Download Voyage to Europa! - Harvard-Smithsonian Center for Astrophysics

Voyage to Europa!
1. Introduction:
Is there life on one of Jupiter's moons?
The Setting
The Challenge
Heads Up!
2. Activities:
Getting the Images
Imaging Jupiter and it's Moons
Scope it Out
Reflecting on your Images
Which moon is Europa?
Using the Images to Investigate Jupiter and Europa
Preparing for the Trip: How far is Jupiter?
Go Figure
A 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 Europa
What 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 experiment
An 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 Room
Timeline
Ideas you'll need
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VOYAGE TO EUROPA
1. INTRODUCTION:
Is there life on one of Jupiter's
moons?
Circling the giant planet, Jupiter, are several moons that may contain
liquid 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 NASA
space probe to Jupiter, this salt-water ocean may be 60 miles deep and as warm as the
waters of Bermuda!
If true, this would make Europa the likeliest place in our solar system to search for extraterrestrial 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 the
prospects 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 any
technologies you may need.
Your challenge is to:
•
Obtain images of the planet Jupiter and its four brightest moons, and identify your
destination, Europa.
•
Evaluate the prospects for setting up
a base camp on Jupiter. Use your
images Ñ and your knowledge of
physics Ñ to find out as much as you
can about Jupiter. If you tried to land
on Jupiter, would you hit a solid
surface, or would you keep on falling
and never be heard from again?
If you could land on Jupiter, how much
would you weigh? Would your body
be able to support you? Would you be
able to take off again?
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•
Evaluate the prospects for a base camp on Europa: Do you expect Europa to have
an atmosphere? How much sunlight does Europa get compared to Earth? If you can
only explore the surface while the sun is up, how long can you explore before you
must get back to base camp? How cold do you expect it to be? How much will you
weigh there, compared to Earth?
•
Evaluate the prospects for life under Europa's icy surface: The ocean on Europa
could contain bizarre life forms Ñ or it could contain no life at all. Make a case for
the likelihood of finding life on Europa. Consider these questions: Does life need an
energy source Ñ and if so, what kinds of energy will do? Does life need light to
exist? Does life need oxygen?
In discussing these questions, you may wish to research and report on one or more of
the 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 things
thrive under those conditions?
This mission guide will take you step by step through the activities you'll need for your
report.
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 lie
along 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, planets
and their moons lie in nearly the same plane. When seen from a point
within that plane, they appear to lie along a line.
Try this dizzying feat: Look up at the sky and picture the plane that the
planets lie in. You'll suddenly become aware that you're standing at an
angle to that plane. When it comes to outer space, which direction is
"up"?!
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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 once
an hour for four or five hours. Amazingly, there will be enough information in these
images for you to determine several properties of Jupiter that you'll need to know for
your mission Ñ such as Jupiter's size, density, and gravity. This information will help
you in deciding whether to establish a base camp on Jupiter.
'SCOPE IT OUT
Selecting the Target: Use the pull-down menu to select Jupiter. (The telescope's computer
will automatically determine Jupiter's location in the sky for the time you selected. Jupiter
does not have a permanent address Ñor RA and DEC Ñ in the sky, because it moves from
night 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 of
view, 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 the
GIF-format image on the Web, without any image processing. Be sure to download both the
image AND its Image Info file, because this contains the information about how and when you
took the image.
It's a good idea to also download the FITS file for each image as well for your records. (Click
and 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 an
image processing program to INVERT your image Ñ that is, to reverse black and white. Then
when you print, Jupiter and its moons will appear black against a white background. That's
much 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.
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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 fly
off into space?
Speed of the moons. Which moons appear to have moved, from image to image? Why
have 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 might
that be?
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WHICH MOON IN YOUR IMAGE IS EUROPA?
You'll probably see up to four of Jupiter's moons in your image. Can you figure out
which moon is Europa?
The moon closest to Jupiter in nature is called Io, followed by Europa, Ganymede and
Callisto. But the moon that looks closest to Jupiter in your image need not be Io. That's
because, from Earth, we're looking at the moons edge-on to the plane of their orbit.
