Science1 publisher2 min readPublished
A new model has Venus's 243-day spin pull a hypothetical moon down onto the planet
Stephen Kane checked his tidal code against the measured Earth-Moon system, then ran Venus with moons of half to ten lunar masses, and most of those runs ended with the moon on the surface. Whether Venus ever had one remains open.
The Scientist · Science desk
What happened
- UC Riverside astrophysicist Stephen Kane, writing in The Astrophysical Journal, finds that Venus's own gravity and rotation rate would have brought a moon down onto the planet with no catastrophe involved.
- The two standing explanations were that a massive impact obliterated a Venusian moon, or that Venus never had a moon-forming collision in the first place, and Kane reports that neither fits.
- The work does not prove Venus ever had a moon, and Kane leaves that open; what it shows is that such a moon could not have gone on existing indefinitely.
- Seismic work has found odd deep structures inside Earth that may be debris from the Moon-forming collision, and comparable measurements at Venus could indicate whether it absorbed a satellite.
Compiled by The ScientistSomething wrong?How this is made
Why it matters
- constraint A large moon can no longer be treated as a durable feature of an Earth-mass world in habitability modelling; the planet's rotation rate becomes a condition on whether the moon is still in orbit.
- contradiction The press framing over the study says Venus swallowed its moon. The modelling only bounds how long a moon could have lasted.
- decision With most of Venus's surface record reset about a billion years ago, the test has to be made inside the planet. That raises the value of interior geophysics on any Venus mission payload.
The first thing Kane's model did was reproduce something already measured: the evolution of the Earth-Moon system [8]. The Earth side of the comparison has a hard number attached. Apollo 11 left mirrors on the lunar surface, so the distance can be measured precisely [4]. "We know that the moon is slowly moving away from the planet, at a rate of around four centimeters (1.6 inches) per year," Kane said [5]. Earth turns once every 24 hours, and energy from that spin goes into the Moon's orbit and pushes it outward [6]. Four centimeters a year is 40,000 kilometers over a billion years, if the rate held steady [3].
Venus needs 243 Earth days for one rotation [7], which is 243 Earth turns for a single Venusian one [2]. In the model, that slow spin and the planet's gravity drag a satellite inward until it hits [7]. Kane varied the rotation rate and the satellite's mass, from half the Moon's mass up to ten times it, a twentyfold range, and most runs ended the same way, with heavier moons crashing sooner [9][1]. "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" [10]. The outcome does not depend on the mass chosen. The phys.org account does not say how many simulations Kane ran, or what fraction of them ended in collision [19].
A moon falling onto Venus would have delivered a large amount of energy and angular momentum, with possible consequences for the planet's rotation, geology and climate [14]. According to the phys.org account, that is also why the scenario bears on whether Venus was ever capable of hosting life: if the planet once had oceans or other favorable conditions, an impact on that scale could have changed the course of its evolution [20].
Kane's result suggests a planet able to form a moon may not be able to keep it, and slowly rotating worlds are the ones whose moons spiral in [17]. Searches for habitable worlds often treat the presence of a moon as one factor influencing habitability [15]. Whether a large moon is necessary for life is not known [15], and Kane does not claim it is. "My feeling is there are benefits to having a moon, but it isn't required for habitability," he said. "The moon has definitely changed the way Earth has evolved through time, but we don't fully know how important that role is" [16].
What to watch
- The full Astrophysical Journal paper, which should give the number of simulations and the rotation rates at which a Venusian moon survived.
- Any Venus mission concept that carries interior geophysics capable of looking for deep remnants of an absorbed satellite.
- Whether exoplanet habitability models begin treating host-star-driven or primordial rotation rate as a condition on moon retention.