TITLE: Could Cirrus Clouds Have Warmed Early Mars?
AUTHORS: Ramses M. Ramirez, James F. Kasting
FIRST AUTHOR INSTITUTION: Carl Sagan Institute, Cornell University, Ithaca, NY
There is plenty of evidence to suggest that the surface of Mars was once covered with a liquid ocean. Specifically, the valleys and other similar features visible today on the Martian surface were very likely created by flowing water over an extended period of time, similar to the geological features we see on Earth. However, it still remains an open question as to how Mars had such conditions that were conducive to the existence of liquid water on it surface, and how that changed over time.
In this paper, the authors use a computational climate model to simulate the atmospheric conditions required for Mars to be warm enough to harbor liquid water. Specifically, they test the idea that sufficient cloud cover may have been enough to trigger a greenhouse effect by acting like a blanket and trapping heat absorbed from the Sun. This paper describes modeling the heat transfer between different layers of the atmosphere, and examine the conditions under which cirrus clouds (the most common form of high altitude clouds, composed mainly of ice crystals) are able to form and cover a significant fraction of the planet. The authors test this for a variety of different parameters: percentage of cloud cover, CO2 content in the atmosphere, ice crystal sizes, and cloud thickness.
After trying an extensive set of conditions and parameters, the authors find that it it is possible for cirrus clouds to trap enough heat for liquid water to survive on the Martian surface. Fig. 1 shows a set of examples of these conditions, which include the IR flux radiated away from the planet, flux received from the Sun, and percentage of cloud cover over the whole planet.
First, the results suggests that at least 75% of the planet must be covered with clouds for liquid water to exist. Other studies have suggested that ~50% is probably the maximum reasonable amount of cloud cover for Mars during its early years. In addition, there are numerous parameters (ex. cloud height, particle size, etc.) that affect the ability of clouds to trap heat, and this model only succeeds in reproducing a liquid water-friendly environment when the most optimal of conditions are met. While clouds provide a means by which a planet can retain its heat, there also needs to be a mechanism by which that heat was supplied to begin with. Other researchers have suggested that the energy from early primordial impacts by asteroids or comets could have generated this initial heating.
Even though this study suggests that cirrus cloud cover probably did not provide all of the requisite warmth for liquid water on Mars, it is still likely that cloud cover played a partial role. It remains to be discovered as to what extent did clouds factor into warming Mars and what other mechanisms (ex. the presence of other greenhouse gases, such as methane and sulfur dioxide) contributed to the greenhouse effect on Mars millions of years ago.