For example, the image at right shows how
the moons might look if you were to look
DOWN on the plane of their orbit. (The
image shows the relative size of the orbits, to
scale.)
The edge-on view below it shows how this
scene would appear through the telescope.
Can you label which moon is which?
When you look at your images, you won't
have the benefit of a top-down view to
compare them to. But can you think of a
way to figure out which moon is which by
following the moons through several
images?
If you could follow the moons for long
enough, you could see the furthest distance
that each one gets from Jupiter. That would
tell which moon is which.
A second
way to tell
the moons
apart is to
see which
moon moves FASTEST -- i.e., moves the furthest in
several successive images taken, say, an hour apart.
The moons closest to Jupiter orbit the fastest, as
predicted by Newton's laws of motion. But even here
you'll have to be careful: Take a look at the figure on
the next page, where the x's mark equal time intervals.
As seen from edge-on, the moon seems to speed up and
slow down. At what part of the orbit will the moon
appear to moving fastest? Where will it seem to be
moving slowest?
After the Image and Analysis Team has examined the
images and is confident they can identify the moons,
label the images with the team's results.
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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 the
distance 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 and
return 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 closer
to the sun than Earth is? By how much? (See the Briefing Room: Ideas You'll
Need)
•
The Earth is about 93 million miles from the Sun. How far is Jupiter from the
Sun?
Record your results on the handy DATA SHEET for the mission to Jupiter. You'll need
them later.
Using your results, what is the closest that Jupiter gets to Earth? What is the furthest? (It
helps to draw a diagram with Earth and Jupiter on the SAME side of the Sun, and then
one with the planets on OPPOSITE sides of the Sun.)
Scale drawing. In the scale drawing below, how far from the sun should you draw
Jupiter? In the drawing, 6 inches represents 100 million miles.
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REFLECTING ON YOUR RESULTS
About how long will it take you to get to Jupiter? Assume that your spacecraft can
average 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 should
they have?
A BASE CAMP ON JUPITER?
What are the prospects for landing on Jupiter? Can you use it to set up a base camp from
which to explore Europa? How much would you weigh on Jupiter? Is there a solid
surface, 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 applying
the physics you've learned. Here are some measurements and estimates you'll need to
make. 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?
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WHAT IS THE SCALE OF MY IMAGE?
How many miles does one pixel represent in your image? You'll need the following
information:
•
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. (See
the activity, "A Wrangle with Angles.")
•
In your telescope image, one degree spans 720 pixels. So 1 pixel = 1 / 720
degree.
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 does
Jupiter'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 than
Earth's?)
Jupiter has about____________________ times the volume of Earth.
Record your results on the mission DATA PAGE.
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REFLECTING ON YOUR RESULTS
Does the size of Jupiter alone tell you anything about
how strong the gravity will be at Jupiter's surface? What
other information would you need?
SIZE MATTERS...
Could there be a planet ten
times larger than Jupiter?
Strangely enough, Jupiter may
be about the largest a planet
can get... and still be a planet!
Although planets two or three
times as massive Jupiter have
been detected orbiting other
stars beyond our Sun,
astronomers believe that an
object much larger than Jupiter
would collapse under its own
weight to form a star.
In a star, the temperature and
density of matter is so great
that the matter undergoes
nuclear reactions, liberating
enormous amounts of energy
and causing the star to shine.
You can learn more about stars
in the investigation, "To the
Stars!"
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.
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HOW MUCH DOES JUPITER "WEIGH"?
You can actually "weigh" Jupiter Ñ that is, determine
its mass ÑÊjust from observations of the motions of its
moons. Remember that the mass of a planet influences
how fast a moon will revolve around it. (See the Box
at right, and the Briefing Room in the appendix.)
To determine Jupiter's mass, you'll need to know the
DISTANCE from Europa to Jupiter, and the TIME it
takes Europa to orbit once.
