Mars water ice distribution detection method based on SHARAD radar

Abstract

The invention relates to a Mars water and ice distribution detection method based on an SHARAD radar, and the method comprises the following steps: obtaining SHARAD radar data and topographic data, and carrying out the preprocessing: carrying out the clutter simulation of each SHARAD radar map, removing a radar map without sub-surface echo through comparison, and extracting the information of a sub-surface reflector; constructing a three-layer reflection model, and calculating a loss tangent and a dielectric constant based on SHARAD radar data and topographic data; respectively calculating water ice indication values based on the dielectric constant and the loss tangent; mars water-ice distribution is estimated based on the water-ice indication value. Compared with the prior art, the method overcomes the limitation that a traditional model does not comprehensively consider a surface weathering layer and an electromagnetic wave attenuation effect, innovatively proposes a water ice indication value for evaluating the possibility that underground water ice is used as a main component of Mars sediment, and is beneficial to deeply understanding the geological evolution of Mars. And an important reference is provided for future Mars landing site selection and mining and utilization of in-situ resources.

Description

Technical Field

[0001] The present invention relates to the field of deep space exploration, and in particular, to a method for detecting the distribution of water ice on Mars based on the SHARAD radar. Background Art

[0002] As one of the terrestrial planets closest to the Earth, Mars has attracted much scientific attention due to its characteristics similar to those of the Earth, such as its atmosphere, seasonal changes, and day-night alternation. Among the many research fields of Mars, the problem of water ice distribution is of crucial importance. Exploring water ice on Mars helps to deepen the understanding of the climate change and geological evolution of Mars. At the same time, water ice, as an important energy resource, can support future human activities and serve as a raw material for fuel production. Therefore, detailed detection of the water ice situation on Mars, especially the investigation of suitable landing areas, has important scientific and practical significance.

[0003] Currently, some studies have evaluated the dielectric properties of materials based on reflection models. For example, CN116500690A discloses a method for detecting the distribution and content of water ice at the lunar south pole based on LOLA and M 3 -assisted Mini-RF, including the following steps: analyzing the multiple water ice polarization properties based on Mini-RF radar images, and combining topographic factors for water ice detection and identification; combining M 3 ineral mapper to supplement and cross-validate the water ice detection results for the data blank part of the radar image; establishing an m–χ hybrid polarization decomposition model to analyze the radar scattering characteristics of total internal reflection at the water ice point; establishing a dielectric constant inversion model based on the backscattering coefficient to obtain the dielectric constant inversion results; establishing a fitting model between the water ice content and the dielectric constant based on the measured values of the dielectric constant of simulated lunar regolith with different water ice content ratios; obtaining the water ice content of each water ice point at the lunar south pole based on the fitting model, and analyzing the average water ice content of the water ice points in the permanently shadowed areas of the craters. However, this method is for the moon rather than Mars, and it fails to comprehensively consider the influence of the surface weathering layer and radar signal attenuation in the model, and lacks a systematic index to quantify the relationship between water ice and the dielectric properties of materials, making it difficult to give an intuitive detection result of the water ice distribution on Mars. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for detecting the distribution of water ice on Mars based on the SHARAD radar, to solve the problems of defects in the reflection model during the water ice inversion process and the failure to systematically quantify the relationship between water ice and the dielectric properties of materials, and to achieve high-confidence detection of water ice on Mars.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A method for detecting the distribution of water ice on Mars based on the SHARAD radar, comprising the following steps:

[0007] Obtain SHARAD radar data and terrain data and perform preprocessing;

[0008] Construct a three-layer reflection model, and calculate the loss tangent and dielectric constant based on the SHARAD radar data and terrain data;

[0009] Calculate the water-ice indication values respectively based on the dielectric constant and the loss tangent;

[0010] Estimate the distribution of water ice on Mars based on the water-ice indication values.

[0011] The preprocessing of the terrain data is specifically: use the terrain data to simulate the echo power map of the reflected signals received by the radar from the regional surface terrain at different times.

[0012] The preprocessing of the SHARAD radar data includes: comparing the radar map and the simulated echo power map to remove clutter, and screening out the radar maps with subsurface reflectors; extracting subsurface reflector information based on the screened radar maps.

[0013] The subsurface reflector information includes the time delay and echo power of the subsurface reflector.

[0014] The specific form of the three-layer reflection model is: assume that both the surface and the subsurface interface are planar and parallel, and divide Mars into three layers in sequence from the surface to the subsurface interface, namely the weathered layer, the subsurface layer, and the deep layer.

