voyage to europa! - center for astrophysics

From the Ground Up!: Jupiter v. 031301 -1- Harvard-Smithsonian Center for Astrophysics

Voyage to Europa!

1. Introduction:

Is there life on one of Jupiter's moons?The SettingThe Challenge

Heads Up!

2. Activities:

Getting the ImagesImaging Jupiter and it's Moons

Scope it OutReflecting on your ImagesWhich moon is Europa?

Using the Images to Investigate Jupiter and EuropaPreparing for the Trip: How far is Jupiter?

Go FigureA Base Camp on Jupiter?What is the scale of my image?How large is Jupiter?How large is Europa?How much does Jupiter weigh?How dense is Jupiter?How strong is Jupiter's gravity?Mission assessment: Jupiter as a base camp?

Search for Life on EuropaWhat is Europa Like?Does Europa have an Atmosphere?What is Europa's Environment?Can We Communice to Earth from Europa?Artificial Gravity?

Try this/Tabletop Experiment: Mysterious motion experimentAn Ocean on Europa?

Try this/Tabletop Experiment: Cooool Heat!Did you know/fun fact: whales

Is there Life at the extremes?

3. Wrap-Up & Reference:

Data Page

Briefing RoomTimelineIdeas you'll need


VOYAGE TO EUROPA

1. INTRODUCTION:

Is there life on one of Jupiter'smoons?

Circling the giant planet, Jupiter, are several moons that may containliquid water. One of those moons Ñ named Europa Ñ may even be

covered by a vast ocean. In fact, according to recent evidence from a NASAspace probe to Jupiter, this salt-water ocean may be 60 miles deep and as warm as thewaters of Bermuda!

If true, this would make Europa the likeliest place in our solar system to search for extra-terrestrial life. The problem: Europa's ocean is hidden under a thick layer of ice.

THE CHALLENGE:

You and your firm, Investigation Inc., have been asked by NASA to report on theprospects for sending an expedition to search for life beneath Europa's frozen surface.NASA will provide the spacecraft to get your crew to Jupiter, and will develop anytechnologies you may need.

Your challenge is to:

• Obtain images of the planet Jupiter and its four brightest moons, and identify yourdestination, Europa.

• Evaluate the prospects for setting upa base camp on Jupiter. Use yourimages Ñ and your knowledge ofphysics Ñ to find out as much as youcan about Jupiter. If you tried to landon Jupiter, would you hit a solidsurface, or would you keep on fallingand never be heard from again?

If you could land on Jupiter, how muchwould you weigh? Would your bodybe able to support you? Would you beable to take off again?


• Evaluate the prospects for a base camp on Europa: Do you expect Europa to havean atmosphere? How much sunlight does Europa get compared to Earth? If you canonly explore the surface while the sun is up, how long can you explore before youmust get back to base camp? How cold do you expect it to be? How much will youweigh there, compared to Earth?

• Evaluate the prospects for life under Europa's icy surface: The ocean on Europacould contain bizarre life forms Ñ or it could contain no life at all. Make a case forthe likelihood of finding life on Europa. Consider these questions: Does life need anenergy source Ñ and if so, what kinds of energy will do? Does life need light toexist? Does life need oxygen?

In discussing these questions, you may wish to research and report on one or more ofthe following:

-What kind of life lives near deep-sea volcanic vents on Earth?

-What are the most extreme conditions on Earth, and what kinds of living thingsthrive under those conditions?

This mission guide will take you step by step through the activities you'll need for yourreport.

HEADS UP!

Next time you find yourself under the stars, look for the planet Jupiter.When it's above the horizon, Jupiter is easy to spot, even in the city,because it often appears brighter than even the brightest stars.

If the other planets or the Moon are also visible, you'll see that they all liealong a nearly straight path across the sky. If extended below the horizon,this line would also pass through the Sun. That's because the Sun, planetsand their moons lie in nearly the same plane. When seen from a pointwithin that plane, they appear to lie along a line.

Try this dizzying feat: Look up at the sky and picture the plane that theplanets lie in. You'll suddenly become aware that you're standing at anangle to that plane. When it comes to outer space, which direction is"up"?!


'SCOPE IT OUT

Selecting the Target: Use the pull-down menu to select Jupiter. (The telescope's computerwill automatically determine Jupiter's location in the sky for the time you selected. Jupiterdoes not have a permanent address Ñor RA and DEC Ñ in the sky, because it moves fromnight to night relative to the background stars. In fact, the word "planet" means "wanderer.")

