Closest planet to sun

  Temperature extremes

Earth’s temperature is relatively stable and close in range because of its rapid rotation speed which releases large amounts of heat, and its thick atmosphere that also helps to store heat absorbed from the sun

                Mercury has a slow rotation and a thin atmosphere, so the range is incredible

                -At noon the temperature is 700 K

                -At terminator temperature is about 425 K

                -On the night side temperature Is about 100 K

                Daily range of 600K

Thin atmosphere (unprotected from impacts and deadly solar radiation)

Mass of planet is only 5.5% of Earth’s mass

                Gravity is too weak to hold permanent atmosphere

                Has 5 elements; hydrogen, helium, sodium, potassium, and oxygen

                Hydrogen and Helium belived to come from the sun

                Sodium and potassium gas escape from rocks inside planet (outgassing)

                Oxygen comes from polar ice that is evaporating

Can only be seen from Earth for a few hours before sunrise and after sunset (because angle from sun elongation is less than 28 degrees)

Sometimes appears as brightest figure in sky—actually as dark as coal

Coasted past Mercury in 1974 to reveal 45% of Mercury’s surface

                -Resembles moon with craters

                -Discovered huge circular region, called Caloris Basin

                -Showed us long cliffs called scarps

                -Mercury has a weak magnetic field

                -Possible evidence of Ice within some craters

Impacted Mercury in first 800 million years after it condensed from solar nebula

                Debris remaining after planet formed pounded on the surface (evidence from dating moon rocks)

Many craters

Similarities

                -Has many craters (densely packed) like the moon

                -Low albedo of .12 12% scattering of incoming light (similar to moon’s low albedo)

Because of this Mercury sometimes appears bright, when it is really just scattering sunlight

                Differences

                -Lacks extensive craterless region

                -Has plains sparsely filled with small craters

1300 km in diameter

                Lies along the terminated (border between day and night)

                Surrounded by 2km high rings of mts, beyond are smooth plains

                Occurred during the end of the major crater-making period

Jumbled, hilly region (appear as tiny wrinkles)

                ½ million square kilometers

                Believe energy from Caloris’ impact became focused and passed through mercury pushing the hills on the far side up

                Similar effect on Moon’s Mare Orientale

Probably formed Mercurian plains

                When craters hit, lava flowed up from interior to flood low-lying areas

                Formed 3.8 billion years ago at the end of the era of heavy bombardment

Long cliffs

                Developed as planet cooled (things that cool contract)

                As molten iron core cooled and contracted surface moved inward and wrinkled (because crust      was solid and could not simply collapse)

Proved that planet’s interior is solid to significant depth because otherwise lave would have leaked out as scarps formed

Density is similar to the Earth’s (but slightly smaller)

                Densities of both planets are caused by dense interiors

                Expect Mercury to contain a lower percentage of Iron because of its lesser density (compared to Earth)---but it is actually the most iron rich planet

                Earth only has a greater density due to its greater mass that presses inward; compressing the    interior

Iron core is 42% of planet’s volume

                Surrounding core is thick rocky mantle

                (Earth’s iron core only 17%)

1.       Inner regions of the primordial solar nebula were incredibly hot, and only iron was able to stand the heat

2.       Outflow or particles from Sun stripped mercury of its low-density mantle leaving iron

3.       Mercury was struck by a large plantestimal which ejected much of the lighter mantle

Sun creates a significant tidal bulge on Mercury that changed the planet’s rotation (Mercury is super close to the sun)

                However, Mercury’s highly eccentric rotation rate (e = .21) caused a 3 to 2 spin-orbit coupling (undergoes three sidereal rotations while undergoing two revolutions around the Sun)

                Consequences of 3 to 2 spin orbit coupling: one of Mercury’s regions of high tide is facing the sun when at the perihelion

1 day on Mercury is two years on Earth

                2. If you set a camer up at high tide as the planet passed through perihelion see sun rise to east, stop, go back towards east, then start to set in west (retrograde motion)

                3. Mercury rotates 59 times more slowly than Earth (can’t generate magnetic field—too slow)

Highlands ~ 3.8 byrs old

Intercrater plains ~ older than 3 byrs old

Launched Aug 2004

Will arrive in 2011