Lecture 20

Outline For Today

• Venus review• Mars

The length of one solar day on Venus is

• A) about the same as that on Earth. • B) much longer than that on Earth. • C) much shorter than that on Earth, about

an hour. • D) about half as long as that on Earth,

about 10 hours. • E) about one month.

Tectonic activity on Venus differs from that on Earth in that

• A) active crustal deformation appears to be completely absent.

• B) the lithosphere appears to be softer or more plastic and cannot support the creation and motion of solid plates.

• C) the lithosphere appears to be cooler and thicker and is therefore too rigid to break up into moving plates.

• D) mantle convection appears to be more vigorous and has broken the lithosphere into a multitude of small plates instead of a few large ones.

The so-called greenhouse effect, which produces very high temperatures on the

surface of Venus, is

• A) the absorption by the CO2 gas of the planet's atmosphere of infrared radiation emitted by its surface.

• B) the absorption of solar visible radiation by the CO2 gas of the Venusian atmosphere, thereby heating this gas.

• C) the trapping of hot gases ejected by continuously active volcanoes under the dense cloud cover.

• D) due to the loss of ozone about its poles.• E) the melting of its polar ice caps.

Why is the surface of Venus hotter than that of Mercury, even though Mercury is much closer

to the Sun? • A) Chemical reactions within the thick clouds and

dense atmosphere are continuously supplying heat to the surface.

• B) Continuous volcanic activity releases large quantities of hot lava onto the surface.

• C) Venus rotates rapidly, thereby ensuring that its entire surface is being heated regularly and uniformly.

• D) The thick CO2 atmosphere prevents re-emission into space of the heat absorbed from sunlight.

• E) Mercury has a lower albedo.

• If an observer says it is noon in at point D, what time is it for the observer at point B if the planet has an orbital period of 100 days and a rotational period (protograde) of 100 days?

• A) Noon B) Dawn C) Dusk D) Midnight E) 9:00 am

A

B

C

D

Outline For Today

• Venus review• Mars

Observations

• Observing Mars• Moons• Surface• Atmosphere

Earth-based observations of Mars are best made during favorable oppositions

The best Earth-based views of Mars are obtained when Mars is simultaneously at opposition and near perihelion

Observations

• Observing Mars• Moons• Surface• Atmosphere

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

Observations

• Observing Mars• Moons• Surface• Atmosphere

Earth-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 changes

Landers have explored the surface of Mars

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

• 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

Observations

• Observing Mars• Moons• Surface• Atmosphere

Earth’s Atmosphere

Mars Atmosphere

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

What causes the seasons on Mars?

• A) its elliptical orbit.• B) its spin axis being tilted with respect to

its orbital plane.• C) its day being about 24 hours in length. • D) its year being about 700 days in length. • E) its distance from the sun varies in its

orbit.

On Mars, the initial and very rapid recession of the edge of the white polar cap region toward the poles in

springtime is caused by

• A) the melting and evaporation of CO2 ice. • B) the increased growth of vegetation toward the

poles from mid-latitudes. • C) the change in color of the rocks by

photochemical action, similar to bleaching. • D) the melting of H2O ice and subsequent runoff

of water.• E) the shrinking of the ozone hole above the

polar caps.

On the basis of appearance and general properties, which planetary body could be

described as Earth's twin?

• A) Pluto—similar in size and density, with a large moon and probably life on its surface

• B) Venus—about the same mass and diameter, with a dense and cloud-shrouded atmosphere

• C) Mars—somewhat smaller but with a similar surface, a thin atmosphere, and clouds

• D) the Moon—somewhat smaller but with the same average density and geology

• E) Io—similar in size and density with 70 percent of its surface covered in water