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The cratering records of very old surfaces , such as Mercury , the Moon , and the southern highlands of Mars , record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago .
 Mercury, Moon, Mars, Solar System
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Mercury, Moon, Mars, Solar System