Venus is an enigma. It's the planet next door and yet reveals little about
itself. An opaque blanket of clouds smothers a harsh landscape pelted by acid
rain and baked at temperatures that can liquify lead.
Now, new observations from the safety of Earth are lifting the veil on some
of Venus' most basic properties. By repeatedly bouncing radar off the
planet's surface over the last 15 years, a UCLA-led team has pinned down the
precise length of a day on Venus, the tilt of its axis and the size of its
core. The findings are published today in the journal Nature Astronomy.
"Venus is our sister planet, and yet these fundamental properties have
remained unknown," said Jean-Luc Margot, a UCLA professor of Earth,
planetary and space sciences who led the research.
Earth and Venus have a lot in common: Both rocky planets have nearly the
same size, mass and density. And yet they evolved along wildly different
paths. Fundamentals such as how many hours are in a Venusian day provide
critical data for understanding the divergent histories of these neighboring
worlds.
Changes in Venus' spin and orientation reveal how mass is spread out within.
Knowledge of its internal structure, in turn, fuels insight into the
planet's formation, its volcanic history and how time has altered the
surface. Plus, without precise data on how the planet moves, any future
landing attempts could be off by as much as 30 kilometers.
"Without these measurements," said Margot, "we're essentially flying blind."
The new radar measurements show that an average day on Venus lasts 243.0226
Earth days—roughly two-thirds of an Earth year. What's more, the rotation
rate of Venus is always changing: A value measured at one time will be a bit
larger or smaller than a previous value. The team estimated the length of a
day from each of the individual measurements, and they observed differences
of at least 20 minutes.
"That probably explains why previous estimates didn't agree with one
another," Margot said.
Venus' heavy atmosphere is likely to blame for the variation. As it sloshes
around the planet, it exchanges a lot of momentum with the solid ground,
speeding up and slowing down its rotation. This happens on Earth too, but
the exchange adds or subtracts just one millisecond from each day. The
effect is much more dramatic on Venus because the atmosphere is roughly 93
times as massive as Earth's, and so it has a lot more momentum to trade.
The UCLA-led team also reports that Venus tips to one side by precisely 2.6392
degrees (Earth is tilted by about 23 degrees), an improvement on the precision
of previous estimates by a factor of 10. The repeated radar measurements
further revealed the glacial rate at which the orientation of Venus' spin axis
changes, much like a spinning child's top. On Earth, this "precession" takes
about 26,000 years to cycle around once. Venus needs a little longer: about
29,000 years.
With these exacting measurements of how Venus spins, the team calculated
that the planet's core is about 3,500 kilometers across—quite similar to
Earth—though they cannot yet deduce whether it's liquid or solid.
Venus as a giant disco ball
On 21 separate occasions from 2006 to 2020, Margot and his colleagues aimed
radio waves at Venus from the 70-meter-wide Goldstone antenna in
California's Mojave Desert. Several minutes later, those radio waves bounced
off Venus and came back to Earth. The radio echo was picked up at Goldstone
and at the Green Bank Observatory in West Virginia.
"We use Venus as a giant disco ball," said Margot, with the radio dish
acting like a flashlight and the planet's landscape like millions of tiny
reflectors. "We illuminate it with an extremely powerful flashlight—about
100,000 times brighter than your typical flashlight. And if we track the
reflections from the disco ball, we can infer properties about the spin
[state]."
The complex reflections erratically brighten and dim the return signal,
which sweeps across Earth. The Goldstone antenna sees the echo first, then
Green Bank sees it roughly 20 seconds later. The exact delay between receipt
at the two facilities provides a snapshot of how quickly Venus is spinning,
while the particular window of time in which the echoes are most similar
reveals the planet's tilt.
The observations required exquisite timing to ensure that Venus and Earth
were properly positioned. And both observatories had to be working
perfectly—which wasn't always the case. "We found that it's actually
challenging to get everything to work just right in a 30-second period,"
Margot said. "Most of the time, we get some data. But it's unusual that we
get all the data that we're hoping to get."
Despite the challenges, the team is forging ahead and has turned its sights
on Jupiter's moons Europa and Ganymede. Many researchers strongly suspect
that Europa, in particular, hides a liquid water ocean beneath a thick shell
of ice. Ground-based radar measurements could fortify the case for an ocean
and reveal the thickness of the ice shell.
And the team will continue bouncing radar off of Venus. With each radio
echo, the veil over Venus lifts a little bit more, bringing our sister
planet into ever sharper view.
Reference:
Jean-Luc Margot et al. Spin state and moment of inertia of Venus, Nature
Astronomy (2021). DOI:
10.1038/s41550-021-01339-7
Tags:
Space & Astrophysics