Mineral equilibria and the high radar reflectivity of Venus mountaintops

Abstract

We studied the relationship between altitude and microwave emissivity (the complement of power reflectivity) in 10 highland regions of Venus, using the Magellan data set. Above a critical altitude that ranges from 4.75 km on Maxwell Montes to 2.49 km on Sapas Mons, emissivity undergoes an abrupt decrease to values so low (<0.6) that the surface mineralogy on these mountaintops must differ from that at lower altitudes in such a way as to enhance the bulk dielectric constant of the highland surface material. This emissivity effect probably is produced not by basalt minerals but by a secondary, weathered mineral assemblage. We developed the phase diagrams descriptive of equilibrium secondary mineral assemblages on Venus, as a function of altitude and atmospheric redox state. The mineral responsible for low emissivity on mountaintops appears to be the electrical semiconductor pyrite (FeS2). Variation from one highland area to another in the altitude at which pyrite becomes the stable Fe mineral (as opposed to magnetite at lower altitudes) may be attributable to local modulation by topography of the atmospheric thermal profile. The altitudes at which the magnetite/pyrite phase boundary is encountered vary with the redox slate of the atmosphere, and observed values of this altitude can be used to estimate an oxygen fugacity at plains level of ∼10−21 bars. Maat Mons, a volcanic peak almost as lofty as Maxwell Montes, is the only high mountaintop on Venus that has not weathered to a low‐emissivity mineral assemblage. Presumably, this means flows at the highest altitudes are negligibly weathered and the volcano is relatively young.

Internal crater modification on Venus: Recognizing crater‐centered volcanism by changes in floor morphometry and floor brightness

“…Nevertheless, differences in elevation can produce variations in modification. Laboratory data indicate that both olivine and pyroxene are more unstable in the Venusian lowlands than in the highlands [Fegley et al, 1992;Klose et al, 1992]. Thus, while basaltic minerals can weather at all elevations on Venus, such weathering should be both faster and more intense at lower elevations, and it may diminish significantly with increasing altitude.…”

“…On Venus this process seems to be dominated by chemical reactions between atmospheric SO2 or CO2 and iron-bearing silicates [Fegley et al, 1992;Klose et al, 1992]. The primary result is a disintegration of rock surfaces into fine-grained dust which slowly accumulates in local lows to soften radar returns .…”

Stabilization of Venus’ climate by a chemical-albedo feedback

“…This observed mixing ratio of SO 2 is close to the equilibrium mixing ratio over pyrite-magnetite assemblage (e.g., Zolotov, 1991;Klose et al, 1992) calculated from the observed CO concentration and thermochemical data (Robie et al, 1979;Chase et al, 1985). In the following we call this as the pyrite-magnetite buffer.…”

“…An increase in atmospheric SO 2 raises the surface temperature. This positive feedback is suggested by Klose et al (1992), and they concluded that the climate system would not be stabilized. We call this as a chemical-greenhouse feedback.…”

“…The catastrophe is initiated by the uptake of atmospheric SO 2 through reaction (2), because Venus' atmospheric SO 2 abundance is much higher than the equilibrium value produced by reaction (2) (e.g., Fegley and Treiman, 1992;Klose et al, 1992). Since the SO 2 greenhouse effect decreases with the decrease of atmospheric SO 2 abundance, the surface temperature drops.…”