New Research Suggests Venus May Have Swallowed Its Moon

California. Scientists have long speculated why Venus, with a size, mass, and structure similar to Earth, has no moon. A new study by the University of California, Riverside (UCR) suggests that this seemingly twin planet may have possibly swallowed its moon. In the past, scientists had proposed theories that Venus once had a moon which was destroyed by a massive impact. Other theories focused on the idea that Venus never had a large enough impact to form a moon. The new research, published in 'The Astrophysical Journal,' shows that both these theories do not hold true. 'My study shows that Venus did not need a catastrophic event to reach its current state,' said Stephen Kane, an astronomer at UCR and lead researcher. 'The combination of the planet's own gravity and its rotation speed naturally caused the moon to fall onto it.' NASA's Apollo 11 landing left mirrors that allow scientists to accurately measure the distance between Earth and the Moon. 'We know the Moon is slowly moving away from Earth at a rate of about four centimeters (1.6 inches) per year,' Kane said. According to Kane, this distance is increasing because Earth completes its orbit relatively quickly every 24 hours. The energy generated by this rotation is transferred to the Moon, pushing it away. The situation with Venus is the opposite. It takes Venus 243 Earth days to complete one rotation. Instead of moving away, the Moon is pulled inward by the planet's slow rotation and gravity, eventually colliding with it. To test this idea, Kane developed computer models based on physics of how celestial bodies interact through gravity. He first modeled the evolution of Earth and its moon to ensure the model accurately represented known systems. Then, he varied the rotation rate of Venus and the size of its hypothetical moons. During this process, he tested moons ranging from half to 10 times the mass of Earth's Moon. In most simulations, the result was the same: the moon collided with Venus. The larger the moon, the faster it collided. 'I was surprised by this finding,' Kane said. 'I thought the different scenarios I was studying would surely lead to different outcomes. But all the results pointed in almost the same direction.' This conclusion does not prove that Venus ever had a moon. Kane believes Venus may have had a moon, but the question of whether a moon formed or not remains unanswered. However, the study shows that even if Venus had a moon, it could not have lasted forever. It is very difficult to find physical evidence of such a collision. About 80 percent of Venus's surface is of uniform age. A major resurfacing event about a billion years ago erased much of its older geological history, which is why the surface age is uniform. Instead, its evidence may lie beneath the surface. Scientists believe that a giant impact early in Earth's history led to the formation of the Moon. Seismic studies have found anomalous structures deep within the Earth that could be remnants of that event. Similar measurements on Venus could provide clues as to whether it once incorporated a moon. Such a collision could also help scientists understand another long-standing mystery about Venus: whether the planet, Earth's closest neighbor, was once habitable. The collision of a moon with Venus must have transferred immense energy and angular momentum to the planet, altering its rotation, geology, and climate. If Venus once had oceans or other conditions conducive to life, such an impact could have changed the course of the planet's evolution. Its effects are not limited to Venus alone. Scientists searching for potentially habitable worlds around other stars often look for Earth-like planets. They also consider the presence of a moon as a factor in determining whether a planet is habitable. Earth's Moon generates tides, may have helped keep the planet geologically active, and has profoundly influenced Earth's evolution. However, scientists do not know if a large moon is necessary for life. 'I think there are benefits to having a moon, but it's not essential for habitability,' Kane said. 'The Moon has certainly changed the way Earth has evolved over time, but we don't fully know how significant that role is.' Kane's findings suggest that planets capable of forming moons may not be able to sustain them. Planets that rotate slowly could push their moons towards the surface in a spiral path, and planets could change dramatically in the process. 'When people think about Earth-like planets around other stars, they ask, does it have a moon?' Kane said. 'My study shows a disappointing scenario in many of those cases. If those planets don't rotate fast enough, the moon will collide with the surface, and that will change the course of those planets' history.'