44th Lunar and Planetary Science Conference (2013)
2606.pdf
RECURRING SLOPE LINEAE (RSL) AND SUBSURFACE CHLORIDE HYDRATES ON MARS. Alian Wang1, Yanli
Lu1, and I-Ming Chou2, 1Dept. of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington
University in St. Louis, One Brookings Drive, St. Louis, MO, 63130, USA; 2U.S. Geological Survey, 954 National Center,
Reston, VA 20192, USA ([email protected]).
Recurring slope lineae (RSL): RSL is an imThis hypothesis is based on three lines of reasonportant phenomenon revealed by HiRISE-MRO obsering: (1) chlorine (Cl) was found to be broadly distribvations on Mars [1]. The RSL formed and grew on
uted on Mars (GRS-ODY) [8] and has been detected in
some equator-facing slopes during warm seasons on
the chemistry of every surface sample during all Mars
Mars when Tafternoon was in the range of ~ -23 to 27 ºC
surface exploration missions (Vikings, Pathfinder,
(with higher Tpeak). Thousands RSLs have been obSpirit, Opportunity, and Phoenix). In addition, the exserved, and they are narrow (0.5–5 m) and long (over
istence of chlorides in the martian southern hemisphere
100 m). They occurred repeatedly on steep slopes
was suggested by a set of THEMIS-ODY data analyses
(>25°) with growth rates of ~ 0-20 m per day. RSLs
[9]. In terrestrial saline playas, large amounts of chlowere found mostly in the southern hemisphere in rerides invariably appear in the precipitates from salty
gions with moderate thermal inertia, but without any
brines [10-11], even though the precipitation sequence
direct association with surface mineralogy and also
of chlorides on Mars might be different from that on
rarely associated with gullies. RSL itself does not have
Earth [12-13]. (2) A subsurface layer, when enriched
any distinct spectral feature in the Vis-NIR range. The
with ice or hydrous sulfates or chloride hydrates (all
HiRISE team has noted that the distribution of RSL
have high thermal inertia) and covered by a dry layer
bears some similarity with putative chloride deposits in
of surface soils (very low thermal inertia), will be able
the southern hemisphere and they proposed that salty
to maintain a very low Tmax and a much smaller T
brines (chlorides or Fe sul- Figure 1. Chloride hydrates have distinctive Raman, MIR, Vis-NIR spectra that are not affected by the
fates) could be the source of
large temperature variaRaman
RSL [1].
tions at the surface during
MgCl2.6w
Sources of RSL: Our ladiurnal and seasonal cyboratory investigations in past
cles [14]. (3) Chloride
CaCl2.2w
MgCl2.6w
few years have demonstrated
hydrates
(such
as
that sulfates (Mg, Fe, Ca, Al
CaCl2.2w
MgCl
12H
O,
2
2
AlCl3.6w
,Na, K, etc.), hydrous or anhyFeCl26H2O, CaCl2.6H2O,
drous, crystalline or amoretc.) formed from ClFeCl2.4w
phous, all have characteristic
bearing brine at low T
FeCl2.4w
spectral peaks in Vis-NIR
contain large amounts of
spectral range [2-5] that could
H2O (up to 69 wt%), and
be seen by OMEGA or
they are stable in median
AlCl3.6w
CRISM, which is an obvious
T range (some > room T)
FeCl3.6w
mismatch with the observation
until deliquescence comFeCl3.6w
of “RSL does not have any
mences abruptly at an
3800 3600 3400 3200 3000 2800 2600
distinct spectral feature in the
elevated temperature, as
Vis-NIR range”. Furthermore,
Raman shift (cm-1)
Wavelength (nm)
indicated by the laser Raunder the current martian atmospheric conditions (low
man studies of briny fluid inclusions in terrestrial mintemperature [T] and low relative humidity [RH]), hyerals [15].
drous sulfates would dehydrate through solid-solid
To test this hypothesis, we need to investigate the
phase transitions [6,7], i.e., a phase with high degree of
pathway of phase transformations of various chloride
hydration transforms to the one with low degree (e.g.,
hydrates (and their mixtures with water ice) under the
epsomite to starkeyite or to kieserite). Thus hydrous
environmental conditions relevant to Mars subsurface.
sulfates are unlikely to be the source of RSL.
Feasibility of experimental investigation: Most
We hypothesize that chloride hydrates (and their
chlorides have no Vis-NIR, MIR, or Raman peaks,
mixtures with water ice) may exist in the subsurface of
because of the metal-Cl ionic bonding in their strucsome areas in southern hemisphere on Mars. When
tures. However, the vibrational modes of H2O in chlotemperature rises during the warm season, the deliride-hydrates do contribute Vis-NIR, MIR, and Raman
quescence of these chloride hydrates may have propeaks. Figure 1a shows the Raman active modes of
duced large quantity of brine that show up as RSL,
H2O in five chloride hydrates; Figure 1b shows the
while any remaining chlorides in RSL would not proovertone and combinational modes of these H2O in
duce spectral peaks in Vis-NIR range detectable by
Vis-NIR spectral range.
CRISM.
