1. a b c d Figure 1. Laboratory set-up for Experiment #3. CaCl2 CO2
Sample cup
CaCl2
Dry ice (195K) beneath the red foam-cover a
CO2
H2O vapor supply b c
Dry ice
Raman objective d

2. Optical fiber feedthrough
Raman probe
Z stage
Electric Feedthrough
Rotation stage
Motor-hub
X stage
cold plate
Vacuum pump
& pressure meter
Laser Raman
Spectrometer
computer
PEACh
Optical fiber feedthrough
feedthrough
Quartz window
RH meter
Needle valve
Extra-dry CO2
water bath at 30 C
H2O
CO2 flow control
Extra dry CO2
mix
CO2 & H2O supply
T-sensor & control
Cooling system
LN2
Figure 2. Laboratory set-up for Experiment #4
Laboratory set-up for Experiment #4

4. Figure 3a of Wang et al., 2017
Figure 3b of Wang et al., 2017

5. Figure 3c of Wang et al., 2017
Figure 3d of Wang et al., 2017

6. green circle represent unknown solid phase with mass > 6w
Figure 3e of Wang et al., 2017
Figure 3f of Wang et al., 2017
green circle represent unknown solid phase with mass > 6w

7. Figure 3g of Wang et al., 2017

8. Figure 3h of Wang et al., 2017
Figure 3i of Wang et al., 2017

9. a. AlCl3·6H2O Fe2+-sulfate·7H2O CaCl2·4H2O Mg-sulfates·7-6 H2O
MgCl2·6H2O
FeCl3·6H2O
AlCl3·6H2O a.
Fe3+-sulfates·9-20H2O
Mg-sulfates·7-6 H2O
Fe2+-sulfate·7H2O
Figure 4a of Wang et al., 2017
Figure 4. Deliquescence phase boundaries of HyCOS compared with those of hydrous sulfates. The deliquescence boundary of each HyCOS was plotted, as a colored band, through the middle space between two sets of RH values in T-RH field, where deliquescence was seen at the higher RH values (right side of band) but not seen at the lower RH values. The width of colored bar reflects the uncertainty of phase boundary in RH. Filled orange data points indicate occurrence of deliquescence. Unfilled points indicate dehydration, or non-change, or rehydration, against the deliquescence boundary at the lowest RH (e.g., CaCl2·4H2O for chlorides). Similar deliquescence boundaries for hydrous sulfates were drawn based on data in Figure S1, S2, S3 and relevant publications in supporting document.

10. b. Mg(ClO4)2·6H2O Ca(ClO4)2·4H2O NaClO4·H2O Fe3+-sulfates·9-20H2O
Mg-sulfates·7-6 H2O
Fe2+-sulfates·7H2O
Figure 4b of Wang et al., 2017

11. Tlow-end ~ 208K Figure 5a of Wang et al., 2017 T range of RSL sites
RH range of 7 deliq-exp = %
Average of the means of 7 different RH
172 sols
Deliquescence of MgCl2·6H2O
Tlow-end ~ 208K
278K
294K
323K
Figure 5a of Wang et al., 2017
Figure 5. Time range t (from t>30% to t100%) of the observed macroscopic manifestation of deliquescence of a HyCOS, as the function of T and RH. (a) in the case of MgCl2.6H2O, the time ranges t for its deliquescence at a specific T (e.g., 278K) but different RHs (e.g., 7 RH values from 45% to 100%) are heavily overlapped. For each t, a mean value was calculated, then an average over the means of all  t was taken, tave(T) (the red dot). It represents an average time for MgCl2.6H2O deliquescence to occur at a T (e.g., 278K) over a wide RH range (45-100%). An exponential regression line (black dotted line), in form of tave(T) = a*eb*T, pass through tave (323K), tave(294K) and tave (278K). The value of R2 reflects the quality of regression. We use 172(=688/4) sols (green dotted line) to set a virtual limit for a season on Mars, in order to have a time basis for comparison. This line intersects with the regression line and set the Tlow-end of a gray-shaded region. This region suggests that if the environmental T of a subsurface MgCl2.6H2O layer rises to be higher than 208 K, its deliquescence would occur in less than 172 sols.

