Planum Australe (
Latin
Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
: "the southern plain") is the southern polar plain on
Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
. It extends southward of roughly 75°S and is centered at . The geology of this region was to be explored by the failed
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
mission
Mars Polar Lander
The Mars Polar Lander, also known as the Mars Surveyor '98 Lander, was a 290-kilogram uncrewed spacecraft lander launched by NASA on January 3, 1999, to study the soil and climate of Planum Australe, a region near the south pole on Mars. It fo ...
, which lost contact on entry into the
Martian atmosphere.
In July 2018, scientists reported the discovery, based on
MARSIS radar
Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...
studies, of a
subglacial lake on Mars, below the
southern polar ice cap, and extending sideways about , the first known stable body of water on the planet.
Ice cap

Planum Australe is partially covered by a permanent
polar ice cap
A polar ice cap or polar cap is a high-latitude region of a planet, dwarf planet, or natural satellite that is covered in ice.
There are no requirements with respect to size or composition for a body of ice to be termed a polar ice cap, nor a ...
composed of frozen
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
and
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
about 3 km thick. A seasonal ice cap forms on top of the permanent one during the Martian winter, extending from 60°S southwards. It is, at the height of winter, approximately 1 meter thick. It is possible that the area of this ice cap may be shrinking due to localized
climate change
Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...
. Claims of more planetwide
global warming
Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...
based on imagery, however, ignore temperature data and global datasets. Spacecraft and microwave data indicate global average temperature is, at most, stable, and possibly cooling.
In 1966, Leighton and Murray proposed that the Martian polar caps provided a store of CO
2 much larger than the atmospheric reservoir. However it is now thought that both polar caps are made mostly of water ice. Both poles have a thin seasonal covering of CO
2, while in addition the southern pole has a permanent residual CO
2 cap, about 8 to 10 metres thick, that lies on top of the water ice. Perhaps the key argument that the bulk of the ice is water is that CO
2 ice isn't mechanically strong enough to make a 3 km thick ice cap stable over long periods of time. Recent evidence from
SHARAD ice penetrating radar has revealed a massive subsurface CO
2 ice deposit approximately equal to 80% of the current atmosphere, or 4–5 mbar, stored in Planum Australe.

Data from
ESA's
''Mars Express'' indicates that there are three main parts to the ice cap. The most reflective part of the ice cap is approximately 85%
dry ice
Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...
and 15% water ice. The second part, where the ice cap forms steep slopes at the boundary with the surrounding plain, is almost exclusively water ice. Finally, the ice cap is surrounded by
permafrost
Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...
fields that extend for tens of kilometres north away from the scarps.
The centre of the permanent ice cap is not located at 90°S but rather approximately 150 kilometres north of the geographical south pole. The presence of two massive
impact basins
Impact may refer to:
* Impact (mechanics), a large force or mechanical shock over a short period of time
* Impact, Texas, a town in Taylor County, Texas, US
Science and technology
* Impact crater, a meteor crater caused by an impact event
* Im ...
in the western hemisphere –
Hellas Planitia
Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the fourth- or fifth-largest known impact crater in the Solar System. The basin floor ...
and
Argyre Planitia
Argyre Planitia is a plain located within the impact basin Argyre in the southern highlands of Mars. Its name comes from a map produced by Giovanni Schiaparelli in 1877; it refers to Argyre, a mythical island of silver in Greek mythology.
A ...
– creates an immobile
area of low pressure over the permanent ice cap. The resulting weather patterns produce fluffy white
snow
Snow consists of individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water througho ...
which has a high
albedo
Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...
. This is in contrast to the
black ice
Black ice, sometimes called clear ice, is a coating of glaze ice on a surface, for example on streets or on lakes. The ice itself is not black, but visually transparent, allowing the often black road below to be seen through it and light to be ...
that forms in the eastern part of the polar region, which receives little snow.
Features
There are two distinct subregions in Planum Australe –
Australe Lingula and
Promethei Lingula. It is dissected by canyons
Promethei Chasma,
Ultimum Chasma,
Chasma Australe and
Australe Sulci. It is theorised that these canyons were created by
katabatic wind
A katabatic wind (named ) is a downslope wind caused by the flow of an elevated, high-density air mass into a lower-density air mass below under the force of gravity. The spelling catabatic is also used. Since air density is strongly dependent o ...
. The largest crater in Planum Australe is
McMurdo Crater.
Geysers on Mars
The seasonal frosting and defrosting of the southern ice cap results in the formation of spider-like radial channels carved on 1 meter thick ice by sunlight.
Then, sublimed CO
2 (and probably water) increase pressure in their interior, producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.
This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology – especially for Mars. The
Mars Geyser Hopper lander is a concept mission that would investigate the geysers of Mars.
Saltwater lakes
In September 2020, scientists confirmed the existence of several large
saltwater lakes under the
ice
Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...
in the south polar region of the planet
Mars
Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
. According to one of the researchers, “We identified the same body of water
s suggested earlier in a preliminary initial detection but we also found three other bodies of water around the main one ... It’s a complex system.”
See also
*
Climate of Mars
*
List of plains on Mars
*
Martian polar ice caps
*
Planum Boreum, the northern polar plain
References
External links
Accumulation and erosion of south polar layered deposits in the Promethei Lingula region, Planum Australe, MarsPlanum Australe on Google Mars
Stratigraphy and Topography of McMurdo Crater area, Planum Australe, MarsUSGS map of Planum Australe
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Plains on Mars
Extraterrestrial plateaus
Polar regions of Mars
Mare Australe quadrangle