Venus Swallowed Its Own Moon, Stephen Kane Says
A new study by astrophysicist Stephen Kane of the University of California, Riverside argues that Venus once had a moon it destroyed with its own gravity, with no outside collision involved.
The University of California, Riverside released a statement on September 15 and 16 about a study by astrophysicist Stephen R. Kane, arguing that Venus may once have had a moon, only to destroy it with its own gravity, with no outside collision involved. The paper, titled “Tidal Demise: The Evolution and Fate of a Hypothetical Venus Moon,” was published in The Astrophysical Journal, volume 1009, issue 1, page 31, after first being posted to the arXiv server in early September. Kane’s co-authors are Franck Selsis and Jérémy Leconte of the University of Bordeaux, and Sean N. Raymond, also of that university.
Why Venus, a planet close to Earth in size, mass and structure, has no natural satellite at all has been an open question in planetary science for decades. Explanations so far have mostly split between two possibilities: that Venus once had a moon later destroyed by a massive impact, or that it never experienced the kind of collision that would have created one in the first place. Kane’s study offers a third path, in which the moon disappears without any catastrophe, simply as a result of the combination of Venus’s gravity and its rotation.
“My study shows that Venus didn’t need a catastrophe to arrive at what we see today. It turns out that the planet’s own gravity, combined with the speed of its rotation, naturally caused the moon to collapse into it,” Kane said in the university’s statement, carried by ScienceDaily.
The team fed Venus’s rotation speed, the mass of the hypothetical moon, its distance from the planet and internal tidal friction into a computer model, then ran simulations spanning 4.5 billion years, roughly the age of the entire solar system. They tested moon masses ranging from half to ten times the mass of Earth’s Moon. The paper was first posted to arXiv on September 2, when, according to phys.org, it had not yet passed peer review, before the version carrying the volume and page number was published in the journal itself.
Why the moon collapsed instead of drifting away
Venus completes one rotation on its axis in about 243 Earth days, slower than it takes to orbit the Sun, an orbit that lasts 225 days. For Earth, the ratio runs the other way: our planet rotates once every 24 hours, and part of that rotational energy is transferred to the Moon, which is why it slowly drifts away from us, by about four centimeters a year. For Venus, according to the model, the math runs in reverse.
“The moon transfers angular momentum inward, rather than outward. That causes a catastrophic collapse of the moon’s orbit,” Kane told EarthSky.
According to the model, the moon would break apart once it reached the so-called Roche limit, roughly 9,321 miles, or about 15,000 kilometers, above Venus’s surface. At that point the planet’s gravity would overpower the moon’s own gravity and tear it apart.
How long such a moon could have survived depended mostly on its mass and on Venus’s initial rotation speed. If Venus, after the hypothetical collision that created the moon, had a day shorter than about 12 hours, a moon with a mass similar to our own could have lasted for billions of years. If the planet rotated more slowly, destruction, according to the model, would have come between 30 million and 1.7 billion years after the moon formed. According to EarthSky, which spoke with Kane, a moon half as massive as Earth’s would have lasted about 1.7 billion years, while one twice as massive would have lasted only about 30 million years. If it formed early in the solar system’s history, Venus would have lost it, under that scenario, about 2.5 billion years ago.
Direct evidence would be hard to come by. About 80 percent of Venus’s surface is roughly the same age, which scientists read as a sign of a major resurfacing event around a billion years ago, one that could have erased most older geological traces, including any consequences of the moon’s fall.
“It’s extremely difficult to test this idea because it left behind almost nothing,” Paul Byrne, a planetary scientist at Washington University in St. Louis who was not involved in the study, told EarthSky.
Kane believes traces could still remain deep inside the planet, similar to the structures seismological measurements have found deep beneath Earth’s surface and that are linked to the collision that created our own Moon. A comparable measurement on Venus, which would require a probe with a seismometer on the surface itself, does not exist for now.
We recently wrote about Venus in connection with its current brightness in the night sky: the planet is at its brightest on September 18, and it will not shine that brightly again until 2028.
The study does not prove that Venus ever actually had a moon, nor does its author claim that it does. The simulations only show that, if such a moon existed, its disappearance would be possible without an outside collision, purely through the combination of Venus’s gravity and the planet’s slow rotation. Whether such a moon ever really existed, and whether its disappearance could explain why Venus is today a hot and inhospitable place rather than a potentially habitable planet, remains an open question that current data cannot answer.
