Transcript 1B11 Foundations of Astronomy Star names and magnitudes

1B11
Foundations of Astronomy
The Earth as a planet
Liz Puchnarewicz
[email protected]
www.ucl.ac.uk/webct
www.mssl.ucl.ac.uk/
1B11 The Earth as a planet
This is an image of
London and the
Home Counties taken
from the Space
Shuttle.
The Earth is the third
planet from the Sun.
radius = 6380km
mass = 6 x 1024 kg
mean density =
5.5g/cm3
1B11 Cross-section through the Earth
The outer crust is
about 30km thick.
asthenosphere lithosphere
(partially molten)
(includes
crust)
mantle silicates –
slow convection
outer core
liquid (Fe, Ni)
inner core
solid (?)
7000K
6380km
5000km
5000K 3800K
2900km
1300K
250km
100km
1B11 Basic parameters
The cross-section through the Earth shows that it is internally
differentiated.
Metal core density = 10 - 13 g cm-3
Silicate mantle density = 3.3 - 5.5 g cm-3
Silicate crust density = 2.7 – 3.0 g cm-3
Atmosphere – P~105 N m-2 (1 bar) 78% N2 ; 21% O2
One natural satellite
Hydrosphere
Biosphere
unique in the Solar System?
1B11 The fluid Earth
Continental masses are mostly granite and float on the
basalt.
Surrounding the mantle is the crust, mostly rocks which
have solidified from molten lava. These are basalt and
comprise the ocean basins and the subcontinent sections of
the crust. It floats on the mantle.
The mantle is rock made of iron and magnesium combined
with silicon and oxygen. The density is about 4 g cm-3 and at
these temperatures and pressures, it flows like a liquid.
1B11 Seismology
Seismology is the study of the passage of waves through the
Earth. Earthquake seismology reveals much about the
structure of the Earth.
Body waves –
Surface waves –
Travel through the “body” of
the Earth.
Both are transverse
P-waves
(pressure or
primary) are
compressional
waves. Move
fastest, ~
6km/s
S-waves
(shear or
secondary)
transverse
waves, travel
at ~2km/s
Rayleigh
waves
describe the
vertical
motion.
Slowest
waves.
Love waves
describe the
horizontal
motion.
ground
motion
1B11 Seismic wave transport
p-wave
surface
wave
s-wave
seismometer
time
1B11 Properties of seismic waves
P-waves move much faster (~ 2x) than S-waves
S-waves cannot propagate through a fluid
Rayleigh wave velocity is ~0.9x S-waves
Love waves travel faster than Rayleigh waves
1B11 Lecture schedule (probably)
Today – Earth and the Moon
Thurs/Tues – terrestrial planets (mercury, mars, venus)
Wednesday – Silvia – Jovian systems (3rd Dec)
Tues?Thurs – asteroids, planetessimals,etc
Weds – extrasolar planets (10th)
Thurs – problem class
1B11 Rock types
1. Igneous rocks – formed from molten lava (magma);
eg, basalts (oceanic crust) and granites
(continental crust).
2. Sedimentary rocks
produced by the erosion and re-deposition of
igneous rocks (generally underwater), eg
sandstone
3. Metamorphic rocks
igneous or sedimentary rocks altered by high
temperatures and/or pressures
1B11 Dating rocks
100
If n0 is the original number of
parent atoms and n the number
remaining at time t, then:
50
n
  0.693t 
 exp

n0



0 12½ 25
%age of parent isotope
remaining
Most rocks contain trace quantities of radioactive elements.
Radioactive isotopes have a half-life – which is the time taken
for 50% of the material to decay into daughter isotopes.
0 1 2 3 4 5 6
where  is the half-life.
1B11 Cross-section through the Earth
The outer crust is
about 30km thick.
asthenosphere lithosphere
(partially molten)
(includes
crust)
mantle silicates –
slow convection
outer core
liquid (Fe, Ni)
inner core
solid (?)
7000K
6380km
5000km
5000K 3800K
2900km
1300K
250km
100km
1B11 The outer layers & upper mantle
continental shelf
0km
sea level
oceanic crust, r~2.9g
5km
Lithosphere
(“rigid”)
40km
cm-3
Asthenosphere
(“plastic”)
continental
crust
30km thick,
continental root r~2.7g cm-3
r~3.3 g cm-3
250km
base of upper mantle 400km below
1B11 Lithosphere as a “condensate”
Temp (K)
2000 3000
4000
melting temp
temp
lower mantle
0
lithosphere
1000
asthenosphere
0
500
1000 1500 2000
depth (km)
1B11 Plate tectonics
The lithosphere is divided into roughly 10 large “plates” which
move in response to convection in the mantle.
