Swiss cheese features are curious pits in the south polar ice cap of Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 (Mare Australe quadrangle

Mare Australe quadrangle

The Mare Australe quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Mare Australe quadrangle is also referred to as MC-30 ....

) named from their similarity to the holes in Swiss cheese

Swiss cheese

Swiss cheese is a generic name in North America for several related varieties of cheese which resemble the Swiss Emmental. Some types of Swiss cheese have a distinctive appearance, as the blocks of the cheese are riddled with holes known as "eyes". Swiss cheese has a piquant, but not very sharp,...

. They were first seen in 2000 using Mars Orbiter Camera

Mars Orbiter Camera

The Mars Orbiter Camera or Mars Observer Camera was a scientific instrument on board the Mars Observer and Mars Global Surveyor spacecrafts...

 imagery. They are typically a few hundred meters across and 8 metres deep, with a flat base and steep sides. They tend to have similar bean-like shapes with a cusp pointing towards the south pole, indicating that insolation

Insolation

Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...

 is involved in their formation. The angle of the sun probably contributes to their roundness. Near the Martian summer solstice

Summer solstice

The summer solstice occurs exactly when the axial tilt of a planet's semi-axis in a given hemisphere is most inclined towards the star that it orbits. Earth's maximum axial tilt to our star, the Sun, during a solstice is 23° 26'. Though the summer solstice is an instant in time, the term is also...

, the sun can remain continuously just above the horizon; as a result the walls of a round depression will receive more intense sunlight, and melt much more rapidly, than the floor. The walls melt and recede, while the floor remains the same.
As the seasonal frost disappears, the pit walls appear to darken considerably relative to the surrounding terrain. The SCFs have been observed to grow in size, year by year, at an average rate of 1 to 3 meters, suggesting that they are formed in a thin layer (8m) of carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

ice lying on top of water ice.