FINDING JUPITER'S MASS
(see Briefing Room)
The period (T) of a moon's orbit
depends on its distance (d) from the
planet and the mass (M) of the planet.:
T2 ~ d 3 / M
You can find the planet's mass if you
know the distance and period of a
moon.
STEP 1: DETERMINE EUROPA'S DISTANCE
FROM JUPITER.
You could determine the distance from Europa to Jupiter (that is, the radius of Europa's
orbit) by following Europa's motion and measuring the maximum separation between
Europa and Jupiter. Then, knowing the scale of your image in miles per pixel, you could
determine the radius of the orbit.
To save you time, the
drawings at left show the
maximum distance for
the four inner moons of Jupiter Ñ at the same
scale as the MicroObservatory zoomed-out and
zoomed-in images. Can you use this guide to
determine the distance from Europa to Jupiter?
(Hint: Use the scale of your image in miles per
pixel 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" or
about a month!) How long would you guess it would take Europa to orbit Jupiter, given
that 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. (A
ruler that measures millimeters is best for this.)
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Europa moved _________ (pixels? millimeters?) over _________ hours
(Don't remember the time interval? Check the Image Info headers on your images to
see when they were taken!)
•
Compare the distance you measure with the maximum width of Europa's orbit, as
shown in the drawing above. What fraction of the orbit does Europa move in the time
interval you've observed.
If Europa takes, say, two hours to move some fraction of the orbit, then how many
hours will it take to move all the way around Jupiter?
IMPORTANT: Discuss with your team how
you will make this estimate. It's tricky: As
you can see from the drawing at right, Europa
will appear to move more slowly near the
turning points of its orbit (furthest from
Jupiter) and will APPEAR to move fastest as
it crosses the face of Jupiter. How will you
correct for this?
Also, don't forget that the period is the time to
go all the way around Europa and back to the
starting 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 to
go around the Earth?
If Europa is at roughly the same distance from Jupiter as the Moon is from Earth, then
why does Europa orbit so much faster than the Moon?
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PUTTING IT ALL TOGETHER:
HOW MUCH DOES JUPITER WEIGH?
"WEIGHING" JUPITER
Use your measurements for the orbital period and
distance of Europa to determine the mass of
Jupiter. (See box at right.)
Use your results for the orbital period and
distance of Europa to determine the mass
of Jupiter. See the Briefing Room for
details.
M ~ d3 / T2
The mass of Jupiter is about
_________________________ (grams)
M = (2¹)2 d3
G
T2
How many times more massive is Jupiter compared
to the Earth?
TIP: You can compare Jupiter's mass to the Earth's
just by knowing the orbital period and distance of
one satellite for each planet.
M is the mass of Jupiter
d is the distance from Europa to the center of
Jupiter
T is the period of Europa's orbit
G 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.
MJupiter / MEarth = (T2 d3 )Europa / (T2 d3 )Earth's Moon
Use your results for Europa, and the results for the Moon from "The Incredible Shrinking
Moon" 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 or
gas? One line of evidence to use is the average density of Jupiter, which is the mass per
unit 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 normal
pressure):
Liquefied natural gas: 1 gram/ cubic centimeter
Water: 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 Jupiter
be solid rock?
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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)? To
answer this question, discuss with your team what factors influence the gravitational pull
you 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 your
exploration of the planet's moon, Europa?
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Searching for Life on Europa
What kind of environment will you find when you get to Europa? And what are the
prospects for life? Try some or all of the activities on the following pages and include
your 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 miles
Mass (Earth = 1): .0083
Density 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 and
dot are in the lower right portion of
this image of Europa?
How does this image compare to
Earth's moon? What might
account 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 support
your hypotheses?
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The surface of Europa is very
bright. What might this indicate?
What might the cracks on the
surface indicate? Where are on
Earth might you see cracks like
this?
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DOES EUROPA HAVE AN ATMOSPHERE?
Your environment team has been asked to assess the chances that Europa has an
atmosphere. What determines whether a moon or planet has an atmosphere?
Consider the following:
•
Our planet Earth has an atmosphere, but our Moon does
not. Why not?
