PATHFINDER RESULTS PREDICTED BY GSFC SCIENTISTS!!!

Barnacle Bill and mini-rover Sojourner
"Barnacle Bill" and mini-rover Sojourner

The recent exciting result of an "andesitic" composition for the rock "Barnacle Bill" on Mars, and the resulting conclusion that Mars is probably geologically more like the Earth than like the Moon, was actually anticipated years ago by GSFC scientists.

In 1989 Paul Lowman, a senior geologist in the Geodynamics Branch, published a paper in Precambrian Research (vol 44, pp. 171-195, 1989) on "Comparative Planetology and the Origin of Continental Crust". In this paper he described a general model for the origin of the Earth's earliest continental crust. This theory combined terrestrial geological and geochemical information with comparative planetary analogies (mostly from the Moon and Mars) and suggested the Earth's earliest (continental) crust was formed by andesitic volcanism, the result of global melting and differentiation (not, originally, by plate tectonics). Lowman pointed out that the ancient highland crust of Mars ("a petrologically Earth-like planet") should likewise be andesitic. Determination of such a composition for martian highland rocks, he suggested, would be the best way to test this theory.

Eight years later, the martian mini-rover Sojourner, in its first analysis of the composition of a martian rock, apparently recorded just the expected composition.

Previous work by Lowman as part of the Mariner 9 IRIS (InfraRed Interferometric Spectrometer) team, led by now-retired GSFC PI Rudy Hanel, had provided some of the information on which this theory of early planetary crustal evolution was based. Spectra obtained by IRIS in 1972 during the global dust storm indicated a composition of about 60% (+/- 10%) silica for the dust, very similar to the 58% silica composition found by Sojourner.

Mini-Rover Sojourner analyzes the composition of Barnacle Bill
Mini-Rover Sojourner analyzes the composition of "Barnacle Bill"

Definition

Andesite - A fine grained intermediate volcanic igneous rock characterized by the presence of oligoclase or andesine. Their chemistry and mineralogy are closely similar to those of the diorites. Porphyritic varieties are fairly common, both feromagnesian minerals and feldspars occurring as phenocrysts - the latter commonly showing zoning. Hypersthene and enstatite are more common in andesites than in diorites. There is little doubt that some pyroxene andesites have been called olivine-free basalts and vice-versa. Pure glassy andesites are rare, but glass is of frequent occurrence in the groundmass of andesites, commonly in a devitrified state.
Rocks which display characteristic basaltic features (especially by containing olivine), except that they contain oligoclase or andesine, may be called mugearite and hawaiite respectively, but they are more properly classified as trachyandesites. With increasing silica content and the development of free quartz, andesites pass into dacites (Rhyolite). Andesites occur as extensive lava flows always associated with continental masses. The 'Andesite Line' is the boundary between the 'continental' andesitic association and the 'oceanic' basaltic association.