The absence of moons around Mars and Venus is primarily due to a combination of factors related to their formation, gravitational interactions, and the dynamics of their respective environments.
Formation conditions: The formation of moons typically occurs through the process of accretion, where smaller objects come together to form larger bodies in the early stages of a planet's formation. The conditions for moon formation around Mars and Venus might not have been conducive to the formation of large natural satellites. Various factors such as the availability of protoplanetary material, gravitational interactions, and impacts from other bodies could have influenced this process.
Gravitational interactions: Moons are often captured by the gravitational pull of their parent planet or formed from material in a planet's circumstellar disk. The gravitational interactions between planets and potential moons are complex, and the circumstances for capturing or forming moons may have been less likely for Mars and Venus compared to other planets in our solar system.
Dynamical environment: The dynamics of a planet's environment, including its proximity to other large bodies, can affect moon formation and stability. Mars and Venus are relatively close to the massive gravitational influence of the Sun, which can perturb the orbits of potential moons. These gravitational interactions and tidal forces from the Sun might have prevented the formation or long-term stability of moons around these planets.
It's important to note that the absence of large moons around Mars and Venus does not mean they are entirely devoid of any satellites. Mars, for example, has two small moons named Phobos and Deimos, which are irregularly shaped and relatively small compared to Earth's moon or the larger moons of gas giants like Jupiter and Saturn.
Our understanding of moon formation and dynamics is still evolving, and ongoing research and future missions might provide further insights into the specific reasons behind the lack of moons around Mars and Venus.