Venus‘ sluggish spin may be the key to why it doesn’t have a moon — anymore, at least, new research says.
The study, led by University of California, Riverside astrophysicist Stephen Kane, appears in The Astrophysical Journal. Kane’s team built computer simulations of how gravity ties a planet to a moon in space, then ran Venus through them again and again.
Nearly every run ended the same way. An ancient Venusian moon crashed onto its host planet — sometimes within 30 million years, sometimes after 1.7 billion. At any rate, it was destined for demise.
That may crack one of the solar system’s oldest cases: why Earth and Venus, near-twins in size, weight class, and composition, turned out nothing alike. Though scientists don’t yet know whether a world needs a moon to be habitable, they do know that our moon plays many crucial roles for Earth, such as steadying its tilt and controlling ocean tides. It’s not far-fetched to suppose the moon may also help keep the planet geologically active. If Venus once had a moon but lost it, its absence could have bent the arc of that scorched world’s history.
“Venus didn’t require a catastrophe to arrive at what we can see today,” Kane said in a statement.
The physics comes down to spin. Earth turns once every 24 hours — fast enough to fling energy outward into the moon‘s orbit. NASA astronauts left mirrors on the lunar surface during Apollo 11, and by bouncing lasers off them, scientists have been able to measure the moon retreating about four centimeters per year — roughly the rate your fingernails grow.
Venus does the opposite. It takes 243 Earth days to turn around once. At that pace, the planet’s gravity wins the tug-of-war and reels in an orbiting moon instead of pushing it away.
Credit: NASA / APL / NRL
Kane tested moons ranging from half the mass of ours to 10 times heavier. The heavier the moon, the faster it spiraled in.
“When I made this discovery, I was shocked,” Kane said. “I thought surely the broad range of scenarios I was exploring would lead to a variety of results. But it all went pretty much in the same direction.”
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Earlier explanations of why Venus doesn’t have a moon leaned on drama. Perhaps something enormous smashed Venus’ ancient moon apart. Perhaps Venus never suffered the kind of collision that builds a moon in the first place. Kane’s research undercuts both — and the reason centers on how moons get made.
Moons don’t come from nowhere. Ours formed when a massive object struck the young Earth and threw debris into orbit, where it clumped together. That same blow did something else: It set how fast the planet spun for billions of years afterward.
On Venus, those two outcomes work against each other. A big moon lasts only if the collision leads to the planet spinning once every 10 to 12 hours. But impacts that leave a planet spinning that fast scatter their debris too close, where the rubble falls back to the surface before it can coalesce into anything in space. The crashes that throw debris far enough away to build a moon leave the planet turning slower than that.
If there had been a moon-forming collision in Venus’ ancient history, it resulted in the planet turning once every 12 hours at best — most likely slower. That means one blow could hypothetically give the planet a satellite, along with the power to destroy it, with nothing else having to go wrong.
But proving whether Venus swallowed a moon is hard. A resurfacing event about a billion years ago paved over roughly 80 percent of the planet, wiping out older scars. Any evidence now lies deep below the surface.
Scientists have detected strange structures within Earth that may be leftovers from the collision that created our own moon; similar measurements at Venus could turn up something comparable. NASA’s planned DAVINCI mission, which could drop a probe through the Venusian atmosphere in the early 2030s, may also catch chemical traces of a moon that broke apart.
The researchers stop short of claiming Venus once had a moon. Though Kane suspects it did, he insists that remains an open question. What his team’s work shows is that a massive moon could not have lasted — something he believes astronomers should keep in mind as they hunt the galaxy for habitable exoplanets, the rocky worlds around other stars.
“My feeling is there are benefits to having a moon, but it isn’t required for habitability,” Kane said. “The moon has definitely changed the way Earth has evolved through time, but we don’t fully know how important that role is.”