500
1000
1500
2000
2500
44th Lunar and Planetary Science Conference (2013)
2606.pdf
Figure 2. Comparison of dehydration rates of
We have been using two nonabove chloride hydrates at three temdifferent chloride hydrates (52° C, 9± 3% RH)
invasive tools, gravimetric measperatures (50, 21, and 5 °C) and ten
100
urements and laser Raman spectrosRH levels (from 6 to 100%) has been
AlCl3.6w
copy, to monitor the phase transirun for 90 days. One primary finding
KMgCl3.6w
tions of hydrous salts. We anticipate
is the extremely large deliquescence
MgCl2.6w
80
the application of these tools will
field of chloride hydrates compared
CaCl2.2w
make the experimental study on the
with those of hydrous sulfates. We
phase transition pathway of chloride
found that deliquescence can happen
FeCl3.6w
hydrates feasible.
60
at quite low RH level, some < 20%
Experimental study of chloRH at 5 °C. Figure 3 shows a potenFeCl2.4w
ride hydrates: We have started a
tial range for solid-deliquescence
systematic laboratory investigation
40
0
2000 4000 6000 8000 10000 12000 boundary of CaCl2.2H2O, located at
on the thermodynamic and kinetic
reaction time (minutes)
quite low RHs in 50 to 5 °C range.
properties of chloride hydrates. The
Figure 3. Large field of CaCl2.2H2O deliquescence
RSL were observed on Mars at
60 CaCl .2H O
goals are to determine: (1) the stadeliquescence
2+
3+
Tafternoon ~-23 to 27 °C. Longer durability fields of Mg-, Fe -, Fe -,
tions of above experiments and addiCa-, Al-, Na-chloride hydrates in
tional low T experiments are needed
RH-T space, specifically the bound40
to define the trend of deliquescence
Potential
boundary
for
aries of hydrate-deliquescence; (2)
deliquescence
as function of temperature.
the rate of their dehydration and
High deliquescence rates of
deliquescence as function of T, P,
20
chloride hydrates compared with
and PH2O; (3) the RH level that each
the rates of dehydration: Figure 4
chloride hydrate can maintain in an
enclosure at Ts relevant to those
shows the deliquescence of a
0
0.0
20.0
40.0
60.0
80.0
100.0
within the martian subsurface. We
CaCl2.2H2O
sample
completed
Relative Humidity (%)
report here some preliminary re(shown as hollow triangles) in 4 - 25
sults.
hours (RH>31%, 50 °C) whereas
Figure 4. Comparison of dehydration and
Bonding strength of
dehydration during 90 days (5deliquescence rates of CaCl2.2H2O
metal-H2O in chloride
11% RH) only lost 32 - 23% of
250%
300%
hydrates: We conducted a
structural water (shown as solid
set of dehydration experidiamonds). The rate ratios of
200%
250%
ments of six chloride hydeliquescence over dehydration
drates,
including
of other chloride hydrates at
150%
MgCl2.6H2O, AlCl3.6H2O,
200%
similar conditions are in the same
KMgCl3.6H2O,
order of magnitude.
100%
CaCl2.2H2O, FeCl2.4H2O,
150%
Conclusion: Preliminary reand FeCl3.6H2O, at 52 °C
sults
from a systematic experi50%
and 9 (±3) % RH. Figure 2
100%
0
200
400
600
800
mental
investigation support our
shows the results from
hypothesis
that subsurface chlogravimetric measurements,
50%
LiBr (6% RH)
LiCl (11% RH)
MgCl2 (31% RH)
ride
hydrates
could possibly be
the remaining % of strucMg(NO3)2 (45% RH)
NaBr (51% RH)
KI (64% RH)
NaCl (74% RH)
KCl (81% RH)
KNO3 (85% RH)
the
source
of
water
for RSL.
tural water in each chloH2O (100% RH)
0%
Acknowlegement:
This
study was supride hydrate vs. experi0
50000
100000
150000
ported by NASA MFRP project
mental duration. These
Experiment duration (minutes)
NNX10AM89G (AW). We thank Robert
data revealed how tightly the water molecules are held
Seal and Harvey Belkin for their constructive reviews.
in the structure of each chloride hydrate, i.e. the bonding strength of metal-H2O. We found the rate of dehyReferences: [1] McEwen et al., 2011, Science; [2] Wang et al.,
dration is directly linked to the size of octahedra of M2006, GCA; [3] Ling & Wang, 2010, ICARUS; [4] Liu & Wang,
(H2O)6. MgCl2.6H2O, AlCl3.6H2O, and KMgCl3.6H2O
2009, LPSC; [5] Zhou & Wang, 2012, LPSC; [6] Wang et al., 2009,
show great resistance to dehydration because of the
JGR; [7] Wang et al., 2012, Icarus; [8] Keller et al., 2007, JGR; [9]
small sizes of Al-(H2O)6 and Mg-(H2O)6. For the same
Osterloo et al., 2008, Science; [10] Zheng & Wang, 2009, LPSC;
reason, CaCl2.6H2O shows a moderate resistance,
[11] Wang & Zheng, 2009, LPSC; [12] Tosca et al., 2008, JGR; [13]
FeCl2.4H2O and FeCl3.6H2O have very low resistance,
McLennan et al., 2011, MEPAG; [14] Mellon et al., 2004, JGR;
and KCl hydrate does not exist.
[15] Baumgartner & Bakker 2009, Chem. Geology; and many other
Large field of deliquescence of chloride hypapers.
drates: A set of stability field experiments on the
Remained structural water (%)
Temperature (C)
2
2