Although it is considered the sister planet of Earth, the fundamental properties of Venus, such as its internal mass distribution and variations in day length, were until then unknown. But from radar measurements made between 2006 and 2020, scientists were able to determine some of these properties, which allowed them to estimate the approximate size of the planet’s core.
Venus has some similarities with Earth. Apart from the fact that they are two rocky planets, their masses and dimensions are comparable and they are both relatively close to the Sun; 41 million kilometers separate them from each other. And according to this first estimate, Venus has a core about 7,000 kilometers in diameter, roughly the same size as that of Earth.
However, the two “sister” planets also differ radically in other aspects: the magnetic field of Venus is much weaker (it does not have a true magnetosphere), it is much hotter (462 ° C on average) and its atmosphere, composed of more than 96% of carbon dioxide, is much denser than the Earth’s atmosphere: the atmospheric pressure at the surface of this planet is 92 times greater than that of Earth!
Much more precise measurements
This particularly thick atmosphere, which conceals surface details, makes observations from Earth much more difficult. Thus, radars and other specific techniques are necessary to probe this neighboring planet. Jean-Luc Margot and his colleagues at the University of California at Los Angeles examined Venus from 2006 to 2020, using the Goldstone Solar System Radar (GSSR), which has already made it possible to study the planets of the solar system on several occasions. Note that the radar of the Arecibo observatory, now destroyed, was the only installation comparable to the GSSR.
The craft includes a 500 kW (8500 MHz) X-band transmitter and low noise receiver located on the 70 meter antenna of the Goldstone Deep Space Communications Complex, located in the Mojave Desert in California. For years, scientists have struck the planet with radio waves repeatedly and then followed the echoes of the waves as they returned to Earth, using the GSSR, but also the Green Observatory telescope. Bank, located some 3,000 kilometers away in West Virginia.
The complex reflections on Venus erratically intensify or attenuate the return signal, which sweeps across the Earth. The Goldstone antenna detects the echo first, then Green Bank picks it up about 20 seconds later. The exact delay between reception at the two facilities provides a snapshot of how fast Venus spins, while the time window in which the echoes are most similar reveals the tilt of the planet. The observations required precise timing to ensure that Venus and Earth were correctly positioned.
This long-term follow-up allowed Margot and her team to spot very small changes in the orientation of the spin axis, the moment of inertia and the length of the day. Their results have just been published in the journal Nature Astronomy. They found in particular that the axis of rotation of Venus, inclined 2.6392 ± 0.0008 ° with respect to its orbital plane, oscillated very slightly according to a pattern which, according to their calculations, would be repeated every 29,000 years; a phenomenon that scientists attribute to the attraction of the Sun.
Characterize the nucleus to better understand the planet
They also found that the length of one day on Venus, roughly equivalent to 243 Earth days – Venus is the slowest rotating planet in the solar system around its axis – fluctuated up to 21 minutes over the course of 15 years of observation. ” This probably explains why previous estimates did not agree with each other. ”Said Margot. The main factor behind this fluctuation is the planet’s thick atmosphere, which as it moves around the planet exchanges a lot of energy with the surface, speeding up and slowing down the rotation. The phenomenon also occurs on Earth, but due to the fact that our atmosphere is much less dense, the exchange of energy is less and has only a tiny impact (of the order of a millisecond) on the duration of One day.
But the team believes that the core of the planet is also responsible for the observed variation; thus, they set out to use this data to calculate the size of a nucleus that would cause such fluctuations. Result: they estimate the radius of the nucleus to be about 3,500 kilometers. However, their analyzes did not allow us to deduce whether this nucleus was liquid or solid. Previous theoretical studies suggest that it is mainly composed of iron and nickel, like that of the Earth. But the researchers would have liked to know more: ” It is not known whether Venus has an inner solid core and an outer liquid core, like Earth, or whether this core is fully solid or fully liquid. », Regrets Margot.
This estimate of the size of the nucleus turns out to be consistent with previous models, but having a real, measurable value opens the way to more precise studies of Venus. Knowing the size and density of the nucleus can be particularly useful in understanding the history of this planet: its formation, its volcanic activity and how its surface has evolved over time. ” Almost everything about the evolution of a planet is dictated by the size of its core », Confirms Margot. The specialist adds that only a good image of the internal structure makes it possible to fully understand the history of a planet.