Abundant martian brines would have important implication for current theories of volatile migration on Mars, since, although the presence of metastable brines is quite plausible, any brine in the reasonably near-surface should be completely depleted on a timescale short in relation to the age of Mars. It is important to determine whether brines exist in the martian subsurface, for the current paradigm for understanding martian volatile regime requires substantial alteration if they are found to exist. It is determined, however, that the prospect for detection of a subsurface brine via atmospheric water vapor measurements is marginal. Four reasons are given for this conclusion

Fanale, Fraser P.

Postawko, S. E.

Zent, Aaron P.

English

NASA Technical Reports Server

48 87 
ATMOSPHERIC HgO AND THE SEARCH FOR MA" BRINES Zent, A. P., F. P. 
Fanale and S. E. Postawko, Planetary Geosciences Division, Hawaii Institute of Geophy- 
sics, University of Hawaii, Honolulu, Hi. 96822 
Abundant martian brines would have signiflcant implication for current theories of 
volatile migration on Mars, since, although the presence of metastable brines is quite 
plausible, any brine in the reasonably near surface should be completely depleted on a 
timescale short in relation to the age of Mars. Their presence would strongly imply 
either large-scale subsurface mass transport, or juvenile outgassing rates significantly 
in excess of those believed currently possible. Any H2O transport mechanism with 
sufficient capacity to maintain a near-surface brine in the martian regolith must be the 
dominant transport mechanism in the martian subsurface. As such, it is important to 
determine whether brines exist in the martian subsurface, for our current paradigm 
for understanding martian volatile regime will require substantial alteration if they are 
found to exist. 
There are no &,&& brine systems on Mars today. The net annual escape flux of 
H20 would be significant at all latitudes where the subsurface temperature exceeds the 
eutectic of chemically reasonable brines (2). Thus, if any brines do exist, they are con- 
stantly losing H20 molecules to the atmosphere at a rate that depends upon the depth 
at which the melting occurs, the local thermal regime, and the porosity and mean pore 
size of the overlying regolith. The loss of H20 molecules from a regolith source to the 
atmosphere, which supports only a very low ambient H20 abundance (controlled by 
polar temperatures), may provide a means of detecting Martian brines, if they exist. 
We will discuss, in qualitative terms, some of the factors which would affect the utility 
of atmospheric HzO observations as tools for detecting, or refuting the existence of, 
Martian brines. That is, it is important to understand whether unreasonable near- 
surface stratigraphy needs to be postulated to prevent the water flux to the atmo- 
sphere from any putative brine system from exceeding that allowed by the MAWD data. 
These factors may be broadly divided into two categories: those peculiar to the brine 
and its immediate environment, and those that are determined by the position of the 
system within the global environment. 
The Brine. In our initial analysis (l), we found that for most chemically reasonable 
brines that might undergo annual melting, the annual average mass loss rates are on 
the order of g H20 yr-* down to lo-' g H2O yr-'. This range of net annual escape 
fluxes applies to a remarkably large percentage of the plausible shallow, seasonally 
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88 
BRINE DETECTION 
Zent, A. P., et al. 
active brine systems; there are plausible brine systems which are too deeply buried to 
have any annual cyclicity imposed on them at all. We will return to them briefly below. 
The escape of HzO molecules from a melting interface at depth Z falls off a s  Z-l, 
approximately in accordance with Ffck's Law; the escape flux achieves -loe2 g HzO yr-' 
at depths of 30cm or less, for a nominal 50% porosity. The porosity of the regolith also 
plays a significant role in determining the escape rate of H20, with the flux increasing 
almost as the square of the porosity. A brine which releases lo4 g H2O yr-' to the 
atmosphere through a regolith with 10% porosity, will release an order of magnitude 
more &O per year at a porosity of -30%. The mean pore size of the regolith is not of 
great importance in determining escape fluxes, unless the mean free path of the H20 
becomes substantially greater than the pore size. Under such conditions, Knudson 
diffusion is the dominant transport mechanism and the escape flux falls off with pore 
size. With respect to  both of these topics, the natural tendency of groundwater to 
leach materials from its surroundings and precipitate them on evaporation should not 
be ignored. Such a process should produce a kind of duricrust, much like those 
observed by Viking; however, duricrust may also decrease mean pore size and bulk 
porosity of the regolith. In principle, a duricrust cap could allow metastable brine sys- 
tems t o  exist without violating the MAWD data. 
Another factor which plays a role in determining the detectability of subsurface 
&O sources is their areal extent. Very concentrated sources, even though they may 
be supplying H2O at a prodigious rate, are unlikely to be detected because of resolution 
problems. 
There is a confiicting set of requirements involved in atmospheric H2O detection of 
brines, because those systems which are best suited to identification are also the 
shortest-lived. Loss of He0 molecules results in recession of the melting interface until 
the seasonal thermal wave is no longer sufficient to cause melting, unless resupplied 
