Impact craters reveals atmospheric thickness

On a quick note: Impact craters on the surface of planets or moon can tell us how thick or thin the atmosphere is. It can even reveal the atmospheric evolution of the planet or moon and its age too. 

Yes, Impact craters are not to underestimate! Let’s start the story from beginning. 

What are Impact craters? 

Rocky or metal objects (meteoroids), floating around the path of planet or moon, gets attracted, and when it hits the planetary surface, it creates (almost circular) depression on the surface, which we call an Impact crater. 

Meteoroids enter the atmosphere at extremely high speeds (11 to 72 km/s). If the atmosphere is thick, it provides a great deal of air resistance which raises the temperature of meteoroid to the boiling point and it gets vaporized layer by layer. In other words, meteoroid burns up before hitting the ground. Only if the meteoroid is big enough, it will make it through the atmosphere and hit the surface. 

Space rocks smaller than about 25 meters (about 82 feet) will most likely burn up as they enter the Earth’s atmosphere. But If the atmosphere is thin, Air resistance would be very low or negligible. So even small meteoroids would make it through the atmosphere and hit the ground. So here’s the key thing to grab. 

If there are just large impact craters on the surface, it implies that the atmosphere is thick enough to burn up the small meteoroids in the air before they hit the ground.  But if the surface has small craters also, it means the atmosphere is very thin to resist the incoming meteoroid which then hits the surface. 

Craters on Moon, ranges from microscopic size (found in rocks returned to Earth from the Moon) to as big as 290 kilometres (located near the lunar south pole) showing that moon has almost no atmosphere. 

Atmospheric evolution

Over a period of billion years, the atmospheric density of a planet or moon mostly changes. Impact craters can even tell that history i.e how thick or thin the atmosphere was, say a million years ago. 

Here’s how. Scientists can determine the age of impact craters. Paleopressure i.e atmospheric pressure in the past can be determined by the size distribution of ancient craters. 

Counting the craters

Along with other methods to estimate the age of the planet or moon, Counting Impact craters is one more. It can provide insights into how old that surface is. This technique works because the rate at which impacts have occurred has been roughly constant for several billion years in our solar system. Thus simply, more the number of craters on the planet or moon implies, for a longer time it has been exposed and thus it is relatively older than the surface with less number of craters. 

 Geological activities pose a challenge to this technique because they can erase the evidence of ancient craters. 

Thus, impact craters are to be looked upon carefully. They can spit many secrets of that planet or moon. 

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