Underground habitats are required not only due to the radiation threat but also due to the cold temperatures. The average temperature is -55C. Surface temperatures may reach a high of about 20 C (293 K; 68 F) at noon, at the equator, and a low of about 153 C (120 K; 243 F) at the poles. Actual temperature measurements at the Viking landers' site range from 17.2 C (256.0 K; 1.0 F) to 107 C (166 K; 161 F). The warmest soil temperature on the Mars surface estimated by the Viking Orbiter was 27 C (300 K; 81 F).
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 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.