IMPACT CRATERING PROCESSES AND THEIR ENVIRONMENTAL EFFECTS ON EARTH AND MARS

The Chicxulub impact event at the K/T boundary illustrates how impact cratering processes can affect the geologic and, through their effect on the environment, the biologic evolution of a planet. The global effects of a large impact event are manifested largely by the interaction of impact ejecta with the atmosphere. The debris can affect the thermal and chemical state of the atmosphere, for example altering its ability to screen ozone, facilitating acid rain, and, in the case of Earth, when temperatures are high enough, generating wildfires after vegetation had evolved.

Environmental effects can occur without any extant life, but if life exists, impact-generated environmental changes can affect the evolution of life by changing evolutionary pressures, extinguishing life outright, and creating new evolutionary niches. Not all impact events, however, will have evolutionary consequences. Both the environmental and potential biologic consequences of an impact event must be evaluated in terms of ambient environmental conditions and the types of extant ecosystems at the time of impact. For example, while the production of greenhouse warming gases following the Chicxulub impact event may have been significant 65 million years ago, a similar contribution to the atmosphere 3 billion years ago, which was still dominated by CO2, would have been inconsequential, particularly in a microbial world.

The environmental effects of impact cratering may have been particularly dramatic ~3.9 Ga, when the flux of impacting objects on Earth and probably Mars was very intense. Over 20,000 craters with diameters in excess of 20 km were produced on Earth, some the size of modern-continents. Over 6,400 impact craters with these same dimensions were produced on Mars. While destructive, these events also had the capacity to assist life. In particular, large impact events produce vast, long-lived hydrothermal systems that are ideal places for pre-biotic chemistry and potentially the origin and early evolution of life. This episode of impact cratering and the habitats it may have produced may be one reason Earth's earliest geologic record of biologic activity corresponds to the same period. It may also be the most likely type of environment for life on Mars, if life ever evolved on that planet.