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Projects: Mars Climate Sounder Team WebsiteHow Mars Climate Sounder WorksMars Climate Sounder is designed to build up a global view of the temperature, pressure, dust, water and carbon dioxide content of the atmosphere of Mars up to a height of 80 kilometers (50 miles) over the course of a full Martian year. In order to get the most complete possible view of Mars' atmosphere, Mars Climate Sounder operates almost continuously, every second of every day, almost independently of Mars Reconnaissance Orbiter's five other science instruments. An Independent ViewMars Climate Sounder is mounted on an instrument deck on the bottom of the
spacecraft with most of the other science instruments. In orbit, Mars
Reconnaissance Orbiter will usually keep this instrument deck parallel
to the Martian surface, which points the instruments "nadir," or
squarely at the surface of Mars. In order to build up its global view
of Mars' atmosphere, Mars Climate Sounder required the capability to point
in other directions than straight down.
So the instrument's two telescopes and supporting electronics are mounted on an optical bench that is itself mounted inside a yoke. The optical bench and yoke are connected through an elevation actuator that can rotate the bench up and down through 270 degrees (the other 90 degrees of the circle would point it at the spacecraft). The yoke is connected to the spacecraft through an azimuth actuator that can also rotate through 270 degrees. The elevation and azimuth actuators provide Mars Climate Sounder the ability to see the entire hemispheric view beneath the nadir-pointed spacecraft, as well as two calibration targets located on the yoke. What Mars Climate Sounder "Sees"Mars Climate Sounder contains a total of 189 individual detectors mounted in nine linear arrays of 21 detectors each. Six of the arrays lie behind the optics of one telescope, and three behind the other. The detectors are themselves composed of twelve thermopiles, devices that can turn thermal energy into an electrical current. Each detector measures only 480 by 240 micrometers (0.48 by 0.24 millimeters) in size.
When the detectors are exposed to a target like the surface or atmosphere of Mars, radiant energy from the target is converted into electrical voltage -- the more radiant energy, the higher the voltage. Thermopiles are sensitive to a very broad range of the electromagnetic spectrum, from ultraviolet through visible light through near infrared to the thermal infrared. At Mars, it is their ability to detect the thermal radiation from different materials that is the most useful. In order to glean more information about what materials on Mars are radiating
how much energy to the detectors, Mars Climate Sounder employs filters to
slice its view of the electromagnetic spectrum into narrower bands. Each
of the 9 linear detector arrays has a different filter. A filter blocks
radiation of most wavelengths, only allowing a narrow range or band of wavelengths
of energy to pass through. The following table details the wavelength
bands that each of the 9 filters allows to pass to the detectors and what
each is used for.
Mars Climate Sounder reads the voltage from its detectors every 2.048 seconds, continuously. This is referred to as one "integration interval" or simply "integration." Sometimes it is valuable to stare for a longer period of time at an interesting target in order to acquire sufficient signal to get a high-quality measurement. In that case, Mars Climate Sounder stares at the target for several integrations, and the measurements from all of them are averaged together. Where Mars Climate Sounder LooksAlthough Mars Climate Sounder can look nearly anywhere it wants to, in practice it follows and repeats a few preset sequences that are designed to develop the most thorough, even, continuous coverage of the Martian atmosphere that it can, while at the same time acquiring data that will be valuable for comparison both to the results from other Mars Reconnaissance Orbiter instruments and to previous missions' data sets on Mars' atmospheric properties. Limb and Nadir Observations In fact, Mars Climate Sounder is designed specifically to excel at limb observations. When the instrument is pointed at the limb of Mars, the nine linear arrays of detectors line up perpendicular to the surface. The size of the detectors and the telescope optics are designed in such a way that each detector "sees" a 5-kilometer (3-mile) thick region of Mars' atmosphere, sampling the atmosphere in 5-kilometer increments up from the Martian surface to 80 kilometers (50 miles) altitude. The instrument as a whole sees a 105-kilometer-square region of Mars' limb.
In fact, there is a little bit of leeway in the length of the arrays, so that they cover 10 kilometers of the Martian surface and 15 kilometers of the limb above the 80-kilometer mark. This leeway allows Mars Climate Sounder to be able to sample the full 80-kilometer range even when the spacecraft rolls slightly, up to 9 degrees, to accommodate the demands of other science instruments, for instance in order to point its cameras at specific targets on the surface. (However, Mars Reconnaissance Orbiter may roll up to 30 degrees for selected observations, a few times per day; during these moments, the soundings will be compromised.) It's this ability to simultaneously observe 16 different elevations in the atmosphere that will mark Mars Climate Sounder's unique contribution to Mars science. Each observation will produce a complete 16-point profile of the variation of temperature, pressure, water vapor, dust, and ice with altitude. Mars Climate Sounder will also commonly be pointed nadir, or directly downward. In this case, the instrument looks through the entire thickness of the atmosphere all at once, and also sees the surface. Mars Climate Sounder will thus be able to perform similar observations to some of those performed by the Thermal Emission Spectrometer (TES) aboard Mars Global Surveyor, including mapping the temperature and the broadband reflectance of solar energy from the surface, both of which are important inputs to climate models. Because the surface is closer to the spacecraft than the limb is, the detectors sample a smaller area of the Martian surface (that is, they achieve higher resolution) than they see when they look at the limb. Each detector sees a roughly 1-by-2-kilometer patch of the surface, which is finer than the resolution achievable by TES. Limb and nadir observations are performed together in the same sequence, repeated over and over again as Mars Reconnaissance Orbiter orbits Mars. Here's a summary of the basic repeating sequence:
Including the time required to rotate from one view to the next, this three-step process takes 34 seconds. In that time, Mars Reconnaissance Orbiter moves about 107 kilometers (66 miles), or 1.8 degrees of latitude, around Mars. Looking at it another way, the orbiter acquires a set of vertical profiles, nadir measurements, and space calibrations roughly 200 times throughout each and every orbit of Mars. Polar Observations The polar buckshot sequence includes only two standard limb observations, so it is performed at a cost in the along-track resolution of the atmospheric measurements being made by Mars Climate Sounder. However, because Mars Reconnaissance Orbiter is in a near-polar orbit, it crosses its own path repeatedly near the poles, and the decrease in the along-track resolution of the limb measurements is more than compensated for by the fact that the orbits are much more closely spaced near the poles than they are at lower latitudes. |
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