ASTR 330: The Solar System Lecture 4: The Planets: Overview Dr Conor Nixon Fall 2006

ASTR 330: The Solar System

Lecture 4:

The Planets: Overview

Dr Conor Nixon Fall 2006


The Solar System To Scale



Density


• What do we mean by the density of an object?

Density is defined as the amount of mass in a given volume:

Volume

MassDensity =

If mass is in grams (g) and volume is in cubic centimeters (cm3) then density is in grams per cubic centimeter: g/cm3.

This is a very convenient unit, because the density of water in these units is 1.0 g/cm3.

We can then easily compare the densities of different materials to the density of water.


Measuring Densities


• To measure the density of a planet, we need to know:

1. the size (diameter), and 2. the mass.

• To determine the actual size, we need to know:

1(a): the distance (from parallax), and1(b): the angular size of the planet.

• To determine the mass (from Kepler’s third law) we need to know:

2(a): the distance (from parallax),2(b): the angular size of the planet’s orbit,2(c): the orbital period.


Planet Diameter Determination

Dr Conor Nixon Fall 2006Figure credit: Nick Strobel, www.astronomynotes.com


Planet Mass Determination


• In practice, this method is inaccurate for planetary orbits around the Sun, because MS>>mp. Use satellite orbits around planet instead.


Example


• Calculate the diameter of Mars at closest approach to the Earth using the formula D= 2dA/360, given that the distance d (from parallax) = 78x109 m and A=0.005 degrees. You do it!

• Answer: d=6.807x106 m.

• Determine the mass of Mars using the formula: m+M=(5.916x1011) a3/T2

applied to the orbit of Phobos, with a=9.377x106 m, T=27,576 s. Assume that the mass of Phobos m can be ignored.

• Answer: M=6.414x1023 kg.

• Now calculate the volume (V=4r3/3) and density =M/V of Mars.

• Answer: =3885 kg/m3 (a tiny bit too low)


Densities of the Planets


• Compare to:

Cork: 0.2Wood: 0.5Water: 1.0Basalt: 3.3Steel: 7.8Lead: 11.0Gold: 19.0

• Which planet would float in water, if we could find a bath tub big enough to put it in?

• Are the planets likely to be uniform density throughout?• If not, why not?

Figure: Adler Planetarium


Categories of Planets


• From consideration of size and density alone, we can divide the nine planets into three main categories:

TERRESTRIAL PLANETS: characterized by small size, high density, and also found in the inner solar system: Mercury, Venus, Earth and Moon, and Mars.

GIANT PLANETS: characterized by large size, low density, and found in the outer solar system: Jupiter, Saturn, Uranus, Neptune.

DWARF PLANETS: Pluto, its moon, Charon, and Ceres, the largest of the asteroids have been recently named ‘dwarf planets’ as they have enough mass to become round, but do not dominate their orbital regions of space.


Terrestrial Planets



Compression vs Composition: The Inner Planets


• From their densities, the inner planets are likely to be composed of rock and some metal in the cores.

• We might expect that planets less massive than the Earth would have lesser densities, because they are less compressed at the center by gravity, right?

• Amongst the terrestrial planets, this is true for both Mars and the Moon, which are both smaller and less dense than the Earth.

• Venus is roughly the same size and density, as the Earth. So far, so good…

• BUT, Mercury is both less massive, and more dense than the Earth! WHY?

We must conclude that Mercury has a different composition than the Earth.


Giant Planets



Compression vs Composition: The Outer Planets


• What about the densities of the outer planets?

• We would expect the outer planets, which are more massive, to be much more compressed than the inner planets, and so more dense, correct?

• In fact, these heavier bodies are less dense than the inner, terrestrial planets.

• The only composition which we can use to construct such massive bodies with such low densities is a mixture of hydrogen and helium, the two lightest elements.

• The composition of the outer planets is hence more similar to the Sun and stars than to the inner planets!


Densities and Compositions of Other Solar System Bodies


• Among the moons of Jupiter we find a similar trend of decreasing density with distance from the primary body:

Satellite Distance from Jupiter Density• Io 0.42 million km 3.3 g/cm3

• Europa 0.76 million km 3.0 g/cm3

• Ganymede 1.07 million km 1.9 g/cm3

• Callisto 1.88 million km 1.8 g/cm3

Why could this be?

• Less is known about asteroids and comets:

Asteroids: thought to be mainly rock composition.

Comets: mainly ice composition.


Chemistry


• Hydrogen and oxygen are the most important chemical elements: they are both highly abundant, and chemically reactive.

• Each can form compounds with many other elements, including each other! (What is the most common hydrogen-oxygen combination?)

