1. The Outer Layers of the Sun
The Photosphere
The Chromosphere
The Corona
The Heliosphere

2. The Chromosphere
2,500 km think irregular region above photosphere of hot thin gas.
Density is about a million times less than the top of the photosphere and the temperature rises to 10,000 K.
First observed during solar eclipses as a faint red
ring that surrounds the photosphere.

4. Discussion If the chromosphere is so much hotter than the
photosphere, why does it not shine brighter than
the photosphere?
Why do you think it is red?

6. Calcium K line at nm

7. What makes the chromosphere so hot?
The chromosphere is heated from above by the solar corona.

10. Speeds are 0.4 km/s hard to see.

11. Plages
Bright patches surrounding sunspots. Plages are associated with concentrations of magnetic fields.

14. Filaments & prominences are the same thing!

15. Prominences Two types: Quiescent – can last for months
Active – last only for hours. Lie above sunspots and have violent motions.

16. Hedgerow Prominence

20. Flares Sudden violent explosions in magnetically active
regions. Last from 1 to 20 minutes and release as much energy as 2.5 million 100-megaton nuclear bombs in an area about the size of Earth. This is enough energy to cause thermonuclear fusion to take place on the Sun’s surface.

22. The Transitional Region
A thin region (can be as thin as a few 10’s of km) above the chromosphere where the temperature rises from 10,000 K to 1 million K typical of the corona.

25. Corona
Outer part of the Sun’s atmosphere. About as bright as the quarter phase Moon, it is visible only when the light of the photosphere is blocked.
The corona is so thin that on Earth we would consider it a vacuum.

27. The Corona is surprisingly hot
The temperature of the corona is over 1 million K.

28. Discussion
At the distance of the Earth’s orbit the corona, although cooler than near the Sun, is still 0.14 million K. Why don’t astronauts burn up when they go on space walks?

29. Discussion
What causes the helmet shaped structures in the corona? Why isn’t the corona spread out uniformly around the Sun?

30. Coronal Mass Ejection
Giant magnetic bubbles that can hurl 5 to 50 billion tons of matter at speeds of 400 km/sec.
70% of coronal mass ejections are associated with, or followed by, erupting prominences. While 40% are accompanied by solar flares that occur at about the same time and place.
Once a day, Only 5% that lost in solar wind

34. The Sun-Earth connection
Coronal mass ejections and solar flares can be directed at Earth. Luckily for us, Earth has a magnetic field and an atmosphere to protect us.

37. Aurora When high speed particles from the Sun collide
with atoms in Earth’s upper atmosphere. The
electrons are knocked into higher energy
orbitals and emit light when returning to the
ground state.

39. Solar Wind
Although the Sun’s surface gravity is much higher than the Earth’s, it is not able to contain particles with a temperature on over a million K. Thus the hot corona spews matter constantly (not just during flares, CME, and explosive prominences) into space at a rate of about 1 million tons per second.
Sun’s escape velocity is about 600 km/s

41. Exam Next Wednesday essay & multiple choice questions
Covers chapters 1-8, S1 & 14
Allowed one standard sheet of notes with writing on one side only

42. Terrestrial planet geology

43. Terrestrial Planets
All the terrestrial planets are more or less differentiated, i.e. the densest materials have sunk to the core and the lighter materials have floated to the surface.
The terrestrial planets were all completely molten at some time in the past.

45. Discussion
Why do you think all the terrestrial planets were so hot in the past? Isn’t space rather cold?

46. Terrestrial planets interior structure
Core – highest density material, mostly iron and nickel
Mantle – high density silicate rocks
Crust – lower density silicate rocks, granite and basalt.

48. Discussion
What’s a silicate? Give and example of a silicate.

50. Earth’s internal structure
Solid crust – 5 km thick under the oceans, made of basalt: silicates of aluminum, magnesium and iron with a density of about 3.5 g/cm3. Under the continents the crust is 35 to 70 km thick and is made mostly of granite: silicates of aluminum, sodium and potassium with a density of 3.0 g/cm3. The continents float on the basalt.

51. 2. Mantle – solid, but top layer is plastic called the asthenosphere
2. Mantle – solid, but top layer is plastic called the asthenosphere. Is about 2800 km thick and made of compounds rich in iron and magnesium. Density increases from 3.5 g/cm3 at top to 5.5 g/cm3 near the bottom.

52. 3. Outer core – liquid. Is about 2200 km thick and is made of iron, nickel and sulfur.
4. Inner core – solid. Is about 1300 km thick and is composed of nearly pure crystalline iron with a density of 13 g/cm3.

