MAGMATIC PROCESSES ON PLANETARY BODIES: WHAT WE (THINK WE) KNOW AND HOW WE KNOW It

Volcanic behavior is best determined by watching active eruptions, and magma origin is constrained by examining samples of erupted materials. However, remotely located or prehistoric volcanic activity must be inferred by examining the volcanic deposits. Extraterrestrial volcanism is no exception, and researchers use an increasing array of tools to infer the inner workings and past behaviors of other planets.

Laboratory analyses of hand samples returned from the field provides vital information on magma genesis, ascent and eruption. Only the Moon and Mars have provided direct samples for laboratory analyses. Analyses of the lunar rocks revealed a high-temperature (~1400°C), low-viscosity (<10 Pa s) basaltic lava; martian meteorites also have a mafic to ultramafic compositions. These results have been used repeatedly to infer similar compositions on other terrestrial planets on the basis of analogous gross morphologies.

In absence of hand samples, we rely on more equivocal data sets collected by landers, rovers and spacecraft, with advantages and disadvantages for all methods of data collection and analyses. Inevitably, the acquisition of more data causes us to question our earlier assumptions and interpretations.

To date, however, we’ve learned that Mars, the Moon, Earth and Venus display crustal dichotomies. Mafic and ultramafic volcanic products dominate on Earth, the Moon, and are abundant on Mars. Limited compositional information, and interpretations of volcanic morphologies (quantified using laboratory and computer simulations), suggest that mafic and ultramafic products are common on Venus as well. Mercury and Io do not (yet) reveal crustal dichotomies, and the compositions of specific volcanic products on both those planetary surfaces remain enigmatic. Smooth plains, commonly interpreted to be flood lavas, are observed on all the terrestrial planets (including Io); recent investigations suggest, however, that not all plains should be interpreted as volcanic.