Olympus Mons and hence lead to the low gradients on the surface of the planet. Olympus Mons is an unlikely landing location for automated space probes in the near future. The high elevations preclude parachute-assisted landings because the atmosphere is insufficiently dense to slow the spacecraft down. Moreover, Olympus Mons stands in one of the dustiest regions of Mars. A mantle of fine dust obscures the underlying bedrock, possibly making rock samples hard to come by and likely posing a significant obstacle for rovers. Olympus Mons is the result of many thousands of highly fluid, basaltic lava flows that poured from volcanic
Solar System icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune. Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury), and, in