[0015] The method for calculating the dielectric constant is:

[0016] Based on the time delay and echo power, combined with the known dielectric constants of the weathered layer and the deep layer, calculate the dielectric constant of the subsurface layer:

[0017]

[0018] T s = 1 - R s

[0019]

[0020] In the formula, P s is the surface echo power received by the radar, P ss is the echo power of the underground reflector received by the radar, R s is the reflection power, T s is the transmission power, R ssis the subsurface Fresnel reflection coefficient at normal incidence, f is the radar frequency, tanδ is the loss tangent of the medium between the surface layer and the subsurface discontinuity, t is the time delay between receiving the surface layer and subsurface echoes, exp(-2πfttanδ) is the loss during the penetration of electromagnetic waves, ε2 is the dielectric constant of the weathered layer, ε3 is the subsurface dielectric constant to be calculated, and ε4 is the deep dielectric constant.

[0021] The method for calculating the loss tangent is as follows:

[0022] Calculate the power loss based on the difference between the subsurface echo power and the surface echo power, and combine the time delay. Use the linear least squares method for fitting to invert the loss tangent:

[0023]

[0024] In the formula, L is the ratio of the power loss to the time delay, λ is the radar wavelength, c is the speed of light, and t is the time delay.

[0025] The method for calculating the first water ice indication value based on the dielectric constant is as follows:

[0026]

[0027] where ε3 is the subsurface dielectric constant, I DC is the first water ice indication value.

[0028] The method for calculating the second water ice indication value based on the loss tangent is as follows:

[0029]

[0030] where tanδ is the loss tangent of the medium between the surface layer and the subsurface discontinuity, I LT is the second water ice indication value.

[0031] The specific estimation of the water ice distribution on Mars based on the water ice indication value is as follows:

[0032] Based on the first water ice indication value I DC and the second water ice indication value I LT calculate the total water ice indication value I WI :

[0033] I WI = I DC + I LT

[0034] where the total water ice indication value I WI represents the degree of the possibility of water ice on Mars, with a value range from 0 to 1, where 0 indicates the minimum possibility of water ice existence and 1 indicates the maximum possibility of water ice existence.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention constructs a three-layer reflection model considering the surface weathered layer, which can achieve high-precision inversion of the subsurface dielectric constant; the present invention designs a water-ice indication value to quantify the relationship between the dielectric constant and the loss tangent of the angle and water ice, and can realize the judgment of the potential distribution of water ice. Description of the Drawings

[0037] Figure 1 is the flowchart of the method of the present invention;

[0038] Figure 2 is a comparison example of the SHARAD radar map and the echo power map of clutter simulation;

[0039] Figure 3 is a schematic diagram of a three-layer reflection model considering the weathered layer and the electromagnetic wave penetration loss;

[0040] Figure 4 is the distribution result diagram of the dielectric constant and the quantified water-ice indication value I DC in an embodiment;

[0041] Figure 5 is the distribution result diagram of the loss tangent of the angle and the quantified water-ice indication value I LT in an embodiment;

[0042] Figure 6 is the distribution result of the possibility of water-ice existence in an embodiment. Detailed Embodiment

[0043] The present invention will be described in detail below with reference to the drawings and specific embodiments. This embodiment is implemented on the premise of the technical solution of the present invention, and gives detailed implementation manners and specific operation processes, but the protection scope of the present invention is not limited to the following embodiments.

[0044] This embodiment provides a method for detecting the distribution of water ice on Mars based on the SHARAD radar, as Figure 1 shown, including the following steps:

[0045] Step 1) Obtain SHARAD radar data and terrain data and perform preprocessing.

[0046] In this embodiment, the Mars SHARAD radar data is from the SHARAD USRDR data provided by the Mars Reconnaissance Orbiter mission, and the radar diagram example is as Figure 2 (a) shown; the terrain data is from the MEGDR data product of MOLA provided by the Mars Global Surveyor mission.

[0047] The preprocessing of the terrain data is specifically as follows: Using the terrain data to simulate the echo power map of the reflected signals received by the radar from the regional surface terrain at different times, and an example of the echo power map simulated by clutter is as shown in Figure 2 (b) of

[0048] The preprocessing of the SHARAD radar data includes: Comparing the radar map and the simulated echo power map to remove clutter, and screening out the radar maps with subsurface reflectors; Extracting subsurface reflector information based on the screened radar maps, including the time delay and echo power of the subsurface reflectors.

