The first spectrometer data from the Mars Rover Curiosity has made its way back to Earth, analyzing the plasma light captured during laser excitation of rocks and soil on the planet’s surface. Solid-state thermoelectric technology was used to cool the ChemCam (Chemistry Camera) CCD sensors. Maintaining temperature is critical for successful operation, and the Thermoelectric Assembly (TEA) provided a reliable cooling solution.
Marlow Industries designed and built a custom TEA for the Rover’s ChemCam. Together with NASA’s Jet Propulsion Laboratory, they developed an assembly that included three thermoelectric modules (TEMs), one for each CCD in the ChemCam, and a mounting configuration that located the assembly inside the instrument’s body.
The TEMs are two-stage, semiconductor- based devices that function as small heat pumps. Their cooling-mode operation is based on the Peltier effect. Driven by DC power, they pre-cool the CCD before the lasers fire, and maintain a constant temperature across the 1024 x 1024 pixel CCD while the laser is firing.
Thermal sensors on or near the CCDs provide continual feedback to the TEMs’ electrical power input. A feedback loop keeps the temperature at 0 °C. If the temperature outside warms up and the CCD temperature starts to rise above this target, the controls provide more power to the coolers, driving them to cool. The TEMs move heat from the detectors into the ChemCam body via conduction. The heat’s thermal path travels through the frame to the chassis of the rover.
TEMs offer a unique way to provide cooling for this mission. They not only offer the ability to meet the mission’s thermal requirements, but do so within the project’s limited power, electrical, and physical space constraints. Without requiring special accommodations, TEMs offer rugged cooling capacity that can withstand the high-vacuum environment during the cruise to Mars, and last during operation in the Martian atmosphere. Since TEMs are solid-state devices with no moving parts, they can withstand the mechanical shock, vibration, and acceleration requirements during the critical moments from a mechanical loading standpoint launching from Earth and deploying on Mars. They also provide reliable operation as Curiosity moves around on the planet, offering the ability to complete potentially thousands of tests over the twoyear mission.
On Mars, the ChemCam will primarily gather samples during the day, and the TEA is designed for daytime operation. The worst-case operating condition for the TEA’s cooling mode is a hot environment during the Martian summer at its equator. Even in these extreme conditions, temperatures will remain under 27 °C, far below the space qualification temperatures for TEAs that range up to 85 °C.
Thermoelectric solutions are designed to operate in Argon, Xenon, or Nitrogen backfilled environments, in both high- and no-vacuum atmospheres. While each gas and environmental condition results in different heat conduction and convection effects, the expected environments on Mars did not create a design impediment.