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Projects: Mars Climate Sounder Team WebsitePlacing Mars Climate Sounder's Scans Into Context at MarsUpdate: July 2, 2007 Data acquired October 1, 2006 and February 10, 2007
These two movies show how and where Mars Climate Sounder acquires its data on the atmosphere of Mars. As Mars Reconnaissance Orbiter circles Mars in its two-hour orbit, Mars Climate Sounder gazes forward along the spacecraft's path toward the "limb," the edge of the planet's disk. Mars Climate Sounder is oriented so that its linear arrays of 21 detectors are aligned perpendicular to the planet's surface; each detector "sees" a 5-kilometer vertical slice of the atmosphere, from the surface at the edge of the disk up to more than 80 kilometers (50 miles) altitude. These movies show what Mars Climate Sounder "sees" as it takes successive views of the atmosphere while orbiting the planet. Because Mars turns beneath the moving orbiter, each scan is taken while flying over a different part of the planet. The day side of each orbit is on the right, and the night side is at the left. For clarity, the atmospheric thickness is greatly exaggerated on these plots; if produced to the correct scale, the Mars Climate Sounder data would only extend one or two pixels above Mars' surface. The two movies represent different seasons (the first was taken in late northern summer, and the following near autumnal equinox). There are a number of major differences between them. The first one was obtained using the full elevation-scanning capability of the telescopes. The scans all extend from very near the surface to about the same altitude above the planet. The second movie, on the other hand, was obtained while staring in one fixed orientation, while the instrument team at the Jet Propulsion Laboratory addressed problems with the elevation actuator motor. Without the elevation scanning capability, the shape of the Mars Climate Sounder data reflects the elliptical shape of Mars Reconnaissance Orbiter's orbit. The orbit has its periapsis at the South Pole and apoapsis at the North Pole (altitudes 255 and 320 kilometers / 158 and 199 miles, respectively). Scanning using the elevation actuator normally corrects for the associated changes in the direction of Mars’ limb as seen from the spacecraft. Because of the fixed orientation, Mars Climate Sounder's view climbs slightly above the limb at the north pole and drops slightly below the limb at the south pole, so data from low in the atmosphere over the north pole and high in the atmosphere over the south pole was not obtained during these scans.
A second significant difference between the two scans is the presence of a number of drop-outs, or data gaps, in the first movie. These were caused when the Mars Climate Sounder telescopes were commanded to view the surface over the polar regions (“buckshot” observations), and when it was performing calibration measurements by pointing at its calibration target or black space. The calibration target and black space have thermal properties that are very well understood. Comparing measurements of Mars made by Mars Climate Sounder to measurements made at nearly the same time of the calibration target and black space improves the accuracy of the Mars data. The second movie has many fewer gaps, because the instrument was looking continuously at the limb. Although the continuous data looks nicer, one negative consequence is that the calibration of this data is not as good, due to the absence of the blackbody and space views in the operational sequence. The basic data used to generate these movies is the observed brightness or intensity observed by the A4 detector, which “sees” mid-infrared radiation with a wavelength of 11.8 microns. (For comparison, human eyes are sensitive to light with wavelengths from about 0.4 to 0.8 microns). This wavelength was chosen for sensitivity to dust and condensates (ice clouds, for instance) in the Mars atmosphere. The parameter plotted is brightness temperature, which is measured in units of degrees Kelvin. (Brightness temperature is the temperature of a blackbody that gives the same signal radiance. Although it is related to atmospheric temperature, it is not the same.) The brightness temperature of the Mars atmosphere (as displayed in these movies) ranges from about 90 Kelvin in the coldest darkest regions, to more than 200 K in the warmest ones on the day side of the planet. (Note that the freezing temperature for ordinary water is 273 K). The movies show broad-scale variability of the brightness temperatures as the planet turns beneath the spacecraft. Brightness temperatures tend to be warmer on the day side of the planet and cooler on the night side. Some part of the increased temperature signal on the day side probably results from the interaction of incoming solar radiation with clouds and dust in the atmosphere. (Warming of the atmosphere due to the absorption of solar radiation by dust has long been known to be an important factor in Martian atmospheric dynamics). The boundary of the light and dark blue colors approximates the highest levels of the clouds present in each scan. The warmest region shifts significantly southward between the two movies, tracking the seasonal change in the latitude of maximum insolation.
Some of the smaller-scale features in these plots are artifacts of the data acquisition method that have not been corrected for. In particular, we occasionally see extended bright loops rising into the upper reaches of the atmosphere. These are not loop-shaped features in reality, but instead result when a bright cloud feature is seen in several successive limb scans over time. When first seen, such a feature will appear low in the atmosphere, just above the surface. As the spacecraft approaches the cloud, the apparent altitude of the cloud will increase, until the time when the tangent point of the spacecraft view and the location of the feature approximately coincide. Following that time, the spacecraft motion will bring the feature closer and closer, until it passes out of view. During this interval, the cloud will appear at lower and lower apparent altitudes, until it disappears. This produces the loops that are occasionally seen in these movies. These data are fresh and new, and have not been comprehensively processed to reach their final form. The calibration is not yet final, and some of the shortcomings of the current calibration are visible in these movies. For instance, in the second movie there is extensive high-altitude streaking in light blue and purple colors that is suggestive of high-altitude cloud decks. However, those features are expected to disappear when the calibration improves. Mars Climate Sounder simultaneously collects similar data in eight other wavelength ranges. Together these data will allow scientists to determine atmospheric pressure and temperature, atmospheric composition, and the density of clouds and dust -- for regions all around the planet -- for one Mars year. From these solutions, we will obtain new insights into large-scale flows and dynamics that can potentially help us fill in some of the gaps in our understanding of this mysterious atmosphere. |
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