Universe Today
One of our solar system's biggest puzzles has long been why Venus has no moon. Often referred to as Earth's twin, Venus is almost the same mass and size as our own planet. But it does have an incredibly bizarre 243-day retrograde rotation.
In a new paper just being published by The Astrophysical Journal, longtime Venus researcher Stephen Kane, a planetary astrophysicist at the University of California, Riverside and colleagues, show that a moon around Venus would have been doomed from the start.
The team’s modeling shows that in most plausible scenarios, any moon Venus once had would spiral inward and be torn apart by the planet’s own gravity. A scenario that would likely happen within the first billion years or so of the moon’s formation.
This means that Venus's lack of a moon may be a natural outcome of ordinary tidal physics, not evidence of some exotic catastrophe, Kane, the paper’s lead author told me via email. Using a model that tracks the gravitational tides between Venus, a hypothetical moon, and the Sun over billions of years, we found that a moon survives only under a narrow set of conditions: a fast-spinning early Venus and a moon no heavier than our own, he says.
As for the team’s modeling?
The team’s individual simulations, systematically varied Venus's initial spin, the moon's mass, Venus’ orbital eccentricity, and the properties of Venus's interior.
We built a computer model that follows the slow gravitational tug-of-war between a planet and a moon over billions of years, says Kane. The same physics is gradually pushing our own Moon away from Earth by a few centimeters a year, he says.
But the so-called Roche limit is a Venus moon deal breaker.
The Roche limit is the distance below which a planet's tidal gravity becomes stronger than a moon's own gravity holding it together. Inside that distance, the moon is literally pulled apart; creating a ring of debris.
It's the same physics that gives Saturn its rings, says Kane. For a moon around Venus, we calculate that limit at about 2.85 Venus radii, which is roughly 17,000 km from the planet's center, he says.
Any moon driven inward past that boundary doesn't survive and eventually rains down onto the planet.
Yet large moons like our own natural satellite can do wonders for making a given planet more habitable.
A large moon acts like a gyroscopic stabilizer, keeping a planet’s tilt steady over long timescales, says Kane. Without one, a planet's tilt can wander chaotically, which could cause dramatic swings in climate, he says.
Thus, losing a moon removes one source of long-term climate stability.
A Testable Hypothesis
NASA’s planned DAVINCI mission scheduled for launch to Venus by the end of this decade could possibly find in situ atmospheric evidence of any such long-destroyed Venus moon.
Yet a late delivery of material from a disrupted moon would have a distinct signature, although disentangling this signal from the effects of subsequent volcanic outgassing and atmospheric escape remains a substantial challenge, they write.
If a moon was destroyed and its material fell onto Venus, it could have altered the chemistry of the surface and atmosphere, says Kane. And noble gas abundances and isotopic ratios are exactly the kind of sensitive tracers that could carry such a fingerprint, he says.
The DAVINCI mission is designed to make exactly those in-situ atmospheric measurements during its descent. And these results will likely predict what planetary scientists find when studying extrasolar Venus-like planets circling other sunlike stars.
Our work predicts that "Venus-like" worlds, rocky planets that orbit close to their stars and spin slowly, should generically be moonless too, says Kane. That's a testable prediction for the next generation of telescopes, and it feeds directly into how we assess which distant worlds might be hospitable to life, he says.
As For Our Own Earth-Moon System?
Earth formed spinning fast enough that the Moon migrates outward and continues on that trajectory, says Kane. But if Earth had formed spinning significantly more slowly, then the Moon could have been driven inward to destruction instead, he says.
Our large, stabilizing Moon isn't guaranteed by having a big impact, it also depended on Earth ending up with the right spin, as Kane points out.
Venus wasn’t so lucky. Its incredibly slow rotation seems to have sealed its fate as a moonless body. Such fate may be a key part of why earth and Venus took such divergent evolutionary paths. Thus, a rocky planet’s slow rotation may be a key factor in its endgame habitability.
Venus shows us the other branch of that story: what happens when a planet lands on the wrong side of the rotational spin boundary, says Kane.