E.g.: hydrogen with carbon can form CH4 (methane)oxygen with carbon can form CO2 (carbon dioxide).

• If hydrogen atoms predominate, as in the giant planets, then most elements form compounds with hydrogen. This is known as a reducing environment.

• If oxygen atoms predominate, many oxygen compounds form. This is called an oxidizing environment. This is the case in the inner solar system.

• Why is hydrogen less abundant in the inner solar system?


Types Of Matter


• We can categorize the forms of matter in various ways:

• To the ancient Greeks there were 4 ‘elements’: Fire, Air, Water, Earth.

• We now know that there are in fact ~92 naturally occurring elements.

• Elements and compounds may be found in one of 4 physical states, depending on temperature and pressure: solid, liquid, gas and plasma.

• However, it will be useful for our investigation of the planets to define four types of common physical-chemical combinations:

gas, ice (solid), rock, metal.

• In what physical state will rock and metal be most commonly found?


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• New materials on website: Lecture 5 - Planetary Astronomy


Quick Quiz


1. Define density. What is the density of water?

2. What are the (i) least dense and (ii) most dense planets?

3. What differences are there between the terrestrial and giant planets?

4. Why does Pluto not fit in either of the previous categories?


Gas


• Gas is what makes up the atmosphere of a planet.

• For the Earth, we have a unique atmosphere of nitrogen (N2 78%) and oxygen (O2 20%). Oxygen gas would not exist without plants!

• For Mars and Venus, the main atmospheric gas is carbon dioxide, CO2. Without life on Earth, our atmosphere would have a lot more CO2.

• For Jupiter and Saturn, the largest gas giants, the atmosphere is mostly hydrogen gas (H2) and helium gas (He). In this sense, they are like the Sun. The outer parts of the Sun most resemble the original solar nebula in composition, out of which the solar system formed.

• Uranus and Neptune also have large amounts of hydrogen and helium gas, but their densities are greater, indicating more heavy elements as well.


Ice


• Volatiles are molecules that are liquid or gaseous at moderate temperatures, but form solid crystals, called ices, at low temperatures.

• They may melt or sublime (vaporize from solid), and later re-freeze at moderate temperatures, hence phase changes will be common.

• The main volatile species on the Earth is … what?

• On Mars, CO2 is the main volatile, but recently water ice has been found.

• Other volatiles include carbon monoxide (CO), ammonia (NH3), and methane (CH4).

• Volatiles are the main component of comets and most planetary satellites.


Polar Caps



Rock


• At higher temperatures, the volatiles are completely evaporated and the remaining matter is typically rock.

• The Moon is almost entirely rock, as is most of the Earth.

• The most common rocks are silicates: oxides of silicon, aluminum and magnesium.

Image: Apollo 17 Astronaut Harrison Schmidt; NASA JSC Archives


Metal


• At still higher temperatures, rock itself is transformed.

• The metallic elements in the rock may separate out: typically iron, magnesium and nickel.

• The core of the Earth is metallic: as is 3/4 of Mercury!

• Some asteroids are nearly pure nickel and iron.

Image: Gibeon Meteorite, FMM Museum, Russian Academy of Sciences


Chemical Trends


• Let us re-cap some of our conclusions so far.

• Trends from the inner to the outer solar system:

Quantity Inner Planets Outer Planets

Proportions of H and He lower (escaped) higher (retained)

compared to Sun

Proportion of oxygen higher lowercompared to Sun

Atmospheric chemistry oxidising reducing

Atmospheres Heavy volatiles Light volatiles; H2; He

Cores Rock; metal Rock; ice

Composition compared more evolved less evolvedto original nebula (more changed) (less changed)


Rocks and Minerals: Definitions


• A rock is an assembly of compounds or elements, called minerals.

A mineral is a single substance (homogenous).

A rock is a mixture of different mineral pieces (inhomogeneous).

A elemental mineral contains only one element, e.g. Gold (Au), graphite or diamond (C) , or sulphur (S).

A compound mineral is comprised of multiple elements bonded together: e.g. quartz (SiO2), hematite (Fe2O3).

• Pure elemental minerals are rare in nature hence most minerals are compounds.


Types Of Rocks


• Rocks may be divided into three main categories depending on origin:

1. Igneous: rocks that have formed directly by cooling from a molten state, e.g. basalts. Igneous rocks make up 2/3 of the Earth’s crust.

2. Sedimentary: composed of rocks or shells that have been ground down and then re-deposited again in layers, e.g. limestone (from shells) and sandstone (from silicates).

3. Metamorphic: originally igneous or sedimentary rocks which were buried far beneath the Earth, processed by high pressures and temperatures, and then re-exposed again at the surface, e.g. marble can be made from limestone which is ‘cooked’ at high temperature.


