The surfaces of Mercury, the Moon, and Mars are riddled with craters formed by the impact of space debris. Many of these craters are billions of years old. By contrast, there are only a few conspicuous craters on the Earth's surface, and these are generally less than 500 million years old. What do you suppose explains the difference?

When we gaze at the night sky and marvel at the moon's craters, we're actually looking at the aftermath of tumultuous events. Space debris, which includes a range of objects such as asteroids, meteoroids, and cometary fragments, has been bombarding celestial bodies for billions of years.

Think of space debris as cosmic bullets shooting across the solar system at incredible speeds. When they collide with a planet or moon, the energy released on impact creates craters. The size and frequency of these craters tell us about the history of these impacts on the celestial body. For example, Mercury, the Moon, and Mars, with their surfaces scarred by countless craters, are silent witnesses to a past filled with frequent and intense bombardment by space debris.

While craters serve as historical records of these violent encounters, not all marks of past impacts are visible forever. Factors such as a planet's atmosphere, geological activity, and even human exploration can alter the appearance of craters over time.

Our atmosphere is like a protective shield, guarding against many smaller incoming space rocks. This gaseous envelope causes friction that generates heat, often disintegrating these objects well before they can reach the surface. It's our planet's first line of defense against the relentless pelting of space debris.

For celestial bodies without substantial atmospheres, like Mercury and the Moon, space debris can strike the surface directly, resulting in more preserved craters. In contrast, Earth's atmosphere acts as a barrier, burning up much of this debris in spectacular fireballs called meteors. Thus, many potential craters never form. Over time, atmospheric erosion plays a significant role in reshaping Earth's surface, gradually wearing down and sometimes completely erasing the craters that do form. This natural process contributes to the much fewer visible craters on our planet compared to our lunar and Martian neighbors.

Our planet is unique with its skin of moving plates, a phenomenon known as plate tectonics. The Earth's crust is divided into large plates that glide over the mantle below. This movement causes mountains to rise, continents to drift, and, importantly, craters to be recycled back into the Earth's interior.

Over millions of years, the shifts and collisions of these tectonic plates can deform and even obliterate impact craters. This geological 'recycling' process works continually to renew the Earth's surface, leading to the few identifiable craters compared to static worlds like the Moon or Mars. On these inactive worlds, where plate tectonics is absent, their crater-covered faces are stark reminders of cosmic collisions unchanged for vast stretches of time.

Every celestial body in our solar system has distinct physical characteristics that influence the appearance and longevity of impact craters. Mercury's craters have endured for billions of years because of its lack of atmosphere and geological activity. The Moon, too, preserves its craters due to a similar lack of erosional and tectonic processes.

Mars presents a somewhat different story—it has a thin atmosphere which is not as effective at burning up space debris as Earth's, and signs of past geological activity, but nothing on the scale of Earth's plate tectonics. These characteristics mean that while it does have weathering and erosion, its craters are also well-preserved. Understanding the various factors that influence each celestial body's surface—from atmospheric composition to geological dynamism—allows us to decipher the history and evolution of not only the bodies themselves but also the solar system at large.