There has been much speculation concerning the origin of lunar craters. One hypothesis is that they are the results of the impact of heavy meteors on the surface of the moon while still soft. The most probable explanation is that they were produced by the gases liberated from the rocky matter. "While solidification was taking place these gases and water vapours steadily escaped through viscous surface, raising giant bubbles". The reader can easily visualize the process that took place by watching frying of pancakes and noticing the formation of bubbles and craters on their surface. Which one of the following actually helps us to determine the origin of lunar craters?