ASTRO 101 Principles of Astronomy

Instructor: Jerome A. Orosz (rhymes with “boris”) Contact: Telephone: WWW: Office: Physics 241, hours T TH 3:30-5:00

Homework/Announcements Homework due Tuesday, March 12: Question 4, Chapter 5 (Why is Earth’s surface not riddled with craters as is that of the Moon?).

Comets! Comet PANSTARRS should be viewable in the western skies starting March 7. It will be close to the crescent Moon on March Comet ISON might be very spectacular in December, 2013, provided it survives its close encounter with the Sun.

Coming up: Chapter 5 (The Earth) Chapter 6 (Other Planets and Moons)

Planetology Some things we want to know about a planet: 1)What are the surface features like? 2)What is the interior like? 3)What is the atmosphere like (if any)? 4)How did it form? 5)Is there (or was there) life? An understanding of other planets may lead to a better understanding of our own Earth.

Next: The Terrestrial Planets

Two Types of Planets Planets come in two types: –Small and rocky. –Large and gaseous. Or –Terrestrial –Jovian

The Terrestrial Planets The terrestrial planets are Mercury, Venus, Earth (and Moon), and Mars. Their densities range from about 3 grams/cc to 5.5 grams/cc, indicating their composition is a combination of metals and rocky material.

The Earth

The Earth’s Atmosphere The Earth’s atmosphere is useful in at least three ways: 1)It keeps the Earth warmer than it would otherwise be. 2)It keeps most of the harmful UV and X-ray radiation from reaching the ground. 3)It allows us to breathe.

The Earth’s Atmosphere Note that the Earth’s atmosphere is not in “normal” chemical balance: –There is more methane, ozone, and oxygen than expected –These species are very reactive, and would normally disappear in a relatively short time –Biological activity keeps renewing these gasses.

The Earth’s Atmosphere The temperature of the atmosphere has a complex dependence on the height:

The Greenhouse Effect The Sun heats the Earth. Some of the energy is scattered, and some heats the ground and water. The Earth tries to cool at night, but the atmosphere traps much of the radiation.

The Interior of the Earth We can use earthquakes to study the interior of the Earth. There are two types of waves: –P-waves, which travel through solids and liquids. –S-waves, which only go through solids. Image from Nick Strobel (

The Interior of the Earth We can use earthquakes to study the interior of the Earth. There are two types of waves: –P-waves, which travel through solids and liquids. –S-waves, which only go through solids. Image from Nick Strobel (

The Interior of the Earth There is a solid inner core of iron, surrounded by a liquid iron core, surrounded by the mantle composed of silicates. A thin crust is on the outside.

The Interior of the Earth Natural radioactivity provides the energy source that heats the Earth’s interior. The thin crustal “plates” float on top of the liquid mantle. The motion of the crustal plates (a few cm per century) causes earthquakes and volcanoes.

The Interior of the Earth Convection causes motions of the crustal plates.

The Interior of the Earth Convection causes motions of the crustal plates. The spreading apart of two plates created this mountain range under the Atlantic.

The Interior of the Earth Convection causes motions of the crustal plates. The continental land masses constantly move relative to each other.

The Interior of the Earth Areas near plate boundaries are prone to earthquakes.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet. This field helps protect us from the “solar wind”.

The Magnetic Field of the Earth The Earth has a “dipole” magnetic field, much like a bar magnet. This field helps protect us from the “solar wind”. The interaction between the solar wind and the field can produce the northern lights.

Next: The Moon

The moon is relatively small: –Mass = 1/81 of Earth’s –Radius = 1/4 of Earth’s –Gravity = 1/6 of Earth’s It appears large in the sky because it is so close, about 240,000 miles, compared to 93,000,000 miles to the Sun.

The Moon The albedo of the Moon is less than 10%.

The Moon The albedo of the Moon is less than 10%. The surface is composed of very dark rock (as dark as coal).

The Moon The albedo of the Moon is less than 10%. The surface is composed of very dark rock (as dark as coal). But why is the Moon so bright?

The Moon The albedo of the Moon is less than 10%. The surface is composed of very dark rock (as dark as coal). But why is the Moon so bright? The Moon is nearby, and we view it contrasted against a dark sky.

The Moon The surface gravity on the Moon is 1/6 of the Earth’s.

The Moon The surface gravity on the Moon is 1/6 of the Earth’s. This is too weak to retain an atmosphere.

The Moon The surface gravity on the Moon is 1/6 of the Earth’s. This is too weak to retain an atmosphere. There is no water on the Moon, nor is there weather of any kind.

The Moon The surface gravity on the Moon is 1/6 of the Earth’s. This is too weak to retain an atmosphere. There is no water on the Moon, nor is there weather of any kind. The surface features we see are very old.

What do we see on the Moon? We always see basically the same side of the Moon facing us, although there is some wobble.

What do we see on the Moon? The rotational period is equal to the orbital period.

What do we see on the Moon? Craters and mountains have been seen since the time of Galileo. We know they are craters by looking at the illumination patterns.

What do we see on the Moon? Craters are best seen during the crescent phases since surface features cast shadows.

