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46th Lunar and Planetary Science Conference (2015)
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FORMATION OF CHLORIDE HYDRATES VIA VAPOR-SOLID REACTION AT LOW-T -IMPLICATION FOR A H2O-RICH CRYOSPHERE IN MARS SUBSURFACE AND ON OTHER ICY
PLANETARY BODIES. Alian Wang, Jie Wei, Lily Lu, and Kathryn Connor, Dept. Earth and Planetary Sciences
and McDonnell Center for Space Sciences, Washington University in St. Louis ([email protected], Rudolph
Hall, One Brookings Drive, St. Louis, MO, 63130, USA).
A H2O-Rich Cryosphere: On Mars, ground ice was
designed and conducted two sets of experiments. The
identified in Polar Region by Phoenix lander [1], and
results from first set of experiment was reported early
was implied at high latitude region by the color change
[13], which demonstrated: (1) the deliquescence of
of “white” impact ejecta through orbital imaging [2].
Mg-, Ca-, Fe-, Al-chloride hydrates are heavily deIn equatorial region, a H2O-rich cryogenic environment
pendents of temperature; (2) under the T and partial
in subsurface sulfate-rich layer was suggested through
water pressure (PH2O) relevant to Mars subsurface, the
the observed color change of subsurface ferric sulfates
rates of deliquescence of these chloride hydrates allow
(Tyrone site at Gusev Crater), which indicates dehydrathe process to begin and to accomplish (to exhaust the
tion happened after excavation and exposure to Mars
tested quantity of chloride hydrates) within hours and
current surface condition [3, 4].
days. These results support that the deliquescence of
Cl was found existing in every sample of every surchloride hydrates can generate RSL phenomena within
face exploration mission to Mars [5]. Putative chlothe time duration of RSL observed on Mars [14].
rides were suggested to exist in broad region of southThis abstract reports the second set of experiments,
ern hemisphere on Mars [6]. Similar to sulfates, a thick
that was designed to test if the rehydration of chlorides
layer of chlorides at subsurface could maintain a lowcan happen through vapor-solid reaction at low-T reltemperature environment because their high thermal
evant to Mars subsurface, i.e., if the recharging of RSL
inertia, on the basis of a thermal profile model [7]. Fursource materials can happen during a local winter perithermore, chloride hydrates are the stable forms of
od on Mars.
chlorides at low temperature, such as CaCl2.6H2O
Vapor-solid reaction at -78 °C: The experimental
(antarcticite) [8].
setup is shown in Figure 1. MgCl2.6H2O was used as
We would hypothesize a H2O-rich cryosphere withthe starting material. It was baked in a 200 °C oven for
in the subsurface of Mars, made of ground ice layer at
48 hours. Gravimetric measurements before and after
polar and high latitude regions, and of salt- or saltbaking confirmed the loss of 6.5 H2O per molecule
enriched-regolith-layers at low latitude regions. One
(including adsorbed H2O). Raman spectroscopic measpiece of puzzle in this hypothesis is the source materiurements before and after baking confirmed the total
als of Reoccurring Slope Lineae (RSL) that has been
transformation from MgCl2.6H2O to MgCl2, shown as
observed during local summer season with large quanthe loss of Raman spectral features in 3000-4000 cm-1
tity and broad spreading at low-latitude region and
for
hydrates.
Visually,
original
transparent
equatorial area on Mars [9, 10].
MgCl2.6H2O grains (Fig. 2a) all transformed to
We report here the results from a set of experiments
opaque-white grains of MgCl2 (Fig. 2b) after baking.
that has proved a critical concept in above RSL related
For the low-T experiment, a reaction vial containstudy, i.e., the recharging of RSL source material at
ing about 100 mg of MgCl2 was buried in dry ice (- 78
low-temperature (T) during local winter on Mars. The°C) in a foam box. This box-vial set was put into a
se results have significant implication for
Pressure
Meter
the process that might happen on all lowCO w/controlled P
T planetary bodies, i.e., Mars, Moon, icy
satellites, asteroids, and comets.
Vacuum Chamber
RSL & 1st set of Experiments: Based on
the observations made by HiRISE and
RH logger
HO
Flow
CRISM, i.e., the tight correlation of RSL
Dry Ice
water bath at 35°C
Control
(-78°C)
occurrence with temperature (T), the time
duration of its occurrence and annually reExtra-dry CO
Valves
CO w/ wellcontrolled P
occurrences, the regional correlation with
Flow
To Vacuum Pump
putative chloride deposits, and the lack of
Control
VIS-NIR spectral features at RSL sites [9,
Extra10, 11], we hypothesized the source matedry CO
mix
rials of RSL to be the subsurface chloride
hydrates [12]. To test this hypothesis, we Fig. 1. Vapor-Solid Reaction at -78 C Experiment al Setup
H2O
Sample
2
2
2
2
H2O
2
46th Lunar and Planetary Science Conference (2015)
2483.pdf
Thus,
the
recharging of RSL
source
materials
annually
during
local
Opaquewinter
period
is
alwhite grains
TransH2O ice
lowed by thermodycrystals
namics and kinetics.
