In the December 7, 2001 issue of the journal Science, members of the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) investigation team describe an observation with broad and important implications for the study of Mars. Their work illustrates the process by which scientific inquiry proceeds from observation through theory, then to proposed test and observation, eventually to confirm or refute the original theory, or to propose additional tests. Scientific knowledge progresses through this cycle of rigorous testing of each hypothesis.
The observation made by the MOC team, illustrated in the images below, is that pits and other depressions within the part of the south polar cap on Mars that survives each summer (variously termed the perennial, permanent, or residual polar cap), enlarged by the "retreat" of their bounding escarpments.
To understand the implications of this observation, we must travel back 35 years to 1966. In that year, two Caltech scientists---physicist Robert Leighton and geologist Bruce Murray---published a paper (not coincidentally also in the journal Science) describing a theoretical framework for an important observation made by the Mariner 4 spacecraft during its flyby of Mars in 1965. That measurement had shown that the atmospheric pressure of Mars was very low, only about a few thousandths of the pressure on Earth (4-5 millibar). Leighton and Murray had known from telescopic observations that Mars was very cold, and recognized that the low pressure and low temperature together could be explained if the atmosphere was composed mostly of carbon dioxide that was in equilibrium with dry ice on the surface. If the temperature rose just a little bit, the solid CO2 on the surface would evaporate and raise the pressure a little bit; if the temperature fell, so would the atmospheric pressure. Leighton and Murray developed a computational model--one of the first of its kind--to study the range of pressures, temperatures, and other conditions that might occur on Mars. Their paper included a number of specific predictions, including that the atmospheric pressure of Mars would vary cyclically every year by as much as 25% of the total pressure (by comparison, the largest hurricanes on the Earth represent variations about one-half that percentage), and that solid CO2 must exist somewhere on the planet, presumably in the polar caps. Most of Leighton and Murray's predictions have been confirmed by later measurements. The CO2 composition of the very topmost material (the "white stuff") on the permanent south polar cap was surmised based on Viking Orbiter measurements of its temperature, and both Viking Landers measured large, cyclic seasonal pressure variations. What has not been seen is a "reservoir" or large, recognizable volume of CO2.
The recognition of layered materials in the martian polar regions in Mariner 9 images acquired in 1971-1972 led to the realization that environmental conditions on Mars might vary cyclically on other than seasonal or annual timescales. Using advancements on the Leighton and Murray approach, numerous scientists began investigating "climate change" on Mars. An important factor in all of these investigations, or models, was that the equilibrium examined by Leighton and Murray also operated on these longer timescales. There is today a well-developed theoretical framework for studying long-term climate change on Mars, but direct evidence of such change has not been found. Until now.
The pits in the perennial south polar cap were first seen in MOC images more than two years ago. At the time, they were recognized as being very unusual and interesting. We suspected that they were formed in solid CO2, but had no way to demonstrate that. The walls of some of the pits, and of some of the mesas surmounting the adjacent smooth surfaces, show layering, suggesting cyclic deposition. The pits and remnant mesas argue that erosion has been occurring most recently. Thus, these layers and pits together indicate that Mars has experienced environmental change. The questions are, "What was the magnitude of the changes, how rapidly did (do?) they occur, and when was the last change?"
This, then, brings us to our observation. Owing to the global dust storm that obscured most of Mars during July, August, and September 2001, we devoted a large amount of the high resolution imaging during that time to the south polar cap (the only part of the planet that wasn't hidden by atmospheric dust). In mid-August, we began seeing images of places previously viewed a Mars year earlier. The changes were impressive. Escarpments had retreated by between 0 and 8 meters, with 3 m being about average. We initially wanted to know if such rapid changes (these changes are larger than anything we've previously seen on Mars using the MOC) were consistent with water ice or carbon dioxide ice. Using a very simplified version of the approach used by Leighton and Murray, we were able to easily eliminate water ice as a candidate--only CO2 is sufficiently volatile to have changed so much in so short a time. The energy we calculated would be needed to sublime CO2 was very close to (but importantly less than) the energy available from sunlight. Thus, the magnitude of the change, and its occurrence only in pits and mesas in the remnant south polar ice cap, indicates that the ice cap consists of layers of CO2 (probably mixed with small amounts of water ice and dust from occasional global dust storms). The amount of CO2 visible through surface exposure is probably equivalent to 5-10% of the present atmospheric mass. There may be more CO2 buried, but we can't tell that from the present observations.
More important than the admittedly limited information we have about the amount of CO2 buried beneath the surface is how fast the surface CO2 is being eroded. At the present rate, a layer 3 m thick can be completely eroded away in a few tens of martian years. Since each layer is equivalent to about 1% of the mass of the present atmosphere, if sufficient CO2 is buried in the south polar cap, the mass of the atmosphere could double in a few hundred to a thousand Mars years. That could lead to profound changes in the environment. For example, it would change how much and where wind erosion would occur, and where and for how long liquid water could survive at or near the surface.
The observation of these changes and the rate at which
they are occurring is akin to the observations of the "ozone hole"
over Antarctica, or the steady increase in atmospheric CO2 in the Earth's
atmosphere. Although the implications of these observations is often hotly
debated (with most scientists convinced that they represent evidence for
the impact of humans on the terrestrial climate), everyone agrees that they
are evidence of contemporary climate change. Mars, too, is experiencing
climate change today. We don't know what it means, or how extensive
the changes may be, but we can now propose tests for future observations
that can begin to address these questions.
Malin Space Science Systems and the California Institute of Technology built the MOC using spare hardware from the Mars Observer mission. MSSS operates the camera from its facilities in San Diego, CA. The Jet Propulsion Laboratory's Mars Surveyor Operations Project operates the Mars Global Surveyor spacecraft with its industrial partner, Lockheed Martin Astronautics, from facilities in Pasadena, CA and Denver, CO.
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