The supply of magma from the interior of the planet controls where and how volcanic features form. The number, variety, and size of Venus’ volcanic structures has led researchers to suggest that very large magma reservoirs fueled them. Such large reservoirs could have accumulated from material delivered by multiple mantle plumes (otherwise known as hot spots) rising from deep in the Venusian mantle. Why did Venus produce so many plumes? How did such a large magma reservoir accumulate before the eruptions? We don’t know.

The locations of the volcanoes are another clue that something different may be happening inside Venus. On Earth, most volcanoes form along the boundaries between tectonic plates. On Venus, volcanic features are distributed globally in a pattern that seems almost random (though it’s actually not quite random). The volcanoes that we can see on Venus today didn’t happen because of plate tectonics. It’s possible that Venus never had Earth-like plate tectonics at all, which begs the question – what else can drive volcanism on a planet, and how did it get so colossal?

Impact craters are another key

Venus also doesn’t do impact craters the same way the Earth or the Moon do. Like volcanoes, impact craters are distributed nearly randomly on its surface. What’s weirder is that almost all of them (85 percent) appear pristine, apparently unmodified by the massive amounts of volcanic activity recorded in the surface features around them. Thus, these craters likely formed after most of the volcanic activity on the surface died down. There are also not very many impact craters, which suggests a youthful surface (on average, 200 to 700 million years old), unlike the ancient surfaces of the Moon, Mercury, and Mars.

What could make a young surface, covered with volcanoes and pristine craters? Some scientists have suggested that Venus’ old impact craters were erased by a global volcanic resurfacing event, a massive catastrophe that left it with no visible impacts craters at all. Other scientists disagree.

Different surfaces, different interiors

The differences in Earth and Venus’ surfaces indicate they are also different on the inside. The surface features of Venus are related to volcanism and/or fracturing from internal stress. Volcanism and tectonism are both driven by the circulation of material (and heat) within a planet’s interior. Therefore, scientists think such circulation inside Venus has developed differently from what happens inside the Earth.

Trying to make sense of it all: catastrophic versus steady state

If the entirety of the Venusian surface was replaced by a worldwide volcanic event, how long did it take? Was it dramatic and catastrophic, completed in a hundred thousand years or less, or could it have been an incremental, yet still globally effective process stretched over more than a hundred million years? The planetary geology community does not agree on this point. Different scientists have proposed different volcanic resurfacing models, all of them consistent with the young impact cratering record but disparate in their approach to explaining the evolution of the planet. These models act as well-developed, complex hypotheses, incorporating each of the necessary geologic processes from the bingo card of what’s possible in planetary science. However, because geologic processes can be as variable as the planets in our solar system, there’s a lot of room for interpretation.

For scientists discussing Venusian history, there are two extreme points of view: catastrophic and steady-state. These terms refer to the primary difference between the two models, which is how long the resurfacing of Venus actually took. The catastrophic resurfacing model suggests that planetary resurfacing on Venus occurred through infrequent, planet-wide volcanic events, large enough to bury all earlier material. In other words, resurfacing was a near-instantaneous change to Venus relative to the slow flow of geologic time.

Steady-state, on the other hand, implies numerous smaller, discrete processes, stretched out across time. For Venus, the steady-state model posits more frequent resurfacing episodes, more similar to the pace of volcanism on Earth, that obliterate older craters at the local scale over much longer timescales. If the location and timing of the volcanism work out, a slower pace might still yield a young age for the surface overall.

Hypothesis testing, and solving the tectonic puzzle

Which model is correct—or, at least, closer to correct? To answer that question, researchers first have to understand what drives Venus' volcanoes. On Earth, volcanism happens because of plate tectonics. However, there does not appear to be plate tectonics on Venus. Without actively moving plates, the production, movement, and eruption of magma is likely a fundamentally different process on Venus than it is on Earth.