Camera: Use the MAIN camera, ZOOMED IN. (If some of the moons are out of the field ofview, you can use ZOOMED OUT instead.)

Filter: Try using the grey filter ("ND-40") to cut down on Jupiter's glare.

Exposure time: Use a 10 second exposure if you are using the grey filter.

Downloading Your Image: You should be able to see Jupiter and its moons clearly in theGIF-format image on the Web, without any image processing. Be sure to download both theimage AND its Image Info file, because this contains the information about how and when youtook the image.

It's a good idea to also download the FITS file for each image as well for your records. (Clickand HOLD on the underlined link, then select "Save As...SOURCE" and download.

Printing the Image: The simplest way to compare your images is to print them. TIP: Use animage processing program to INVERT your image Ñ that is, to reverse black and white. Thenwhen you print, Jupiter and its moons will appear black against a white background. That'smuch easier to measure, and you'll be saving your printer's ink as well!

Making Measurements: You can make measurements directly from your computer monitor,or from printed images. For an image printed at 100% scale, 1 inch = 72 pixels.

2. ACTIVITIES

Getting the Images

IMAGING JUPITER AND ITS MOONS

Your first challenge is to use the telescope to get good images of your destination:Jupiter and its moons.

In this challenge, you'll investigate the motion of the moons by taking images about oncean hour for four or five hours. Amazingly, there will be enough information in theseimages for you to determine several properties of Jupiter that you'll need to know foryour mission Ñ such as Jupiter's size, density, and gravity. This information will helpyou in deciding whether to establish a base camp on Jupiter.


REFLECTING ON YOUR IMAGES:

Size of Jupiter. Why does Jupiter appear so small, compared to the Moon?

Point of view. Why do we see the moons arranged on a more or less straight line?

Forces and motion. What keeps the moons in orbit around Jupiter? Why don't they flyoff into space?

Speed of the moons. Which moons appear to have moved, from image to image? Whyhave some moons moved more than others?

Getting the big picture. Jupiter and its moons look like a miniature "solar system."How does the plane of the moons compare to the plane of the solar system? Why mightthat be?


WHICH MOON IN YOUR IMAGE IS EUROPA?

You'll probably see up to four of Jupiter's moons in your image. Can you figure outwhich moon is Europa?

The moon closest to Jupiter in nature is called Io, followed by Europa, Ganymede andCallisto. But the moon that looks closest to Jupiter in your image need not be Io. That'sbecause, from Earth, we're looking at the moons edge-on to the plane of their orbit.

For example, the image at right shows howthe moons might look if you were to lookDOWN on the plane of their orbit. (Theimage shows the relative size of the orbits, toscale.)

The edge-on view below it shows how thisscene would appear through the telescope.Can you label which moon is which?

When you look at your images, you won'thave the benefit of a top-down view tocompare them to. But can you think of away to figure out which moon is which byfollowing the moons through severalimages?

If you could follow the moons for longenough, you could see the furthest distancethat each one gets from Jupiter. That wouldtell which moon is which.

A secondway to tellthe moonsapart is tosee whichmoon moves FASTEST -- i.e., moves the furthest inseveral successive images taken, say, an hour apart.The moons closest to Jupiter orbit the fastest, aspredicted by Newton's laws of motion. But even hereyou'll have to be careful: Take a look at the figure onthe next page, where the x's mark equal time intervals.As seen from edge-on, the moon seems to speed up andslow down. At what part of the orbit will the moonappear to moving fastest? Where will it seem to bemoving slowest?

After the Image and Analysis Team has examined theimages and is confident they can identify the moons,label the images with the team's results.


Getting the Facts on Jupiter and Europa

PREPARING FOR THE TRIP: HOW FAR IS JUPITER?

How far is Jupiter anyway Ñ and how long will it take you to get there?

You can use your knowledge of physics, and a few basic observations, to estimate thedistance to Jupiter. Knowing this distance will also help you interpret your images.

GO FIGURE

Start with this observation: Jupiter takes about 12 years to orbit once around the sun andreturn to the same part of the sky (same background constellations of stars).

• How long does it take the Earth to orbit once around the sun?

• Based on the periods of their orbits around the sun, is Jupiter further or closerto the sun than Earth is? By how much? (See the Briefing Room: Ideas You'llNeed)

• The Earth is about 93 million miles from the Sun. How far is Jupiter from theSun?

