Mars Pathfinder

Geological data:
First analysis of Barnacle Bill rock
All graphics NASA.
 
Igneous rocks are usually classified according to the minerals they contain. In the absence of mineralogic data, volcanic rocks can be classified using their chemical compositions. Using this system, Martian rock Barnacle Bill (labelled here as APXS site A-3) is classified as an andesite. The term "andesite" derives from the Andes Mountains in South America, where this type of lava is particularly abundant. Barnacle Bill's andesitic composition could indicate that it is a volcanic rock (a true andesite) or a physical mixture of particles of rocks such as granite and basalt. Such a mixture could have been formed as the particles were transported and deposited as a sediment, or could have been produced during a large impact that pulverized and mixed the target rocks in andesitic proportions.
 

Magnesium/Silicon versus Aluminum/Silicon

SNC meteorites are thought to be Martian samples, based primarily on the fact that they contain trapped gases that have the composition of the Martian atmosphere. All the known SNC meteorites, as well as Viking soils, lie well to the left of the trend for Earth rocks in this diagram. Weight ratios of elements tend to be more precise than absolute element abundances, so ratios are used to plot these preliminary APXS data. Martian rock Barnacle Bill (labelled as APXS site A-3), plots near the line defined by SNCs, perhaps providing support for the idea that SNC meteorites are actually derived from Mars. 
 

Calcium/Silicon versus Iron/Silicon 

These three elements are especially well suited for APXS analysis. The compositions of SNC meteorites, as well as Viking soils, have higher iron/silicon ratios than terrestrial rocks. The APXS soil analysis A-2 plots directly on top of Viking soils in this diagram. However, Barnacle Bill's composition (A-3) plots to the left, because of its high silicon content. 
 
  This image shows the location of Barnacle Bill rock (left of the Sojourner rover) and the approximate location of the full-resolution "multispectral spot" acquired on Barnacle Bill. Lossless (no compression) images were taken in all geology filters using the IMP camera to study in detail the variation of brightness in each filter, which provides information regarding the mineralogy of the material sampled. Spectra were extracted from several study regions (shown to the right of the high resolution view). The green area represents soil found behind the rock. Red patches represent brighter areas on the rock that are interpreted as accumulations of wind-blown dust found in small holes, or vesicles, on the rock. Blue patches represent darker rock faces not contaminated by a soil deposit. The spectra of these materials are shown in the accompanying figure. 
 
  Preliminary data acquired from the multispectral spot image sequence for Barnacle Bill rock. Images were acquired with no compression in all geology filters. Reflectance spectra (that is, the variation of brightness with wavelength, or color) are shown for background soil (green), soil-like deposits found on and within small holes in the rock (red), and dark portions of the rock face (blue). Comparison of the spectra of these three types of materials demonstrates that the rock has relatively homogeneous composition at the spatial resolution of the patches sampled (about 1-3 cm). That is, all soil-like deposit and rock face spectra cluster in both their overall brightness (reflectance) and shape of their reflectance curves. A more heterogeneous rock would show variable spectral characteristics across its face. Note that the spectra of the soil-like deposit is intermediate to that of the background soil and rock face spectra. This is consistent with the interpretation that the soil-like deposit is a relatively thin layer in which portions of the rock are also sampled within the patches selected. 

Also shown are laboratory spectra of oxidized and unoxidized volcanic rocks from Earth. Scientists will compare spectra of terrestrial materials such as these to help determine the composition of the rocks observed at the landing site in combination with data returned by other instruments such as the APXS.