This is the cause of continental drift and seismic and
volcanic activity.
volcanoes
midoceanic
ridge
(new crust)
melting due
to release
of pressure
oceanic
lithosphere
Mantle
convection
continental
crust
earthquakes
1B11 Plate boundaries
1. Spreading ridges
The rise of molten material from the mantle creates new
oceanic crust in the lithosphere.
2. Convergent boundaries
Plates are subducted back into the mantle in subduction
zones. At a continental boundary, this causes folding
and the creation of mountains, and volcanism.
3. Translational boundaries
Plates slide past each other along transform faults.
1B11 Evidence for plate tectonics
1.
2.
3.
4.
5.
Continental shapes
Biological and fossil evidence
Earthquake and volcano distribution
Topography of the ocean floor
Direct measurement:
satellite
radio
telescope
1B11 Radiogenic heating
The main source of the Earth’s internal heat comes from the
decay of radioactive isotopes.
The most important isotopes are:
238U, 235U, 232Th
and 40K
and together these provide approx. 28 x 1012 W.
Other possible heat sources:
original heat from planet formation
growth of the inner care (latent heat, gravitational potential
energy)
gravitational contraction
1B11 Geothermal heat flow
The average geothermal heat flow is 0.06 W m-2.
Over the whole Earth, this is 30 x 1012 W
which is in good agreement with estimated radiogenic values.
BUT:
There are sources of heat loss, eg hydrothermal vents at
ocean ridges, so taking these into account, the output may be
as high as 40 x 1012 W.
This would then imply a significant non-radiogenic heat
source, which is most likely to be residual heat from the
Earth’s rotation.
1B11 The Earth is cooling
Note that radiogenic heat must be decreasing with time:
Today – 28 x 1012 W
4.5 billion years ago: 120 x 1012 W
So there must have been much more vigorous geological
activity (ie plate tectonics) in the past.
1B11 The age of the Earth
Radioactive dating indicates an age for the Earth of 4.6 billion
years.
The oldest rocks on the Earth’s surface are younger – about
4.0 billion years. These are igneous rocks – ie they have
formed out of molten material. It is estimated that it would
have taken 0.5 billion years for these first rocks to form.
Meteorites are generally 4.55 billion years old and the Moon
is 4.6 billion years old (from radioactive dating).
This is similar to the age of the Sun – thus it seems that the
solar system formed at the same time – about 4.6 billion
years ago.
1B11 Useful isotopes for dating rocks
87Rb
238U
40K
235U
-> 87Sr
48 x 109 yrs
-> 206Pb
4.5 x 109 yrs
-> 40Ar
1.3 x 109 yrs
-> 207Pb
0.71 x 109 yrs
By measuring the relative proportions of these isotopes in
rocks it is possible to fate them.
Note however that melting resets the clock so the ages
relate to the time that a rock was last molten.
1B11 How old is the Earth?
17th Century: Archbishop Ussher – 4004 BC
1788: James Hutton – “The abyss of time… no vestige of a
beginning, no prospect of an end”
1859: Darwin – more than 300 million years old
1900: Best estimates were about a billion years
1956: Patterson – 4.6 billion years from radiogenic lead
isotopes. This agrees with astronomical estimates for the age
of the Sun (estimate independently from the H-R diagram)
and with meteorites.
Note that most surface rocks are much younger, with ages
less than 600 million years.
1B11 Surface processes
Plate tectonics
Continental drift, fold mountains,
volcanism and earthquakes
Weathering
Wind, rain and ice form new
sedimentary rocks
Biology
Some erosion and sedimentation
processes. Atmospheric evolution.
Meteorite
impacts
More important in the past –
evidence removed by weathering
1B11 Structure of the atmosphere
150
ionosphere dissociation and
heating by solar
UV and X-rays
Height (km)
thermosphere
100
mesosphere
50
ozone layer
(heating)
stratosphere
15
troposphere
200
240
280
Temp (K)
320
Ground –
heated by
sunlight
1B11 Atmosphere cont.
The ground is heated by sunlight to a temperature of approx
300OK.
In the troposphere, the temperature gradient falls off by about
6O per km.
Pressure:
Ph  Ph0 e
where P(h0) = 1.01 x 105 Pa
h
8km
1B11 The magnetic field
The Earth’s
magnetic field
is a dipole (bar
magnet)
inclined at 12O
to the rotation
axis. Field
strength
B = 4 x 10-5 T
(small toy
magnets are ~
0.02T).
1B11 Origin of the magnetic field
The metallic core of the Earth is a conducting fluid. The
(nonuniform) rotation and convection currents in the core are
believed to generate “organized” currents, and thus a
magnetic field.
