Global atmospheric chemistry of CFC-123

THE compound 1,1-dichloro-2,2,2-trifluoroethane (CFC-123), which is potentially usable as a foam-blowing agent in the plastics industry, an aerosol propellant and a refrigerant, has been proposed as an industrial substitute for trichlorofluoromethane (CFC-11), the use of which is increasingly restricted because of its effects on the ozone layer and on climate(1-3). It is expected that CFC-123, although like CFC-11 an absorber of infrared radiation, will be less stable in the atmosphere because of its expected reaction with OH radicals in the troposphere. Using a three-dimensional global model of the atmosphere, we have calculated the chemical destruction rates of CFC-123 by various processes, confirming that the chief sink is destruction by OH radicals below 12 km, which accounts for 88% of its loss. The calculated destruction rate is greatest in the equatorial region below 2 km. The calculated steady-state lifetime of CFC-123 is 1.5 years, based on the best available estimate of the rate constant of the reaction with OH. This lifetime is very much shorter than that of CFC-11, the destruction of which is largely confined to the stratosphere. For equal rates (by mass) of CFC-123 and CFC-11 emission to the atmosphere, the molar content in the atmosphere and the injection rate of chlorine into the stratosphere are, respectively, 48 and 14 times greater for CFC-11 than for CFC-123 in steady-state.     

Assessment of Mars Exploration Rover landing site predictions

Comprehensive analyses of remote sensing data during the three-year effort to select the Mars Exploration Rover landing sites at Gusev crater and at Meridiani Planum correctly predicted the atmospheric density profile during entry and descent and the safe and trafficable surfaces explored by the two rovers. The Gusev crater site was correctly predicted to be a low-relief surface that was less rocky than the Viking landing sites but comparably dusty. A dark, low-albedo, flat plain composed of basaltic sand and haematite with very few rocks was expected and found at Meridiani Planum. These results argue that future efforts to select safe landing sites based on existing and acquired remote sensing data will be successful. In contrast, geological interpretations of the sites based on remote sensing data were less certain and less successful, which emphasizes the inherent ambiguities in understanding surface geology from remotely sensed data and the uncertainty in predicting exactly what materials will be available for study at a landing site.     

Aeolian processes at the Mars Exploration Rover Meridiani Planum landing site

The martian surface is a natural laboratory for testing our understanding of the physics of aeolian (wind-related) processes in an environment different from that of Earth. Martian surface markings and atmospheric opacity are time-variable, indicating that fine particles at the surface are mobilized regularly by wind. Regolith (unconsolidated surface material) at the Mars Exploration Rover Opportunity's landing site has been affected greatly by wind, which has created and reoriented bedforms, sorted grains, and eroded bedrock. Aeolian features here preserve a unique record of changing wind direction and wind strength. Here we present an in situ examination of a martian bright wind streak, which provides evidence consistent with a previously proposed formational model for such features. We also show that a widely used criterion for distinguishing between aeolian saltation- and suspension-dominated grain behaviour is different on Mars, and that estimated wind friction speeds between 2 and 3 m s(-1), most recently from the northwest, are associated with recent global dust storms, providing ground truth for climate model predictions.     

Planetary science: bedrock formation at Meridiani Planum

The Mars Exploration Rover Opportunity discovered sulphate-rich sedimentary rocks at Meridiani Planum on Mars, which are interpreted by McCollom and Hynek as altered volcanic rocks. However, their conclusions are derived from an incorrect representation of our depositional model, which is upheld by more recent Rover data. We contend that all the available data still support an aeolian and aqueous sedimentary origin for Meridiani bedrock.