Chap 8 notes/Lecture 14 outline

planetesimals

"Pieces of planets"

Caloris Basin

Caloris Basin is the largest impact crater on Mercury. Multiple rings inside from an intense surface shaking few craters within formation after the heavy bombardment period.

Chance events may have played a role in what?

Chance events may have played a large role in determining how individual planets turned out. No one knows how different out solar system might be if the process started over.

The process of planet formation also explains the origin of

the many asteroids and comets that populate our solar system (including those large enough to qualify as dwarf planets): They are "leftovers" from the era of planet formation. Asteroids are the rocky leftover planetesimals of the inner solar system, while comets are the icy leftover planetesimals of the outer solar system.

Inside the frost line, only metal and rock could condense into solid "seeds" which is why

the terrestrial planets ended up being made of metal and rock.

8.3 The age of the solar system.- How do we measure the age of a rock?

'Radiometric dating' is based on carefully measuring the proportions of radioactive isotopes and their decay products within rocks. The ratio of the isotopes changes with time in a steady and predictable way that we characterize an isotopes 'half-life', the time it takes for half the atoms in a collection to decay.

Materials in the Solar Nebula

-Hydrogen and Helium Gas, an example would be hydrogen or helium, they don't condense in the nebula, and their relative abundance by mass in the solar nebula is 98% -Hydrogen Compounds, an example would be water, methane and ammonia. They condense at temperatures higher then 150 K degrees. Their relative abundance by mass in the solar nebula is 1.4% -Rock, an example could be various minerals, they condense at temperatures of 500-1,300 K. Their relative abundance by mass in the Solar nebula is 0.4% -Metal, an example would be iron, nickle, or aluminium. They condense at temperatures of 1,000 to 1,600 K and their relative abundance by mass would be 0.2%

Lunar History: Maria

-Lunar maria (latin for "seas") are dark, round planes, formed by volcanic eruptions. They cover 17% of the lunar surface. -Maria are composed of basalt, similar to Earth's ocean floors, or to Earth's volcanic lava. They were created later than highlands and are much less cratered. Major volcanism on the Moon ended ~3.3 billion years ago.

Luna History

-Lunar samples brought by the Apollo astronauts were dated using radioactive method. They solidified 3.3-4.4 billion years ago, older than most of Earth's rocks. -83% of the Moon consists of silicate rocks located in lunar highlands. Highlands were formed early and are heavily cratered by early bombardment in the forming of the Solar system.

What have we learned?

-Mercury is a totally solid planet that shows signs of shrinking while cooling. -Mercury's large iron core may be due to an impact. -Venus' surface cannot be viewed from space because of a thick atmosphere made of CO2. -Venus has many volcanoes, but no plate tectonics. -Its retrograde rotation is a result of an impact. -Most craters on the Moon's surface appeared in the heavy bombardment period, maria are lava fields that erased earlier craters.

Mercury:Basic Facts

-Mercury is the closest planet to the Sun (0.38 AU). -It is smaller than Ganymede (Jupiter's moon) and Titan (Saturn's moon) Mass: 1/50th of Earth's mass. Size: 0.38 Earth's size. Gravity: 38% of Earth's gravity Temperature: day side +450 C (723 K= 840 F), night side 183 C(= 90 K=360 F Atmospheric pressure: 2 trillionth of the Earth's Length: of the day- 58.7 Earth's days, year- 87.9 Earth days.

Crater Counts

-No erosion on the Moon. -No internal activity during the past 3 billion years. -Therefore, the number of impact craters can be used to estimate the surface age. -Lunar maria or the number of potential projectiles can be used for this purpose. -A 1-km size crater should be formed every 200,000 years, a 10-km crater every few million years, and 1-2 100-km craters every billion years. -The result is similar to that from the radioactive method: 3.3-3.8 billion years.

Mercury's Evolution

-The surface of Mercury has land forms that indicate its crust may have contracted (reason cooling). -Puzzle: Large iron core, but a very weak magnetic field (~1% of the Earth's)

Venus' Retrograde Rotation

-Venus spins in the opposite direction to Earth (the Sun rises in the West from Venus' surface). Two main theories: 1.) A huge chunk of debris crashed into the planet a long time ago, and sent it spinning in the opposite direction. 2.) most of the boulders that formed Venus were originally spinning in an opposite direction to the rest. - Venus rotates very slowly: a Venusian day (243 Earth days) is longer than a Venusian year (225 Earth days). As a result, there is almost no magnetic field.

