# Some Approaches for Light and Color on the Surface of Mars
## 1 Introduction  
Both the Earth and Mars receive light from the Sun, but total (direct + diffuse) spectral irradiances on the surfaces of both planets are different for several reasons. For example, the average distances to the Sun from Earth and Mars are 0.98-1.02 and 1.38-1.67 AU (1 AU = 149.6 million km), respectively; temperatures (120-293 K) and atmospheric pressures (6-10 hPa) on Mars' surface are much lower than those on Earth; the thickness and chemical composition of the atmospheres of these two planets are very different, with the main gasses being N\({}_{2}\) (78%) on Earth and CO\({}_{2}\) (96%) on Mars. In particular, dust is ubiquitous in the Martian atmosphere, which plays a key role in climate. Experimental measurements of the spectral power distribution of light on the surface of Earth (usually known as daylight) have been performed [1; 2], and the results indicateddependencies on geographical location, time of day, season, atmospheric conditions, etc. From measurements of spectral distributions of 622 samples of daylight performed at three different locations [1], the International Commission on Illumination (CIE) has proposed a model of daylight (see clause 4.1.2 in [3]). However, daylight remains an active research topic because it is most important for the right conditions for the development of life on Earth and many different applications. For example, the CIE Technical Committee 3-60 currently analyzes new daylight experimental measurements, the model for daylight proposed by the CIE has been revisited [4], and the CIE has proposed indoor and smoothed daylight illuminants for applications [3]. In contrast, our knowledge about light on Mars' surface is scarcer. Experimental measurements of light on the surface of Mars (sometimes designated as sol-light, because'sols' is the name for Martian days) are much more difficult to obtain than those on the surface of Earth. Limitations in optical communications with very long distances [5], such as the available optical power at the transmitter, the size of optical apertures, and the sensitivity of the receiver, together with the limited number of filters in cameras installed on rovers, imply that current missions on Mars' surface provide irradiance data only at a few wavelengths in the visible and near-infrared range [6]. From a reduced set of 50 spectral irradiance samples provided by rovers on the Spirit and Opportunity missions, [PERSON] proposed five preliminary Mars illuminants with correlated color temperatures (CCTs) in the range of 4600 K-5400 K [7]. [PERSON] tried to collect a higher number of spectral irradiances on Mars' surface to develop definitive sol-light illuminants from a given CCT using the method employed by [PERSON] et al. to obtain daylight illuminants [1], but, as far as we know, his goal was not completed. When looking for the computation of true colors of objects on the surface of Mars, we must consider several questions. First, the CIE explicitly recommended that, in the interest of standardization, the D65 illuminant (a phase of daylight with a CCT of approximately 6500 K) be used whenever possible [3], but it seems that lights on the surface of Mars have CCTs that are considerably lower than 6500 K. Therefore, the assumption of D65 to compute colors on Mars surface is not realistic. Second, the CIE recommended performing color computations using spectral data in the range of 360-830 nm in steps of 1 nm [3], but on Mars, the total number of narrow spectral ranges sampled currently is only about 10-20, because of the reasons stated above. To solve this problem we can consider different approaches, like the use of inter- and extrapolation from measured spectral data [8], or the adoption of theoretical models providing spectral power distributions of light on Mars' surface [9].