Venus's slow spin drags simulated moons down into the planet

The physics turns on which way energy moves between a planet's spin and its satellite's orbit. Earth turns once in about 24 hours, and some of that rotational energy is passed to the Moon, which drifts outward. "We know that the moon is slowly moving away from the planet, at a rate of around four centimeters per year," Kane said ^[6]. That rate is measurable to high precision because Apollo 11 left mirrors on the lunar surface ^[7]. Venus turns once in about 243 Earth days ^[8], roughly 243 times Earth's rotation period ^[16]. Kane said the result for a Venusian moon follows from the planet itself: "It turns out the gravity of the planet itself combined with the rate at which it spins naturally caused the moon to collapse on top of it." ^[9]

He varied Venus's rotation rate as well as the satellite, so the sweep covered both the planet's spin and the moon's mass, running from half the mass of Earth's Moon to ten times it ^[3], a twenty-fold range ^[17].

The University of California, Riverside release describes the crash outcome as holding across most of those runs, but does not say how many runs there were or which combinations ended otherwise ^[10]. Kane described the uniformity as a surprise. "When I made this discovery, I was shocked," he 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." ^[18][19]

The simulations stop short of showing that Venus ever had a moon ^[4]. The argument the simulations support is narrower, and Kane put it this way: "My study shows Venus didn't require a catastrophe to arrive at what we can see today." ^[11]

Direct evidence of an impact would be hard to find. About 80 percent of Venus's surface is similar in age, which scientists read as a large resurfacing event roughly a billion years ago that may have erased much of the older geological record ^[5]. On Earth, seismic studies have found unusual structures far below the surface that could contain remnants linked to the collision thought to have formed the Moon ^[12]. Comparable measurements of Venus might someday reveal whether the planet absorbed one of its own, according to the release ^[13].

The design is the part I would trust. Reproducing the Earth-Moon system before touching Venus is a genuine control ^[2], and without it an inward spiral in the Venus runs would be indistinguishable from a coding error. The control cannot test whether there was a moon there to begin with. Venus is close to Earth in size, mass and overall structure and has no moon ^[14]; whether it ever did is a question Kane leaves open ^[4].