archive: SETI FW: [ASTRO] Mars Pathfinder APXS Science Results

SETI FW: [ASTRO] Mars Pathfinder APXS Science Results

Larry Klaes ( )
Thu, 8 Oct 1998 09:27:52 -0400

From: Ron Baalke
Sent: Wednesday, October 07, 1998 7:34 PM
Subject: [ASTRO] Mars Pathfinder APXS Science Results

Mars Pathfinder APXS Science Results

The Alpha Proton X-Ray Spectrometer on the rover measured the compositions
of nine rocks. The silicon content of some of the rocks is much higher than
that of the martian meteorites, our only other samples of Mars. The martian
meteorites are all mafic and ultramafic igneous rocks, volcanic and
intrusive rocks that are relatively low in silicon and high in iron and
magnesium. Such rocks would be expected to form by partial melting of the
upper mantle of Mars. The melt rises up though the crust and solidifies at
or near the surface. The mafic volcanic martian meteorites, referred to as
basalts, are the most common rock on Earth and have also been found on the
Moon. Based on the composition of the martian meteorites and the presence of plains and mountains that look like features produced by basaltic volcanism
on Earth, geologists expected to find primarily basalts on Mars.

The rocks analyzed by Pathfinder, however, are not basalts. If they are
volcanic - as suggested by their pitted surface texture, presumably formed
when gases trapped during cooling left small holes in the rock - their
silicon content classifies them as andesites. One way that andesites can
form is when a basaltic melt from the mantle intrudes deep within the crust.
Crystals rich in iron and magnesium form and are separated from the melt,
leaving a more silicon-rich melt that erupts onto the surface. The andesites
were a great surprise, but because we do not know where these rocks came
from on the martian surface, we do not know the full implications of this
discovery. If the andesites are representative of the highlands, they
suggest that ancient crust on Mars is similar in composition to continental
crust on Earth. This similiarity would be difficult to reconcile with the
very different geologic histories of the two planets. Alternatively, the
rocks could represent a minor fraction of high-silicon rocks on a
predominately basaltic plain.

Intriguingly, not all the rocks appear to be volcanic. Some have lineations
that may be layers like those in terrestrial sedimentary rocks, which form
by deposition of smaller fragments of rocks in water. Indeed, rover images
show many rounded pebbles and cobbles on the ground. In addition, some
larger rocks have what look like embedded pebbles and shiny indentations,
where it looks as though rounded pebbles that were pressed into the rock
during its formation have fallen out, leaving holes. These rocks may be
conglomerates formed by flowing liquid water. The water would have rounded
the pebbles and deposited them in a sand, silt and clay matrix; the matrix
was subsequently compressed, forming a rock, and carried to its present
location by the flood. Because conglomerates require a long time to form, if
these martian rocks are conglomerates they suggest that liquid water was
once stable and that the climate was therefore warmer and wetter than at
present. A possible alternative explanation for the bumpy textures is weathered lithic fragments and crystals in volcanic rocks.