Record your results on the handy DATA SHEET for the mission to Jupiter. You'll needthem later.

Using your results, what is the closest that Jupiter gets to Earth? What is the furthest? (Ithelps to draw a diagram with Earth and Jupiter on the SAME side of the Sun, and thenone with the planets on OPPOSITE sides of the Sun.)

Scale drawing. In the scale drawing below, how far from the sun should you drawJupiter? In the drawing, 6 inches represents 100 million miles.


REFLECTING ON YOUR RESULTS

About how long will it take you to get to Jupiter? Assume that your spacecraft canaverage about 100,000 miles per hour. (This is 3 times faster than current spacecraft.)

How would you choose your crew members for a trip this long? What qualities shouldthey have?

A BASE CAMP ON JUPITER?

What are the prospects for landing on Jupiter? Can you use it to set up a base camp fromwhich to explore Europa? How much would you weigh on Jupiter? Is there a solidsurface, or would you sink in? Would you be able to blast off again?

Before planning a landing, you'd better find out as much as you can about the planet.Amazingly, you'll be able to tell a lot just from the images you took Ñ and by applyingthe physics you've learned. Here are some measurements and estimates you'll need tomake. Each one builds on one or more of the previous measurements.

• What is the scale of my image?• How large is Jupiter?• How large is Europa?• How far is Europa from Jupiter?• How long does Europa take to orbit Jupiter?• How much does Jupiter weigh?• How dense is Jupiter?• How much would you weigh on Jupiter?


WHAT IS THE SCALE OF MY IMAGE?

How many miles does one pixel represent in your image? You'll need the followinginformation:

• The distance to Jupiter is ________ miles. (Use your result from activity above.)

• An object that is 57 times further than it is wide will appear 1 degree wide. (Seethe activity, "A Wrangle with Angles.")

• In your telescope image, one degree spans 720 pixels. So 1 pixel = 1 / 720degree.

So 1 pixel represents ___________ miles wide (for an object at Jupiter's distance).

HOW LARGE IS JUPITER?

Now that you have the scale of your image, you can determine how large Jupiter really is.

In your image, how many pixels wide is Jupiter? How many miles is this? How doesJupiter's size compare to Earth's?

Width of Jupiter = _____________________ miles

Width of Earth = 8000 miles

How many Earths would fit inside Jupiter? (How much larger is Jupiter's volume thanEarth's?)

Jupiter has about____________________ times the volume of Earth.

Record your results on the mission DATA PAGE.


SIZE MATTERS...

Could there be a planet tentimes larger than Jupiter?

Strangely enough, Jupiter maybe about the largest a planetcan get... and still be a planet!Although planets two or threetimes as massive Jupiter havebeen detected orbiting otherstars beyond our Sun,astronomers believe that anobject much larger than Jupiterwould collapse under its ownweight to form a star.

In a star, the temperature anddensity of matter is so greatthat the matter undergoesnuclear reactions, liberatingenormous amounts of energyand causing the star to shine.You can learn more about starsin the investigation, "To theStars!"

REFLECTING ON YOUR RESULTS

Does the size of Jupiter alone tell you anything abouthow strong the gravity will be at Jupiter's surface? Whatother information would you need?

STORM ON JUPITER

URL:

Knowing the size of Jupiter,

students use digital movies of

Jupiter to estimate the wind

speeds in a violent storm there.


FINDING JUPITER'S MASS(see Briefing Room)

The period (T) of a moon's orbitdepends on its distance (d) from theplanet and the mass (M) of the planet.:

T2 ~ d3 / M

You can find the planet's mass if youknow the distance and period of amoon.

HOW MUCH DOES JUPITER "WEIGH"?

You can actually "weigh" Jupiter Ñ that is, determineits mass ÑÊjust from observations of the motions of itsmoons. Remember that the mass of a planet influenceshow fast a moon will revolve around it. (See the Boxat right, and the Briefing Room in the appendix.)

To determine Jupiter's mass, you'll need to know theDISTANCE from Europa to Jupiter, and the TIME ittakes Europa to orbit once.

STEP 1: DETERMINE EUROPA'S DISTANCEFROM JUPITER.

You could determine the distance from Europa to Jupiter (that is, the radius of Europa'sorbit) by following Europa's motion and measuring the maximum separation betweenEuropa and Jupiter. Then, knowing the scale of your image in miles per pixel, you coulddetermine the radius of the orbit.

