. Between these two large rocks is a smaller rock (
about 0.20 m wide, 0.10m high, and 6.33 m from the lander) that was observed close-up with
the Sojourner rover.
This rover image of
"Shark" (upper left center), "Half Dome" (upper right), and a small
rock (right foreground) revealtextures and structures not visible in lander camera images.
These rocks are interpreted as conglomerates because their surfaces have rounded
protrusions up to several centimeters in size. It is suggested that the protrusions are
pebbles and granules.
This close-up Sojourner rover image
of a small rock shows that weathering has etched-out pebbles to produce sockets. In the
image, sunlight is coming from the upper left. Sockets (with shadows on top) are visible
at the lower left and pebbles (with bright tops and shadowed bases) are seen at the lower
center and lower right. Two pebbles (about 0.5 cm across) are visible at the lower center.