Sun-Scorched MercuryCloud covered VenusThe Red planet MarsChapters 11,12,13

88 days

Earth-based optical observations of Mercuryare difficultAt its greatest eastern and western elongation, Mercury is never more than 28 from the sunIt can be seen for only brief periods just after sunset or before sunrise

Solar Transit

There will be a transit on November 8, 2006Transits occur about twelve times a century when the sun, Earth and Mercury are aligned

Best Earth-based Views of Mercury

Difficulties observing Mercury from Earth led early astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbitNote phases like the moon

Radio telescope observations from sites such as Arecibo gave evidence of a non-synchronous orbit

Mercury rotates slowly and has an unusualspin-orbiting coupling

Strong tidal effects, Mercurys slightly elongated shape and its very eccentric orbit cause this strange 3-to-2 orbitA day of solar light on Mercury would be 88 earth days

Images from Mariner 10 reveal Mercurys heavilycratered surfaceMost of our detailed information about Mercurys surface is from this fly-by mission in 1974/1975.Mariner only saw one side of the planet.There are presently three planned missions to Mercury with data return beginning in 2009.

Heavily cratered surfaceLess dense cratering than moonGently rolling plainsScarpsNo evidence of tectonics

Note how much more densely the craters occur on the moons surface.

The Caloris Basin is evidence of a large impact

Mercury has an iron core and a surprisingmagnetic fieldMost iron-rich planet in the solar system with a core that is 75% of the diameterThe earths core is 55% of its diameter and the moons core is 20% of its diameterHighest density for the planetsWeak magnetic field indicating part of the core is liquidMagnetic field causes a magnetosphere similar to Earths but weaker

The magnetosphere blocks the solar wind from reaching the surface of the planet

The surface of Venus is hidden beneath a thick,highly reflective cloud coverVenus is similar to the Earth in its size, mass, average density, and surface gravityIt is covered by unbroken, highly reflective clouds that conceal its other features from Earth-based 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 are approximately two Venusian solar days in a Venusian year.

Venus has a hot, dense atmosphere and corrosivecloud layersSpacecraft measurements reveal that 96.5% of the Venusian atmosphere is carbon dioxideMost of the balance of the atmosphere is nitrogen.Venuss clouds consist of droplets of concentrated sulfuric acid.The surface pressure on Venus is 90 atm, and the surface temperature is 460CBoth 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 responsiblefor Venuss cloudsVenuss clouds consist of droplets of concentrated sulfuric acidActive 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 differentevolutionary path from that on EarthVenuss 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 vaporThis caused a runaway greenhouse effect that evaporated Venuss oceans and drove carbon dioxide out of the rocks and into the atmosphereAlmost 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 Earths oceans and chemically bound into its rocks

The surface of Venus shows no evidenceof plate tectonicsThe surface of Venus is surprisingly flat, mostly covered with gently rolling hillsThere are a few major highlands and several large volcanoesThe surface of Venus shows no evidence of the motion of large crustal plates, which plays a major role in shaping the Earths surface

Earth-based ObservationsA solar day on Mars is nearly the same length as on EarthMars has polar caps that expand and shrink with the seasonsThe Martian surface undergoes seasonal color changes

Earth-based observations were once thoughtto show evidence of intelligent life on MarsA few observers reported a network of linear features called canalsThese observations, which proved to be illusions, led to many speculations about Martian life

Unmanned spacecraft found craters, volcanoes,and canyons on MarsThe Martian surface has numerous craters, several huge volcanoes, a vast rift valley, and dried-up riverbeds but no canalsMartian 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 lavaMars 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 lowlandsThe origin of this crustal dichotomy is not completely understood

Surface features indicate that water once flowed on MarsFlash-flood features and dried riverbeds on the Martian surface indicate that water has flowed on Mars at least occasionallyNo liquid water can exist on the Martian surface today

Polar Ice CapsMarss polar caps contain frozen water, a layer of permafrost may exist below the Martian regolith, and there may be liquid water beneath the surfaceThe 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 atmospheresthat evolved very differentlyMarss primordial atmosphere may have been thicker and warmer than the present-day atmosphereIt 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 planets surfaceThe present Martian atmosphere is composed mostly of carbon dioxideThe 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

Earths Atmosphere

Mars Atmosphere

Landers have explored the surface of Mars

The Martian atmosphere changes dramaticallywith the seasonsGreat dust storms sometimes blanket MarsFine-grained dust in its atmosphere gives the Martian sky a pinkish-orange tintSeasonal 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 planetThey may be captured asteroids or may have formed in orbit around Mars out of solar system debris