Venus has no moon, but a modelling study suggests it probably did have one for nearly 2 billion years – before the planet ate it. Understanding this could help us learn about Venus’s previous habitability.
Many large objects probably hit Venus in the early solar system, which could have formed a moon, similar to the giant impact that is thought to have formed Earth’s moon. “There’s a very good chance that Venus did have a moon-forming impact,” says Stephen Kane at the University of California, Riverside.
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The lack of a moon today is therefore quite unusual, but Kane and his colleagues might have an answer. They modelled what would have happened if Venus did have a moon, and found that in almost all scenarios, it would have been consumed by the planet, long erased from existence by the present day.
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Venus rotates quite strangely with respect to the sun, completing a rotation once every 243 days, compared with its orbit of 225 days. This is possibly a result of its thick atmosphere slowing its spin.
In contrast, Earth’s much faster spin transfers angular momentum to our moon, meaning it is moving away a few centimetres each year. For Venus, the opposite would have been true. “The moon is transferring angular momentum inward instead of outward,” says Kane. “It causes the moon’s orbit to catastrophically collapse.”
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The researchers found that at the upper limit of their models, a moon about half the mass of our own could have survived around Venus for 1.7 billion years. At twice the mass of our moon, it would have survived just 30 million years, because of the stronger tidal effect.
Once the moon reached a distance of about 15,000 kilometres, it would have encountered a boundary known as the Roche limit. “That’s where Venus’s gravity rips the moon apart and consumption occurs,” says Kane, with the moon’s debris temporarily forming a ring around Venus until it fell into the atmosphere.
Paul Byrne at Washington University in St Louis, Missouri, says the model is interesting but hard to prove. “It’s exceedingly hard to test this idea because there is basically nothing left behind.” This is because Venus’s surface has been repeatedly resurfaced by volcanism, he says.
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Kane, however, says a future mission to the surface of Venus could include a seismometer, a device that measures seismic waves to study the interior of the planet. This could look for evidence of a consumed moon buried underground. “That would tell us a lot,” he says.
If Venus did have a moon, it may have played a role in the planet’s habitability, with Venus once thought to have had oceans on its surface. It is possible a moon could be linked to the planet’s runaway greenhouse effect, in which it lost all its water for reasons unknown and developed a thick, deadly atmosphere.
“If there was an early habitable period of Venus, then the big question is what brought that to an end,” says Kane. A moon crashing into the planet could have evaporated much of its water. “It could definitely be connected to Venus’s runaway greenhouse state,” says Kane.
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