GALILEO SSI Education





PLANETARY CURRICULUM MODULE:

PLANETS AND SATELLITES


Cynthia Phillips

University of Arizona / NASA Spacegrant





Written for teachers
Target level: middle school


To print whole module, click here and then print

GOALS: USE FACTS ABOUT THE SOLAR SYSTEM TO DEMONSTRATE


This is the first in a planned series of educational modules relating to the Galileo satellite science goals. The target grade level is 6-9, but activities can be adapted either up or down for use with different levels. This module consists of an introductary section which exposes teachers and students to basic concepts in solar system exploration, a table containing various solar system parameters, and a series of four activities which use parameters from the table to demonstrate concepts such as graphing, ratios, and analyzing and drawing conclusions from data. The activities are mostly independent, but later ones do refer to results from earlier exercises, so it is recommended that they be done in order.

Contents:

PART I: INTRODUCTION

Our solar system is composed of 9 planets which orbit the sun. All the planets have different sizes, densities, and compositions, and some have satellites orbiting them like our Moon orbits the Earth. We can compare properties such as these for the different planets, and deduce how the solar system formed, look for patterns in its structure, and contrast conditions on other planets to those on Earth.

Information we want to know about planets:

Measuring these values:



Discussion:

It is relatively easy to measure the physical properties of the Earth, since, after all, we live here. It's much harder to measure them for the other planets, though, since the only other world people have traveled to so far is the Moon. We have sent computerized spacecraft to all the planets except Pluto, however, and while computers can't do everything that humans can do, they don't mind being cooped up in a tiny spaceship for the months or years it takes to get to another planet. They don't need to eat, either! Robot spacecraft can also survive in the hostile conditions on other planets -- we take for granted the fact that the Earth has air we can breathe, food we can eat, and temperatures that don't let us burn up or freeze to death. Conditions on other planets aren't nearly as nice. Computerized spacecraft can land on other planets and not have to worry as much about the temperature or the fact that there's no air to breathe. Probably, people will travel to other planets someday, but until then, we will continue sending robot probes to gather information and help future astronauts know what to expect.

Although our resources for studying the other planets are limited compared to those available for the Earth, we have refined our measurements over the years and currently have fairly good data for most of the solar system. By comparing information about the other planets to the Earth, which we know the most about, we can try to discover what the other planets are made of (so far, we only have pieces of the Moon and Mars to measure in labs on Earth), how they formed, and what their history has been like compared to that of the Earth. We can also look at pictures of the surfaces of other planets, and try to determine what the geology and weather are like -- so far we've seen exciting things like volcanoes on Jupiter's moon Io and the Great Red Spot, a giant storm in Jupiter's atmosphere. The science of looking at other planets and using this information to learn more about them, and about Earth, is called comparative planetology, and it helps us find out how the Earth formed, what it was like long ago, and what it might be like in the future.



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ACTIVITY 1: DISTANCE VS. SIZE

Concepts:




Activities: Using the attached table of values, make three graphs:

Sample plot:



Interpretation:


Scientific context: Scientists use graphs like the ones you just made to try to determine how the solar system formed. The terrestrial planets are all small and rocky, and the outer planets (except for Pluto) are large and gaseous. Why? Scientists think that the planets all formed out of leftover material that was in a huge disk of gas and dust around the sun in the early days of the solar system. As material got farther from the sun (see exercise 3), the temperature got cooler, and some of the components began to condense and rain out. Heavy materials like iron and other metals were the first to condense out, and scientists think that this is why the terrestrial planets have high densities and are made up mostly of rock and metal. It wasn't until out near the orbit of Jupiter that the temperature got cool enough to allow volatiles like water and other ices to condense out of the disk, and this could be why the outer planets are made up mostly of gases and ices. This theory is still being developed, and scientists still aren't sure of all the details. It seems, however, that formation of planets is a natural by-product of star formation. We can see billions of stars in the sky, so it seems likely that at least some must have planets around them. Since planets are so much smaller (and therefore dimmer) than stars, they're hard to detect, but recently we've had the first confirmed discoveries of planets around other stars than the sun. As our telescopes and detectors get better and better, we seem sure to detect even more. So maybe someday we'll be able to see how our theories of planetary formation apply to other solar systems!



