Measuring Moons

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Measuring Moons
In order to work out the density of any object,
we need to know two things about it. What are
those two things?
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Measuring Moons
In order to work out the density of any object,
we need to know two things about it. What are
those two things?
Density mass ? volume
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Measuring Moons
In order to work out the density of any object,
we need to know two things about it. What are
those two things?
Density mass ? volume
First, the easy one How can we work out the
volume of a moon?
Answer gtgt
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Measuring Moons
Volume
When we look a t a moon through a telescope, we
can measure the size of the image. Knowing how
far the telescope is from the moon, we can use
angles to work out the diameter of the moon.
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Measuring Moons
Volume
When we look a t a moon through a telescope, we
can measure the size of the image. Knowing how
far the telescope is from the moon, we can use
angles to work out the diameter of the moon.
Moons are roughly spheres. Whats the formula for
working out the volume of a sphere, if we already
know the radius?
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Measuring Moons
Volume
When we look a t a moon through a telescope, we
can measure the size of the image. Knowing how
far the telescope is from the moon, we can use
angles to work out the diameter of the moon.
Moons are roughly spheres. Whats the formula for
working out the volume of a sphere, if we already
know the radius?
Volume 4/3 ? r3
Now for mass gtgt
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Measuring Moons
Mass
This is much more tricky to measure we cant
just weigh it
But gravity depends on the mass of an object. How
could we use this to work out the mass? Bear in
mind that we need to be able to do thiswithout
actually going there!
Answer gtgt
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Measuring Moons
Mass
This is much more tricky to measure we cant
just weigh it
But gravity depends on the mass of an object. How
could we use this to work out the mass? Bear in
mind that we need to be able to do thiswithout
actually going there!

• There are two main methods
• See how much the moons gravitational pull
  deflects our spacecraft as we fly by
• There are many objects in the Saturn system. We
  can watch how they affect each others orbits
  over time.

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Measuring Moons
So we can work out the mass and the volume of a
moon, and from those we can work out the density.
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Measuring Moons
So we can work out the mass and the volume of a
moon, and from those we can work out the density.
Many of the tiny moons of Saturn are icy. We know
this because of the way they reflect light, and
the wavelengths that they reflect.
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Measuring Moons
So we can work out the mass and the volume of a
moon, and from those we can work out the density.
Many of the tiny moons of Saturn are icy. We know
this because of the way they reflect light, and
the wavelengths that they reflect.
Ice has a density of just under 1 gram per cm3 So
wed expect these moons to have the same density
as ice.
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Measuring Moons
So we can work out the mass and the volume of a
moon, and from those we can work out the density.
Many of the tiny moons of Saturn are icy. We know
this because of the way they reflect light, and
the wavelengths that they reflect.
Ice has a density of just under 1 gram per cm3 So
wed expect these moons to have the same density
as ice.
However, some of the moons have a density
between0.4 and 0.6 grams per cm3 How can this
be?
Ideas gtgt
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Measuring Moons
Perhaps theyre not solid, like hailstones they
might be porous, more like snowballs
Or perhaps theyve been shattered by some ancient
impact, and the fragments have drifted back
together but not fitted too well, leaving gaps
inside.
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Measuring Moons
Perhaps theyre not solid, like hailstones they
might be porous, more like snowballs
Or perhaps theyve been shattered by some ancient
impact, and the fragments have drifted back
together but not fitted too well, leaving gaps
inside.
We think that a similar thing has happened to
some asteroids. Measuring the density of rocky
asteroid fragments that have landed on Earth, and
comparing the density of these to the density of
some asteroids, we think that this is a likely
explanation.
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