To save you time, thedrawings at left show themaximum distance for

the four inner moons of Jupiter Ñ at the samescale as the MicroObservatory zoomed-out andzoomed-in images. Can you use this guide todetermine the distance from Europa to Jupiter?(Hint: Use the scale of your image in miles perpixel from the activity above.)

STEP 2: DETERMINE HOW FAST EUROPA ORBITS JUPITER.

Our own Moon takes a little more than 27 days to orbit the Earth once (one "moonth" orabout a month!) How long would you guess it would take Europa to orbit Jupiter, giventhat they are roughly the same distance apart as our Earth and Moon?

Use your images to find out. To estimate Europa's period:

• Measure the distance that Europa moved in the first and last images that you took. (Aruler that measures millimeters is best for this.)


Europa moved _________ (pixels? millimeters?) over _________ hours

(Don't remember the time interval? Check the Image Info headers on your images tosee when they were taken!)

• Compare the distance you measure with the maximum width of Europa's orbit, asshown in the drawing above. What fraction of the orbit does Europa move in the timeinterval you've observed.

If Europa takes, say, two hours to move some fraction of the orbit, then how manyhours will it take to move all the way around Jupiter?

IMPORTANT: Discuss with your team howyou will make this estimate. It's tricky: Asyou can see from the drawing at right, Europawill appear to move more slowly near theturning points of its orbit (furthest fromJupiter) and will APPEAR to move fastest asit crosses the face of Jupiter. How will youcorrect for this?

Also, don't forget that the period is the time togo all the way around Europa and back to thestarting point.

The period of Europa's orbit is about_________________ hours.

How does the period of Europa's orbit compare with the time it takes our own Moon togo around the Earth?

If Europa is at roughly the same distance from Jupiter as the Moon is from Earth, thenwhy does Europa orbit so much faster than the Moon?


"WEIGHING" JUPITER

Use your results for the orbital period anddistance of Europa to determine the massof Jupiter. See the Briefing Room fordetails.

M ~ d3 / T2

M = (2¹)2 d3

G T2

M is the mass of Jupiterd is the distance from Europa to the center ofJupiterT is the period of Europa's orbitG is Newton's gravitational constant(G = 6.7 x 10-8 cm3 / g sec2 ) .

If d is in centimeters and T is in seconds,then M will be in grams.

PUTTING IT ALL TOGETHER:HOW MUCH DOES JUPITER WEIGH?

Use your measurements for the orbital period anddistance of Europa to determine the mass ofJupiter. (See box at right.)

The mass of Jupiter is about_________________________ (grams)

How many times more massive is Jupiter comparedto the Earth?

TIP: You can compare Jupiter's mass to the Earth'sjust by knowing the orbital period and distance ofone satellite for each planet.

MJupiter / MEarth = (T2 d3 )Europa / (T2 d3 )Earth's Moon

Use your results for Europa, and the results for the Moon from "The Incredible ShrinkingMoon" challenge, or from a published source.

HOW DENSE IS JUPITER?

Can you land on Jupiter, or would you sink right in? Is Jupiter solid rock? Is it liquid orgas? One line of evidence to use is the average density of Jupiter, which is the mass perunit volume.

Use your findings for the mass and volume of Jupiter to determine its average density.Then compare your result with densities for the following materials (under normalpressure):

Liquefied natural gas: 1 gram/ cubic centimeterWater: 1 g/ cc.Rock: 3 g/ cc.Iron: 5 g/ cc.

How does the density of Jupiter compare to Earth density (about 3 g/ cc.)? Could Jupiterbe solid rock?


Is it safe or unsafe to attempt to land on Jupiter?

HOW STRONG IS JUPITER'S GRAVITY?

How much stronger is Jupiter's gravity than Earth's (at the surface of each planet)? Toanswer this question, discuss with your team what factors influence the gravitational pullyou feel from a planet. Then use any of your previous results that you need.

How much would you weigh at the surface of Jupiter?

MISSION ASSESSMENT: JUPITER AS A BASE CAMP?

Given your results, how does your team rate Jupiter as a potential base camp for yourexploration of the planet's moon, Europa?


Searching for Life on Europa

What kind of environment will you find when you get to Europa? And what are theprospects for life? Try some or all of the activities on the following pages and includeyour findings in your mission report.

WHAT IS EUROPA LIKE?

A good place to start: Data and images from Europa sent by NASA's Galileo spacecraft.What can you learn from this evidence about what Europa is like?

