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.[117] The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters."[118] Cave entrances measure from 100 m to 252 m wide and they are believed to be at least 73 m to 96 m 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 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.[119]
Atmosphere
Main article: Atmosphere of Mars
The tenuous atmosphere of Mars, visible on the horizon in this low-orbit photo
Mars lost its magnetosphere 4 billion years ago,[120] 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,[120][121] and this atmospheric loss will be studied by the upcoming 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).[122] The highest atmospheric density on Mars is equal to the density found 35 km[123] above the Earth's surface. The resulting mean surface pressure is only 0.6% of that of the Earth (101.3 kPa). The scale height of the atmosphere is about 10.8 km,[124] which is higher than Earth's (6 km) 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. The atmosphere of Mars consists of about 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen, carbon dioxide, and water.[6][125] 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
