Lesson2e
Lava Tubes and Density
Lava tubes
Lava pouring out of Lava Tube
Collapsed Lava Tube on Earth
Lava flows in
channels
also
Lunar Rille (means “groove”)
Topographical map of Mars
Olympus
Mons
Lava Tubes on Mars – Pavonis Mons
Solidified
Lava flow
at the
base of
Olympus
Mons
“A”
“B”
Flat Plain
at bottom
of slope
Which
surface
do you
think
might be
older,
Surface A
or
Surface
B?
A
Which
surface
do you
think
might be
older?
A or B?
B
• Surface A is likely to be older. We can see a
number of impact craters in surface A. But
surface B seems to be missing craters.
• The area in the image is about the same for
both regions.
• We must assume that lava has covered up any
craters that might have been present in
surface B. This makes B younger than A.
A very big question to answer
• We see that the Moon and Mars have both had
active volcanism.
• When a planet is still very hot inside various
tectonic processes occur that allows the heat to
escape the interior. We say the planet is
geologically active.
• When a planet cools to the point that tectonism
stops, we say the planet is geologically dead.
• If we don’t see tectonic processes occurring at
the moment, what do we have to do in order to
determine when a planet was geologically active?
• We need to determine the age of the features
we see.
• Radiometric data of lava would be the best
but we can not currently travel to Mars and
return rock samples. Or the other planets or
moons.
• An important tool to use is crater density on
the surface. The greater the density of craters
the older the surface must be.
Density -- an important parameter
• There are two types of densities we will use in
this class.
• Mass density (mass/volume) and Number
density (number/volume).
• A density doesn’t have to be a volume density.
• It can be an area density. (kg/m2)
• And it doesn’t have to be mass.
Population Density
• Alaska has a population of 686,000 people
• It has an area of 663,000 square miles.
• That is roughly 1 person/square mile.
• What does this tell us about the lives of
Alaskans?
• Not much. It only tells us there is a lot of
room for people to expand into.
• This is an average density. In fact most
Alaskans live near or in cities where the
population density is much higher.
White blood cell counts.
• When a doctor checks your white blood cell
counts for an infection, they draw some blood
and then use a very small volume of blood to
count the number of white cells.
• This is a number of white blood cells/volume.
• This density tells them if you have an infection
or not.
• What assumptions is built into this analysis?
Let’s compute the mass density of the
Earth.
•
•
•
•
•
Earth mass = 5.97 x 1024 kg.
Earth radius = 6.96 x 106 m.
Density = mass/volume
The Earth is approximately a sphere.
So all we need is the volume equation for a
sphere.
A volume is 3-D
R
L
L
L
V = L x L x L = L3
V = 4πR3/3
• The surface area of a sphere is: 4πR2
• The surface area of a cube is: 6L2
• The volume of a sphere is (4/3)πR3
• The volume of a cube is L3
Let’s compute the mass density of the
Earth.
•
•
•
•
•
Earth mass: M = 5.97 x 1024 kg.
Earth radius: R = 6.96 x 106 m.
Density = mass/volume = M/V
The Earth is approximately a sphere.
So all we need is the volume equation for a
sphere.
• V = (4/3) π R3
• V = (4/3)π(6.96 x 106 m)3
• Dearth = (5.97 x 1024 kg)/(1.09 x 1021 m3)
• Dearth = 5,477 kg/m3
• Dearth = 5.48 g/cm3
We can test this by comparing to rocks we find
on the surface.
This is easy to do.
• Weigh the rock to find its mass.
• Find the volume of the rock by the amount of
water it displaces.
Result
• The average density of rocks on the surface of
the Earth is Drock = 3 g/cm3
• The density we calculated for the Earth was
Dearth = 5.48 g/cm3
This is very different. How can we explain
this?
• The density we calculated for the Earth was
the average density. It doesn’t mean that the
density is the same everywhere inside.
• Here is what we have:
• Average density of Earth is 5.48 g/cm3
• Average density of surface rocks is 3 g/cm3
• Inside the Earth there must be something that
is much more dense than the surface rock in
order to increase the average.
• Research on the composition of meteorites
shows that many are composed of high
fractions of iron.
• The Earth and the other planets are built up
from the accumulation of meteors and
asteroids.
• The planets should have a lot of iron.
• The density of iron is about 8 gm/cm3
• You can see that mixing iron densities with
rock densities will give us something closer to
the average.
• The iron has to be in the interior of the Earth
and not much on the surface.
• In fact, the core of the Earth is mostly iron.
• Why is the majority of Earth’s iron in the
core?
Differentiation
• When the Earth was forming it was suffering
many impacts from meteors and asteroids.
• These impacts heated the Earth so that it was
completely molten.
• When it was in liquid form, it was possible for
the dense elements (like iron) to sink, and the
least dense elements (like silicon) to rise.
• This is why the surface rocks are low density.
Compressed vs. Uncompressed
The average density we have been discussing is
the Earth’s compressed density. It depends on
the material in the Earth and how much that
material is compressed by the Earth’s gravity.
If we want to know what planets are made of,
we do not want to take into account the force
of gravity. This is the uncompressed density.
It only depends on the material in the planet.
• http://www.mesacc.edu/~khealy/flash/planet
-density.html
• Let’s see what we can deduce about the four
Galilean moons, Io, Europa, Ganymede, and
Callisto.
Io – Density = 3.6 gm/cm3
Europa – Density = 3.0 g/cm3
Ganymede – Density = 1.9 g/cm3
Callisto – Density = 1.8 g.cm3
Other evidence
• Surface of Europa, Ganymede and Callisto is
composed of water ice. (Density = 1 g/cm3)
• What can you conclude about the Galilean
moons?
Io
Ganymede
Europa
Callisto