•
The planet Mercury does NOT have an atmosphere, but
Titan Ñ a moon of Saturn that's the same size as
Mercury Ñ DOES have atmosphere. Why?
DISCUSSION: What factors do you think influence whether a planet or moon has an
atmosphere?
If the Earth had been much smaller than it is, would it have kept its atmosphere? If the
Earth had been much smaller, would you be here now?!
From your telescope images, can you say anything about the size of Europa? Do you
expect it to have an atmosphere?
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WHAT IS EUROPA'S ENVIRONMENT?
What kind of environment can your mission team expect on Europa? Can you use light
from the sun as your power source?
Solar energy on Europa. How much light does Europa get, compared to Earth? Use
your findings from a previous activity about the relative distances of Earth and Jupiter
from the Sun.
Temperature on Europa. Using your results for the amount of sunlight and the lack of
an atmosphere on Europa, how would you expect the temperature to compare to that on
Earth?
Gravity on Europa. How does gravity on Europa compare to Earth? (Use data from
preceding pages.) How much would you weigh on Europa? If you can carry 70 pounds
maximum on Earth, how heavy a space suit could you wear on Europa?
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COMMUNICATING FROM EUROPA
From Earth, we always see the same side of the Moon. That's because our Moon rotates
on its axis at the exactly the same rate (once every 27.5 days) as it revolves around the
Earth. This used to be thought a coincidence Ñ but now we know it always happens
when 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 one
side to Jupiter.
Suppose you wanted to have a communications satellite in orbit above Europa that
always remained above a particular spot on the surface (e.g., above your base camp
on Europa). How far above the surface of Europa should the orbit be? Use your
finding for Europa's period of revolution (and therefore rotation). Apply Kepler's law to
figure out how high the satellite should be to have a period equal to Europa's rotation
rate.
Is this satellite feasible? How does its orbit compare to the distance from Europa to
Jupiter? 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 have
before you are plunged into darkness? (You'll need to draw a model showing the
location of the Sun, Jupiter, and Europa, and take into account the motions of Europa.)
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CAN YOU DESIGN ARTIFICIAL GRAVITY?
When humans are deprived of Earth's gravity for too long, their bodies start to feel the ill
effects. 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 to
provide one Earth gravity? How large a vehicle should it be, and how fast should it
spin?
Some of your colleagues want a box-shaped orbiting vehicle. Trying the following
experiment, report on the dangers of a box-shaped vehicle and the importance of shape.
MYSTERIOUS MOTION EXPERIMENT:
Using an empty cereal box, try the following
experiment. Toss the box in the air so that it
spins around an axis through its largest side (1).
Can you get it to spin WITHOUT wobbling?
Try it again, this time spinning around an axis
through its smallest side (2). Is it again stable?
Now try spinning it about the axis through its
medium side (3). What happens?
The motion you observe is an example of
"chaotic motion." Newton's laws can predict that
such motion will take place, but cannot describe
in detail the motion itself!
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IS THERE AN OCEAN ON EUROPA?
Scientists have evidence that a vast ocean exists under the surface ice on Europa. But if
Europa is so cold, then where does the heat come from to keep the underground ocean
from freezing solid?
Simple friction may be at work: As Europa orbits
around Jupiter, the enormous gravity from Jupiter (and
the other moons) alternately stretches and squeezes
Europa. The friction created by this process is enough to
heat the interior and melt the ice on Europa, from the
inside out. (The giant volcano on Jupiter's nearest moon,
Io, is thought to be powered in the same way. Image at
right.)
To see how this continual stretching might provide an
energy source to melt Europa's water, try this
experiment:
EXPERIMENT: COOOOL HEAT!
Materials: A wire coat hanger.
SLOWLY bend the coat hanger back and forth at one
point along its length. CAREFULLY and BRIEFLY
touch 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 hanger
cool faster? Slower? The same?
DISCUSSION: Objects can generate heat through their entire volume, but they can lose
heat only through their surface. What happens to the volume/ surface ratio as objects get
larger? What examples in nature can you think of where size plays a role in an object's
temperature?
