Over the past five decades, human exploration and robotic missions have significantly expanded our understanding of Earth’s celestial companion. However, there is still much to discover about the Moon, and one important aspect of lunar science is understanding its thermal behavior. By building a first-of-its-kind thermophysical model, combining simulations with laboratory experiments, Dr. Durga Prasad, from the Physical Research Laboratory, India, has made significant strides in comprehending the spatial and temporal variations in temperature within the Moon’s shallow surface and subsurface.
Why Do We Need to Know About the Moon’s Surface?
Radiation can adversely affect human spaceflight operations, causing carcinogenesis in astronauts, and thermal cycling of the lunar surface can cause thermal fatigue in any habitat built. Therefore, studies of the Moon’s thermal dynamics aid in mission planning by helping select suitable landing sites, identifying stable thermal conditions for equipment and habitats, and optimizing power generation and thermal management systems. Additionally, studies like these play a crucial role in allowing scientists to locate potential resources like water ice and develop strategies for extraction. Such information also provides insight into the Moon’s geology, regolith properties, and internal processes, contributing to scientific research and our broader understanding of celestial bodies.
Figure 1. A photograph of a full Moon. Image by Gregory H. Revera and licensed under CC BY-SA 3.0 via Wikimedia Commons.
To gain further insights into the temperature distribution and thermal behavior of the Moon, Dr. Durga Prasad and his team set out to build a comprehensive 3D thermophysical model to help predict realistic thermal behavior, to simulate geophysical problems, and to assist in planning future experiments on the Moon.
Developing a Thermophysical Model of the Lunar Surface
Dr. Durga Prasad detailed the efforts that followed in a 2022 paper published in Earth and Space Science. At the time, the existing knowledge and measurements of lunar thermophysical behavior and heat flow, were limited. Whatever information known is only for equatorial and mid-latitudes. It is understood that the lunar surface consists of a porous layer with low thermal conductivity followed by a denser layer, which significantly influences surface and subsurface temperatures. The topography of the Moon also plays a critical role in heat transport and was considered in this study.
To enhance their understanding, Dr. Durga Prasad proposed employing laboratory experiments and numerical simulations as potential approaches. The objective of this analysis was to take the initial steps toward developing such a comprehensive model by deriving lunar surface and subsurface temperatures to predict the realistic thermal behavior of the Moon.
The model development process involved the creation of two-layer cross-section models to explore the behavior of temperature and heat flux. The researchers implemented the COMSOL Multiphysics® software environment and the Heat Transfer Module add-on product, utilizing a three-dimensional finite element approach. This approach enables accurate representation of the lunar surface’s complex geometries and ensures the model’s suitability for small-scale to large-scale simulations.
Considering Key Parameters and the Influence of Topography
To accurately simulate the thermophysical behavior of the lunar surface and subsurface, it was crucial to consider the appropriate parameter values and boundary conditions. These parameters, such as density, thermal conductivity, and specific heat, are not constant and are interdependent. Key parameters, including density, were defined based on relationships derived from previous studies. The thermal conductivity and specific heat were derived using a temperature-dependent function (a theoretical curve-fit). Additionally, a semi-sinusoidal function was used to represent the diurnal variation of solar heat flux.
One essential aspect of Dr. Durga Prasad’s model is the incorporation of the Moon’s topographic variations and their impact on heat exchange and thermophysical behavior. Traditional one-dimensional models offer a global perspective but fail to capture localized and regional scale phenomena. Incorporating the actual topography of the lunar surface through digital elevation model (DEM) data (Figure 2) allowed for a more realistic representation of the Moon’s thermal behavior.
Figure 2. (a) Taurus–Littrow Valley and Apollo 17 landing site (b) region of interest considered for regional scale simulations (c) artificial DEM geometry created for local scale simulations (d) meshed geometry and y‐z cutplane.
Figure 3. Three-dimensional plots of surface temperatures derived from the model for local scale simulations for selected times of a lunar day.
The model developed by Dr. Durga Prasad, a unique one and first of its kind globally, successfully incorporates topographic variations, enabling the simulation of temperature distributions at different locations on the lunar surface. The results of the model were validated using laboratory experiments and Apollo 17 in-situ data, highlighting the significance of lunar surface thermal structure, including the thickness of the uppermost layer, as a key parameter influencing surface and subsurface temperature variations.
The Way Ahead for Lunar Research
Dr. Durga Prasad’s research represents a significant step forward in advancing our understanding of the localized thermophysical behavior of the Moon and in planning targeted investigations during future lunar missions. By developing a comprehensive 3D thermophysical model, he has contributed valuable insights into the temperature variations within the Moon’s surficial layer and subsurface. This research has practical implications for future lunar missions, aiding in the selection of suitable landing sites, optimizing thermal management systems, and facilitating resource utilization. Furthermore, the model enhances our understanding of the Moon’s geology, regolith properties, and internal processes.
Reference
- K.D. Prasad, V.K. Rai, and S.V.S. Murty, “A comprehensive 3D thermophysical model of the lunar surface,” Earth and Space Science, vol. 9, 2022; https://doi.org/10.1029/2021EA001968.
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