Scientists believe that the dark areas are regions of bare rock. Then bright areas are regions with deposits of weathered material.
At certain times of the year like spring and summer when Mars is closest to the sun, great dust storms appear as yellow clouds. The largest of these storms can cover the globe of Mars and last for months. At other times white clouds of water vapor are visible. Scientists now believe that the canals people observed on Mars during the 19th century are actually another optical illusion, caused by the tendency to draw connections between irregular patches in a fuzzy image.
The Hubble Space Telescope (HST) provides the clearest Earth-based views of Mars. Astronomers use it to study the weather on the planet. The HST has provided images of dust storms with such detail that scientists can pinpoint the areas on the planet in which the storms started. The telescope also makes general studies of the atmosphere. Using HST images, astronomers have determined that the atmosphere of Mars has change and became clearer and drier than it was in the mid-1970s.
Scientists believe that Mars's interior consists of a crust, mantle, and core like Earth's interior, but they do not know the relative sizes of these parts. No spacecraft has ever brought instruments that can study Mars's interior to the planet. The only real data that scientists have about the planet's structure are its mass, size, and the structure of the gravity field. From that data scientists can learn some things about density in different parts of the planet.
Mars is thought to have a relatively thick crust. Beneath the Tharsis bulge, an area of volcanic activity in the northern hemisphere, it may be as thick as 80 miles. Also it may be as thin as 9 mi. The core is guest to be mostly iron, with a small amount of nickel. If the core is made up of these things it may be quite large. Mars unlike Earth does not have a significant magnetic field, so scientists believe that Mars's core is probably solid.