Discovering a completely different Moon beneath the surface through microwave technology
When we gaze at the Moon from Earth, we see a brilliant celestial companion reflecting sunlight in a display of stark contrasts between bright highlands and dark maria. Yet, this familiar view represents merely the surface of our lunar neighbor. Hidden beneath the visible layer lies a completely different Moon—one that can only be perceived through the special lens of microwave technology.
China's Chang'e-2 lunar orbiter, launched in 2010, carried a sophisticated Microwave Radiometer (CELMS) that for the first time allowed scientists to peer beneath the lunar surface using passive microwave radiation3 . This technology revealed an unexpected phenomenon: mysterious "cold spots" that defied conventional explanation1 .
These enigmatic features don't always correlate with surface characteristics, suggesting they hold secrets about the Moon's geological history and composition that have remained hidden until now.
The CELMS instrument aboard Chang'e-2 represented a revolutionary approach to lunar exploration. Unlike optical cameras that capture reflected sunlight or infrared sensors that measure surface temperature, microwave radiometers detect natural thermal emission coming from beneath the surface.
The key to this subsurface vision lies in the instrument's four frequency channels, enabling scientists to create a vertical profile of the Moon's subsurface properties.
Microwave emissions are influenced by both the physical temperature and the dielectric properties of the lunar regolith—characteristics directly tied to composition and density3 .
| Frequency (GHz) | Penetration Depth | Primary Information Source |
|---|---|---|
| 3.0 GHz | 1-2 meters | Deep subsurface layer |
| 7.8 GHz | 38.5-75 cm | Middle subsurface layer |
| 19.35 GHz | 15.5-31 cm | Shallow subsurface layer |
| 37.0 GHz | 8.1-16.2 cm | Immediate subsurface |
As researchers analyzed the CELMS data, they noticed something peculiar: certain regions exhibited unexpectedly low brightness temperatures, especially during the lunar night. These "cold spots" presented a compelling scientific mystery1 3 .
The curious aspect of these cold spots was their inconsistent behavior across different microwave frequencies2 . This variable behavior with depth suggested that the causes were related to subsurface properties rather than surface characteristics alone.
Perhaps most intriguing was the discovery that these cold spots don't always correlate with high rock abundance1 . This finding challenged conventional wisdom, pointing toward more complex explanations, potentially including hidden geological structures.
The significance of these cold spots becomes particularly evident when examining specific locations like Mare Orientale, one of the Moon's youngest and best-preserved multi-ring impact basins2 .
Despite having low (FeO+TiO₂) content, it showed distinctly different microwave emissions at high versus low frequencies2 .
Researchers identified three specific regions with pronounced microwave anomalies despite their low (FeO+TiO₂) abundance2 .
These anomalies appeared to relate to the temperature gradient deep within the regolith, potentially revealing information about thermal evolution2 .
To understand what causes these cold spots, we need to delve into the fascinating physics of how microwave radiometry works and what influences lunar subsurface temperatures.
The "brightness temperature" measured by the CELMS isn't the same as the physical temperature we might measure with a conventional thermometer. Instead, it represents the microwave thermal emission from the lunar regolith, which depends on both the physical temperature and the dielectric properties of the material3 .
| Factor | Effect on Brightness Temperature | Scientific Significance |
|---|---|---|
| Rock Abundance | Generally decreases TB, but not always | Challenges assumptions about rock-dominated areas |
| (FeO+TiO₂) Content | Increases TB in most cases | Correlates with mare basalts composition |
| Regolith Density | Variable effect based on depth and composition | Reveals compaction history and impact effects |
| Hidden Structures | Creates anomalous patterns | Indicates buried geological features |
| Temperature Gradient | Affects TB difference between day and night | Provides clues about thermal properties |
Some of the most exciting discoveries from the CELMS data have been the detection of hidden linear and circular structures beneath the visible surface1 . These features don't always manifest in optical imagery but appear clearly in microwave data, suggesting they represent ancient geological formations buried by later regolith development.