12. Tlow-end ~ 238K Figure 5b of Wang et al., 2017 T range of RSL sites
RH range of 8 deliq-exp = %
Average of the means of 8 different RH
172 sols
Deliquescence of CaCl2·4H2O
t100%
t>30%
t :
Tlow-end ~ 238K
278K
294K
323K
Figure 5b of Wang et al., 2017

13. Tlow-end ~ 231K Figure 5c of Wang et al., 2017 T range of RSL sites
RH range of 7 deliq-exp = %
Average of the means of 7 different RH
172 sols
Deliquescence of FeCl3·6H2O
t100%
t>30%
t :
Tlow-end ~ 231K
278K
294K
Figure 5c of Wang et al., 2017

14. Tlow-end ~ 224K Figure 5d of Wang et al., 2017 T range of RSL sites
172 sols
Deliquescence of AlCl3·6H2O
RH range of 7 deliq-exp = %
Average of the means of 7 different RH
t100%
t>30%
t :
Tlow-end ~ 224K
278K
294K
323K
Figure 5d of Wang et al., 2017

15. Tlow-end ~ 257K Figure 5e of Wang et al., 2017 T range of RSL sites
RH range of 7 deliq-exp = %
Average of the means of 7 different RH
172 sols
Deliquescence of NaClO4·H2O
t100%
t>30%
t :
Tlow-end ~ 257K
278K
294K
323K
Figure 5e of Wang et al., 2017

16. Tlow-end ~ 232K Figure 5f of Wang et al., 2017 T range of RSL sites
172 sols
Deliquescence of Ca(ClO4)2·4H2O
RH range of 8 deliq-exp = %
Average of the means of 8 different RH
t100%
t>30%
t :
Tlow-end ~ 232K
278K
294K
323K
Figure 5f of Wang et al., 2017

17. Tlow-end ~ 240K Figure 5g of Wang et al., 2017 T range of RSL sites
RH range of 7 deliq-exp = %
Average of the means of 7 different RH
172 sols
Deliquescence of Mg(ClO4)2·6H2O
Tlow-end ~ 240K
278K
294K
323K
Figure 5g of Wang et al., 2017

18. a
CaCl2.2H2O
CaCl2.4H2O +
CaCl2.6H2O
CaCl2.2H2O +
3800
3600
3400
3200
3000
Raman Shift (cm-1)
MgCl2
MgCl2.xH2O
(x=2, 4, 6, 8, 12)
2800
3550
3500
1000
980
960
940
NaClO4
NaClO4.xH2O
(x=1, 2, more) b
CaCl2
3600
3400
3200
3000
Raman Shift (cm-1)
1040
1020
1000
980
960
Ca(ClO4)2
Ca(ClO4)2 +
Ca(ClO4)2.xH2O
(x=2, 4)
940
920
3800
Mg(ClO4)2 +
Mg(ClO4)2.2H2O +
Mg(ClO4)2.6H2O
Mg(ClO4)2 d e c
Figure 6. Evidences of HyCOS rehydration at 195 K. (a). Rehydration of CaCl2 at 195 K, revealed by the appearance of H2O Raman peaks (marked by dotted lines); (b) Rehydration of MgCl2 at 195K, revealed by the appearance of H2O Raman peaks; (c). Rehydration of NaClO4 at 195 K, revealed by the appearance of multiple H2O Raman peaks, and by a new Raman peak at 942 cm-1, in addition to 952 cm-1 peak of NaClO4 (both are ClO4 vibrational modes); (d). Rehydration of Ca(ClO4)2 at 195 K, revealed by the appearance of multiple H2O Raman peaks, and by a new Raman peak in cm-1; (e). Rehydration of Mg(ClO4)2 at 195 K, revealed by the appearance of multiple H2O Raman peaks, and by two new Raman peaks in cm-1.
Wang et al., 2017

19. 3800
3600
3400
3200
Raman Shift (cm-1)
Started as CaCl2
CaCl2·4H2O
(4 min 50 s)
a. CaCl2
1000
950
900
850
started as Ca(ClO4)2
Ca(ClO4)2 ·4H2O (14 min)
Ca(ClO4)2 ·2H2O (10 min)
b. Ca(ClO4)2
Wang et al., 2017
Figure 7. Evidence of extremely fast rehydration of chlorides and perchlorates measured at different T and RH in a Mars chamber in experiment #4. (a) rehydration of CaCl2 at 251 K, 23.6 %RH, and 16.4 mbar. The H2O peak (3445 cm-1) of CaCl2·2H2O appeared at 4 minutes 50 seconds after the starting of this experiment. (b). rehydration of Ca(ClO4)2 at 296 K, 4.8 %RH, and 19.7 mbar. The H2O peak (3491 cm-1) of Ca(ClO4)2 ·2H2O and the H2O peak (3550 cm-1) of Ca(ClO4)2·4H2O appeared at 10 and 14 minutes, respectively, after the starting of this experiment.

20. High obliquity period
Current obliquity period
2H2O
4H2O
6H2O
10H2O
12H2O
Figure 8. Calculated increase of hydration degree for seven anhydrous HyCOS in 172 sols, as the function of input H2O vapor volume. For current obliquity period, the assumed input H2O vapor volume range is pr-µm (Jakosky, 1985). For high obliquity period, the assumed the input H2O vapor volume range is pr-µm. Five dotted lines indicate the highest hydration degree of a HyCOS, with color code matches with the line. For example, the highest hydration degree of NaCl and NaClO4 (green color) is two H2O.