Scientists have no idea what made this huge hole on Mars
A HUGE, deep depression on Mars has left astronomers baffled.
It was discovered by NASA's Mars Reconnaissance Orbiter, which has been studying the Martian surface for 11 years.
The vast pit, estimated to be hundreds of feet across and surrounded by frozen carbon dioxide, is located on the south pole of Mars - sticking out among the Swiss-cheese terrain of Earth's closest neighbour.
"This pattern is created when there is relatively high, smooth material that is broken up into these circular-shaped depressions forming the 'Swiss cheese' terrain," NASA explained.
"The depressions are thought to be caused by sublimation, which is when a material goes directly from a solid to a gas phase.
"Repeated images are taken of areas like this so the changes in depression size and where they form can be monitored through the seasons."
While people might be quick to suggest alien life, meteorite impacts, collapsing lava tubes and ancient floods are all responsible for holes formed on Mars.
As it is currently summer for Mars' South Pole, the subtle features of the hole pop right out because the sun is much lower in the sky to accentuate shadows over the landscape.
The depression was discovered by the orbiter's High Resolution Imaging Science Experiment, or HiRISE camera, which allows NASA to see Martian objects larger than one metre from about 200 to 400 kilometres above.
The camera takes repeated images throughout Mars' seasons to monitor the terrain's changes.
NASA's MRO has been in Martian orbit since March 2006 and has successfully completed all of its primary goals, plus two mission extensions.
Earlier this year, NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter has spotted iron, magnesium and sodium ions - electrically charged atoms - high up in the Red Planet's atmosphere.
"MAVEN has made the first direct detection of the permanent presence of metal ions in the ionosphere of a planet other than Earth," study lead author Joseph Grebowsky, of NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement at the time.
"Because metallic ions have long lifetimes and are transported far from their region of origin by neutral winds and electric fields, they can be used to infer motion in the ionosphere, similar to the way we use a lofted leaf to reveal which way the wind is blowing."
As Earth has a strong global magnetic field and Mars does not, the ions behave differently from the metal swirling on our planet.
Mr Grebowsky said studying the two different systems will offer scientists a better understaning of dust impacts and atmospheric dynamics throughout the solar system.
"Observing metal ions on another planet gives us something to compare and contrast with Earth to understand the ionosphere and atmospheric chemistry better," he said.