For the next few weeks: Terrestrial Planets, their Moons, and the Sun
Announcements Reading Assignment Section 9-1 (pp 186-189), 9-5 and 9-6 (pp 199-203) 3 rd Homework is now posted on the course website (due Thursday 3/1) Term paper details are now posted on the website (due 4/17) Results from first exam will be briefly discussed today Solutions will be posted soon Public Lecture next Tuesday (2/27) at 7:30PM in this auditorium Prof. Bob Strom: Global Warming There will be a sign-up sheet for students in this class. Sign in and get 5pts extra-credit added to your in-class activities score for each one of these public lectures that you attend (including the first one, if you attended it)
First Exam Results Average: 69.1 Median: 70 High: 102-½ (5 scores over 99) Total tests taken: 130 89 and above 18 (13.8%) 78 88 19 (14.6%) 67 77 38 (29.2%) 56 66 26 (20.0%) 55.5 and below 29 (22.3%)
First mid-term exam grade distribution 25 20 15 Series1 10 5 0 95-100 90-95 85-90 80-85 75-80 70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-40 30-35 25-30
Approximate Curve > 87.5 A (15.4%) 74 87 B (21.5%) 62.5 73.5 C (30.8%) 45 62 D (21.5%) < 45 E (10.8%)
Today: Planetary Interiors and Surfaces
Review: Formation of terrestrial planets Small dust particles accreted to make planetesimals Planetesimals accreted (and collided with other planetesimals) to form protoplanets The protoplanets were at least partially molten denser iron-rich material fell to the center, bringing heavier metals with it, making an iron-rich core (differentiation) A terrestrial planet!
Internal structure of a terrestrial planet
All of the terrestrial planets, and Earth s moon have a similar internal structure, but the relative sizes are different depending on how hot the interior got, how rapidly the object cooled, and how much mass it has The largest factor in determining this structure is the object s size
Sources of heat Accretion Conversion of gravitational energy of incoming material to kinetic energy, into thermal energy Chemical Differentiation Conversion of gravitational energy of falling denser materials within the interior to thermal energy Radioactive decay of elements within the body s interior leads to a slight mass difference between initial and final elements which is converted to energy E=Δmc 2
Cooling processes in terrestrial planets Mantle convection in the interior Hot material rises, cold material falls (like boiling water on a stove) Thermal conduction in the lithosphere Like when a metal plate is heated at one end the other end will soon get hot too Radiative loss through the surface into space Volcanic eruptions
Probing the interiors of planets The only reliable way to accurately probe the interior of a planet is by analyzing seismic activity like Earthquakes Another reasonable approach is to measure how the body rotates (by measuring its libration) The existence of a planetary magnetic field provides some basic information Theoretical and numerical models can be constructed, but these are not as definitive
Seismic Waves P and S waves (Primary and Secondary) Move through the Earth s interior Provide information about interior s structure Surface Waves The rolling waves that are felt on the surface Like water waves
P- and S-waves P-waves, or compression waves P-waves Compressional (or longitudinal) waves Can travel through solids or liquids S-waves Shear (or transverse) waves Cannot travel through liquids S-waves, or transverse waves Water waves
S-waves do not travel through the Earth s core (creating a shadow zone as shown at below right) This proves that part of the core is liquid
Planetary Magnetic Fields Another important tool for probing the interior of a planet Mercury has a global magnetic field this is somewhat of a puzzle! Venus does NOT have a global magnetic field Slow rotation rate Earth has a very strong global magnetic field Mars does not have a global magnetic field, but there is evidence that it had one in the past
Permanent magnets lose their field if raised to a temperature above about 500 o C The Earth is hotter than this nearly everywhere The Earth s Dynamo Earth s field is also known to change periodically Pole reversals It must be generating its own internal magnetic field Need a circulating electric current Circulation and convection of electrically conductive molten iron in the Earth's outer core PTYS/ASTR produces 206 the magnetic field
Planetary Surfaces The appearance of a terrestrial planet s surface is determined by internal geological activity, impacts with asteroids and comets, and erosion Examples of internal activity include: Plate tectonics and volcanism Atmospheres can give rise to both aeolian processes (wind erosion) and water erosion through precipitation and water flow, and other surface processes, such as the flow of glaciers
Evidence of Internal Activity: Volcanoes Volcanoes either active or extinct indicates whether the interior is currently active, or was so in the past Earth active volcanoes active interior Venus possibly active volcanoes, (most are probably extinct) active interior Mars extinct volcanoes no internal activity Mercury no volcanoes probably no internal activity Olympus Mons, Mars
Evidence of Internal Activity: Plate Tectonics Plate tectonics is the shifting around of large crustal plates on the surface of a planet. It is the result of convection in the interior Currently, only Earth has plate tectonics One result of plate tectonics huge mountain ranges
Aeolian processes Dust devils on Mars Wind and water erosion have effected the surfaces of Earth, Mars, and Titan Sand Dunes exist on all three bodies Dust devils exist on Mars and Earth Mars rover Sprit is still working because of a dust devil! Dust devils are much larger on Mars Sand dunes on Titan
Impact Cratering The dominant geological process in the solar system The number of craters on a body is a good indicator of the age of the surface Impacts have also affected the atmospheres of the gas giants
History of Craters Less than 100 years ago scientists felt that cratering due to impacts was unrealistic and highly improbable! Volcanism was the explanation (can see similar features associated with terrestrial volcanoes) 1960 s consensus that impacts caused craters in solar-system bodies Crater Elegante (Pinacate balsatic fields in Mexico, just below Arizona border)
Shock effects in quartz (also seen in lunar rock samples) due to shock waves produced by the explosions associated with impacts Evidence for Craters due to Impacts Craters are nearly all circular in outline raised rims, concentric inner terraces, and other features that are seldom found in volcanic craters Circular also because of the shock waves associated with the explosion not elongated! Volcanic craters are usually very small and occur in a clearly volcanic environment.
Earth s Impact Craters The Earth s surface has probably received more impact events than the Moon Earth is larger (but its atmosphere burns up smaller ones). Only about 160 surviving craters have been found thus far. So few because geological activity erases the evidence (Earth s surface is a young one) Earth s atmosphere provides a shield against smaller asteroids Manicougan crater, Quebec, Canada (5 th largest crater in the world)
Earth s Impact Craters