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Solar System Parameters


MercuryVenusEarthMarsJupiterSaturn UranusNeptunePluto
Mass (1024 kg)
0.33024.8695.9750.64191,898.6568.4686.83 102.430.0125
Eq. Radius (km)
243960526378339371,49260,26825,559 24,7661137
Mean Density (kg/m3)
5,4275,2045,5203,9331,326687 1,3181,6382,050
Orbital Distance (106km)
57.9108.2149.6227.9778.31427.02869.6 4496.64913.5
Orbital Period (days)
87.969224.7365.25686.984330.610,747 30,58859,80090,591
Rotational Period (hours)
1407.65832.5 (ret.)23.93424.629.9210.517.24 (ret.)16.11153.3 (ret.)
Ave. Surf. Temp. (K)
44073728821012997585850
Surface Pressure
10-15 bars92 bars1.014 bars0.008 bars>>100 bars>>100 bars>>100 bars>>100 bars3 microbars
Atm. Comp.
98% He 2% H296.5% CO2, 3.5% N278% N2, 21% O2, 1% H2O95.32% CO2, 2.7% N289% H2, 11% He89% H2, 11% He89% H2, 11% He89% H2, 11% He meth, N2

Satellite Parameters (Jupiter, Earth, Pluto)


Jupiter: IoJupiter: EuropaJupiter: GanymedeJupiter: CallistoEarth: MoonPluto: Charon
Mass (1020 kg)
893.3479.71482107673517
Radius (km)
1,821.31565263424031738586
Mean Density (kg/m3)
3,5302,9901,9401,8513,3401,800
Orbital Distance (103 km)
421.6670.910701883384.419.4
Orbital Period (days)
1.773.557.1516.6927.36.39

Other Useful Parameters

MaterialDensity
Air1.2 kg/m3
Water or Ice1000 kg/m3
Typical Rocks3000 kg/m3
Metal at High Pressure10,000 kg/m3

ACTIVITY 2: ORBITS

Concepts:




Activities: Using the attached table of values, make two graphs:

Interpretation:



Scientific context: No one really knows why Venus rotates so slowly. One suggestion is that early in its formation, it was hit by a huge body coming from a direction opposite its rotation, and that this slowed down its rotation immensely, as well as making it spin backwards.

Try this out with a beach ball and your hand: spin a beach ball (or a globe with a fixed base), and see if you can push it with your hand in the opposite direction from its spin. The spin slows down, and if you push hard enough it'll start spinning in the other direction.

Uranus is another anomaly: it not only spins backwards, but it also spins on its side, like it was hit from the side by a giant impact. The study of giant impacts and their importance in the early days of the formation of the solar system is a current field, and many scientists use clues like rotation rates and direction of spin axes to make guesses about what happened long ago.



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ACTIVITY 3: TEMPERATURES AND ATMOSPHERES

Concepts:



Activity:

Interpretation:

Scientific context: The general trend of decreasing temperature with increasing distance from the sun works pretty well, but what about Venus? Why do you think its temperature is so much higher than Mercury's, even though Mercury is so much closer to the sun? It's because Venus has a thick atmosphere that traps the heat, while Mercury has barely any atmosphere at all.

Compare the surface pressure on Mercury, Venus, the Earth, and Mars. The surface pressure is also called the atmospheric pressure -- it's how hard the atmosphere pushes down on us. We don't notice on Earth that there's a thick atmosphere above us pushing down because we're used to it, but if we went to Venus, where the pressure is almost 100 times as high as on Earth, we'd notice that! (we wouldn't be able to breathe -> no oxygen, but also the pressure is too high) One of the effects that Venus' thick atmosphere has is that it traps the heat inside, and doesn't let much of it escape.



Activity:

Interpretation:

Scientific context: One reason that the surface temperature on Venus is so high is atmospheric thickness, but atmospheric composition is also important. Venus' atmosphere contains lots of carbon dioxide, which helps keep heat from sunlight from radiating back into space. Some people have called Venus an example of a runaway greenhouse effect: a greenhouse has glass walls that let light and heat in, but don't let much of it out. Carbon dioxide acts in much the same way, making a barrier around Venus that lets heat in, but doesn't let much out. So a combination of the fact that Venus has a thick atmosphere, and the fact that most of that atmosphere is made up of carbon dioxide, produces the high surface temperatures. While the main component of the Martian atmosphere is also carbon dioxide, the atmosphere is just so thin that even the carbon dioxide can't keep enough heat around to warm up the planet.

Extra: relate the discussion above of carbon dioxide in Venus' atmosphere to current environmental concerns about "greenhouse gases" in Earth's atmosphere, and the apparent trend of global warming.

