In a first set of experiments, Ca(ClO 4 ) 2 • 4H 2 O is exposed to a CO 2 atmosphere saturated with water vapor. In a second set of experiments, Ca(ClO 4 ) 2 • 4H 2 O is placed in direct contact with ice. By comparing the number of decomposed spectral peaks, their wave number, and their width in both sets of experiments with those of the references summarized in Table 1 , hydrated crystalline salts can be distinguished from solutions.

Decomposed Raman spectra of Ca(ClO• 4HO, water ice, and liquid water: (a) The decomposed O‐H vibrational spectrum of hydrated Ca(ClOat −50°C, 800 Pa, and 100% RH shows eight Gaussian components, at 3446, 3471, 3487, 3515, 3542, 3564, 3603, and 3628 cm, with. (b) Water ice at −80°C and 800 Pa indicating five Gaussian components, at 3046, 3115, 3227, 3336, and 3399 cm, with. (c) Decomposition of the O‐H vibrational region of the spectrum of liquid water [], indicating four broad Gaussian components, at 3230, 3420, 3540, and 3620 cm, with

Figure 1 shows the decomposed reference spectra for the single salt, water ice, and liquid water. Figure 1 a shows that the Gaussian decomposition of Ca(ClO 4 ) 2 • 4H 2 O at −50°C contains spectral peaks similar to those shown in literature [ Nuding et al ., 2013 ]. It contains eight narrow peaks at 3446, 3471, 3487, 3515, 3542, 3564, 3603, and 3628 cm −1 , with all peaks except for one exhibiting a full width at half maximum (FWHM) ≤50 cm −1 . Figure 1 b shows that the decomposition of the spectrum of pure water ice contains five peaks at ~3046, ~3115, ~3227, ~3336, and 3399 cm −1 , all with FWHM > 50 cm −1 . Figure 1 c shows the Gaussian decomposition for liquid water with peaks at 3230, 3420, 3540, and 3620 cm −1 from data obtained from literature [ Zhang and Chan , 2003 ], indicating that all spectral peaks have FWHM > 50 cm −1 . These spectral features are summarized in Table 1 . The 3230 and 3420 cm −1 peaks for liquid water are ice‐like components (C 1 and C 2 ), which explain their proximity to the 3227 and 3399 cm −1 peaks in ice.

3.2 Spectra of Brine Formation Experiments

Figure 2 shows results for the first set of deliquescence experiments. Ca(ClO 4 ) 2 at a constant temperature of −50°C (well above its eutectic point at −74°C) is exposed to a CO 2 atmosphere at a pressure of 800 Pa saturated with water vapor. Figure 2a shows eight narrow peaks and shoulders in the O‐H vibrational band, indicating the presence of hydrated crystalline salts throughout the experiment. Figure 2b shows the decomposition of the spectrum of the O‐H vibrational band, taken 205 min after the beginning of the experiment whose results are shown in Figure 2a. It shows that the O‐H spectrum can be decomposed into Gaussians representing the crystalline salt, with the same number of peaks and similar positions and widths (narrow) as the crystalline salt (Figure 1a). This indicates the presence of Ca(ClO 4 ) 2 hydrates only. Indeed, spectral peaks in the O‐H band indicating the occurrence of a solution were not detected within the limit of detection of the Raman spectrometer, even after the samples had been kept at T = −50°C (about 25°C above T E ) and RH = 100% for almost 3.5 h. Since the O‐H spectrum remains unchanged over the duration of the experiment, whereas a small peak appears in the perchlorate band at 936 cm−1 (Figure 2a), and Ca(ClO 4 ) 2 • 4H 2 O is not stable under the above mentioned experimental conditions, we conclude that a partial change in the hydration state from tetrahydrate to octahydrate occurs.

Figure 2 Open in figure viewer PowerPoint 4 ) 2 • 4H 2 O exposed to saturated air. The spectra do not show evidence for deliquescence even after the sample has been kept at T = −50°C (about 25°C above T E ≈ −74°C) and RH = 100% for 205 min. The values shown in the figure correspond to the spectral peaks of the Gaussian decomposition of the 205 min curve. Analysis of this decomposition indicates that all significant spectral peaks in the O‐H stretching region correspond to hydrates. The appearance of a small peak at 936 cm−1 indicates a partial change to Ca(ClO 4 ) 2 • 8H 2 O. (b) Decomposed O‐H vibrational spectrum of the 205 min curve. It shows eight Gaussian components at 3447, 3470, 3486, 3516, 3541, 3562, 3604, and 3628 cm−1, all except one of them with FWHM ≤ 50 cm−1. Comparison with Figure 4 ) 2 . . (a) Raman spectra of Ca(ClO• 4HO exposed to saturated air. The spectra do not show evidence for deliquescence even after the sample has been kept at= −50°C (about 25°C above≈ −74°C) and= 100% for 205 min. The values shown in the figure correspond to the spectral peaks of the Gaussian decomposition of the 205 min curve. Analysis of this decomposition indicates that all significant spectral peaks in the O‐H stretching region correspond to hydrates. The appearance of a small peak at 936 cmindicates a partial change to Ca(ClO• 8HO. (b) Decomposed O‐H vibrational spectrum of the 205 min curve. It shows eight Gaussian components at 3447, 3470, 3486, 3516, 3541, 3562, 3604, and 3628 cm, all except one of them with FWHM ≤ 50 cm. Comparison with Figure 1 a and Table 1 shows that these components indicate the presence of crystalline hydrated Ca(ClO