Diameter of Europa: 1882 milesMass (Earth = 1): .0083Density of Europa: 3.01 (grams/ cubic centimeter)

For full-color images:http://www.jpl.nasa.gov/galileo/images/europa/eurimages.html

What do you think the bright spot anddot are in the lower right portion ofthis image of Europa?

How does this image compare toEarth's moon? What mightaccount for the lack of craters?

What might the dark, reddish-brown material be?What are possible sources of this material? What evidence would you need to supportyour hypotheses?


The surface of Europa is verybright. What might this indicate?

What might the cracks on thesurface indicate? Where are onEarth might you see cracks likethis?


DOES EUROPA HAVE AN ATMOSPHERE?

Your environment team has been asked to assess the chances that Europa has anatmosphere. What determines whether a moon or planet has an atmosphere?

Consider the following:

• Our planet Earth has an atmosphere, but our Moon doesnot. Why not?

• The planet Mercury does NOT have an atmosphere, butTitan Ñ a moon of Saturn that's the same size asMercury Ñ DOES have atmosphere. Why?

DISCUSSION: What factors do you think influence whether a planet or moon has anatmosphere?

If the Earth had been much smaller than it is, would it have kept its atmosphere? If theEarth had been much smaller, would you be here now?!

From your telescope images, can you say anything about the size of Europa? Do youexpect it to have an atmosphere?


WHAT IS EUROPA'S ENVIRONMENT?

What kind of environment can your mission team expect on Europa? Can you use lightfrom the sun as your power source?

Solar energy on Europa. How much light does Europa get, compared to Earth? Useyour findings from a previous activity about the relative distances of Earth and Jupiterfrom the Sun.

Temperature on Europa. Using your results for the amount of sunlight and the lack ofan atmosphere on Europa, how would you expect the temperature to compare to that onEarth?

Gravity on Europa. How does gravity on Europa compare to Earth? (Use data frompreceding pages.) How much would you weigh on Europa? If you can carry 70 poundsmaximum on Earth, how heavy a space suit could you wear on Europa?


COMMUNICATING FROM EUROPA

From Earth, we always see the same side of the Moon. That's because our Moon rotateson its axis at the exactly the same rate (once every 27.5 days) as it revolves around theEarth. This used to be thought a coincidence Ñ but now we know it always happenswhen a smaller moon orbits close enough to a larger planet.

Europa also rotates exactly as fast as it revolves around Jupiter Ñ so it always shows oneside to Jupiter.

Suppose you wanted to have a communications satellite in orbit above Europa thatalways remained above a particular spot on the surface (e.g., above your base campon Europa). How far above the surface of Europa should the orbit be? Use yourfinding for Europa's period of revolution (and therefore rotation). Apply Kepler's law tofigure out how high the satellite should be to have a period equal to Europa's rotationrate.

Is this satellite feasible? How does its orbit compare to the distance from Europa toJupiter? Will the satellite risk crashing into Jupiter at this distance?

If you leave base camp to explore Europa, what's the maximum daylight you'll havebefore you are plunged into darkness? (You'll need to draw a model showing thelocation of the Sun, Jupiter, and Europa, and take into account the motions of Europa.)


CAN YOU DESIGN ARTIFICIAL GRAVITY?

When humans are deprived of Earth's gravity for too long, their bodies start to feel the illeffects. For example, bones start to dissolve, and muscles lose their fitness.

Can you design vehicle that orbits Europa, and that spins on its axis just enough toprovide one Earth gravity? How large a vehicle should it be, and how fast should itspin?

Some of your colleagues want a box-shaped orbiting vehicle. Trying the followingexperiment, report on the dangers of a box-shaped vehicle and the importance of shape.

MYSTERIOUS MOTION EXPERIMENT:

Using an empty cereal box, try the followingexperiment. Toss the box in the air so that itspins around an axis through its largest side (1).Can you get it to spin WITHOUT wobbling?

Try it again, this time spinning around an axisthrough its smallest side (2). Is it again stable?Now try spinning it about the axis through itsmedium side (3). What happens?

The motion you observe is an example of"chaotic motion." Newton's laws can predict thatsuch motion will take place, but cannot describein detail the motion itself!


IS THERE AN OCEAN ON EUROPA?

Scientists have evidence that a vast ocean exists under the surface ice on Europa. But ifEuropa is so cold, then where does the heat come from to keep the underground oceanfrom freezing solid?