Having an atmosphere not only helps keep heat, it also smoothes out the temperature differences between day, when the sun is shining directly down and providing heat, and night, when there's no solar heating at all. On the Earth on a typical day, the temperature variation between day and night is at most 40 or 50 degrees F, since the atmosphere helps save some of the daytime heat to keep us warmer at night. On Mars, which has a very thin atmosphere, the temperatures are not only very cold on average, because there's not much atmosphere to trap heat, but they are also highly variable: the temperature can range from -220 degrees F on a cold winter night to a maximum of 70 degrees F at the equator on a hot summer day. Imagine trying to dress for temperature differences like those!

Bonus activities:



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ACTIVITY 4: THE JOVIAN SYSTEM

Concepts:




Activity: The planet Jupiter and its satellites are often called a "mini solar system". Jupiter has over 15 moons, but we'll just consider its four largest moons at this point.

Interpretation:


Scientific context: Remember the discussion on the formation of the solar system from a cooling disk of gas and dust. Scientists call the Jovian system a "mini solar system" because they think that these four large satellites could have formed in a similar way, through the cooling of a disk of leftover material surrounding Jupiter right after it formed. The same decrease in density with increased distance from the center is there: Io, like Mercury, is dense and consists of lots of rock, while Ganymede and Callisto are farther out and less dense, and are probably made of mostly ice.



Activity:

Interpretation I: Solar system analogues

Interpretation II: Satellites and Planets


Scientific context: How do planets get moons? It depends on the planet. As discussed above, scientists think that Jupiter's four largest satellites formed in place around it out of leftover material, like the planets formed around the sun. Not all moons were formed this way, however. Mars has two tiny satellites that look a lot like members of the asteroid belt (an area full of small bodies (house-sized to city-sized) that orbits the sun between Mars and Jupiter). Since Mars is located right next to the asteroid belt, it seems likely that it captured these two satellites from there when their orbits got too close to Mars. Jupiter, on the other side of the asteroid belt, has two groups of moons that orbit outside the four major ones. Once group orbits prograde (forwards), the other retrograde (backwards). Scientists think these bodies might have been captured, like Mars' satellites. They might even have been captured as two larger bodies, each of which then broke apart into a number of pieces.

The Earth's Moon is harder to explain. It's so large in comparison to the Earth (as you showed above) that it's very unlikely that the Earth could have captured it unchanged, or formed it in place from leftover material. We also know from rocks brought back from the Moon by the Apollo astronauts that the Moon's composition is quite similar to Earth's in most respects, but that it has much less metal than the Earth does. This makes the capture theory even more unlikely, since a body that formed somewhere else in the solar system is unlikely to have a similar composition at all. The currently favored theory is that a giant impact soon after the Earth formed splattered a large amount of molten and rocky material into orbit. Most of the material would have fallen back to the Earth, but if the size and direction of the impact were within a certain range, enough material could have remained in orbit to clump together and form the Moon. Since metals are heavier than plain rocks, the metal would be more likely to fall back to Earth, while the less dense rock could stay in orbit long enough to form the Moon. So this could explain why the Moon's composition is so similar to the Earth's in most respects, but depleted in metals. Scientists are still working on this theory, and all the details have yet to be fully understood.

Pluto, and its moon Charon, are another question. As described above, the planets can roughly be divided into the four inner terrestrial planets, which are small and rocky, and the four outer gas giants, which are huge and gaseous. Pluto doesn't fit either category: it's far from the sun, near the gas giants, but it seems to be a small body made of rock and ice, rather than a huge ball of gas. Some scientists have thought that Pluto might originally have been a satellite of Uranus or Neptune, which escaped somehow (maybe due to a collision) and began orbiting the sun on its own. More recently, scientists have begun discovering icy bodies out beyond the orbit of Pluto. These bodies make up something called the Kuiper Belt. Not much at all is known about them, since they're so small, dark, and far from the sun, but some scientists think that these objects might be leftover remnants of rock and ice from the formation of the solar system, which were too far out to become part of a planet. Pluto might be one of the largest and closest of this class of objects (it's been called the "King of the Kuiper Belt"), and would therefore be a very interesting sample of what the material which formed the solar system was like over 4 billion years ago. We've never sent a space probe to Pluto, so we know very little about it. There have been some recent proposals for such a mission, however, and if one of these is selected for funding, we may soon know much more about this tiny, cold world so far from the sun.



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This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.

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