Early Mars »
Like the Earth, the climate of Mars has changed over time. Today, Mars is cold and dry and liquid water is not stable on the surface. However, very early in the planet’s history (more than 3.5 billion years ago) climatic conditions appear to have been favorable for the presence of liquid water on the surface. The evidence for this comes from the presence of fluvial features on ancient terrains, such as valley networks and open lake systems, that likely indicate precipitation and runoff.
Other compelling reasons to believe early Mars was warm and wet come from the detection of phyllosilicates (clays) in localized outcrops on these same ancient surfaces (see above figure). Phyllosilicates are the end product of basaltic weathering and their presence suggests water flowed on the surface for an extended period of time.
So the major outstanding question for early Mars is how did its climate produce conditions favorable for rainfall and runoff?
This has been a major challenge for the climate community. The main problem is dealing with the faint young sun. The sun was 25-30% less luminous during its first billion years than it is today making it difficult to generate warm wet conditions. By far, most of the effort has been focused on boosting the greenhouse power of the early Martian atmosphere by changing it mass and composition.
Carbon dioxide, methane, and SO2 get the most attention, but each has its own unique issues and problems. For example:
- » Clouds form in CO2 atmospheres that may limit the greenhouse potential, and vast beds of carbonates have not yet been detected as would be expected when CO2 and liquid water coexist for extended periods of time.
- » Methane is an excellent greenhouse gas, but it requires a large continuous source to counter its removal by photolysis.
- » And SO2, also a good greenhouse gas, may not build up to high enough levels to be effective since it readily oxidizes to sulfate aerosols which can block sunlight from reaching the ground and cool the surface.
One part of our collaboration with Francois Forget and his group at LMD is to study the climate of thick CO2 atmospheres on early Mars. These studies indicate that surface conditions can be favorable for seasonal liquid water at low elevations in the northern hemisphere, but having such conditions exist continuously all year long at every location on the planet is problematic.
Another possibility for explaining wet conditions on early Mars is that they were created by large impact events, which induced significant climate change (Segura et al., 2001; 2008). It is obvious that impactors have pummeled the Martian surface in the distant past and these could have temporarily altered the climate and produced episodes of intense rainfall.
Our group has been using a special version of Ames GCM to study the post-impact environment on early Mars to better understand the consequences and processes involved in impact-generated climate change on early Mars (Colaprete et al., 2005). These preliminary simulations do show that impacts can produce heavy rainfall for relatively short periods of time.
Post Noachian Mars »
Though the apparently warmer and wetter conditions on early Mars did not extend much beyond the end of the Noachian epoch (~ 3.5 Gya), the gradual increase in the Sun’s luminosity with time, and the large quasi-periodic variations in Mars’ orbit parameters must have led to significant climate variations since then. Variations in Mars’ obliquity, eccentricity, and longitude of perihelion alter the latitudinal and seasonal distribution of sunlight, which control the climate system (Figure below).
Some of the consequences of these oscillations include:
- » the collapse of the atmosphere at low obliquity (Kieffer et al., 1992)
- » an increase in atmospheric dustiness at high obliquity (Haberle et al., 2003)
- » and the mobilization and redistribution of surface water ice reservoirs at all obliquities (Haberle et al., 2000; Mischna et al., 2003; Levrard et al., 2004; Forget et al., 2005).
Our group at Ames is working on improving our representation of clouds and their radiative effects to better determine the nature of orbtially-forced climate change on Mars.