These subsurface structures may create "thermal shadows"—patterns of anomalous cooling that result from differences in how heat flows through varying subsurface materials. Dense, rocky layers might conduct heat more efficiently than porous debris, creating localized cold spots that persist through the lunar night.
The operation of Chang'e-2's microwave radiometer represented one of the most sophisticated experiments ever conducted in lunar orbital science.
The instrument simultaneously collected data at all four frequency channels, enabling depth profiling.
From its 100-km altitude orbit, the satellite gathered data across the entire lunar surface, including the far side never visible from Earth4 .
Observations were collected at different local times to track diurnal variations in thermal emissions3 .
Data was validated against measurements from the earlier Chang'e-1 mission to ensure consistency and accuracy5 .
The CELMS worked as a high-sensitivity microwave receiver, detecting the natural thermal emission from the lunar subsurface at each of its frequency channels. Since higher-frequency microwaves originate from shallower depths while lower frequencies penetrate deeper, scientists could effectively "peel back" layers of the lunar subsurface without digging a single hole.
The 37 GHz channel data proved particularly revealing for studying surface and immediate subsurface features. Analysis of this data showed that certain regions, particularly in lunar maria, exhibited unexpected thermal behavior3 .
Some regions showed reduced brightness temperatures due to identifiable factors like composition or topography.
Other areas defied conventional explanation, suggesting previously unknown subsurface conditions or materials.
| Characteristic | Traditional Explanation | CELMS Findings |
|---|---|---|
| Correlation with rocks | High rock abundance causes lower TB | Many cold spots show low rock abundance |
| Frequency dependence | Consistent behavior across frequencies | Variable with depth, suggesting layered causes |
| Geographic distribution | Concentrated in specific geological units | Scattered, sometimes isolated patterns |
| Diurnal variation | Predictable day-night differences | Anomalous variations in some regions |
| Association with features | Linked to visible craters or structures | Sometimes reveal hidden buried structures |
Understanding the Moon's cold behavior required specialized instruments and analytical approaches.
A multi-channel microwave radiometer measuring four frequency bands that provided the primary brightness temperature data for analysis.
Aboard NASA's Lunar Reconnaissance Orbiter, it measured surface temperature in thermal infrared bands and enabled cross-validation3 .
Provided detailed information on (FeO+TiO₂) abundance and helped correlate microwave features with surface composition2 .
Numerical simulations of microwave emission through layered media that enabled interpretation of observed brightness temperatures3 .
Methods to ensure consistency between Chang'e-1 and Chang'e-2 data, accounting for instrumental differences and observation timing5 .
The discovery of the Moon's mysterious cold spots has fundamentally expanded our understanding of lunar geology and opened new avenues for research.
The hidden linear and circular structures detected by CELMS may provide crucial evidence about the Moon's volcanic history and impact chronology1 .
The variable behavior of these cold spots with depth adds another dimension to this investigation. The layered nature of the anomalies suggests that the lunar regolith preserves a historical record of deposition and modification processes.
Understanding the Moon's cold behavior has practical implications for future exploration. The thermal environment of the lunar subsurface affects everything from the stability of potential habitats to the design of scientific instruments.
Additionally, the same microwave techniques could be used to search for water ice deposits in permanently shadowed polar regions—a critical resource for sustained human presence on the Moon1 .
New analytical approaches, including machine learning algorithms, are being developed to extract more information from the CELMS dataset. These methods can identify subtle patterns that might escape conventional analysis, potentially revealing additional cold anomalies and advancing our understanding of their origins.
The cold behavior of the Moon's surface, as revealed by Chang'e-2's microwave radiometer, represents a perfect example of how new sensing technologies can transform our understanding of familiar celestial bodies. What initially appeared as anomalies in datasets has blossomed into a compelling scientific mystery that challenges conventional wisdom about lunar geology.
These cold spots remind us that planetary surfaces can be deceptively complex, with subsurface stories dramatically different from surface appearances. As analysis of the CELMS data continues, each discovery brings us closer to understanding the Moon's complete geological history—a narrative written not just on its cratered surface, but hidden in the chilly depths beneath.