NASA Ames Mars Climate Modeling Group

Climate Cycles

The seasonal cycles of carbon dioxide (CO2), dust and water (H2O) couple to radiative and dynamical processes to produce the climate of Mars.

“One of our group’s main research goals is to further our understanding of carbon dioxide, dust and water cycles, the interactions between them, and how they have evolved throughout the history of Mars.”

The major constituent of Mars’ atmosphere is carbon dioxide (CO2).

During winter, the temperatures in the polar regions are cold enough for the CO2 in the atmosphere to condense into ice on the surface. The CO2 then sublimates off the ice cap in the spring and summer, returning to the atmosphere.

NASA JPL Malin Space Science Systems
Viking Lander 1

In the northern hemisphere, the CO2 ice cap completely vanishes in the summer, uncovering a large perennial H2O ice cap. During the southern hemisphere summer, a small CO2 covered ice cap survives; this perennial ice cap is offset from the south pole. This cycling of CO2 into and out of ice on the surface changes the atmospheric mass by tens of percent over the course of a Martian year.

The Viking Landers measured a highly repeatable annual cycle of surface pressure, which suggests that the CO2 cycle has very little interannual variability. This is intriguing and not yet well understood given the highly variable nature of the dust cycle and the reasonable assumption that interactions between dust and CO2 frost affect the CO2 cycle.

Understanding the CO2 seasonal cycle and how it is coupled to both the dust and water cycles is likely key to understanding its repeatable nature.
Dust is a critically important component of Mars’ climate. NASA JPL Malin Space Systems
(click to enlarge)

Dust is lifted from the surface, mixed and transported by the atmosphere, and pulled back to the surface by gravity. When dust is airborne, it affects the radiative balance of the atmosphere by absorbing and scattering visible light and absorbing and radiating in the infrared. Martian dust events have been observed to range in size from just meters across to hemisphere- or planet-encircling.

Smith, M.D., 2004

A low-level background haze of dust characterizes the atmosphere dust loading during northern spring and summer, while higher dust loadings dominate during northern fall and winter. Although the global atmospheric dust loading generally exhibits cyclic behavior, the dust cycle has the most year-to-year variability of the three climate cycles. The most dramatic example of this interannual variability is the presence of global dust storms that occur during some years and not others.

At the temperatures and pressures in the atmosphere and at the surface of Mars, water transitions directly from the ice to the vapor state.

The main source of atmospheric water is the north residual water ice cap , which is exposed during northern summer when the CO2 ice sublimates. Water sublimates off of the north residual cap, producing the annual local maximum in atmospheric water vapor at high northern latitudes during summertime. This water is then mixed and transported by the atmosphere and is deposited back to the surface either directly as frost or as snow.

Water ice clouds form in the presence of atmospheric dust when cloud nucleation and condensation is thermodynamically favored. These clouds affect the radiative balance of atmosphere and tend to cool or warm the atmosphere and surface depending on their location and altitude.

Although the north residual cap is likely the most important source for water on Mars today, other sources may play a role as well. For example, the regolith may adsorb and release water on daily or possibly seasonal timescales.

Smith, M.D., 2004

Outstanding questions remain about the water cycle, including understanding the sources and sinks for water, the role of water ice clouds and atmospheric dynamics in controlling the spatial distribution of water, the variability of water on daily and seasonal timescales, and the role of water ice clouds in the variable nature of the dust cycle.