What do we see on the Moon? More craters as seen from a NASA probe.

How did the Craters Form? The craters on the Moon were caused by impacts of large bodies, not by volcanoes. In many cases you can see trails of lighter- colored matter thrown out by the impacts.

How did the Craters Form? The lunar craters are caused by impacts, and not by volcanoes.

What do we see on the Moon? We also see large darker areas called “maria” (Italian for “seas”).

What do we see on the Moon? We also see large darker areas called “maria” (Italian for “seas”).

What do we see on the Moon? We also see large darker areas called “maria” (Italian for “seas”).

How did the Maria Form? An impact of a very large body and subsequent lava flows may have formed the maria.

The Lunar Interior The inside of the Moon can be studied using seismic equipment left by the Apollo astronauts, and by tracking the orbits of spacecraft near the moon. The moon has an uneven interior, with mass concentrations near the maria. The interior is cold, and there are no active volcanoes today.

The Earth and Moon The Earth has an atmosphere The Earth is hot inside The Earth has lots of water The Earth has lots of iron The Moon does not have an atmosphere The Moon is cold inside The Moon has very little water The Moon has little iron

Next The formation of the Moon

The Formation of the Moon The density of the Moon is about 3.3 grams/cc, somewhat less than the Earth.

The Formation of the Moon The density of the Moon is about 3.3 grams/cc, somewhat less than the Earth.  This density is similar to the density of the Earth’s Mantle.

The Formation of the Moon The density of the Moon is about 3.3 grams/cc, somewhat less than the Earth.  This density is similar to the density of the Earth’s Mantle.  The Moon is deficient in iron.

The Formation of the Moon The density of the Moon is about 3.3 grams/cc, somewhat less than the Earth.  This density is similar to the density of the Earth’s Mantle.  The Moon is deficient in iron.  The surface composition of the Moon is similar, but not exactly like that of the Earth.

The Formation of the Moon In the late 1960s, there were three main theories on how the Moon formed:

The Formation of the Moon In the late 1960s, there were three main theories on how the Moon formed: 1)Fission. The Earth somehow spun up and flung off the Moon, leaving behind a depression that would become the Pacific Ocean.

The Formation of the Moon In the late 1960s, there were three main theories on how the Moon formed: 1)Fission. The Earth somehow spun up and flung off the Moon, leaving behind a depression that would become the Pacific Ocean. 2)The Moon was captured by the Earth’s gravity.

The Formation of the Moon In the late 1960s, there were three main theories on how the Moon formed: 1)Fission. The Earth somehow spun up and flung off the Moon, leaving behind a depression that would become the Pacific Ocean. 2)The Moon was captured by the Earth’s gravity. 3)The Moon was formed nearby the Earth at the same time.

The Formation of the Moon In the late 1960s, there were three main theories on how the Moon formed: 1)Fission. The Earth somehow spun up and flung off the Moon, leaving behind a depression that would become the Pacific Ocean. 2)The Moon was captured by the Earth’s gravity. 3)The Moon was formed nearby the Earth at the same time. All three ideas are probably not correct.

The Formation of the Moon The currently most accepted model is the ejection of material caused by a giant impact.

The Formation of the Moon The currently most accepted model is the ejection of material caused by a giant impact. A large body (perhaps bigger than Mars) collided with the young Earth and ejected a considerable amount of material from the Earth’s upper layers.

The Formation of the Moon The currently most accepted model is the ejection of material caused by a giant impact. A large body (perhaps bigger than Mars) collided with the young Earth and ejected a considerable amount of material from the Earth’s upper layers. This material condensed and formed the Moon.

The Formation of the Moon Computer simulations and chemical analysis of Moon rocks supports the collision/ejection theory.

The Formation of the Moon Computer simulations and chemical analysis of Moon rocks supports the collision/ejection theory. This collision could have caused the Earth’s rotation axis to become tilted.

Newton’s Laws and Tides If the tides are caused by the Moon pulling on the oceans, then why is there usually two high tides per day?

Newton’s Laws and Tides If the tides are caused by the Moon pulling on the oceans, then why is there usually two high tides per day? Actually tides are caused by differences in the gravitational forces.

Newton’s Laws and Tides Spring tides are when the Sun and Moon are roughly aligned (e.g. new and full moon). The tides tend to be higher at these times. Local conditions can also effect the height of the tides.

Next: Chapter 5: Other Planets and Moons.

Mercury Mercury is the closest planet to the Sun. It is never seen against a dark sky, and it is never far above the horizon.

Mercury as Seen From the Earth Here is the best ground-based image of Mercury.

Mercury as Seen From Earth Mercury is hard to study from the ground since it is close to the Sun.

Mercury as Seen From Earth Mercury is hard to study from the ground since it is close to the Sun. We can measure its average density. We find the density is 5.4 grams/cc, much like the Earth.

Mercury as Seen From Earth Mercury is hard to study from the ground since it is close to the Sun. We can measure its average density. We find the density is 5.4 grams/cc, much like the Earth. We can measure the albedo, and we find it is about 10%, much like the Moon.