[0049] Step 2) Construct a three-layer reflection model, and calculate the loss tangent and dielectric constant based on the SHARAD radar data and the terrain data.

[0050] In order to accurately calculate the dielectric constant of the material, this embodiment constructs a three-layer reflection model. This model comprehensively considers the effects of the surface weathered layer and electromagnetic wave attenuation, and can directly estimate the dielectric constant of the material without knowing the depth of the reflector. As shown in Figure 3 , the model assumes that both the surface and the subsurface interface are flat and parallel. The Mars is divided into three layers in sequence from the surface to the subsurface interface, namely the weathered layer, the subsurface layer, and the deep layer. The top layer is the weathered layer, whose thickness is lower than the vertical resolution of SHARAD (15 meters). The middle layer is the subsurface layer, and the bottom layer is called the deep layer. By measuring the echo power and time delay obtained, and combining the dielectric constants of the weathered layer and the deep layer, the dielectric constant of the subsurface layer can be directly calculated.

[0051] Specifically, based on the time delay and echo power, and combining the known dielectric constants of the weathered layer and the deep layer, calculate the dielectric constant of the subsurface layer:

[0052]

[0053] T s = 1 - R s (3)

[0054]

[0055] In the formula, P s is the surface echo power received by the radar, P ss is the echo power of the underground reflector received by the radar, R s is the reflection power, T s is the transmission power, R ssis the subsurface Fresnel reflection coefficient at normal incidence, f is the radar frequency, tanδ is the loss tangent of the medium between the surface layer and the subsurface discontinuity surface, t is the time delay between receiving the surface layer echo and the subsurface echo, exp(-2πfttanδ) is the loss during the penetration of the electromagnetic wave, ε2 is the dielectric constant of the weathered layer, ε3 is the dielectric constant of the subsurface to be calculated, and ε4 is the dielectric constant of the deep layer.

[0056] The dielectric constant distribution results in the southern part of a certain place (17°-30°N, 115°-135°E) are as Figure 4 shown in (a) of, indicating that the dielectric constant distribution of the overall result area is between 3 and 6, and there are some areas with relatively small dielectric constant values.

[0057] To further constrain the dielectric properties of the material, the loss tangent of the material was calculated.

[0058] Specifically, based on the difference between the subsurface echo power and the surface echo power, the power loss was calculated. Combining with the time delay, the linear least squares method was used for fitting to invert the loss tangent:

[0059]

[0060] In the formula, L is the ratio of the power loss to the time delay, λ is the radar wavelength, c is the speed of light, and t is the time delay.

[0061] The loss tangent distribution results in the southern part of a certain place are as Figure 5 shown in (a) of, indicating that the loss tangent values of the overall result area are distributed between 0.0013 and 0.0374, with large fluctuations, and there are some areas with relatively small loss tangent values.

[0062] Step 3) Calculate the water ice indication values based on the dielectric constant and the loss tangent respectively.

[0063] In order to obtain high-confidence water ice distribution results using radar data, this embodiment proposes a water ice indication value to quantify the relationship between the dielectric properties (dielectric constant and loss tangent) of the material and water ice.

[0064] Since the dielectric constant of the atmosphere is often considered to be 1 on Mars, the dielectric constant of pure water ice is 3.15, and the dielectric constant of bedrock is relatively high, possibly in the range of 6-10. Therefore, the first water ice indication value I DC The conversion function with the dielectric constant is as follows:

[0065]

[0066] The water ice indication value I in the southern part of a certain place DC The distribution results are as Figure 4As shown in (b), it indicates that there is a high possibility of large water ice distribution in two sub-regions.

[0067] Since the loss tangent values of water ice currently found at the north and south poles of Mars are both less than 0.006, while the maximum loss tangent value of the substance that may contain water ice found in the mid-latitudes and low-latitudes can reach 0.01. Therefore, the second water ice indicator value I LT The conversion function between and the loss tangent is as follows:

[0068]

[0069] where tanδ is the loss tangent of the medium between the surface and the subsurface discontinuity, and I LT is the second water ice indicator value.

[0070] The water ice indicator value I in the southern part of a certain place LT The distribution result is as Figure 5 shown in (b), indicating that there is a high possibility of large water ice distribution in two sub-regions.

[0071] Step 4) Estimate the water ice distribution on Mars based on the water ice indicator value.

[0072] Based on the first water ice indicator value I DC and the second water ice indicator value I LT Calculate the total water ice indicator value I WI :

[0073] I WI = I DC + I LT

[0074] where the total water ice indicator value I WI represents the degree of possibility of water ice existence on Mars, with a value range of 0 to 1, where 0 indicates the lowest possibility of water ice existence and 1 indicates the highest possibility of water ice existence.

