Earth and Venus are rocky planets of similar size and rock chemistry, so they should be losing their internal heat to space at the same rate. The way in which Earth loses its heat is well known, but the mechanism of heat flow from Venus has long been a mystery. A study using three-decade-old data from NASA’s Magellan mission has taken a fresh look at how Venus cools and found that thin regions of the planet’s top layer may hold the answer.
Our planet has a fiery core that heats the surrounding mantle, which transmits that heat to Earth’s solid, rocky outer layer, or lithosphere. The heat is then lost to space, cooling the upper region of the mantle. This mantle convection drives tectonic processes at the surface, keeping a mosaic of moving plates in motion. Venus has no plate tectonics, so how the planet loses its heat and the processes that shape its surface have long been questions in planetary science.
The study explores this mystery using observations that the Magellan spacecraft made in the early 1990s of quasi-circular geological features on Venus called coronae. By making new corona measurements visible in the Magellan images, the researchers concluded that coronas tend to be located where the planet’s lithosphere is thinnest and most active.
This radar image from NASA’s Magellan mission shows circular fracture patterns surrounding the Aine corona, located in the southern hemisphere of Venus. The corona is about 200 kilometers (124 miles) across and shows various features that could be associated with volcanic activity.
Credits: NASA/JPL-Caltech
“For a long time we’ve been locked into this idea that the Venusian lithosphere is stagnant and thick, but now our view is evolving,” said Suzanne Smrekar, principal research scientist at the Jet Propulsion Laboratory (JPL). English) at NASA in Southern California, who led the study published in Nature Geoscience.
Just as a thin sheet releases more body heat than a thick duvet, a thin lithosphere allows more heat to escape from the planet’s interior through floating plumes of molten rock that rise toward the outer layer. Typically, where there is greater heat flux, there is greater volcanic activity below the surface. Therefore, the coronas are likely to reveal places where active geology is shaping the surface of Venus today.
The researchers focused on 65 previously unstudied coronas that can reach several hundred kilometers in diameter. To calculate the thickness of the surrounding lithosphere, they measured the depth of the trenches and ridges around each corona. What they found is that the space between the ridges is smaller in areas where the lithosphere is more flexible or elastic. By applying a computer model of how an elastic lithosphere bends, they determined that, on average, the lithosphere around each coronal is about 11 kilometers (7 miles) thick, which would make it much thinner than previous studies suggest. These regions have an estimated heat flux that is greater than the Earth average, suggesting that the coronas are geologically active.
“While Venus does not have similar tectonics to Earth, these regions of thin lithosphere appear to be allowing significant amounts of heat to escape, similar to areas where new tectonic plates are forming on Earth’s seafloor.” said Smrekar.
This composite radar image of the Quetzalpetlatl corona was created by superimposing data from about 70 orbits from NASA’s Magellan mission on an image obtained by the Arecibo Observatory radio telescope in Puerto Rico. The edge of the corona indicates possible tectonic activity.
Credits: NASA/JPL-Caltech
A window into Earth’s past
To calculate the age of material on the surface of a celestial body, planetary scientists count the number of visible impact craters. On a tectonically active planet like Earth, impact craters are obliterated by subducting continental plates and covered by molten rock from volcanoes. If Venus lacks the tectonic activity and regular churning of Earth-like geology, it should be covered in ancient craters. But by counting the number of Venusian craters, scientists estimate that the surface is relatively young.
Recent studies suggest that the youthful appearance of the Venusian surface is likely due to volcanic activity, which is driving regional renewal of the surface today. This finding is supported by the new research, which indicates increased heat flux in coronal regions, in a state that Earth’s lithosphere might have resembled in the past.
“What is interesting is that Venus offers a window into the past to help us better understand what Earth looked like more than 2.5 billion years ago. It is in a state that is predicted to occur before a planet forms tectonic plates,” said Smrekar, who is also the principal investigator for the upcoming Venus Emissivity, Radioscience, InSAR, Surveying and Spectroscopy (VERITAS) mission. English) from NASA.
VERITAS will pick up where Magellan left off, improving on that mission’s data, which is low-resolution and contains large margins of error. With a scheduled launch within a decade, the mission will use state-of-the-art synthetic aperture radar to create 3D global maps and a near-infrared spectrometer to discover what the surface is made of. VERITAS will also measure the planet’s gravitational field to determine the structure of Venus’ interior. Together, the instruments will complete the history of the planet’s past and present geological processes.
“VERITAS will be an orbiting geologist, able to identify where these active areas are and to better resolve local variations in the thickness of the lithosphere. We may even be able to capture the lithosphere in the act of deforming,” Smrekar said. “We will determine if volcanism is really making the lithosphere soft enough to lose as much heat as Earth, or if Venus has more mysteries in store.”
Ian J. O’Neill
Jet Propulsion Laboratory, Pasadena, California
Read this story in English here.