Despite sharing similar mass, size, and structure with Earth, Venus is unique in that it has no moons. Previously, scientists theorised that Venus may have had moons in the past, which were subsequently lost following a colossal impact from a celestial body. Alternatively, another hypothesis suggested that Venus had never encountered an object large enough to form a moon.
New Insights from Recent Research
However, a recent study conducted by researchers at the University of California, Riverside (UCR), presents a different explanation. This new theory posits that Venus’s moons were not obliterated by a catastrophic external force, but rather were gradually drawn in and ultimately consumed by the planet’s gravity and its slow rotation. “This research demonstrates that no catastrophic event was necessary for Venus to become what it is today,” explains Stephen Cain, an astrophysicist at UCR and the lead author of the study. “It is believed that the combination of gravity and the planet’s rotational speed caused the moon to naturally spiral inwards towards Venus.”
The Role of Rotational Speed in Satellite Fate
At present, our Moon is drifting away from Earth at an approximate rate of 4 cm per year. This phenomenon occurs because Earth rotates on its axis relatively quickly, completing a rotation every 24 hours. The energy from this rotation is transmitted to the Moon through tidal forces, which stretch celestial bodies due to variations in gravitational intensity, pushing its orbit outward. This was evidenced by precise distance measurements taken by a reflector placed on the lunar surface during NASA’s Apollo 11 mission.
In stark contrast, Venus has an extremely slow rotation period, taking 243 times longer than Earth to complete one rotation, making it one of the slowest spinning planets in the solar system. When this slow rotation is coupled with Venus’s own gravity, even if a satellite were to exist, it would not be pushed outward but rather pulled inward in a spiralling motion.
Modelling Gravitational Interactions
To explore this mechanism, Cain and his research team developed a computational model capable of simulating gravitational interactions between celestial bodies. After confirming the model’s accuracy by successfully replicating the evolution of Earth and its Moon, they repeated the simulation while varying Venus’s rotation period from 5 to 100 hours and the mass of a hypothetical satellite from 0.01 to 10 times that of the Moon.
The findings revealed that the fate of any satellites would depend on Venus’s rotational speed, the satellite’s mass, and the shape of its orbit. Under numerous conditions, the satellite’s orbit would spiral inward until it could no longer withstand Venus’s tidal forces and ultimately disintegrated. Conversely, in scenarios where Venus had an initial rotation period of 10-12 hours, and the satellite was comparable in mass to the Moon with a nearly circular orbit, some satellites could have survived for 4.5 billion years.
The Uncertain Existence of Venusian Moons
It is crucial to clarify that the research does not conclusively prove that Venus ever had a moon. Whether a satellite ever formed remains an open question. Even if such a satellite existed, it would not have survived if Venus had rotated more slowly from the beginning. Given the planet’s current extremely slow rotation, that condition is indeed more plausible. The significance of this discovery lies in demonstrating this possibility.
The Quest for Evidence Beneath the Surface
Despite the historical collisions that may have occurred, finding physical evidence is challenging. The surface of Venus is geologically young, with its age estimated at only a few hundred million years based on the number of craters. Much of the previous geological evidence is believed to have been lost due to extensive surface changes.
If any evidence does exist, it could be hidden beneath the surface. It is thought that Earth’s Moon formed from a giant impact during the early stages of planetary formation, and seismological studies have uncovered anomalous structures deep within Earth that appear to be remnants of this event. If similar measurements could be conducted on Venus, it might be possible to find traces indicating that it absorbed a satellite in the past.
Implications for the Search for Life
These collisions may also provide insights into another long-standing mystery: whether Venus ever possessed conditions suitable for life. Collisions with satellites could have unleashed vast amounts of energy and angular momentum on the planet, potentially altering its rotation, geology, and climate. If Venus did once have conditions conducive to life, such as oceans, these impacts could have significantly influenced its evolutionary path.
This consideration is not exclusive to Venus. Researchers searching for habitable planets often regard the presence of a moon as a critical factor influencing habitability. However, the necessity of a large moon for the existence of life remains unclear. “Personally, I see advantages in having a moon, but I don’t believe it is essential for habitability,” Kane asserts. “The moon has undoubtedly shaped the evolution of Earth, but we do not fully understand the importance of that role.”
New Perspectives on Exoplanet Research
The discovery that planets with slower rotation rates are less likely to retain their moons offers a fresh perspective in the search for Earth-like exoplanets. Even if a “twin planet” to Earth were to exist, if its rotational speed were insufficient, it could swallow its moon, drastically altering the planet’s history.