The Sun is at the center of our Solar System. • The Sun makes up 99.85% of the mass of the solar system.
The Planets: An Overview The Solar System The terrestrial planets are planets that are small and rocky—Mercury, Venus, Earth, and Mars. The Jovian planets are the huge gas giants—Jupiter, Saturn, Uranus, and Neptune. Pluto does not fit into either the Jovian or the terrestrial category.
Terrestrial Planets Mercury Mars Venus Earth
Jovian Planets Neptune Uranus Saturn Jupiter
The terrestrial planets are also called the Rocky or the Inner planets. • The Jovian planets are also called the Giant or the Outer or the Gas planets.
Orbits of the Planets The Sun’s gravitational pull guides the elliptical orbits of the planets in our Solar System.
The Planets: An Overview The Solar System • Size is the most obvious difference between the terrestrial and Jovian planets. • Earth’s mass is 1/17th of the mass of Neptune (the smallest Jovian planet). Density, chemical makeup, and rate of rotation are other ways in which the two groups of planets differ.
Interior Planetery Composition • The planets are made of three different substances: gases, rocks, and ices.
Gases (hydrogen and helium) are those with melting points near absolute zero (-273⁰ C).
Rocks are mostly silicates (silicon and oxygen) and metallic iron which have melting points above 700 ⁰ C.
Ices are ammonia (NH3), methane (CH4), carbon dioxide (CO2) and water (H2O) which have intermediate melting points.
The terrestrial planets are dense with mostly metallic and rocky substances. The Jovian planets are less dense with large amounts of gases and ices.
The Atmosphere of the Planets • The Jovian planets have very thick atmospheres of hydrogen, helium, methane, and ammonia.
By contrast, the terrestrial planets, including Earth, have thin, meager atmospheres at best.
Gases can escape from its planet’s gravity if it reaches escape velocity. • For Earth, escape velocity is 11 km/s. • For Jovian planets, the escape velocities are between 21-60 km/s. So it is more difficult for gases to escape a Jovian planet’s gravity.
Formation of the Solar System The Solar System Nebular Theory • A nebula is a cloud of gas and/or dust in space. • A nebula often consists of 92% H, 7% He, and <1% heavier elements.
According to the nebular theory, the sun and planets formed from a rotating disk of dust and gases. • As the speed of rotation increased, the center of the disk began to flatten out and become more concentrated. This eventually became our sun!!
The Hubble Telescope captured this picture of a nebula near a dying star (450 light years away in the constellation Aquarius) (these are two gases colliding)
Formation! • The Solar System was formed from the contraction of a pre-solar nebula, a vast cloud of gas and dust. • The center of the nebula became the Sun and the outer parts accreted to form the planetesimals.
Formation of the Solar System The Solar System • Planetesimals are small, irregularly shaped bodies formed by colliding matter. • Over time, these planetisimals will collect enough material/mass to have their own gravitational pull. • The inner planets (close to the sun) were too hot for ice to form and only rocky material accreted. • The outer planets (far from the sun) had both solid matter and ice to accrete.
Mercury: The Innermost Planet The Terrestrial Planets
Mercury is the innermost and second smallest planet; it is hardly larger than Earth’s moon.
Because Mercury is so small, the gravity on Mercury is 38% of the gravity on Earth. • A 100 pound person would weigh only 38 pounds on Mercury. To calculate your weight on Mercury, just multiply your weight by 0.38
Orbit: very elliptical with no seasons (because no tilt) Mercury orbits the sun every 88 days and has a rotation of 59 days.
Surface Features • Mercury has cratered highlands, much like the moon, and vast smooth terrains that resemble maria.
Surface Temperatures • Mercury has a very, very thin atmosphere. • This means that it has no shielding from the sun’s energy (on the light side) and no way to hold this energy (on the dark side). • Mercury has the greatest temperature extremes of any planet.
Since the atmosphere is so thin, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day.
Mercury has a daytime temperature of about 400⁰ C and a nighttime temperature of about -200 ⁰ C, a mere 73 degrees above absolute zero.
If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth!
Venus Mercury was visited by NASA's Mariner 10 in 1973 and 1974 (robotic space probe that visited both Venus and Mercury). Mariner is STILL orbiting the Sun, but is no longer transmitting (and is probably horribly damaged by the Sun’s energy.)
Currently, the MESSENGER space probe is orbiting Mercury and sending messages back to Earth. Probe launch in 2004 First image of Mercury sent back to Earth (2008)