EXPERIMENTAL CONSTRAINTS ON INTERPRETING BASALTIC ALTERATION ON THE SURFACE OF VENUS: EFFECTS OF CHEMISTRY, CRYSTALLINITY, AND ATMOSPHERIC COMPOSITION

Experiments simulating the surface P-T-atmospheric conditions of Venus are needed for future missions, which will use features in the 1 µm region to define surface compositions remotely. Prior work has used the presence or absence of observed alteration to estimate Venus’ surface age by assuming alteration occurs via bulk equilibration or diffusion. With sparse data on the surface composition of Venus currently, experimental constraints are necessary to better comprehend how alteration progresses and the effects that may have on composition and age calculations.

Experiments simulating Venus surface conditions have been run on basaltic compositions to determine the potential alteration pathways and resulting phase changes. In all experiments, two foci of alteration were observed: 1) formation of phases at the surface-atmosphere boundary, and 2) changes in glass chemistry from the surface into the sample’s interior. Samples run in CO₂-only atmospheres were observed to have nanometer-scale near-surface alteration rinds extending into the original sample that are interpreted to have formed due to interactions at the surface-gas interface, including the occasional formation of discrete grains of an Fe-rich phase and CaCO₃. Samples run in CO₂+SO₂ atmospheres show rapid deposition of surficial coatings, consisting of nanoscale mixtures of Fe-oxides and sulfates (either CaSO₄ or Na₂SO₄ depending on the starting composition) that form in two-week duration runs. Coatings hamper further surface-atmosphere interactions, potentially limiting the progression of diffusion-driven alteration of the original sample. These results serve as a reminder that invoking wholesale alteration of the Venus surface primarily via simple diffusion or mass breakdown in order to interpret ages is complicated. The processes by which alteration occurs (i.e., diffusion vs. advection), the length scales over which each process is active, the effects of addition of crystalline phases common to natural samples, and the rate-limiting steps of each of the alteration pathways interact in complex ways. The experiments described here provide a first step into constraining the alteration processes and chemistries on Venus.