The Terrestrial Planets Mercury, Venus, Earth, Mars

Mercury Closest to the sun Visible above the horizon for up to two hours before sunrise or after sunset High average density which indicates a large iron core Similar to Earth’s moon

Mercury Discovered over 3000 years ago Named for the Roman God of commerce and travel No atmosphere Orbits faster than any other planet Axial rotation is faster than orbit

Mercury Mercury’s orbital and rotational motions combine to produce a solar day that is two Mercury years long

Mercury Hydrogen and helium are temporarily trapped by solar wind, giving the appearance of an atmosphere High surface temperature and low mass  700 K  4.5 times the mass of the moon Temperature falls to 100 K due to no atmosphere Poles possibly covered with extensive thin sheets of ice

Mercury Surface features are similar to the moon Craters are a result of meteoritic bombardment Contains cliffs (scarps) that were likely formed about 4 billion years ago

Mercury Caloris Basin  Immense crater formed by the impact of a large asteroid  Helped demonstrate that volcanism was mainly responsible for inter-crater plains  Measures about 1400 km across  Ringed by mountain ranges that reach more than 3 km high  Spans more than half of Mercury’s radius

Mercury Magnetic field is about 1/100 of Earth’s field  Details of how such a relatively strong field can be produced by a slowly rotating planet remain to be resolved Large, iron rich core  Radius = 1800 km  60% of planets mass Seems to have been geologically dead for the past 4 billion years  Results from the mantle being solid

Mercury Formed 4.6 billion years ago Dense, largely metallic overall composition Over half a billion years the planet melted and differentiated Intense meteoritic bombardment Lava erased craters As the core formed the planet began to shrink  This compression caused the scarps

Mercury Mariner 10 was the first spacecraft to travel to Mercury Three flybys during 1974 and 1975 Messenger was the second spacecraft to travel to Mercury Three flybys during 2008 Scheduled to go into orbit in 2011

Venus 2 nd Planet from the sun Visible up to 3 hours just before sunrise or just after sunset 3 rd brightest object in the sky  Only the sun and moon are brighter  Shines by reflected sunlight Almost all sunlight hitting the planet is reflected by dense clouds enveloping it  Can be seen in the daytime

Venus Clouds make it impossible to discern any surface features Retrograde Spin 243 day rotation period Rotation axis is perpendicular to its orbital plane Slow rotation thought to be caused by an impact shortly after formation.

Venus Atmosphere  More massive  Extends further from surface  96.5% Carbon dioxide  3.5% Nitrogen  Upper Level winds exceed 400 km/hr  Air is clear below 30 km Polar Vortex  Stable, long-lived wind flow circling the poles  Study of vortex helps scientists understand the global circulation Surface Temperature – 730 K Surface Pressure – 90 times the pressure at sea level on Earth

Venus Radio astronomy allows us to get a picture of the surface features U.S. Magellan provided the most recent radar images Mostly Smooth  Resembles rolling plains  Two continents 8% of total surface area  Mountains similar to the size on Earth Volcanic Activity  Shield volcanoes are most common  Coronae Largest volcanic features

Venus No Magnetosphere  Due to slow rotation Earth-like Composition Partially molten iron- rich core

Venus Runaway Greenhouse Affect  A process in which the heating of a planet leads to an increase in it’s atmosphere’s ability to retain heat and thus to further heating

Mars Appears to traverse the entire sky  Close to the ecliptic Brighter than any star

Mars Surface features easily visible Axial Rotation – 24.6 hours Tilt – 24.0°  Similar to Earth  Daily and seasonal cycles

Mars Atmosphere  Very Thin  95.3% carbon dioxide  2.7% Nitrogen  1.6% Argon  Small amounts of oxygen, carbon monoxide, and water vapor Average Surface Temperature – 70 K lower than Earth

Mars The Surface of Mars  Polar Caps Frozen Carbon Dioxide Smaller caps consist of water  Planet-wide Dust Storms  Northern Hemisphere Rolling volcanic plains younger  Southern Hemisphere Heavily cratered

Mars Martian Volcanism  Largest known volcanoes in the solar system  Olympus Mons Largest of all 700 km in diameter (slightly smaller than Texas) Shield volcanoes

Mars Evidence of Past Water  Runoff channels Strong resemblance to river systems  Outflow Channels Relics of catastrophic flooding

Mars Where is the water today?  Locked in Permafrost Layer of ice just below the surface Ice may make up 50% of the planets soil  Polar caps Permanent caps are now known to be water ice

Mars Internal Structure

Mars Mars Rover Life?