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tokens: the PCs would contain the changes. Materials that are detached from the surface, such as the surface-independent clouds in the case of the Earth, would also appear in one of the PCs if they have a large influence on the light curve. If these materials can be constrained by the reflection spectra, their fractions can be derived from the magnitude of the corresponding PC. Once the surface information is extracted from light curves, the surface map of the exoplanet can be recovered from the observational geometry without making any spectral assumptions. Aside from orbital elements, which can be determined from light curve observations, the only two geometry assumptions required for constructing the 2D surface map are the summer/winter solstice and the obliquity. The rotation period of a clear or partially cloudy exoplanet can be inferred from the power spectrum of PCs using a Fourier transform (Figure 3 (d)), which requires the observation frequency to be higher than that of the exoplanet’s rotation. Studies have been performed to identify a planet’s rotational period from light curves at different viewing geometries (e.g., Pallé et al. 2008 ). The summer/winter solstice would coincide with the maxima and minima of the PC time series as long as the asymmetry between the northern and southern hemispheres is noticeable, and the reflection changes monotonically with latitude when the sub-stellar point is near extremum. For the Earth, the peak of the time series of PC2 takes place on 2016 June 15 (Figure 3 (c)), which is within one week of the true value. The obliquity of an exoplanet could be derived through its influence on the light curves. A number of recent publications (e.g., Kawahara 2016 ; Schwartz et al. 2016 ) developed methods for deriving the obliquity using its influence on the amplitude and frequency of light curves. Although these inversion methods are mostly based on a cloudless Earth and known surface spectral features, our SVD analysis of separating clouds from the surface could fill the gap. Some issues exist in constructing and interpreting the retrieved Earth surface map. Degeneracy resulting from the convolution between pixel geometry and spectrum is the dominant factor affecting map construction quality. As discussed in Cowan Strait ( 2013 ) and Fujii et al. ( 2017 ), light curves only cover a small portion of the PC plane, which results in a trade-off between the spatial and spectral variation. Although only PC2 is used for constructing the surface map in this work, its time series covers only a small range of all valid values, whose corresponding land fractions are between 0 and 1. Therefore, we introduce a regularization parameter, λ , when constructing the map, to constrain the pixel values of PC2 within a reasonable range; the resulting effect is described in Appendix B . We select the value of λ based on the ground truth of Earth’s surface map. Glints, features that are small in area but contribute significantly to the