The Solar System is a bustling place, filled with swift celestial objects that make quite an impression when they collide with a planet or moon. I'm referring to craters. However, you may wonder why these craters are nearly perfect circles. After all, meteorites and asteroids plummet from the sky at various angles, so shouldn't the craters be more elongated? As it turns out, they occasionally are, and discovering these peculiar elliptical craters on rocky bodies can provide valuable insights into the history of our solar system.
Creating your own elliptical crater by throwing a rock into mud is relatively simple. However, the objects hurtling from space travel much faster than anything you could ever hope to throw, reaching speeds of over 10 kilometers per second. When a meteor or asteroid collides with the ground at such tremendous velocity, the impact releases an immense amount of energy, resembling a detonating bomb. This energy generates a shockwave that displaces the material surrounding the crater. In other words, it is not the space rock itself that displaces the ground. Since shockwaves radiate outward from a central point in all directions, a circular shape is formed, even if the rock does not fall directly downward. However, upon closer examination of the Moon, Venus, or Mars, you will notice that approximately 5% of craters exhibit an elliptical shape. How is this possible?
While the precise mechanics are subject to debate, it is believed that if the rock enters at an extremely shallow angle, different parts of the rock can strike distinct points on the surface, resulting in an elongated shockwave. Computer simulations indicate that the angle can vary depending on factors such as the speed, size, and composition of the space rock, as well as the characteristics of its target. Generally, we are looking at a maximum angle of approximately 15 degrees. Since we know that elliptical craters can only form from shallow impacts, they provide valuable clues about a planet's past. For instance, in 2018, a team of researchers utilized elliptical craters on Mars to infer the extent to which the planet's tilt has changed over the past 3.5 billion years.
Similar to Earth, Mars has a spin axis that is not perpendicular to its orbit around the Sun. Moreover, the precise angle of its tilt undergoes fluctuations over time. If a planet's tilt remains constant, the number of shallow impacts at a given latitude should remain consistent since the majority of space objects originate from the relatively flat asteroid belt. However, if the tilt changes, the number of elliptical craters would either increase or decrease because the ground's angle would differ from the direction of incoming asteroids. Consequently, the research team conducted simulations to determine the number of skewed craters that would form at various locations for different tilt angles, subsequently comparing the results with the actual surface of Mars. Their findings suggested that over the past 3.5 billion years, Mars has oscillated between tilt angles of 10 degrees and 30 degrees for the majority of its existence. This stands in stark contrast to Earth, where the tilt only varies by a few degrees, likely due to the stabilizing influence of our large moon's gravitational pull. Consequently, astronomers still have much to learn from craters and their shapes, making this type of research significantly impactful.
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Bruno Tumuhairwe
As a wordsmith, I have breathed life into the digital era through captivating storytelling. I have mastered the art of weaving words, seamlessly merging creativity and expertise to craft compelling narratives that leave readers spellbound.
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