It means that the
a. MgCl2.6H2O
b. MgCl2
subsurface salt- or
MgCl2.xH2O
salt-rich-regolith
c. Reaction at -78 °C
d. Raman at -78 °C
layers could function
like a “cold-trap”, to
op-wh
MgCl2.yH2O
react with atmospheric H2O vapor
trans
Reaction vial
that is moving from
MgCl2.zH2O
dry ice
Reaction vial
polar region to midlow latitude regions
4000
3800
3600
3400
3200
3000
dry ice beneath a foam
Raman Shift (cm-1)
during local winter
e. Image of reaction products
period
on
Mars.
This
vapor-solid
reaction would rehy(after 2 hours at Room T)
drate the chlorides, which were formed during the local
op-wh
summer period on Mars by dehydration. The chloride
trans
hydrates formed in such way will be the source materials for RSL to reoccurring in next local summer.
Future works: These two sets of experiments are
op-wh
trans
proof-of-concept studies, which demonstrated the corvacuum chamber where a low vacuum near PMars was
rectness of our hypothesis, and the feasibility of our
maintained. H2O vapor generated from a H2O bath
experimental design. A detailed systematic experi(maintained at 35 °C, thus a controlled PH2O) was
mental investigation is designed and will be proposed.
mixed with a flow of dry CO2 and was input into the
It will concentrate on the thermodynamics and kinetics
chamber. A delegate balance between the chamber
properties of specific chloride hydrates that are stable
pressure P and PH2O was maintained. A logger for temat lower T thus having even higher hydration degrees.
perature and relative humidity (RH) was kept in the
The study on reaction rates (for deliquescence and rechamber during the whole experimental duration.
hydration) will be emphasized. Furthermore, experiThe vapor (H2O)-solid (MgCl2) reaction was conments to reveal the PH2O values (i.e., H2O availability)
ducted for about 6 hours, and repeated three times in
at low-T within an environment filled with chloride
three days. Similar results were obtained each day,
hydrates and sulfate hydrates will be conducted, for the
which include: (1) the visual observation during the
purpose to evaluate the habitable potentials of icy-saltexperiment at -78 °C through the transparent wall of
environments, within Mars subsurface and on other icy
vacuum chamber that indicated the formation of some
planetary bodies as well.
transparent grains at the surface of MgCl2 grains in the
Acknowledgements: Thanks for NASA funds,
reaction vial (Fig. 2c); (2) Raman spectroscopic meas#1295053 & NNX13AM22G, and for the discussion
urements at -78 °C (Fig. 2d) of the reaction products
with A. McEwen, J. Head, J. Dubessy, I-Ming Chou.
that show various Raman peaks of chloride hydrates
References: [1] Smith et al., (2009) Science, 325, p58. [2]
(MgCl2.xH2O, x=TBD, Fig. 3); (3) gravimetric measByrne et al., (2008) Science, 325, p1674. [3] Wang et al.,
urements of the reaction products that indicate the
(2008) JGR, 113, E12S40.[4] Wang and Ling (2011) JGR,
gaining of 0.7 to 1.3 H2O molecule per MgCl2 mole116, E00F17. [5] Clark et al., (1978) JGR, 87, p10059. [6]
cule from three repeating experiments; (4) optical miOsterloo et al., (2008) Science, 319, p1651. [7] Mellon et al.,
croscopic images of reaction products (after keeping at
(2004) ICARUS, 169, p324. [8] Dubessy et al.,(1982) Chem.
room T for two hours) that show mixtures of transparGeology, 37, p137. [9] McEwen et al., (2011) Science, 333,
ent grains with opaque-white grains (Fig. 2 e).
p740. [10] McEwen et al., (2013) Nature GeoScience,
These results demonstrated the formations of chloDOI:10.1038; [11] Ojha et al., (2013) GRL, 40, p5621. [12]
ride hydrates via a vapor-solid reaction at a temperaWang et al., (2013) 44th LPSC, abs #2606. [13] Wang (2014)
ture within the T range that can be maintained by a
8th Mars, abs #1058. [14] Wang et al., (2014) GSA, abs #
thick layer of salt at nowadays Mars subsurface.
248005.
Figure. 2
48 h @ 200 °C
Fig. 3 Raman spectra of reaction products at -78 C
(MgCl2.xH2O, where x can be 2, 4, 6, 8, 12 that
needs a systematic experimental study)