Venus, also known as. Earth’s “sister planet” has always been a mystery to astronomers. Despite being the closest neighbor of planet Earth, thanks to its incredibly dense and opaque atmosphere, scientists did not know what the surface of Venus looked like until well into the 20th century. Even in the age of robotic space exploration, its surface was virtually inaccessible to probes and landers.
And so the secrets of Venus remain, not least of which have to do with some of its most basic properties – such as its internal mass distribution and variations over the length of a day. Thanks to observations made by a team at UCLA that have repeatedly ricocheted radar off the planet’s surface over the past 15 years, scientists now know the exact length of a day on Venus, the inclination of its axis, and the size of its core.
The team’s study, titled “The Spin State and Moment of Inertia of Venus,” was recently published in the journal Nature Astronomy. The team was led by Jean-Luc Margot, a professor of Earth and Planetary Sciences and Astrophysics at UCLA. He was joined by researchers from Cornell University, NASA’s Jet Propulsion Laboratory (JPL) and the Green Bank Observatory of the National Radio Astronomy Observatory (NRAO).
Radar measurements of the surface of Venus to determine the spin rate and the axial inclination. Photo credit: Jean-Luc Margot / UCLA and NASA
In summary, Venus and Earth are appropriately considered siblings because they are similar in size, composition, mass, and density. Even so, the separate development paths they have followed have produced very different results. While Earth has an atmosphere that can sustain life-enhancing temperatures, Venus has a super-dense atmosphere that is toxic and hot enough to melt lead!
To understand why and how our two planets had such different histories, scientists need to know the basics – how many hours there are in a Venusian day. Knowing this can provide valuable information about a planet’s spin, orientation, internal structure, and mass distribution. Precise measurements of these properties will ultimately provide information about the formation and volcanic history of the planet, as well as the development of its surface over time.
Accurate data is also crucial for planning missions on the surface, as the rotation of a planet can ward off attempts to land by up to 30 km. “Venus is our sister planet, and yet these fundamental properties have remained unknown,” said Margot in a press release from the UCLA Newsroom. “Without these measurements, we are essentially flying blind.”
To get accurate estimates of Venus’ rotation, Margot and his colleagues used the 70-meter radio antenna from the Goldstone Deep Space Communications Complex, located in the Mojave Desert and part of NASA’s Deep Space Network (DSN). . Between 2006 and 2020, the team took 21 separate measurements of Venus ‘surface by bouncing radio signals off Venus’ surface, which were then received by Goldstone and Green Bank.
The Goldstone Deep Space Communications Complex is located in the Mojave Desert in California. Photo credit: NASA
As Margot explained, the process is similar to lighting (the radio bowl) from millions of tiny reflectors (the landscape of the planet) and measuring the reflections to get a sense of how fast they are moving:
“We use Venus as a giant disco ball. We illuminate it with an extremely powerful flashlight – approximately 100,000 times brighter than your typical flashlight. And if we follow the reflections of the disco ball, we can infer properties of the spin [state]. ”
The complex way in which Venus reflects the radio signals causes them to brighten and dim irregularly before being received back on Earth. The Goldstone antenna intercepts the return signal first, followed by the Green Bank antenna about 20 seconds later. The exact timing of the delay allows scientists to know how fast Venus is rotating, while the specific window of time when the echoes are most similar allows them to measure the axial tilt of the planet.
What they found was pretty interesting. To start with, they found that an average day on Venus is 243,022 Earth days – that’s roughly two-thirds of a year on Earth. In addition, the results showed that the rate of rotation of Venus seems to be constantly changing. This was indicated by how each individual radar reading would be smaller or larger than a previous one, and by a difference of at least 20 minutes per reading.
These deviations, which likely caused previous estimates to be inconsistent, are likely the result of Venus’ strong atmosphere. As it spins around the planet, it will likely exchange a lot of momentum with the surface, causing its rotation to accelerate and slow down. The same phenomenon occurs on Earth, but the lower density of our atmosphere means that there is only a millisecond difference per day.
Margot and his colleagues also obtained far more accurate measurements of the axial tilt of Venus, which is tilted 2.6392 degrees (compared to the 23-degree tilt of the Earth). Your measurements improve by a factor of 10 over previous measurements and also show the speed at which the alignment of the Venus axis changes over time. On Earth, the precession of our axial tilt takes approximately 26,000 years to complete a single cycle, while that of Venus takes approximately 29,000 years.
In addition, this accurate measurement enabled the team to measure the core of Venus and determine that it was approximately 3,500 km in diameter. This is similar to Earth’s estimated 3,485 km in diameter, though they still can’t tell if it’s liquid or solid. The Earth’s magnetic field is the result of a dynamo effect created by the Earth’s molten outer core rotating around its solid inner core.
For this reason, knowing the state of the core of Venus is critical to understanding whether the lack of a global magnetic field contributed to the evolution of Venus. Obtaining accurate measurements with this method presents many challenges, not the least of which is the exceptional timing required to ensure that Venus and Earth are correctly positioned.
At the same time, both observatories must work properly to ensure that they reliably intercept the return signals. “We found that it is actually difficult to get everything working properly in 30 seconds,” said Margot. “Most of the time we get some data. But it is unusual for us to get all of the dates we hope for. “
Jupiter’s moons Europa (left) and Ganymede (right). Both moons likely have subterranean oceans. Photo credit: NASA
Despite the challenges, Margot and his colleagues plan to continue studying Venus using this radio echo technique. With each signal bounced back from its surface, researchers can learn a little more about its surface, its formation, and its intricate history. This knowledge will not only enable us to unravel the secret of our “sister planet”, but also greatly improve our understanding of how habitable planets can turn into true hellholes!
Similarly, Margot and his team hope to use the same method to study Jupiter’s moons Europa and Ganymede. For decades, astronomers have strongly suspected that these moons contain huge oceans of warm water inside them (especially in Europe). Ground-based radar measurements of these moons should amplify the fall of the inner oceans and show how thick their icy shells are – both of which will inform future missions in search of life there.
This research was carried out with support from NASA JPL and the National Science Foundation (NSF).
Further reading: UCLA Newsroom, Natural Astronomy