Fun fact. When whales are about to give birth to their calves, they migrate to warmer
waters. Reason: The smaller babies would lose heat
too fast in the colder waters. As soon as the babies
gain weight, it's back to the frigid waters again!
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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 you
would need to get.
What are the major requirements for life? What energy source could be present under the
ice? Are the chemical elements for life present? Could life survive without light or
oxygen? 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 the
following:
On Earth, is there life near deep-sea
volcanic? 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 under
those conditions?
For a skeptic's view, visit the following site and discuss with your team the reports found
there: http://www.spaceviews.com/1999/08/05a.html
In your view, would it be worth exploring Europa if only microscopic life Ñ but no
larger life forms Ñ existed? Why?
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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.
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BRIEFING ROOM
TIMELINE
Ancient Roman times: The planet Jupiter is named after one of the Roman gods.
1610. Using one of the first telescopes, Galileo
Galilei discovers four moons orbiting the planet
Jupiter. This shows that Earth is not the
only "center of attraction" in the heavens,
and makes it easier to accept
Copernicus' conclusion that the Earth
orbits the Sun.
Some townspeople refuse to look
through Galileo's telescope,
calling it a distortion of reality.
1979. Three scientists from the Jet Propulsion Laboratory
conclude that Jupiter's strong gravity may flex and heat its
nearby moons.
1979. Three days later, the Voyager spacecraft sends the first
images of volcanoes on Io, the closest moon to Jupiter.
1996. NASA's Galileo spacecraft, is launched to explore the
planet Jupiter and its moons.
2000. The Galileo spacecraft arrives at Jupiter sends back images and
data about the planet Jupiter and several of its moons.
2000. Margaret Kivelson and her team from the University of California conclude
that an ocean exists under the surface of Europa. Evidence: Telltale magnetic fields
from 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 5
gods corresponding to the five visible planets, including
Jupiter. The Norse god equivalent to Jupiter was Thor.
So every time you make plans for Thor's day, or
Thursday, you're paying homage to the planet Jupiter!
From the Ground Up!: Jupiter v. 031301
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Harvard-Smithsonian Center for Astrophysics
BRIEFING ROOM
IDEAS YOU'LL NEED
Newton's law of gravity. Newton discovered that the force of gravity between any two
objects depends only on the mass of the two objects and the distance between them. He
showed that the force is proportional to the mass of each object, and inversely
proportional 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 its
gravitational 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 a
light appears more feeble as the square of your distance from it, so does gravity become
more 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 planet
depends 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, the
MORE 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 more
massive a planet, the stronger its gravity, and so an object in orbit must whip around
faster to stay in orbit.
Where does Kepler's law come from? Kepler discovered his law through careful
observations of the motions of the planets around the sun. But the law can also be
derived from Newton's law of gravity and his law of motion. If you're interested in
seeing how, see box X.
From the Ground Up!: Jupiter v. 031301
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Harvard-Smithsonian Center for Astrophysics
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 the
same 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 are
examples of acceleration.
For an object that changes its speed but keeps the same direction, it's easy to see how
we might measure acceleration: For example, we say that a sports car goes "from 0 to 60
miles 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 object
that keeps the same speed but changes its direction Ñ such as a car traveling in a circle
at 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, and
d is the distance to the center of the circle.
To see why this relationship makes sense, think about
what you feel when you're a passenger in a car going
around a curve. The tighter the curve, the more the
acceleration (i.e., small d leads to big a). If the car tries to
take the curve twice as fast (greater v, in the drawing) then the
acceleration 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 on
the car Ñ depends on speed.)
From the Ground Up!: Jupiter v. 031301
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Harvard-Smithsonian Center for Astrophysics
WHERE KEPLER'S LAW COMES FROM.
The nice thing about physics and math is that we can use them to help us find out what
we 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 comes
from 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 is
moving 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 distance
around 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 Jupiter
d is the distance from Europa to the center of Jupiter
T is the period of Europa's orbit
G 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.
From the Ground Up!: Jupiter v. 031301
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Harvard-Smithsonian Center for Astrophysics