1. _____________ craters are much more common than 2. ______________ ones. This is because 3. _______________ bolides are more common than 4. ________________ones, and crater diameter is proportional to bolide size. Smaller bolides would be more common than larger ones if most craters were caused by impacts of 5. ________________ fragments. The 6. ________________is a region of small planetesimals, most of which orbit the sun between the orbits of 7. __________________ and 8. ________________. Asteroids come in size from very small to very large, but the size distribution is not random. Rather, it is governed by the fact that asteroids often 9. _________________ with each other. In such a 10. __________________, both asteroids break into smaller pieces. The size of the pieces follows a 11. __________________________________ (which can be simulated in a laboratory) made up of many small fragments and a few larger one. Over the 12. _________________ time, therefore, asteroids continue to collide and produce many small fragments. These bodies may continue to collide with the 13. ____________________, producing a crater size distribution dominated by smaller craters.
Measurements of crater densities can be used for 14. ____________________ of planetary surfaces. Assuming that bolides strike all regions of a planet at approximately the same rate, all areas of the surface should have the same crater density unless 15._______________________ have removed them during the planet's geologic history. A common way for large numbers of craters to be removed is through 16. ____________. 17. _________________ , in contrast to the ancient Moon, is one of the most geologically active bodies in the solar system today. It's surface has no recognizable impact craters, and is continually being 18. __________________ by volcanic eruptions which cover any craters which might form. Regions of a surface with a higher crater density are 19. ______________ than regions with a lower crater density, and a surface like Io's, with no observable craters, is extremely young and implies current geologic activity.
Crater size distributions can also be used to estimate the absolute age of a surface. The cratering rate decreased with time in the early solar system, beginning when the 20. ________________ finished forming about 21. ____________________ ago and there was much left over interplanetary 22. ________________ to cause impacts. The amount of debris decreased over time as collisions and impacts swept it up, thus decreasing the 23. ___________________ of impacts. By measuring the crater density of different areas of the Moon, and measuring the actual ages of rocks returned from different regions by 24. ______________________________, scientists can calibrate cratering rate to actual 25. _____________________ ages.