Simple friction may be at work: As Europa orbitsaround Jupiter, the enormous gravity from Jupiter (andthe other moons) alternately stretches and squeezesEuropa. The friction created by this process is enough toheat the interior and melt the ice on Europa, from theinside out. (The giant volcano on Jupiter's nearest moon,Io, is thought to be powered in the same way. Image atright.)

To see how this continual stretching might provide anenergy source to melt Europa's water, try thisexperiment:

EXPERIMENT: COOOOL HEAT!

Materials: A wire coat hanger.

SLOWLY bend the coat hanger back and forth at onepoint along its length. CAREFULLY and BRIEFLYtouch the area around the bend. What do you feel?

Why is the metal hot? Why does the hanger get cool again? Would a larger coat hangercool faster? Slower? The same?

DISCUSSION: Objects can generate heat through their entire volume, but they can loseheat only through their surface. What happens to the volume/ surface ratio as objects getlarger? What examples in nature can you think of where size plays a role in an object'stemperature?

Fun fact. When whales are about to give birth to their calves, they migrate to warmerwaters. Reason: The smaller babies would lose heattoo fast in the colder waters. As soon as the babiesgain weight, it's back to the frigid waters again!


REPORT ON: LIFE AT THE EXTREMES

An ocean on Europa could contain bizarre life forms Ñ or it could contain no life at all.Make a case for each of these possibilities, citing existing evidence, or evidence youwould need to get.

What are the major requirements for life? What energy source could be present under theice? Are the chemical elements for life present? Could life survive without light oroxygen? What kinds of creatures could live there?

In support of the view that there may be life, you may wish to research and report on thefollowing:

On Earth, is there life near deep-seavolcanic? E.g., see the Web site:

http://www.discovery.com/stories/science/seavents/archive/entry1.html

What are the most extreme conditions on Earth,and what kinds of living things thrive underthose conditions?

For a skeptic's view, visit the following site and discuss with your team the reports foundthere: http://www.spaceviews.com/1999/08/05a.html

In your view, would it be worth exploring Europa if only microscopic life Ñ but nolarger life forms Ñ existed? Why?


VOYAGE TO EUROPA

DATA PAGE

Distance to Jupiter:

Jupiter is ______________ times further from the Sun than is Earth.

Jupiter is ______________ miles from the Sun.

Jupiter's closest approach to Earth is ____________ miles.

Size of Jupiter:

Jupiter is __________________ miles wide, compared to 8000 miles wide for Earth.

Jupiter's volume is________________ times the volume of Earth.

Jupiter's volume is _______________ cubic centimeters

Mass of Jupiter:

The mass of Jupiter is ________________ grams

Density of Jupiter:

The density of Jupiter is _____________ grams per cubic centimeter

Gravity on Jupiter:

A 100 pound person would weigh______________ pounds at the surface of Jupiter.

Size of Europa:

Europa is less than ________________ miles wide

Radius of Europa's orbit:

The distance from Europa to Jupiter is about ______________ miles

Period of Europa's orbit:

It takes Europa ____________ days (hours) to orbit once around Jupiter.


BRIEFING ROOM

TIMELINE

Ancient Roman times: The planet Jupiter is named after one of the Roman gods.

1610. Using one of the first telescopes, GalileoGalilei discovers four moons orbiting the planet

Jupiter. This shows that Earth is not theonly "center of attraction" in the heavens,

and makes it easier to acceptCopernicus' conclusion that the Earth

orbits the Sun.

Some townspeople refuse to lookthrough Galileo's telescope,calling it a distortion of reality.

1979. Three scientists from the Jet Propulsion Laboratoryconclude that Jupiter's strong gravity may flex and heat itsnearby moons.

1979. Three days later, the Voyager spacecraft sends the firstimages of volcanoes on Io, the closest moon to Jupiter.

1996. NASA's Galileo spacecraft, is launched to explore theplanet Jupiter and its moons.

2000. The Galileo spacecraft arrives at Jupiter sends back images anddata about the planet Jupiter and several of its moons.

2000. Margaret Kivelson and her team from the University of California concludethat an ocean exists under the surface of Europa. Evidence: Telltale magnetic fieldsfrom the sloshing of salt-water inside Europa.

The King of Thursday.

Ever wonder how the days of the week got their names?The Romans named the days after the Sun, Moon, and 5gods corresponding to the five visible planets, includingJupiter. The Norse god equivalent to Jupiter was Thor.So every time you make plans for Thor's day, orThursday, you're paying homage to the planet Jupiter!