The Earth’s magnetic field reaches far beyond the planet
itself, and traps the charged particles which are emitted in the
solar wind. The particles become trapped in the magnetic
field, in the Van Allen belts.
The influence of the magnetic field reaches out even further,
for many hundreds of Earth radii. This region is called the
magnetosphere, which engulfs the Earth and channels most
solar wind particles away from the Earth.
1B11 The Magnetosphere
Origin of multi-celled animals (600M)
Time (x 109 years ago)
Atmospheric O2 increases
1B11 History of life on Earth
1
2
3
4
Eukaryotic cells appear (large,
complex cells containing a nucleus
(with DNA) and organelles which
perform respiration and
photosynthesis). Reproduce sexually.
Oldest micro-fossils / prokaryotic cells
- the simplest form of carbon based
life. Reproduce by cloning.
Origin of life
1B11 Photosynthesis
CO2 + H2O
CH2O + O2
chlorophyll
O2 is a waste product of photosynthesis and is
toxic to most photosynthesising organisms.
However it is probably required for large, multicellular animals.
1B11 The Moon – essential facts
Radius : 1738km (1/4 of the radius of the Earth)
Mass : 1/80 of the mass of the Earth
Mean density : 3.3 g cm-3
Distance from the Earth : 400,000 km
No atmosphere!!
Midday temp : +120OC
Midnight temp : -180OC
Sidereal rotation period = 27.3 days = orbital period
Surface dominated by impact craters
No magnetic field!!
1B11 Exploration summary
1959 – First impact – Luna2
1959 – First far-side images – Luna 3
1966 – First orbiter – Luna 10
1964-65 – 5 “Surveyor” landers
1966-67 – 5 Lunar Orbiters
1969-72 – Apollo
1994 – Clementine
1998 – Lunar Prospector
1B11 Lunar surface features
1. Highlands
2. Maria
3. Impact Basins
4. Regolith
5. Rilles
The highlands are bright and heavily cratered and cover
84% of the surface of the Moon. They are very old (at
least 4 billion years) and are the original lunar crust.
The maria are seen only on the near side. They are dark
regions with fewer craters and cover 16% of the surface.
They are relatively young (3-3.8 billion years old) and are
basaltic flood lavas which have filled impact basins.
1B11 Impact basins
Impact basins are very large impact craters, measuring at
least 300 km in diameter. They are surrounded by concentric
mountain ranges and are found all over the Moon.
They are only flooded on the nearside (by lava to form the
maria). This implies that the near side of the Moon has a
thinner crust.
The basins formed 3.9-4 billion years ago, but the final
flooding occurred up to 800 million years later.
Impact energies:
Meteorite:
D=5km, r=3g cm-3
and v=20 km/s
KE = 4x1022J = 107 MT TNT
=> Crater 50-100 km across
rim
slumping
flat floor
secondary
crater
ejecta
blanket
central peak
1B11 Impact craters
Extent of
“transition
cavity”
10’s of km
1B11 More surface features
A regolith is where the surface is covered by a layer of dust
(“soil”) produced by micro-meteorite impacts.
Approx 0.5mm every million years
Rilles are sinuous valleys cut by flowing lava.
Heavy
bombardment
epoch
flooding
of basins
origin of basins
origin of the Moon
number of craters
1B11 Impact cratering rate
The flux of impacting
meteorites decreased
rapidly in the Moon’s
early history.
Curve calibrated
using dated Apollo
rock samples
4
3
2
1
0
Age of surface (x109 years)
1B11 Basic Lunar geophysics
a. Seismicity
is very low, approx 2 x 1010J/yr (compared to the Earth,
approx 2 x 1018 J/yr)
b. Heatflow
measured at Apollo 15 and 17 sites to be approx 0.02 W
m-2, consistent with radiogenic heating
c. No evidence for current volcanic or tectonic activity
d. No magnetic field, so if a metal core exists, it’s probably
solid (more seismic data are needed)
1B11 Is there ice on the Moon?
In 1998, data from the Lunar Prospector indicated that water
ice is present at both the north and south lunar poles, in
agreement with Clementine results for the south pole
reported in November 1996.
The ice could represent relatively pristine cometary or
asteroid material which has existed on the Moon for millions
or billions of years. Deposits of ice on the Moon would have
many practical aspects for future manned lunar exploration.
Humans need water (!) and could provide hydrogen and
oxygen for rocket fuel.
However in 2003, radar signals beamed from the Arecibo
Observatory in Puerto Rico penetrated to depths of 20ft - but
found no sign of thick layers of ice.
1B11 Lunar cross-section
crust
iron-poor mantle
(density ~ 2.9 g
cm-3)
To Earth
zone of
moonquakes
(homogeneous
material)
iron-rich core (density
~ 3.5 g cm-3)
mare