The surface of Venus is hidden beneath a

The surface of Venus is hidden beneath a thick, highly reflective cloud cover • Venus is similar to the Earth in its size, mass, average density, and surface gravity • It is covered by unbroken, highly reflective clouds that conceal its other features from Earthbased observers

• Venus rotates slowly in a retrograde direction with a solar day of 117 Earth days and a rotation period of 243 Earth days • There approximately two Venusian solar days in a Venusian year.

Venus has a hot, dense atmosphere and corrosive cloud layers • Spacecraft measurements reveal that 96. 5% of the Venusian atmosphere is carbon dioxide • Most of the balance of the atmosphere is nitrogen. • Venus’s clouds consist of droplets of concentrated sulfuric acid. • The surface pressure on Venus is 90 atm, and the surface temperature is 460°C • Both temperature and pressure decrease as altitude increases

The upper cloud layers of the Venusian atmosphere move rapidly around the planet in a retrograde direction, with a period of only about 4 Earth days

The circulation of the Venusian atmosphere is dominated by two huge convection currents in the cloud layers, one in the northern hemisphere and one in the southern hemisphere

Volcanic eruptions are probably responsible for Venus’s clouds • Venus’s clouds consist of droplets of concentrated sulfuric acid • Active volcanoes on Venus may be a continual source of this sulfurous material

The density of craters suggests that the entire surface of Venus is no more than a few hundred million years old. According to the equilibrium resurfacing hypothesis, this happens because old craters are erased by ongoing volcanic eruptions

The climate on Venus followed a different evolutionary path from that on Earth • Venus’s high temperature is caused by the greenhouse effect, as the dense carbon dioxide atmosphere traps and retains energy from sunlight. • The early atmosphere of Venus contained substantial amounts of water vapor • This caused a runaway greenhouse effect that evaporated Venus’s oceans and drove carbon dioxide out of the rocks and into the atmosphere • Almost all of the water vapor was eventually lost by the action of ultraviolet radiation on the upper atmosphere. • The Earth has roughly as much carbon dioxide as Venus, but it has been dissolved in the Earth’s oceans and chemically bound into its rocks

The surface of Venus shows no evidence of plate tectonics • The surface of Venus is surprisingly flat, mostly covered with gently rolling hills • There a few major highlands and several large volcanoes • The surface of Venus shows no evidence of the motion of large crustal plates, which plays a major role in shaping the Earth’s surface

Earth-based Observations • A solar day on Mars is nearly the same length as on Earth • Mars has polar caps that expand shrink with the seasons • The Martian surface undergoes seasonal color changes

Earth-based observations were once thought to 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 life

Unmanned 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 mantle

Olympus 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 past

• 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 understood

Surface 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 today

Polar 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 atmosphere

Earth and Mars began with similar atmospheres that 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’ Mountains

Earth’s Atmosphere

Mars Atmosphere

Landers have explored the surface of Mars

The Martian atmosphere changes dramatically with 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 changes

Afternoon dust devils help to transport dust from place to place

Winter Frost

The 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 debris