A Comparative Study on Continuum Removal for Chang'E-4 VNIS Hyperspectral Data and its Impact on Elements Retrieval

Spectral absorption features are important parameters for lunar mineral identification and its abundance retrieval. The spectral absorption features of lunar regolith around 1000 nm (Band I) and 2000 nm (Band II) are significant weakened due to long-history space weathering. Continuum removal can effectively enhance the features of spectral absorption for lunar surface materials, which is particularly applicable for processing the in-situ hyperspectral data that obtained by the Visible and Near Infrared Spectrometer (VNIS) onboard Chang'E-4 Yutu-2 Rover that landed in Von Karman crater. In this paper, three types of functions, i.e., linear, the second-order parabola polynomial and the piecewise polynomial functions, are used to fit the continuum of VNIS spectra, and further accessing their influences on calculating the parameters of spectral absorption features. The study shows that the piecewise polynomial continuum performs the best in enhancing the spectral absorption features of VNIS data in Bands I and II. Both endpoints of the continuum can be found. The derived absorption features, such as band depth and centers are highly consistent with those obtained with ENVI software, who uses the full convex hull to determine spectral continuum. The difference between the piecewise polynomial method and full convex hull method in absorption band center and band area ratio is less than 1%. In addition, we tested the retrieval of Wo and Fs with the spectral feature parameters from different continuums and find that continuum composed by piecewise linear equation is appropriate.