On top of all that, 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 would be canceled in lieu of a higher-priority request for the facility.
The payoff
The team began observing Venus in 2006. Originally, they asked for 121 separate observing sessions and ultimately got about 50. By 2020, they’d managed to make 21 observations that produced usable data. “It's a challenging measurement to make and it requires both high precision and patience,” Margot reaffirms 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 long. What’s more, the venusian day varies in length by as much as 20 minutes. Margot thinks this is caused in part by Venus’s extremely dense atmosphere, which, unlike the solid surface below, has a rotation period of just four days. As the atmosphere “sloshes about” above Venus, it transfers some angular momentum to the planet itself.
The team also found that Venus’s axis is tilted by 2.6392°, a tenfold increase in accuracy over earlier estimates. The precession rate — how fast Venus’ pole wobbles — is 44.58" per year, which means the pole draws a complete circle on the sky every 29,000 years. A more accurate precession rate gave the team a measure of the planet’s moment of inertia, which in turn gave a rough estimate of the size of Venus’ core, which was previously unknown. Margot’s team found it’s roughly 2,175 miles (3,500 km) across — about the same size as Earth’s.
But do the similarities stop there? Our planet’s core has an outer molten layer and a solid center, but what about Venus’ core? At this point, Margot says, we can’t be certain. Recent computer modeling suggests it could be either solid or molten, or perhaps solid at the center with a molten outer region, like Earth’s. Margot thinks it’s probably entirely liquid, but would love for additional data from ongoing speckle observations to confirm it. Alternatively, he says, direct observational evidence for the core’s size could come from tracking the motion of an orbiter around Venus and measuring tidal deformations induced in the planet by the Sun. “The ultimate best way [to learn about the core] is to have seismometers on the surface; but that's not going to happen soon,” he adds, because of the planet’s hellish conditions.
Working from home
While they continue their work on Venus, Margot’s team is also using radar speckle tracking on Jupiter’s moons Europa and Ganymede. Astronomers strongly believe Europa has a global ocean beneath its icy surface and Ganymede likely may as well. But using this technique to observe these moons is far more challenging than Mercury or Venus because of the distance involved, so the radar echoes received over that vast distance are thousands of times weaker. The team’s early results, though, already suggest Europa “has an exterior layer that is decoupled from the interior of the body,” Margot says, confirming that it does indeed have a subsurface global ocean.
We have become used to the remarkable discoveries made by robotic space probes circling or even driving about on our celestial neighbors. And why not? But Margot and his team remind us that equally astonishing new knowledge of the cosmos, including our sibling planet, is being acquired from right here on Earth.