Mercury's Interior

-We learned about the Mercury's interior structure thanks to the data obtained by Mariner-10 in 1975. -The planet's core is comparable in size to the Moon. The core is likely composed of 60-70% iron by mass.

Four Criteria for the success of a solar system formation theory:

1.It must explain the patterns of motion. 2.It must explain why planets fall into two major categories: small, rock, terrestrial planets near the Sun and large, hydrogen-rich Jovian planets farther out. 3.It must explain the existence of huge numbers of asteroids and comets and and why these objects reside primarily in the regions we called the asteroid belt, the Kuiper belt, and the Oort cloud. 4.It must explain the general patterns while at the same time making allowances for exceptions to the general rules, such as the odd axis tilt of Uranus and the existence of Earth's large moon.

Giant impact

A collision between a forming planet and a very large planetesimals, such as thought to have formed our Moon.

8.1 The search for origins.- How did we arrive at a theory of solar system formation?

A successful theory much explain four major features of our solar system: patterns of motion, the existence of two types of planets (terrestrial and Jovian), the presence of asteroids and comets, and exceptions to the rules. Developed over a period of more than two centuries, the 'nebular theory' explains all four features and also can account for other planetary systems.

8.2 Explaining the major features of the solar system.-What caused the orderly patterns of motion?

As the solar nebula collapsed under gravity, natural processes caused it to heat up, spin faster, and flatten out as it shrank. The orderly motions we observe today all came from the orderly motion of this spinning disk.

8.2 Explaining the major features of the solar system.-Where did asteroids and comets come from?

Asteroids are the rocky leftover planetesimals of the inner solar system, and comets are the ice-rich leftover planetesimals of the outer solar system. These objects will occasionally collide with planets or moons,but the vast majority of impacts occurred during the 'heavy bombardment' in the solar system's first few hundred million years.

Mercury's interior-Why is the core large?

Collision with a smaller body. Impact blew off most of Mercury's mantle. Reformed planet had a huge iron core left behind.

Venus:The closest Neighbor

Earth and Venus were frequently called "twin sisters," because they seem to share many characteristics.

Spinning

Like an ice skater pulling in her arms as she spins, the solar nebula rotated faster and faster as it shrank in radius. This increase in rotation rate represents conservation of angular momentum in action. The rotation of the cloud may have been imperceptibly slow before its collapse began, but the cloud's shrinkage made fast rotation inevitable. The rapid rotation helped ensure that not all the material in the solar nebula collapsed into the center: the greater the angular momentum of a rotating cloud, the more spread out it will be.

Venus:Basic Parameters

Mass: 0.8 Earth's mass Gravity: 0.9 Earth's gravity Size: 0.95 Earth's size Density: Almost the same as Earth's density Temperature: 750 K (surface) First flyby: Mariner 2 in 1962 Landings: 1970 Russian Venera 7 spacecraft recorded by a panoramic view. NASA's Magellan spacecraft: mapping of 98% of the surface (radio location) in 1990's.

Moon:Basic Parameters

Mass: 1/80 Earth's mass Gravity: 1/6 Earth's gravity, no atmosphere Size: 1/4 Earth's size Density: 3/5 Earth's density The moon is the only planet we visited. 6 landings in 1969 and 1972 (NASA) Moon's far side photographed in 1959 by the Russian Luna 3 spacecraft.

Hydrogen compounds (1.4 % of the Solar Nebula)

Materials such as water, methane, and ammonia, can solidify into ices at low temperatures (below about 150 K under the low pressure of the solar nebula).

Metal (0.2% of the solar nebula)

Metals such as iron, nickle, and aluminium are also gaseous a very high temperatures, but condense into solid form at higher temperatures than rock-typically in the range of 1,000K to 1,600K.

8.2 Explaining the major features of the solar system. -How do we explain "exceptions to the rules"?

Most of the exceptions probably arose from collisions or close encounters with left over planetesimals. Our moon is most likely the result of a 'giant impact' between a mars sized planetesimals and the young earth.

8.3 The age of the solar system.- How do we know the age of the solar system?

Radiometric dating of the oldest meteorites tells us that accretion began in the solar nebula about 4.55 billion years ago, with the planets forming by about 4.5 billion years ago.

Rock (0.4% of the solar nebula)

Rocky material is a gaseous at very high temperatures, but condenses into solid bits of mineral at temperatures between about 500 K and 1,300 Km depending on the type of Rock. (A mineral is a type of rock with a particular chemical composition and structure.)

