10.2 The Geology of Venus

Venus' Geological Features and Surface Conditions

Venus is often called Earth's twin because of their similar size and mass, but its geology tells a very different story. Understanding how Venus resurfaces itself and why it lacks plate tectonics helps clarify what makes Earth's geology so unusual among the terrestrial planets.

Surface Features of Venus

About 80% of Venus is covered by volcanic plains, relatively smooth and flat regions formed by extensive lava flows. Two well-known examples are Guinevere Planitia and Lavinia Planitia.

The remaining surface includes highlands, elevated regions with mountains, plateaus, and heavily deformed terrain called tesserae (complex, ridged surfaces that look like crumpled tiles). The two major highland regions are:

• Ishtar Terra in the north, home to Maxwell Montes, the tallest mountain on Venus (about 11 km above the mean surface level)
• Aphrodite Terra near the equator, stretching roughly the size of South America

Coronae are another feature unique to Venus. These are large, oval-shaped structures (typically 100–300 km across) thought to form when hot mantle material pushes up from below, causing the surface to bulge and then partially collapse.

Crater Distribution and Surface Age

Planetary scientists use crater density to estimate how old a surface is. The logic is straightforward: older surfaces have been exposed to impacts longer, so they accumulate more craters. Younger surfaces have fewer.

Venus has a surprisingly uniform distribution of craters across its entire surface, with no region showing significantly more or fewer craters than any other. This tells us something striking: the whole surface is roughly the same age, estimated at about 300–600 million years old.

That's young compared to the heavily cratered surfaces of Mars or Mercury (billions of years old). The best explanation is that Venus underwent some kind of global resurfacing event around that time, where widespread volcanism essentially paved over the entire planet and erased older craters. Whether this happens in a single catastrophic episode or in a more gradual process is still debated.

Venus also has relatively few impact craters overall (around 1,000 total). Small craters are almost entirely absent because Venus' thick atmosphere breaks apart smaller meteoroids before they reach the surface. Notable craters include Mead (the largest, at 280 km across) and Cleopatra.

Venus vs. Earth: Volcanic Activity

Venus and Earth share some volcanic similarities: both have volcanoes, lava flows, and tectonic features driven by internal heat. But the differences are significant.

• No plate tectonics on Venus. Earth's crust is broken into moving plates, but Venus has a single, continuous lithosphere. This means Venus can't release internal heat the way Earth does through mid-ocean ridges and subduction zones.
• Higher surface temperature and pressure (462°C and 90 atmospheres) change how lava behaves. Lava on Venus likely flows farther and stays molten longer because the extreme surface heat slows cooling.
• Venusian volcanoes tend to be broader and fewer than Earth's. Many are large shield-type volcanoes. Some recent data from the Magellan mission suggests volcanic activity may still be occurring today.
• Unique tectonic features like tesserae and coronae have no direct equivalent on Earth and likely result from Venus' different way of managing internal heat.

Extreme Conditions on Venus

Venus' surface is one of the most hostile environments in the solar system:

• Surface temperature averages 462°C (864°F), hot enough to melt lead. This is driven by a runaway greenhouse effect: Venus' dense atmosphere of $CO_2$ traps solar heat extremely efficiently.
• Atmospheric pressure is about 90 times Earth's, equivalent to the pressure roughly 900 meters deep in Earth's oceans.
• Atmosphere composition is 96% carbon dioxide and about 3% nitrogen, with clouds of sulfuric acid droplets. This makes the atmosphere both toxic and corrosive.
• No liquid water exists on the surface. Any water Venus once had was likely lost long ago as the greenhouse effect intensified and ultraviolet radiation broke water molecules apart.
• No global magnetic field means the surface is exposed to solar wind and radiation. This also contributed to the loss of lighter atmospheric gases over time.

Venus' Geological Processes

Without plate tectonics, Venus has to release its internal heat differently than Earth. The leading idea is that heat builds up beneath the thick, rigid lithosphere until it triggers massive volcanic episodes, which is consistent with the evidence for global resurfacing.

The greenhouse effect is central to understanding Venus' geology, not just its climate. The extreme temperatures affect rock strength, lava behavior, and weathering processes at the surface. Venus' lithosphere is thought to be thicker and more rigid than Earth's, which is part of why plates don't form and move the way they do here. Instead, the crust deforms in place, producing features like coronae and tesserae rather than the linear mountain chains and ocean trenches you see on Earth.