[0075] The distribution result of the water ice possibility in the southern part of a certain place is as Figure 6 shown, indicating that there is a very high possibility of water ice distribution in Region II ( Figure 6 (c)), and the possibility of water ice distribution in the remaining sub-regions is not high. Region I ( Figure 6 (a)) does not contain water ice, and the main substance is volcanic rock with a relatively high density; Region II ( Figure 6 (b)) is considered to be porous explosive volcanic rock rather than water ice; Region IV ( Figure 6 (d)) may be a sediment similar to VBF, formed by volcanic ash released during a Mars eruption.

[0076] The preferred specific embodiments of the present invention have been described in detail above. It should be understood that those of ordinary skill in the art can make many modifications and variations based on the concept of the present invention without creative efforts. Therefore, all technical solutions that can be obtained by those skilled in the art in the technical field based on the concept of the present invention through logical analysis, reasoning, or limited experiments on the basis of the prior art should fall within the protection scope determined by the claims.

Claims

1. A method for detecting the distribution of water ice on Mars based on SHARAD radar, characterized in that, It includes the following steps: Obtain SHARAD radar data and topographic data and perform preprocessing; Construct a three-layer reflection model, and calculate the loss tangent and dielectric constant based on the SHARAD radar data and topographic data; Calculate the water ice indication values respectively based on the dielectric constant and the loss tangent; Estimate the distribution of Martian water ice based on the water ice indication values.

2. The method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 1, wherein The preprocessing of the topographic data specifically is: Use the topographic data to simulate the echo power map of the reflection signals received by the radar from the regional surface topography at different times.

3. The method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 2, wherein The preprocessing of the SHARAD radar data includes: Compare the radar map and the simulated echo power map to remove clutter, and screen out the radar maps with subsurface reflectors; Extract the subsurface reflector information based on the screened radar maps.

4. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 3, characterized in that, The subsurface reflector information includes the time delay and echo power of the subsurface reflector.

5. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 1, characterized in that, The three-layer reflection model specifically is: Assume that both the ground surface and the subsurface interface are planar and parallel, and divide Mars into three layers: the weathered layer, the subsurface layer, and the deep layer in sequence from the ground surface to the subsurface interface.

6. The method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 4, wherein, The method for calculating the dielectric constant is: Based on the time delay and echo power, combined with the known dielectric constants of the weathered layer and the deep layer, calculate the dielectric constant of the subsurface layer: T s = 1 - R s Where, P s is the surface echo power received by the radar, P ss is the echo power of the underground reflector received by the radar, r s is the reflection power, T s is the transmission power, R ss is the subsurface Fresnel reflection coefficient at normal incidence, f is the radar frequency, tanδ is the loss tangent of the medium between the surface layer and the subsurface discontinuity, t is the time delay between receiving the surface layer and subsurface echoes, exp(-2πfttanδ) is the loss during the penetration of the electromagnetic wave, ε2 is the dielectric constant of the weathered layer, ε3 is the dielectric constant of the subsurface to be calculated, and ε4 is the dielectric constant of the deep layer.

7. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 4, characterized in that, The method for calculating the loss tangent is: Calculate the power loss based on the difference between the subsurface echo power and the surface echo power, and combined with the time delay, use the linear least squares method for fitting to invert the loss tangent: In the formula, L is the ratio of the power loss to the time delay, λ is the radar wavelength, c is the speed of light, and t is the time delay.

8. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 1, characterized in that, The method for calculating the first water ice indication value based on the dielectric constant is: Among them, ε3 is the subsurface dielectric constant, and I DC is the first water ice indication value.

9. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 8, characterized in that, The method for calculating the second water ice indication value based on the loss tangent is: where tanδ is the tangent of the loss angle of the medium between the surface layer and the subsurface discontinuity, and I LT is the second water ice indication value.

10. A method for detecting the distribution of water ice on Mars based on SHARAD radar according to claim 9, characterized in that, The estimating the distribution of Martian water ice based on the water ice indication values specifically is: Based on the first water ice indication value I DC and the second water ice indication value I LT calculate the total water ice indication value I WI : I WI = I DC + I LT Among them, the total water ice indication value I WI represents the degree of the possibility of the existence of water ice on Mars, and the value range is from 0 to 1, where 0 indicates the minimum possibility of the existence of water ice and 1 indicates the maximum possibility of the existence of water ice.

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