from depth. At depths of -1 m, average mass loss rates of lO-'g H2O cm-2 yr-l, the 
recession rate of a brine would be on the order of los yr m-l. Brines below about 2m 
should be quite well insulated from the annual thermal wave. Such deep brines how- 
ever cannot be invoked to explain seasonal variability in radar reflectivity measure- 
ments. 
There are some brines which have eutectics below the annual average tempera- 
ture at low latitudes. Such brines could conceivably remain liquid throughout the mar- 
tian year at depths greater than a few meters. These putative brines would remain 
Liquid and would be entirely undetectable, with mass loss rates on the order of a frac- 
tion of a percipitable micron per year, and well below the penetration depth of radar 
BRINE DETECTION 
Zent, A. P., et al. 
89 
signals. 
The composition of the brine system is also critical. Melting begins with a brine of 
eutectic composition and continues isothermally until the supply of one of the com- 
ponents is exhausted. As long as ice is present in the system, the vapor pressure over 
the brine will equal the vapor pressure over ice at the temperature of the system. 
However, once all the ice has melted, the vapor pressure over the brine will be lower 
than it would be over ice at the same temperature. Therefore incomplete melting of 
the brine, which results from a bulk composition significantly less saline than the 
eutectic composition, favors higher escape fluxes and hence detectability. 
A brine which has a high evaporative loss to the atmosphere, for whatever reason, 
will be most detectable. High loss rates may be due to a highly porous overburden, a 
water-rich brine system, or a shallow melting interface. High salinity brines, capped by 
a great deal of overburden, or by overburden of low porosity, will hide brines effectively 
from atmospheric detection. 
The Environm& Environmental factors are those which are approximately 
independent of the existence of the brine, such as the local thermal regime, the back- 
ground column abundance of HzO and the regional wind velocities. 
Atmospheric signatures of escaping brines will have greater absolute magnitude at 
low latitudes because the warmer melting interface will support a greater equilibrium 
water vapor abundance. Naturally, escape fluxes become more seasonally dependent 
as latitude increases, and the annual average escape flux decreases. Higher escape 
fluxes favor detection, and low latitude brines should be more easily detectable. 
Detection is also aided by the escape of H20 molecules into a low background 
abundance of water vapor, since it is the difference between the escaping and ambient 
H20 that is indicative of a regolith source. Therefore, in general, the likelihood of 
detecting a brine increases as one moves away from the north pole, although again, 
seasonal variations in local H20 abundances will affect the outcome of any search. 
HzO molecules which escape from a seasonal vapor source in the regolith to the 
atmosphere will be dispersed by the winds. The lower the local wind velocities, the 
more likely that an excess abundance of HzO will accumulate in the atmosphere. 
The most likely location for brine detection, assuming the same brine exists every- 
where on the planet, is one in which the subsurface temperatures are high, while the 
atmospheric HzO abundances are low. On Mars, this is most closely approximated dur- 
ing low southern latitudes near perihelion. Temperatures are warmer at that time than 
in the northern hemisphere summer because perihelion occurs during southern 
BRINE DETECTION 
Zent ,  A. P. et al. 
90 
hemisphere summer. At  most locations on the planet though, peak atmospheric HzO 
abundances are more closely correlated with peak surface temperatures. Unfor- 
tunately, the low southern latitudes also experience their peak annual wind velocities 
near perihelion. 
We conclude that the prospect for detection of a subsurface brine via atmospheric 
water vapor measurements is marginal, for four reasons. 1) There is an inherent inade- 
quacy in using escape to the atmosphere to detect regolith water sources, since one 
does not really observe the source at all, but a product of the system's dissipation. 
Those brines which are best suited for detection are therefore the same ones that are 
most unstable, and hence have the shortest mean lifetimes. 2) Not only the mass loss 
itself, but processes which take place concurrently with evaporation also tend to 
reduce the escape flux; evaporation leads to concentration of salts in the brine which 
depresses the equilibrium water vapor abundance at the melting interface, and hence 
the escaping flux. Continued evaporation then leads to deposition of a duricrust-type 
deposit which may have the effect of decreasing the porosity of the overburden, further 
decreasing the escape flux. For most reasonable putative brine systems, this process 
requires no more than about lo5 years. 3) Those brines which do not suffer from rapid 
degradation, and are consequently longest-lived, are also those least detectable since 
they are either found at great depth, or are capped by a very low porosity overburden. 
4) The environmental factors which are most important in determining the detectabil- 
ity of such a system (i.e. temperature and atmospheric water vapor) tend to be corre- 
lated in a manner opposite that which is best suited to facilitate brine detection. 
REFERENCES 
1. Zent, A. P., and Fanale, F. P., (1986). J. Ceophya. Res., Vol. 91, D430 - D445. 
2 Fanale, F. P., and Clark, R N., (1063), Proc. Fourth ht. Coni. on Permafrost, 28B284, Net. Aced. Press, 
Washington, D.C. 


Atmospheric H2O and the search for Martian brines

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