| 其他摘要 | Inversion of lunar soil thickness using brightness temperature, which obtained in lunar exploration using passive microwave remote sensing, is one of the scientific goals in the Chang’e lunar exploration project, and it is a new attempt in lunar soil thickness exploration by human. Further study in distribution and variation of lunar-surface solar radiation, lunar interior heat flow, and lunar-surface temperature are prior conditions of interpreting remote sensing data and inversing lunar soil thickness. It is also important for further exploring the Moon and exploiting the lunar resource, even for building the lunar base.
In this study, we have constructed an effective solar irradiance real-time model according to the relationship between lunar-surface effective solar irradiance and solar constant, Sun-Moon distance, solar radiation incidence angle, which is expressed as follows:
(1)
Where,
(2)
(3)
By analyzing the parameters in the lunar-surface effective solar irradiance, we have found that the Sun-Earth distance and the solar radiation incidence angle are the key factors which affect the lunar-surface effective solar irradiance. The error analysis of the model shows that the theoretic erroneous percentage of this model is less than 0.28% during the period from 1950 to 2050. It indicates that the model could accurately reflects the variation of lunar-surface effective solar irradiance. The results show that the total solar irradiance on the lunar surface would change form 1321.5 to 1716.6 W•m-2 in 2007, with the average is 1368.0 W•m-2. The minimum variation amplitude wthin a month is about 6.0 W•m-2, the maximum is about 23.6 W•m-2.
Based on the lunar-surface effective solar irradiance real-time model, the conservation of energy, and the law of Stefan-Boltzmann, we have made the lunar-surface temperature distribution model as follows:
(4)
Where, the initial condition is determined by the following equation.
(5)
By comparing with the observations, we found that the computed results are consistent with the observations when the reflectivity, the thermal emissivity and the thermal inertia are 0.127, 0.94 and 125 J•m-2•s-1/2•K-1, respectively. In this case, the model could accurately predict the lunar-surface temperature in the ideal condition.
An important application of lunar-surface thermal parameters is to interpreting brightness temperature data which obtained in lunar exploration using passive microwave remote sensing. In the exploration, the brightness temperature obtained by radiometer reflects the characters of thermal radiation in outermost lunar layer, which is related closely to its thermal condition. With the thermal parameters models have been made in this study, we further analyzed the brightness temperature contribution of outermost lunar layer in different thermal conditions in lunar exploration using passive microwave remote sensing, according to thermal radiation transfer theory; determined the relationship between brightness temperature and lunar-surface temperature, lunar soil thickness; and estimated the possibility and precision of lunar soil thickness exploration by passive microwave remote sensing.
The analysis of thermal radiation transfer in the outermost lunar layer shows that the brightness temperature obtained in the lunar exploration using passive microwave remote sensing is controlled by the thermal radiation of outermost lunar layer. There is an exponential relativity between the brightness temperature and the lunar soil thickness, and the brightness temperature is also affected by the lunar surface temperature. When the complex permittivity is 2.0 + 0.005 j and 9.0 + 1.0 j for lunar soil and lunar rock respectively, and the relative magnetic permeability of lunar materials is 1, the brightness temperature of the outermost lunar layer at 3.0GHz, 7.8GHz, 19.35GHz and 37.0GHz could be expressed as follows:
3.0GHz:
(6)
7.8GHz:
(7)
19.35GHz:
(8)
37.0GHz:
(9)
The brightness temperature would vary within the ranges of 212.5K～252.8K, 207.4K～266.7K, 193.8K～288.6K, and 174.0K～310.9K at those four frequencies, when the lunar soil thickness and the lunar surface temperature vary from 0.5m to 30m, and from 100K to 400K respectively. For the passive microwave remote sensing, brightness temperature increases with lunar soil thickness and shows a steady tendency at lower frequencies; in contrast, it is fluctuant at higher frequencies. Hence, it is disadvantageous to explore lunar soil thickness by a single band in passive microwave remote sensing. Moreover, the higher the frequency is, the larger the brightness temperature gradient would be. And it trends to be 0 in a case of a very small lunar soil thickness. Except for 3.0GHz, the decreasing amplitudes of brightness temperature gradient at 7.8GHz, 19.35GHz, and 37.0GHz are larger when the lunar surface temperature increases, especially at 19.35GHz. Those frequencies are suitable to make accurate explorations in those areas with smaller lunar soil thickness during the lunar night period. And we could explore the lunar soil thickness globally in the whole lunar day and night at 3.0GHz. If the resolution of microwave radiometer is 0.02K, at 3GHz, the exploring precision of lunar soil thickness is about 0.07m when the lunar soil thickness is about 10m, and about 1.4m when the lunar soil thickness is about 20m. When the lunar soil thickness is smaller than 0.5m, brightness temperature decreases firstly and then increases with lunar soil thickness ranging from 0 to 0.5m. It results in that a brightness temperature corresponds possibly to two different lunar soil thicknesses. So, it is difficult to inverse lunar soil thickness with single band brightness temperature in the area which lunar soil thickness is smaller than 0.5m.
In lunar exploration using passive microwave remote sensing, the lunar soil thickness could be determined if it is smaller than 0.5m by the brightness temperature obtain in the lunar night period. The reference values to distinguish if the lunar soil thickness is smaller than 0.5m for 3.0GHz, 7.8GHz, 19.35GHz and 37.0GHz are 212.9K, 207.4K, 193.5K and 174.1K respectively, when the lunar-surface temperature is 100K. And they are 220.8K, 226.8K, 234.1K and 237.2K respectively, when the lunar-surface temperature is 240K. When the brightness temperature is smaller than the reference value, it indicates that the lunar soil thickness is smaller than 0.5m. In the contrast, it indicates the lunar soil thickness is larger than 0.5m. Moreover, we could determine wether the lunar rock is bareness by the influence coefficient of lunar-surface temperature. When the influence coefficient of lunar-surface temperature close to 0.77, 0.82, 0.84 and 0.85 for 3.0GHz, 7.8GHz, 19.35GHz and 37.0GHz respectively, we consider that the lunar rock is bareness. |
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