<P> The large canyon, Valles Marineris (Latin for "Mariner Valleys", also known as Agathadaemon in the old canal maps), has a length of 4,000 km (2,500 mi) and a depth of up to 7 km (4.3 mi). The length of Valles Marineris is equivalent to the length of Europe and extends across one - fifth the circumference of Mars . By comparison, the Grand Canyon on Earth is only 446 km (277 mi) long and nearly 2 km (1.2 mi) deep . Valles Marineris was formed due to the swelling of the Tharsis area, which caused the crust in the area of Valles Marineris to collapse . In 2012, it was proposed that Valles Marineris is not just a graben, but a plate boundary where 150 km (93 mi) of transverse motion has occurred, making Mars a planet with possibly a two - tectonic plate arrangement . </P> <P> Images from the Thermal Emission Imaging System (THEMIS) aboard NASA's Mars Odyssey orbiter have revealed seven possible cave entrances on the flanks of the volcano Arsia Mons . The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters". Cave entrances measure from 100 to 252 m (328 to 827 ft) wide and they are estimated to be at least 73 to 96 m (240 to 315 ft) deep . Because light does not reach the floor of most of the caves, it is possible that they extend much deeper than these lower estimates and widen below the surface . "Dena" is the only exception; its floor is visible and was measured to be 130 m (430 ft) deep . The interiors of these caverns may be protected from micrometeoroids, UV radiation, solar flares and high energy particles that bombard the planet's surface . </P> <P> Mars lost its magnetosphere 4 billion years ago, possibly because of numerous asteroid strikes, so the solar wind interacts directly with the Martian ionosphere, lowering the atmospheric density by stripping away atoms from the outer layer . Both Mars Global Surveyor and Mars Express have detected ionised atmospheric particles trailing off into space behind Mars, and this atmospheric loss is being studied by the MAVEN orbiter . Compared to Earth, the atmosphere of Mars is quite rarefied . Atmospheric pressure on the surface today ranges from a low of 30 Pa (0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa). The highest atmospheric density on Mars is equal to that found 35 km (22 mi) above Earth's surface . The resulting mean surface pressure is only 0.6% of that of Earth (101.3 kPa). The scale height of the atmosphere is about 10.8 km (6.7 mi), which is higher than Earth's, 6 km (3.7 mi), because the surface gravity of Mars is only about 38% of Earth's, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars . </P> <P> The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water . The atmosphere is quite dusty, containing particulates about 1.5 μm in diameter which give the Martian sky a tawny color when seen from the surface . It may take on a pink hue due to iron oxide particles suspended in it . </P>

In 3 to 5 complete sentences describe the environment on mars