BRIEFING ROOM

IDEAS YOU'LL NEED

Newton's law of gravity. Newton discovered that the force of gravity between any twoobjects depends only on the mass of the two objects and the distance between them. Heshowed that the force is proportional to the mass of each object, and inverselyproportional to the square of the distance between their centers:

F ~ mM / d2

Does this relationship make sense?: The more massive (M) a planet, the stronger itsgravitational pull on you. The more massive (m) you are, the STRONGER the pull(that's why a person with twice your mass weighs twice as much). But the more distant(d) you are from the Earth, the WEAKER the pull of gravity. (Just as the brightness of alight appears more feeble as the square of your distance from it, so does gravity becomemore feeble as the square of the distance.)

[The Art of Science: "Laws"... Or Relationships?]

Kepler's law. Johannes Kepler discovered that the time it takes a moon to orbit a planetdepends only on the mass of the planet and the distance between their centers:

T2 ~ d3 / M

Does this relationship make sense? Yes: The more distant you are from a planet, theMORE TIME it takes to go around the planet. (Two reasons: You have further to travel,and you are moving more slowly, because gravity is weaker further out.) And the moremassive a planet, the stronger its gravity, and so an object in orbit must whip aroundfaster to stay in orbit.

Where does Kepler's law come from? Kepler discovered his law through carefulobservations of the motions of the planets around the sun. But the law can also bederived from Newton's law of gravity and his law of motion. If you're interested inseeing how, see box X.


ACCELERATION AND CIRCULAR MOTION

An old puzzle: Name three parts of a car that cause it to accelerate. Any ideas?

One is the gas pedal (causes car to speed up, called acceleration). Second is the brake(causes car to slow down: negative acceleration). But what's the third part? Give up?

Answer: The steering wheel.

According to Newton's description of motion, an object in motion will keep going at thesame speed in a straight line UNLESS a force acts on it, causing it to accelerate.Accelerate means to speed up, slow down, or CHANGE DIRECTION. All of these areexamples of acceleration.

For an object that changes its speed but keeps the same direction, it's easy to see howwe might measure acceleration: For example, we say that a sports car goes "from 0 to 60miles per hour in six seconds." So its acceleration is 60 miles per hour per six seconds,or 10 miles per hour per second. But how do we measure the acceleration of an objectthat keeps the same speed but changes its direction Ñ such as a car traveling in a circleat constant speed?

Your teacher may show you how to derive the simple relationship for the acceleration,the radius of the circle, and the speed of the object:

a = v2 / d

where a is the acceleration, v is the speed of the object, andd is the distance to the center of the circle.

To see why this relationship makes sense, think aboutwhat you feel when you're a passenger in a car goingaround a curve. The tighter the curve, the more theacceleration (i.e., small d leads to big a). If the car tries totake the curve twice as fast (greater v, in the drawing) then theacceleration is four times as great. (Many drivers lose control of a car going into a curve,because they don't realize how strongly the acceleration Ñ and the centrifugal force onthe car Ñ depends on speed.)


WHERE KEPLER'S LAW COMES FROM.

The nice thing about physics and math is that we can use them to help us find out whatwe want to know.

For our mission, we want to weigh Jupiter, knowing the size and period of Europa's orbit.We can use our understanding of gravity and acceleration to help us.

From Newton's description of gravity, we know that the acceleration on Europa comesfrom the force of gravity that Jupiter exerts on it:

a = F / m

This force of gravity is

F = mMG / d2

So the acceleration of Europa is

a = MG/ d2

But we also know how to relate the acceleration of an object to its speed, when it ismoving in a circle:

a = v2 / d

v2 / d = MG/ d2

The time that it takes Europa to make one orbit is just its speed divided by the distancearound the circle. (T = 2¹ d / v, or v = 2¹ d / T)

So replacing the velocity with 2¹d / T, the equation becomes

(2¹) 2 d / T2 = MG/ d2

So the mass of Jupiter is related to the size and period of Europa's orbit:

M = (2¹)2 d3

G T2

M is the mass of Jupiterd is the distance from Europa to the center of JupiterT is the period of Europa's orbitG is Newton's gravitational constant(G = 6.7 x 10-8 cm3 / g sec2 ) .

If d is in centimeters and T is in seconds, then M will be in grams.

voyage to europa! - center for astrophysics

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