53. Why is inner core solid while outer core is liquid?
Isn’t the inner core hotter than the outer core?
The melting point of substance depends on both temperature and pressure. In general, the melting point goes up with pressure. The inner core is hotter than the outer core but is under a greater pressure and thus has a higher melting temperature.

55. Discussion
How can we know anything about the core of the Earth when our deepest mines and bore holes haven’t even made it through the curst?

56. Earthquakes
Earthquakes produce three types of waves that travel through the Earth.
Surface waves
Primary waves, or P waves
Secondary waves, of S waves

58. Wave speed
The speed of seismic waves depends primarily on the density of the material through which they travel. If the density changes, the waves will be refracted, just as light is refracted in a glass lens.

59. Discussion
Which type a wave do you think will travel better through the interior of a substance which is liquid and why?

60. S waves cannot travel far through a liquid
S waves cannot travel far through a liquid. On the opposite side of the Earth from and epicenter of an earthquake, seismographs only detect P waves.
The molten core causes the P waves to bend is such a way that the density of the core can be determined.

62. Discussion
How do we know that the Earth’s inner core is solid?

63. Discussion
Study of seismic waves on Earth indicate that the inner solid core is rotating faster than the rest of the Earth. How do we know the core is rotating faster?
Core rotates once every 400 years wrt Earth’s surface

64. Discussion
What do you suspect would be the result of this difference in rotation rate between the solid and liquid cores?

66. Planetary seismic data?
We know the internal structure of the Moon because astronauts placed nuclear powered seismic stations at the Apollo landing sites. But this information is not available for any of the other planets.

67. Planets internal structure
Two tools without seismic data:
Mean density
Gravitational mapping – mascons

68. Density Mass of the planet divided by the volume of the planet.
Higher density implies a larger percentage of high density materials, such as iron and nickel, lower density implies more silicates.

69. Mapping the gravitational field
By carefully tracking an orbiting space probe, concentrations of denser materials below the surface can be mapped. Space probes are accelerated more toward higher density regions.

70. How the planets got hot Heat of accretion Heat of differentiation
Heat from radioactive decay

72. Discussion
Which of the terrestrial worlds (including the Moon) was likely the hottest during its formation? Why?

73. Heat and planets
All the terrestrial planets started out hot and have been losing heat over time by radiating it into space from their surfaces.

74. 2nd law of Astronomy 201
Larger planets lose heat more slowly than do smaller planets.

75. Discussion
Larger planets have larger surface areas, and a larger surface area should radiate more energy into space? So shouldn’t larger planets cool faster? Why do larger planets cool more slowly than smaller planets?

76. Volume and heat
The greater the surface area, the faster heat will be radiated. But, it is the volume that stores the heat. The greater the volume of a planet the more internal heat it can retain.
Also, more massive planets have more radiative material.

77. It’s geometry
The surface area increases as the square of the radius. But the volume increases as the cube of the radius. Thus, a larger sphere has less square miles to radiate the heat per cubic mile of material.
This is why people get fat!

78. Geologic activity
Internal heat drives geologic activity on the planets’ surfaces.

79. Discussion
Because heat is radiated from the surface of a planet, the surface is cooler than the interior.
How does the heat from the core of the planet get to the surface?

80. How does the heat get out?
Convection in the mantle.
Note that the mantle is not liquid but it is plastic, meaning that it can flow like silly putty or glass.

82. Plate tectonics

83. Lithosphere
The convective cells in the planets do not make it to the surface as on the Sun, but are stopped at the base of the lithosphere. The lithosphere includes the crust and the upper mantel region of cooler, stronger rock which does not flow as easily as the warmer, lower mantel rock.

84. Geologic processes
Impact cratering
Volcanism
Tectonics
Erosion

85. 3rd law of astronomy 201
The more impact craters on a surface, the older that surface is.
“Age” referring to the time since the surface was last molten

86. Discussion
Which area on the Moon is older, the light region to the left or the dark region in the center of the picture?

88. Why?
All the terrestrial planets probably receive about the same number and size distribution of impacts. All the other geologic processes (volcanism, tectonics, and erosion) tend to erase impact craters on the surface.

89. Discussion
Rank the terrestrial planets (include the Moon) in terms of the age of their surfaces from youngest to oldest to try and predict which planets will have the most craters.
Size ranking

90. Earth
Venus
Mars
Mercury
Moon
Smaller planets retain less heat and therefore have less geologic activity.

93. Discussion
Why do you think Earth’s oceans have so few impact craters as compared to Earth’s continents?

95. Venus radar map

96. Mars laser altimeter map