Rock Formation Summary


• This diagram shows the three rock formation processes in action:

Figure: msnucleus.org


Rock Formation Pictures


IGNEOUS ROCK FORMATION: lava flowing on Mauna Loa, Hawaii (picture: R.W. Decker, U.S.G.S.)

SEDIMENTARY ROCK FORMATION: The Grand Canyon, Arizona.(picture: Matthew Nyman, TERC)

METAMORPHIC ROCK FORMATION: These Arizona rocks show sedimentary rocks squeezed into new formations (picture: R.W. Decker, U.S.G.S.)

Web source: earthsci.terc.edu


Primitive Rocks


• The three types of rocks we have just discussed have all been created at some point in the Earth’s history, from either molten rock (magma, lava) or other rocks.

• But, is there such a thing as primitive rock, which formed at the beginning of the solar system and has remained unaltered?

• Yes, in fact there is. But not on Earth! As the primitive rocks which were to become the Earth gravitated together to form the planet, a lot of heat was released, partly from impacts and partly from radioactivity.

• The Earth became hot enough for the center to become molten: the rocks melted and became a liquid. Denser materials then gravitated to the center, a process called differentiation.

•Where might we look to find primitive rocks?


Atmospheric Differences


• When we consider the solar system, we see a wide variation in the planets and moons in terms of atmospheres.

The Earth is much smaller and denser then Jupiter, yet both have an atmosphere, although they are very different.

Photos: The Nine Planets, LPL Arizona


Atmospheric Differences


Titan and Ganymede are similar sizes, yet only Titan has an atmosphere. How can we explain this difference?

Photos: The Nine Planets, LPL Arizona


Acquiring An Atmosphere


• We can imagine two main ways in which a planet may acquire an atmosphere and oceans:

1. Direct capture (primordial): the volatile gases were captured by the planet’s gravity from the original gas nebula.

2. Outgassed (secondary): the gases were released from solids and liquids (e.g. rocks and ices) in the planet after formation was complete.

3. Impacts (secondary): the volatiles were brought by cometary impacts.

• In the case of the outer planets, the most likely explanation is a combination of (1) and (2). For the inner planets, which were most important?


Losing An Atmosphere


• Once a planet has acquired an atmosphere, it must hold on to it, by gravity. But there are forces at work which will tend to remove gases:

1. Impacts: from comets and meteorites may impart enough energy to remove gas from smaller bodies.

2. Thermal escape: if the random motions in the warm gas are great enough to equal the escape velocity, then the molecules of the gas can leave the planet!

3. Charged particles: e.g. the solar wind may scour the outer atmosphere and strip particles from the planet.

• Thermal escape happens in a layer called the exosphere, where the atmosphere is so thin that a molecule moving upward will not encounter another molecule before escaping.


Differential Escape


• For a given temperature (energy), lighter gas molecules will have higher velocities than heavier ones (imagine an eight-ball ricocheting off a bowling ball).

• Also, the escape velocity depends on gravity, i.e. the mass of the object.

• Therefore, smaller and warmer bodies will tend to lose more gas species than larger, cooler ones.

• For example, in the outer planets, we see Jupiter and Saturn holding on to almost all their original hydrogen and helium, which have mass numbers of 2 and 4.

• Whereas, the Earth holds on to heavier gases such as nitrogen and oxygen, with mass numbers of 28 and 32.


Summary of Atmospheric Diversity


• Let us summarize the main reasons for the differences between the atmospheres of the various planets and moons:

1. Initial capture from solar nebula.:- Large outer planets were able to capture and hold all gases from the initial solar nebula, and hence exhibit compositions like the Sun.- Small inner planets were not able to hold on to the lighter gases.

2. Later out-gassing:- Both outer and inner planets out-gassed part of their atmospheres, but for the inner planets, out-gassing is the dominant mechanism.

3. Biological agents: only on the Earth has biology substantially changed the atmosphere, which would otherwise be more similar to the composition of Mars and Venus.


Rock Type Quiz


IGNEOUS

Photos: University of North Dakota, Grand Forks

SEDIMENTARY

METAMORPHIC

• Be a geologist! Can you identify which types these rocks belong to?


Quiz - Summary


1. Give three examples of volatile species in the solar system, and say where they could be found.

2. What is the difference between a rock and a mineral?

3. Name the three main categories of rocks found on Earth, and give an example of each.

4. What is a primitive rock, and where might one be found?

5. What processes lead to the formation of planetary atmospheres?

6. What processes lead to the loss of atmospheres?

Documents

ASTR 330: The Solar System Lecture 4: The Planets: Overview Dr Conor Nixon Fall 2006