Similar results are obtained for the Gaussian decomposition of the other curves shown in Figure 2a. The 3.5 h limit corresponds to an upper bound of the period of time during which conditions at the Phoenix landing site would meet the conditions necessary for deliquescence to occur [Möhlmann, 2011]. As shown next, this result indicates that liquid brines are much less likely to occur on Mars by the absorption of water vapor from the air than when salts are in direct contact with water ice.

Figure 3a shows the Raman spectra for Ca(ClO 4 ) 2 • 4H 2 O on top of water ice as the sample temperature is raised from below the eutectic temperature to values well above it, corresponding to those reached in the shallow subsurface during the warm season at the Phoenix landing site [Smith et al., 2009; Rennó et al., 2009]. Gaussian decomposition of the O‐H vibrational spectrum of the blue curve, representing the spectrum just below the eutectic temperature, is shown in Figure 3b. It contains eight narrow peaks with FWHM ≤ 50 cm−1 and similar positions as those in Figure 1a, indicating the presence of Ca(ClO 4 ) 2 hydrates. It also contains four peaks with similar widths and positions as those in Figure 1b, indicating water ice. Spectral peaks indicating the presence of liquid water are not present in this spectrum.

Figure 3 Open in figure viewer PowerPoint 4 ) 2 • 4H 2 O in contact with water ice. The values shown in the figure correspond to the spectral peaks of the Gaussian decomposition of the −56°C curve. The blue and green curves contain spectral peaks indicating the presence of hydrated salt and water ice, similar to those shown in Figure −1, indicating the formation of liquid brines by melting of the water ice within ~3 h after the beginning of the experiment. (b) Decomposed O‐H vibrational band of the spectrum of Ca(ClO 4 ) 2 in contact with ice at −75°C. It shows 12 Gaussians components: at 3067, 3120, 3236, and 3410 cm−1 indicating the presence of ice and at 3440, 3468, 3487, 3510, 3536, 3577, 3602, and 3630 cm−1 indicating the presence of crystalline hydrated salt. . (c) Decomposed O‐H vibrational band of the spectrum at −56°C. It shows seven Gaussians components at 3060, 3127, and 3356 indicating the presence of ice, at 3250 and 3424 cm−1 indicating the presence of liquid water or ice, and at 3545 and 3605 cm−1 indicating the presence of liquid water, all except one with a width of FWHM > 50 cm−1. . (a) Spectra of Ca(ClO• 4HO in contact with water ice. The values shown in the figure correspond to the spectral peaks of the Gaussian decomposition of the −56°C curve. The blue and green curves contain spectral peaks indicating the presence of hydrated salt and water ice, similar to those shown in Figure 2 (see Table 1 ). The orange and red curves contain broad spectral peaks at ~3545 and 3605 cm, indicating the formation of liquid brines by melting of the water ice within ~3 h after the beginning of the experiment. (b) Decomposed O‐H vibrational band of the spectrum of Ca(ClOin contact with ice at −75°C. It shows 12 Gaussians components: at 3067, 3120, 3236, and 3410 cmindicating the presence of ice and at 3440, 3468, 3487, 3510, 3536, 3577, 3602, and 3630 cmindicating the presence of crystalline hydrated salt.. (c) Decomposed O‐H vibrational band of the spectrum at −56°C. It shows seven Gaussians components at 3060, 3127, and 3356 indicating the presence of ice, at 3250 and 3424 cmindicating the presence of liquid water or ice, and at 3545 and 3605 cmindicating the presence of liquid water, all except one with a width of FWHM > 50 cm

In contrast, Figure 3c showing the Gaussian decomposition of the spectrum when the sample is about 20°C above the eutectic temperature (red curve in Figure 3a) contains the wider spectral peaks characteristic of liquid water. In particular, it shows peaks at 3545 and 3605 cm−1 with widths of 160 and 34 cm−1, indicating that a solution has formed in less than ~1.5 h. The spectral peaks at 3060, 3127, and 3356 cm−1 indicate that ice is still present below the solution.