Moreover, the observation time with the Goldstone antenna was very limited. The dish is part of the Deep Space Network used to communicate with various space probes. Often, the team’s scheduled time with the dish was canceled instead of a higher priority request for installation.
The team started observing Venus in 2006. They originally requested 121 separate observing sessions and eventually got around 50. By 2020, they had managed to make 21 observations that produced usable data. “It’s a difficult measurement to make and it requires both great precision and patience,” reaffirms Margot with a smile.
The results of their patience and precision are remarkable. The team found that the average day on Venus (between 2006 and 2020) was 243.0226 Earth days. In addition, the Venusian day varies in length up to 20 minutes. Margot thinks this is partly due to Venus’ extremely dense atmosphere, which, unlike the solid surface below, has a rotation period of just four days. As the atmosphere “laps” over Venus, it transfers some angular momentum to the planet itself.
The team also discovered that the axis of Venus is tilted by 2.6392°, a tenfold increase in accuracy compared to previous estimates. The rate of precession – the rate at which the pole of Venus wobbles – is 44.58″ per year, which means that the pole draws a complete circle in the sky every 29,000 years. A more accurate rate of precession has gave the team a measurement of the planet’s moment of inertia, which Margot’s team in turn gave a rough estimate of the size of Venus’ core, which was previously unknown.
But do the similarities end there? Our planet’s core has a molten outer layer and a solid center, but what about Venus’ core? At this point, says Margot, we can’t be certain. Recent computer modeling suggests it could be solid or molten, or possibly solid in the center with a molten outer region, like Earth’s. Margot thinks it’s probably entirely liquid, but would like additional data from ongoing speckle observations to confirm this. Alternatively, he says, direct observational evidence for the size of the core could come from tracking the motion of an orbiter around Venus and measuring tidal deformations induced on the planet by the Sun. “The ultimate best way [to learn about the core] is to have surface seismometers; but it won’t happen anytime soon,” he adds, due to the hellish conditions on the planet.
Work at home
As they continue their work on Venus, Margot’s team is also using radar tracking of shimmer on Jupiter’s moons Europa and Ganymede. Astronomers strongly believe that Europa has a global ocean below its icy surface, and Ganymede probably could too. But using this technique to observe these moons is much more difficult than Mercury or Venus due to the distance involved, so the radar echoes received over this vast distance are thousands of times weaker. The team’s initial results, however, already suggest that Europa “has an outer layer that is decoupled from the interior of the body,” says Margot, confirming that there is indeed a global ocean below the surface.
We have grown accustomed to the remarkable discoveries made by robotic space probes circling or even hovering over our celestial neighbors. And why not? But Margot and her team remind us that equally amazing new knowledge about the cosmos, including our sister planet, is being gained right here on Earth.