Liquid water on Mars
Without water, no form
of life as we know it could exist. There is therefore great interest in
detecting liquid water on other planets of our Solar System. Landforms such as
dry river valleys and lakes show that liquid water must have been present on
Mars in the past (1). Nowadays, small amounts of gaseous water exist in the
martian atmosphere, and some water ice is found on the planet’s surface. Water
droplets were seen condensing onto the Phoenix lander (2), and there may be
reoccurring water activity on slopes during the martian summer (3). However,
stable bodies of liquid water have not been found on Mars. Published in
Science’s First Release this week, Orosei et al. (4) report an analysis of
radar data from the Mars Express mission that shows the existence of stable
liquid water below 1.5 km of ice, close to the martian south pole.
Ice caps similar to
those on Earth exist at the martian north and south poles, known as the North
and South Polar Layered Deposits (NPLD and SPLD, respectively). More than 30
years ago, Clifford hypothesized that liquid water might be present below the
martian polar ice caps (5). Despite mean annual air temperatures of around
−60°C, lakes exist below Earth’s Antarctic ice sheet (6). Glacier ice insulates
the bed from the cold surface. Thus, temperatures at the base of the Antarctic
ice sheet, which may be as thick as 4.8 km, can reach the pressure melting
point of water; the melting point is reduced owing to the pressure of the ice
layer above. Water at the ice base reduces basal friction, leading to increased
flow speeds. Finding liquid water below the martian ice caps might solve
ongoing debates about whether the NPLD ice flow is due to ice deformation,
deformation of the bed, or gliding over the bed or whether it is not flowing at
all (7).
Water below the Antarctic
ice sheet has been detected and analyzed by using radar waves that are
transmitted actively above the surface. As the electromagnetic radar waves pass
downward through the ice, they are reflected back at the interfaces between
different materials, such as contacts between ice and bedrock, sediment, or
liquid water (see the figure). Along a flight track, measurements are
continuously carried out to form an image of the subsurface. Such a radargram
shows reflectors from the surface and the base and often multiple weaker
reflections from within the ice body. The radar wave reflection is stronger
from a water interface than from a rock or sediment interface and therefore
shows up as a relatively bright reflector in the radargram.
Orosei et al. now apply
this method to data from Mars. The Mars Advanced Radar for Subsurface and
Ionosphere Sounding (MARSIS) instrument on the Mars Express spacecraft
collected radar measurements over the SPLD. Orosei et al. identify a distinct
20-km-wide bright reflector on multiple profiles collected over 3 years. They
rule out a number of possible explanations for this bright reflector, leaving
the existence of liquid water, either as a distinct water layer or as saturated
sediments, as the only explanation.
It is even colder on
Mars than in Antarctica. Temperatures at the base of the SPLD are estimated to
be around −68°C (7), and thus, pure liquid water could not exist there under
1.5 km of ice. However, liquid water may still exist because the freezing point
of water is far lower if large amounts of salts are dissolved in the water.
Such brine lakes have been found on Earth, with salinities of up to 200
practical salinity units (psu) in the McMurdo Dry Valleys, Antarctica (8).
There, water remains liquid down to temperatures of −13°C. For comparison,
ocean water has a salinity between 32 and 37 psu and freezes at about −2°C.
Salts of sodium,
magnesium, and calcium have been found on the martian surface and can reduce
the melting point of water to −74°C (7, 9); when in contact with ice, the salts
can suppress the freezing point enough for liquid water to form (7, 10). The
droplets observed on the Phoenix lander and the observed reoccurring water
activity on slopes were explained by the presence of such briny water (5, 10).
Briny water can also explain Orosei et al.’s observation of a stable water body
below the SPLD.
In the future, with
higher-resolution data, smaller liquid water bodies that influence the ice flow
might be detectable below martian ice caps. Like Earth’s ice sheets, the
martian ice caps are important climate archives. Depending on the climate, the
ice caps grow and shrink as a result of depositional and erosional events. This
creates a unique stratigraphy within the ice caps, consisting of layers of
equal age that scientists can analyze to derive information about past climate.
Changes in ice flow owing to water at the base can change the appearance of
these englacial layers; this needs to be considered when reconstructing their
age. Analyzing these englacial reflectors, taking the new findings of liquid
water below the SPLD into account, can therefore help unravel the climate
history of Mars.
source:
http://science.sciencemag.org/content/early/2018/07/24/science.aau1829?rss=1
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