The Geologic Histories of Venus

I've worked on several projects trying to understand what the surface of Venus is able to show us about its past.

Venusian Tectonics

Mead Basin (shown below) is the largest impact structure on Venus, at nearly 300 km in diameter. Its two circumferential faults, shown above as the bright circular features, are arguably the basin's most striking features.

SAR Image of Mead Basin

Early in the Solar System, Venus and Earth were likely very similar but they clearly unerwent very different processes since then. Above is a cartoon illustrating how Venus and the Earth may have experienced divergent geologic evolutions, and what that means for the observations we can make about each planet now.

Figure from Hansen (2015)

The harsh atmospheric conditions on Venus make it very difficult, if not impossible, to directly observe the near-surface geologic properties in its crust and upper mantle in the ways we can do so here on Earth. Large-scale impact structures, however, are sensitive to the material properties deep in the subsurface. Because of this relationship, it is possible to model the largest impact basin on Venus--Mead Basin--to determine if the basin's two characteristic ring faults could tell us anything about the geology on Venus when it formed.

We recently published our findings in Nature Astronomy if you would like to learn more:

Bjonnes, E., Johnson, B. C., and Evans, A. J. Estimating Venusian thermal conditions using multiring basin morphology. Nat Astron. (2021). doi: 10.1038/s41550-020-01289-6

This work was submitted to the 51st and 52nd Lunar and Planetary Science Conferences, abstracts linked below.

Determining Venus' Thermal Conditions through Multiring Basin Formation (2020) Bjonnes, E., Johnson, B. C., and Evans, A. J. In Lunar and Planetary Science LI, Abstract # 2511, Lunar and Planetary Institute, Houston.

The Effects of Venus' Thermal Conditions on Multiring Basin Formation (2021) Bjonnes, E., Johnson, B. C., and Evans, A. J. In Lunar and Planetary Science LII, Abstract # 1513, Lunar and Planetary Institute, Houston.

Resurfacing Histories

Pancake domes on Venus.

Venus has a seemingly random distribution of impact craters on its surface and widespread evidence of volcanism...could those observations be related?

Figure 6 from Bjonnes et al. (2012), demonstrating a possible spatial relationship between impact craters and small-scale resurfacing events on the surface of Venus.

Venus' surface is often thought to have undergone at least one planet-wide resurfacing event as a way to explain the relatively low number of craters on its surface and their apparently random distribution across the planet's surface. However, this framework puts some challenging constraints on the internal workings of the planet and may not accurately reflect the actual geologic history of the planet.

We completed a series of Monte Carlo models simulating the evolution of a planet undergoing continuous impact events concurrently with widespread, smaller-scale volcanic resurfacing (i. e. lava flows) to determine if it is possible to recreate the observed impact crater record without a global volcanic event. These statistical models demonstrated that these smaller-scale volcanic resurfacing events are, in fact, consistent with the impact cratering record. These finding opened the door for incorporating more Earth-like volcanism in how we think about Venus' geology.

You can read more about these findings here:

Equilibrium resurfacing of Venus: Results from new Monte Carlo modeling and implications for Venus surface histories E. Bjonnes, V. L. Hansen, B. James, and J. B. Swenson (2012) Icarus 217, 451-461.