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| Recurring slope lineae flowing downhill on Mars (Hale Crater) are inferred to have been formed by contemporary flowing water. Credits: NASA/JPL/University of Arizona |
A large number of space probes and robots are exploring mars
in search of life. Water is the main requirement for the existence of life on
any place. The key role of water in existence of life has caused the scientific
explorations to look for water on Mars Planet. You may have heard earlier that
the water was present in the form of oceans in the ancient Mars and in the form
of ice on the Mars these days. But this time NASA has found evidence of liquid
water on Mars. The liquid water can exist on Mars under certain extreme
conditions. The spectroscopic studies of Mars Reconnaissance Orbiter (MRO)
strongly suggest that the liquid water flows intermittently on the surface of
Mars even today.
MRO has studied the patches of Salts precipitated on the
slope on Mars, this streaks are called “Recurring slope lineae”. These narrow,
dark, 100 meter-long streaks flowing downhill on Mars are supposed to have been
formed by contemporary flowing water. These
streaks have low reflectance compared to the surrounding terrain, thus they
appear dark. The streaks grow incrementally in the downhill during the warm
seasons when temperature reach about 250-300 K then fade in cooler seasons and
resembles a pattern consistent with the transient flow of a volatile species. These
streaks appear in many locations on Mars during warmer seasons and disappears
during cooler seasons.
It has been proposed that the brine flows can explain the formation
of recurring slope lineae, yet no direct evidence for either liquid water or
hydrated salts has been found. The recurring slope lineae found on four
different locations on Mars have been studied using the Compact Reconnaissance
Imaging Spectrometer for Mars instrument onboard the Mars Reconnaissance
Orbiter.
Lujendra Ojha, a scientist at Georgia Tech had first spotted
the lineae back in 2010 when he was a Phd candidate in University of Arizona.
Ojha and his team find evidence for hydrated salts at all four locations in the
seasons when recurring slope lineae are most extensive, which suggests the
source of hydration is recurring slope lineae activity.
The presence of liquid water on Mars has astrobiological,
geologic and hydrologic implications and may affect future human exploration.
Various salts like sulphates, chlorides and perchlorates have been detected on
the surface of Mars from remote and in situ investigations. The presence of
salts can lower the freezing point of water by up to 80 K, lower the
evaporation rate of water and can be hygroscopic. Hygroscopic property of salts
helps in absorption of atmospheric moisture. All this things increase the
possibility of forming and stabilizing liquid water on the surface of present
day Mars.
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| Recurring slope lineae emanating out of the walls of Garni crater on Mars. Credits: NASA/JPL/University of Arizona |
Recurring slope lineae (RSL) are narrow and low-reflectance
features forming on present-day Mars that have suggested to be due to the
transient flow of liquid water. RSL grows incrementally downslope on steep,
warm slopes, fade when inactive, and reappear annually over multiple Mars years.
The average width of RSL is from a few meters (<5m) to the detection limit
for the High Resolution Imaging science Experiment (HiRISE) camera (0.25 m).
The slope where RSL are active typically has temperature more than 250 K and
commonly are above 273 K. These characteristics strongly suggest a possible
role of salts in decreasing the freezing point of water, allowing briny
solutions to flow. The confirmation of this hypothesis for origin of RSL based
on water would require either detection of liquid water absorptions on the
surface of Mars, or detection of hydrated salts precipitated from that water on
the RSL.
MRO takes mid-afternoon observation at 3 p.m. which is the
time of the day when the relative humidity is lowest which minimized the
probability of detecting liquid brines that is present in hydrated salts, and
may even facilitate dehydration of salts. The magnesium perchlorate, magnesium
chlorate and magnesium chloride was found to be the best match for the spectral
mixture model of the team. Sodium perchlorate was proposed to be the best match
at Horowitz crater. Various sulphates were also analyzed but no good match
could be found for spectral mixing model.
The presence of perchlorates on the surface of Mars has been
confirmed at Gale crater by Mars Science Laboratory, the northern plains by the
Phoenix mission, and is suspected at the Viking landing sites. Hydrated calcium
perchlorate is supposed to be the best matching oxychlorine compound and
Magnesium perchlorate and calcium perchlorate were found to be the most likely
perchlorate at the Phoenix landing site. It has been suggested that the thin
film of water have dissolved perchlorate from the surface to the subsurface and
likely to have formed concentrated patches at the landing site of Phoenix. These
detection of perchlorates are clearly consistent with the observation of RSL
sites.
The team has not understood the origin of water forming the
RSL. The water could form by surface or subsurface melting of ice but the presence
of ice near surface at equatorial region is highly unlikely. Alternatively the
RSL could form through deliquescence, but it is still not clear whether the
Martian atmosphere has sufficient supply of water vapor every year to form RSL.
Another hypothesis is seasonal discharge
of a local aquifer, but the lineae extending upward to the tops of local peaks
are difficult to explain. Finally it can be said that the RSL are forming in various
parts of Mars with the help of different formation mechanisms.
This animation simulates a fly-around look at one of the places on Mars where dark streaks advance down slopes during warm seasons, possibly involving liquid water. This site is within Hale Crater. The streaks are roughly the length of a football field.
These results strongly support the hypothesis that the seasonal warm slopes are forming the liquid water on contemporary Mars. The spectral identification of perchlorate associated with RSL also suggests that the water is not pure instead it is briny.
On the Earth, in the hyper-arid core of the Atacama Desert, the
deliquescence of hygroscopic salts provide favorable environment for the active
microbial communities and halophilic prokaryotes. If RSl are really formed due
to the deliquescence of perchlorate salts, then they might provide transiently
wet conditions near the surface of Mars for the life in the form of microbes
although the water activity in the perchlorate solutions may be too low to
support known terrestrial form of life.
The presence of water may be helpful for the future space
mission to Mars. The RSL site may prove to be the good places for explorations
for next Mars missions. If you have any question or suggestion, please comment.
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