kilometers. That area is twice the area of the state of Texas. In July 2018 ESA discovered indications of liquid salt water buried under layers of ice and dust by analyzing the reflection of radar pulses generated by Mars Express. Swiss cheese appearance While the north polar cap of Mars has a flat, pitted surface resembling cottage cheese, the south polar cap has larger pits, troughs and flat mesas that give it a Swiss cheese appearance. The upper layer of the Martian south polar residual cap has been eroded into flat-topped mesas with circular depressions. Observations made by Mars Orbiter Camera in 2001 have shown that the scarps and pit walls of the south polar cap had retreated at an average rate of about 3 meters (10 feet) since 1999. In other words, they were retreating 3 meters per Mars year. In some places on the cap, the scarps retreat less than 3 meters a Mars year, and in others it can retreat as much as 8 meters (26 feet) per Martian year. Over time, south polar pits merge to become plains, mesas turn into buttes, and buttes vanish forever. The round shape is probably aided in its formation by the angle of the sun. In the summer, the sun moves around the sky, sometimes for 24 hours each day, just above the horizon. As a result, the walls of a round depression will receive more intense sunlight than the floor; the wall will melt far more than the floor. The walls melt and recede, while the floor remains the same. Later research with the powerful HiRISE showed that the pits are in a 1–10 meter thick layer of dry ice that is sitting on a much larger water ice cap. Pits have been observed to begin with small areas along faint fractures. The circular pits have steep walls that work to focus sunlight, thereby increasing erosion. For a pit to develop a steep wall of about 10 cm and a length of over 5 meters in necessary. The pictures below show why it is said the surface resembles Swiss cheese; one can also observe the differences over a two-year period. Starburst channels or spiders Starburst channels are patterns of channels that radiate out into feathery extensions. They are caused by gas which escapes along with dust. The gas builds up beneath translucent ice as the temperature warms in the