![\"NASA\u2019s](\"https:\/\/cdn.sci.news\/images\/enlarge14\/image_15066e-Venus.jpg\")
<\/a><\/p>\nNASA\u2019s Mariner 10 spacecraft captured this view of Venus in February 1974. Image credit: NASA \/ JPL-Caltech.<\/p>\n
\u201cOur study shows Venus didn\u2019t require a catastrophe to arrive at what we can see today,\u201d Dr. Kane said.<\/p>\n
\u201cIt 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.\u201d<\/p>\n
Using computer simulations of how a hypothetical Venusian moon\u2019s orbit would evolve under gravitational tides, the astronomers found that a moon\u2019s fate hinges on how fast Venus was spinning after its formation.<\/p>\n
If a moon-forming giant impact left an early Venus spinning rapidly, with a day shorter than about 12 hours, a lunar-mass satellite could actually have survived for billions of years.<\/p>\n
But if the impact left Venus spinning more slowly, any moon would have been dragged inward by tidal forces and torn apart by Venus\u2019 gravity within roughly 30 million to 1.7 billion years.<\/p>\n
The researchers also modeled two different assumptions about how Venus\u2019 rocky interior dissipates tidal energy.<\/p>\n
They found that the two approaches agree at slower spins but diverge for more massive moons at fast spins, underscoring how much the outcome depends on poorly constrained properties of Venus\u2019 interior.<\/p>\n
Crucially, the scientists connected their results to separate simulations of the giant impacts themselves.<\/p>\n
Those impact simulations suggest that collisions violent enough to produce Venus\u2019 actual, slow retrograde rotation tend to produce debris disks that either fall back onto the planet without forming a moon, or leave any resulting moon sitting right at the tidal survival boundary.<\/p>\n
Together, the findings suggest Venus\u2019 moonless state doesn\u2019t require an extra catastrophic event, such as a later impact stripping away an existing moon.<\/p>\n
Instead, ordinary tidal evolution following a single giant impact could account for it.<\/p>\n
The study also implies that any such moon, if it briefly existed, would have been destroyed early in Venus\u2019 history and that similar Venus-like exoplanets orbiting close to their stars may generally be unable to hold onto large moons, with implications for their climate stability.<\/p>\n
\u201cMy feeling is there are benefits to having a moon, but it isn\u2019t required for habitability,\u201d Dr. Kane said.<\/p>\n
\u201cThe moon has definitely changed the way Earth has evolved through time, but we don\u2019t fully know how important that role is.\u201d<\/p>\n
\u201cThe findings suggest that even planets capable of forming moons may not be able to keep them.\u201d<\/p>\n
\u201cSlowly rotating worlds could send their moons spiraling toward their surfaces, dramatically changing the planets in the process.\u201d<\/p>\n
\u201cWhen people think about Earth twins around other stars, one question they ask is, Does it have a moon?\u201d<\/p>\n
\u201cOur study shows a disturbing scenario for many of those cases.\u201d<\/p>\n
\u201cIf these planets don\u2019t rotate fast enough, the moon will crash to the surface, and that would change the course of history for those planets.\u201d<\/p>\n