As shown by the Phoenix Mars Lander’s Thermal and Electrical Conductivity Probe (TECP), contact measurements of thermal conductivity and diffusivity (using a modified flux-plate or line-source heat-pulse method) are constrained by a number of factors. Robotic resources must be used to place the probe, making them unavailable for other operations for the duration of the measurement. The range of placement is also limited by mobility, particularly in the case of a lander. Placement is also subject to irregularities in contact quality, resulting in non-repeatable heat transfer to the material under test. Most important from a scientific perspective, the varieties of materials which can be measured are limited to unconsolidated or weakly-cohesive regolith materials, rocks, and ices being too hard for nominal insertion strengths.
Accurately measuring thermal properties in the laboratory requires significant experimental finesse, involving sample preparation, controlled and repeatable procedures, and, practically, instrumentation much more voluminous than the sample being tested (heater plates, insulation, temperature sensors). Remote measurements (infrared images from orbiting spacecraft) can reveal composite properties like thermal inertia, but suffer both from a large footprint (low spatial resolution) and convolution of the thermal properties of a potentially layered medium. In situ measurement techniques (the Phoenix TECP is the only robotic measurement of thermal properties to date) suffer from problems of placement range, placement quality, occupation of robotic resources, and the ability to only measure materials of low mechanical strength.
A spacecraft needs the ability to perform a non-contact thermal properties measurement in situ. Essential components include low power consumption, leveraging of existing or highly-developed flight technologies, and mechanical simplicity.
This new in situ method, by virtue of its being non-contact, bypasses all of these problems. The use of photons to both excite and measure the thermal response of any surface material to a high resolution (estimated footprint ≈ 10 cm2) is a generational leap in physical properties measurements.
The proposed method consists of spot-heating the surface of a material with a low (<1 W) power laser. This produces a moderate (5-10 K) temperature increase in the material. As the heat propagates in a hemisphere from the point of heating, it raises the temperature of the surrounding surface. The temperature of the heating spot itself, and that of the surrounding material, is monitored remotely with an infrared camera system. Monitoring is done during both the heating and cooling (after the laser is turned off) phases.
Temperature evolution as a function of distance from the heating point contains information about the material’s thermal properties, and can be extracted through curve-fitting to analytical models of heat transport.
In situ measurement of thermal properties of planetary surface materials provides ground-truth for remote sensing observations and high-resolution, site-specific data for any landed spacecraft environment. Thermal properties are necessary parameters for modeling and understanding thermal evolution of the surface and subsurface, climate state and history, and predicting the presence of subsurface water/ice. The applications extend to all solid bodies in the solar system, but with greatest applicability to bodies with thin or tenuous atmospheres where conduction and radiation are the dominant heat-transport properties.
This work was done by Troy L. Hudson and Michael H. Hecht of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. NPO-47390
This Brief includes a Technical Support Package (TSP).
Non-Contact Thermal Properties Measurement With Low-Power Laser and IR Camera System
(reference NPO-47390) is currently available for download from the TSP library.
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Overview
The document outlines advancements in non-contact thermal properties measurement technologies developed by NASA's Jet Propulsion Laboratory (JPL). It focuses on a system that employs low-power laser technology and a Quantum Well Infrared Photodetector (QWIP) to analyze the thermal properties of geologic samples in situ, particularly in extraterrestrial environments.
Key highlights include the successful demonstration of small form-factor diode lasers operating at 532 nm, available in 50 and 100 mW power levels. These lasers are designed to consume less power than existing technologies, such as the Phoenix Thermal and Electrical Conductivity Probe (TECP), which faced several engineering constraints during its operation on Mars. The TECP's limitations included a fixed robotic arm that restricted insertion angles, maximum vertical force constraints, and issues with soil contact that could lead to variability in measurements.
The new non-contact system aims to overcome these limitations by utilizing a laser to deliver a heat pulse to the surface of geologic samples, allowing for precise thermal measurements without the need for physical insertion into the material. This method not only enhances measurement accuracy but also preserves the integrity of the sample being analyzed. The QWIP camera plays a crucial role in monitoring the temperature response of the sample, enabling the assessment of thermal conductivity and diffusivity, which are essential for understanding the thermal properties of natural solid materials.
The document emphasizes the significance of these advancements for future space missions, as they could be applied to any solid body, eliminating the need for complex robotic manipulation. The development of a comparison database and data interpretation algorithms is highlighted as a critical next step in refining these techniques for broader applications.
Overall, the research represents a significant leap forward in remote thermal analysis, providing NASA and JPL with a powerful tool for in-situ measurements on planetary bodies. This technology not only enhances scientific understanding of extraterrestrial materials but also has potential implications for various commercial and technological applications on Earth.
