Earth-based optical observations of Mercury are difficult At its greatest eastern and western elongation, Mercury is never more than 28° from the sun It can be seen for only brief periods just after sunset or before sunrise
Solar Transit There will be a transit on November 8, 2006 Transits 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 orbit Note 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 unusual spin-orbiting coupling
Strong tidal effects, Mercury’s slightly elongated shape and its very eccentric orbit cause this strange 3- to-2 orbit A “day” of solar light on Mercury would be 88 earth days
Images from Mariner 10 reveal Mercury’s heavily cratered surface Most of our detailed information about Mercury’s 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 surface Less dense cratering than moon Gently rolling plains Scarps No evidence of tectonics
Note how much more densely the craters occur on the moon’s surface.
The Caloris Basin is evidence of a large impact
Mercury has an iron core and a surprising magnetic field Most iron-rich planet in the solar system with a core that is 75% of the diameter The earth’s core is 55% of its diameter and the moon’s core is 20% of its diameter Highest density for the planets Weak magnetic field indicating part of the core is liquid Magnetic field causes a magnetosphere similar to Earth’s but weaker
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 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 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 are 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 and 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
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
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
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
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