Mercury as Seen From Earth Mercury is hard to study from the ground since it is close to the Sun. We can measure its average density. We find the density is 5.4 grams/cc, much like the Earth. We can measure the albedo, and we find it is about 10%, much like the Moon. Mercury mass is 5.5% of the Earth’s mass, and its gravity is 38% of the Earth’s.

Mercury as Seen From Earth Mercury is hard to study from the ground since it is close to the Sun. We can measure its average density. We find the density is 5.4 grams/cc, much like the Earth. We can measure the albedo, and we find it is about 10%, much like the Moon. Mercury mass is 5.5% of the Earth’s mass, and its gravity is 38% of the Earth’s. We expect Mercury to be similar to the Moon.

Mercury Seen up Close In 1974 NASA sent a probe to Mercury. It really does look like the Moon.

Mercury Seen up Close. Mercury is covered with craters.

Mercury Seen up Close In many cases you can see rays of material ejected by the impacts.

Mercury’s Interior Mercury has a large iron core. This core is relatively cold. There is very little evidence of present- day geological activity.

Mercury’s Interior Mercury has a large iron core. It is possible that a collision early in the history of Mercury could have stripped off less dense material near the surface, leaving behind the heavier material.

Mercury’s Rotation Since Mercury is so close to the Sun, tidal forces have forced it into a 3-to-2 spin-orbit coupling. As a result, a day on Mercury is 2 Mercury years long!

Mercury Mercury has a very thin atmosphere. There is no water. There is essentially no erosion. It is relatively hot on the day side (up to 800 o F) since it is near the Sun. However, on the night side it can be as low as -280 o F It looks a lot like the moon on the surface, but it is different in its interior.

Venus Venus has a mass and radius similar to that of the Earth.

Venus Venus has a mass and radius similar to that of the Earth. Its gravity is strong enough to retain a substantial atmosphere.

Venus Venus has a mass and radius similar to that of the Earth. Its gravity is strong enough to retain a substantial atmosphere. The albedo is very high, more than 75%.

Venus Venus has a mass and radius similar to that of the Earth. Its gravity is strong enough to retain a substantial atmosphere. The albedo is very high, more than 75%.

Venus Venus has a mass and radius similar to that of the Earth. Its gravity is strong enough to retain a substantial atmosphere. The albedo is very high, more than 75%. We do not see the surface, but rather the tops of the clouds.

Venus Venus is the second closest planet to the Sun. It is never seen against a very dark sky, and it is never far above the horizon.

Venus No surface features are seen from Earth.

Venus The cloud patterns are changing over several hours.

Venus The surface temperature is about 475 o C, compared to about 25 o C for Earth.

Venus The temperature at the surface of Venus is high.

Venus The surface temperature is about 475 o C, compared to about 25 o C for Earth. The atmospheric pressure at the surface of Venus is 90 times that of the Earth.

Venus The surface temperature is about 475 o C, compared to about 25 o C for Earth. The atmospheric pressure at the surface of Venus is 90 times that of the Earth. The composition of the atmosphere is about 96% CO 2, compared to mostly N and O on the Earth.

Venus The surface temperature is about 475 o C, compared to about 25 o C for Earth. The atmospheric pressure at the surface of Venus is 90 times that of the Earth. The composition of the atmosphere is about 96% CO 2, compared to mostly N and O on the Earth. ??????

The Greenhouse Effect Venus has a “runaway” greenhouse effect that heats the planet an extra 375 o C.

The Greenhouse Effect

Venus has a “runaway” greenhouse effect that heats the planet an extra 375 o C. Some visible light from the Sun reaches the surface and heats it.

The Greenhouse Effect Venus has a “runaway” greenhouse effect that heats the planet an extra 375 o C. Some visible light from the Sun reaches the surface and heats it. The surface radiates the energy in the infrared, which the CO 2 in the atmosphere absorbs.

The Greenhouse Effect Some visible light from the Sun reaches the surface and heats it. The surface radiates the energy in the infrared, which the CO 2 in the atmosphere absorbs. The extra heat “bakes out” more CO 2 from the rocks.

The Greenhouse Effect The surface radiates the energy in the infrared, which the CO 2 in the atmosphere absorbs. The extra heat “bakes out” more CO 2 from the rocks. The extra CO 2 leads to more trapping of the surface infrared radiation.

The Greenhouse Effect The extra heat “bakes out” more CO 2 from the rocks. The extra CO 2 leads to more trapping of the surface infrared radiation. The extra trapped heat bakes out more CO 2, and so on…

The Surface of Venus Soviet spacecraft have landed on Venus and recorded close-up pictures. These images show basalt, which is quite similar to lava rock.

The Surface of Venus The Venusian surface has been mapped with radar by the Magellan spacecraft. These maps reveal gently rolling hills, two “continents”, and many volcanoes.

The Surface of Venus The Venusian surface has been mapped with radar by the Magellan spacecraft. There are relatively few impact craters. Perhaps melting of the surface has erased earlier craters.

Venus Summary Although Venus has a similar mass and radius as the Earth, it is a very different place owing to the runaway greenhouse effect: –The temperature at the surface is about 475 o C. –The atmospheric pressure is about 90 times that on the Earth. –The atmosphere is mostly CO 2.

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