Frontier Adventure

How Long is a Day on Venus? We Finally Know the Exact Answer

Venus, aka. Earth’s “Sister Planet,” has always been shrouded in mystery for astronomers. Despite being planet Earth’s closest neighbor, scientists remained ignorant of what Venus’ surface even looked like for well into the 20th century, thanks to its incredibly dense and opaque atmosphere. Even in the age of robotic space exploration, its surface has been all but inaccessible to probes and landers.

And so the mysteries of Venus have endured, not the least of which has to do with some of its most basic characteristics – like its internal mass distribution and variations in the length of a day. Thanks to observations conducted by a team led from UCLA, who repeatedly bounced radar off the planet’s surface for the past 15 years, scientists now know the precise length of a day on Venus, the tilt of its axis, and the size of its core.

The team’s study, titled “Spin state and moment of inertia of Venus,” recently appeared 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 National Radio Astronomy Observatory‘s (NRAO) Green Bank Observatory.

Radar measurements of Venus’ surface, used to determine its rate of spin and axial tilt. Credit: Jean-Luc Margot/UCLA and NASA

To recap, Venus and Earth are appropriately considered siblings, seeing as how they are similar in size, composition, mass, and density. In spite of that, the separate evolutionary paths they have followed have resulted in widely different outcomes. Whereas Earth has an atmosphere that can maintain temperatures conducive to life, Venus has a super-dense atmosphere that is toxic and hot enough to melt lead!

In order to understand why and how our two planets had such divergent histories, scientists need to know the fundamentals – like how many hours there are in a Venusian day. Knowing this can yield valuable information about a planet’s spin, orientation, internal structure, and mass distribution. Having precise measurements for these characteristics will ultimately shed light on the formation and volcanic history of the planet, as well as how its surface evolved over time.

Precise data is also crucial to planning missions to the surface since a planet’s
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