Major Features of Venus

Surface temperature higher than that on Mercury because of a thick cloud layer (96% CO2). Two "continents" (highlands) The highest peak is 12 km above the surface level a few hundred craters, ~1,600 volcanoes The surface made of mainly basalt, volcanic rock. Geology of Venus seems dominated by hot currents and spots beneath the crust.

frost line

The boundary in the solar nebula beyond which ices could condense; only metals and rocks could continue within the frost line.

Condensation

The general process in which solid or liquid particles condense out of the gas. Pressures in the solar nebula were generally too low to allow the condensation of liquid droplets. These particles start out microscopic in size, but they can grow larger with time.

8.2 Explaining the major features of the solar system.- Why are there two major types of planets?

The inner regions of the solar nebula were relatively hot, so only metal and rock could condense into tiny, solid grains; these grains accreted into larger 'planetesimals' that ultimately merged to make the terrestrial planets. Beyond the 'frost line' cooler temperatures also allowed more abundant 'hydrogen compounds' to condense into ice, building ice-rich planetsimals; some of these grew large enough for their gravity to draw hydrogen and helium gas, forming the jovian planets.

8.1 The search for origins.-Where did the solar system come from?

The nebular theory holds that the solar system formed from the gravitational collapse of an interstellar collapse known as the 'solar nebula.' This cloud was the product of recycling of gas through many generations of stars within our galaxy. This material consisted of 98% hydrogen and helium and 2% all other elements combined.

Why did the Nebular theory of the formation of the Solar system gain wide acceptance?

The nebular theory of the solar system formation gained wide acceptance because of its success in explaining the major characteristics of the solar system

Heavy bombardment

The period in the first few hundred million years after the solar system formed during which the tail end of planetary accretion created most of the craters found on ancient planetary surfaces.

Solar Nebula

The piece of interstellar cloud from which our own solar system formed.

accretion

The process by which small objects gather together to make objects larger.

Flattening

The solar nebula flattened into a disk. This flattening is a natural consequence of collisions between particles in a spinning cloud. A cloud may start with any size or shape, and different clumps of gas within the cloud may be moving in a random directions at random speeds. These clumps collide and merge as the cloud collapses, and each new clump has the average velocity of the clumps that formed it. The random motions of the original cloud therefore become more orderly as the cloud collapses, changing the cloud's original lumpy shape into a rotating, flattening disk. Similarly, collisions between clumps of material in highly elliptical orbits reduce their eccentricities, making the orbits more circular.

Heating

The temperature of the solar nebula increased as it collpased. Such heating represents energy conservation in action. As the cloud shrank, its gravitational potential energy was converted to the kinetic energy of the individual gas particles falling inward. These particles crashed into one another, converting the kinetic energy of their inward fall to the random motions of thermal energy. The Sun formed in the center, where temperatures and densities were highest.

half-life

The time it takes for half of the nuclei in a given quantity of a radioactive substance to decay.

Hydrogen and helium gas (98% of the solar nebula)

These gases never condense in interstellar space.

Venus seen from a Spacecraft

Venus is the third brightest object in the sky. Venus is bright due to: 1.) its closeness 2.) high reflectivity (reflects 76% of the solar light)

Solar wind

a stream of charged particles (such as protons and electrons) continually blown outward in all directions from the Sun.

The age of a rock is the time since its atoms

became locked together in their present arrangement, which in most cases means the time since rock that last solidified.

Spontaneous change sometimes called

decay.

The chaotic process that accompanied planet formation, including the many collisions that surely occurred are

expected to have led to at least a few exceptions.

The age of our solar system is

four and a half billion years, from radiometric dating of the oldest meteorites. This age agrees with ages estimated through a variety of other techniques, making it clear that we are recent arrivals on a very old planet.

In the 20th century, so much evidence had been accumulated in favor of the Nebular hypothesis that

it achieved the status of a scientific theory- becoming the Nebular theory on the formation of the solar system.

Some planetesimals probably grew to hundreds of kilometers in size in only a few

million years - a long time in human terms, but only about one-thousandth of the present age of the solar system.

Meteorites

rocks that have fallen to Earth from space.

The idea of Immanuel Kant and Pierre-Simon Laplace became known as

the Nebular Hypothesis. They proposed the solar system formed from the gravitational collapse of an interstellar cloud of gas.

The hypothesis that proposes that the planets represent debris from a near collision between the Sun and another star was called

the close encounter hypothesis.

Most of the general features of the Solar system

were determined by processes that occurred very early in the Solar system's history

We expect flattening to occur anywhere orbiting particles can collide,

which explains why we find so many cases of flat disks, including the disks of spiral galaxies like the Milky Way, the disks of planetary rings, and the accretion disks that surround neutron stars and black holes in the close binary star systems.