New Pictures Of Mars

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 Arsia Mons volcano. The caves, named after loved ones of their discoverers, are collectively known as the "seven sisters." 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 likely 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.
Atmosphere
Mars lost its magnetosphere 4 billion years ago, 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 these ionised atmospheric particles trailing off into space behind Mars. Compared to Earth, the atmosphere of Mars is quite rarefied. Atmospheric pressure on the surface ranges from a low of 30 Pa (0.030 kPa) on Olympus Mons to over 1,155 Pa (1.155 kPa) in the Hellas Planitia, with a mean pressure at the surface level of 600 Pa (0.60 kPa). The surface pressure of Mars is equal to the pressure found 35 km above the Earth's surface. This is less than 1% of the Earth's surface pressure (101.3 kPa). The scale height of the atmosphere is about 10.8 km, 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.