1Red Planet MarsChapter ThirteenGuiding Questions1. When is it possible to see Mars in the night sky?2. Why was it once thought that there are canals on Mars?3. How are the northern and southern hemispheres of Mars different from each other?4. What is the evidence that there was once liquid water on Mars?5. Why is the Martian atmosphere so thin?6. What have we learned about Mars by sending spacecraft to land on its surface?7. What causes the seasonal color changes on Mars?8. As seen from Mars, how do the Martian moons move across the sky?2Earth-based observations of Mars are best made during favorable oppositionsThe best Earth-based views of Mars are obtained when Mars is simultaneously at opposition and near perihelionEarth-based Observations• A solar day on Mars is nearly the same length as on Earth• Mars has polar caps that expand and shrink with the seasons• The Martian surface undergoes seasonal color changesEarth-based observations were once thoughtto show evidence of intelligent life on Mars• A few observers reported a network of linear features called canals• These observations, which proved to be illusions, led to many speculations about Martian life3Unmanned spacecraft found craters, volcanoes,and canyons on Mars• The Martian surface has numerous craters, several huge volcanoes, a vast rift valley, and dried-up riverbeds— but no canals• Martian volcanoes and the Valles Marineris rift valley were formed by upwelling plumes of magma in the mantleOlympus Mons• For reasons that are not understood, the chemical composition of ancient Martian lava is different from that of more recent lava• Mars has no planet wide magnetic field at present but may have had one in the ancient past4• The heavily cratered southern highlands are older and about 5 km higher in elevation than the smooth northern lowlands• The origin of this crustal dichotomy is not completely understoodSurface features indicate that water once flowed on Mars• Flash-flood features and dried riverbeds on the Martian surface indicate that water has flowed on Mars at least occasionally• No liquid water can exist on the Martian surface today5Polar Ice Caps• Mars’s polar caps contain frozen water, a layer of permafrost may exist below the Martian regolith, and there may be liquid water beneath the surface• The Martian polar caps expand in winter as a thin layer of frozen carbon dioxide (dry ice) is deposited from the atmosphere6Earth and Mars began with similar atmospheresthat evolved very differently• Mars’s primordial atmosphere may have been thicker and warmer than the present-day atmosphere• It is unclear whether it contained enough carbon dioxide and water vapor to support a greenhouse effect that would permit liquid water to exist on the planet’s surface• The present Martian atmosphere is composed mostly of carbon dioxide• The atmospheric pressure on the surface is less than 1% that of the Earth and shows seasonal variations as carbon dioxide freezes onto and evaporates from the poles Clouds Above Mars’ Mountains7Earth’s AtmosphereMars AtmosphereLanders have explored the surface of Mars8The Martian atmosphere changes dramaticallywith the seasons• Great dust storms sometimes blanket Mars• Fine-grained dust in its atmosphere gives the Martian sky a pinkish-orange tint• Seasonal winds blow dust across the face of Mars, covering and uncovering the underlying surface material and causing seasonal color changesAfternoon dust devils help to transport dust from place to place9Winter FrostThe two Martian moons resemble asteroids • Mars has two small, football-shaped satellites that move in orbits close to the surface of the planet• They may be captured asteroids or may have formed in orbit around Mars out of solar system debrisKey Words• andesite•cosmic rays• crustal dichotomy• dust devil• favorable opposition• northern lowlands•permafrost• residual polar cap• rift valley• runaway icehouse effect• southern
View Full Document
Unlocking...