1B11 Foundations of Astronomy

The Earth as a planet

Liz Puchnarewiczemp@mssl.ucl.ac.ukwww.ucl.ac.uk/webctwww.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.

inner core solid (?)

outer core liquid (Fe, Ni)

mantle silicates – slow convection

asthenosphere (partially molten)

6380km

lithosphere (includes

crust)

5000km 2900km 250km100km

7000K 5000K 3800K 1300K

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

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.

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.

Continental masses are mostly granite and float on the basalt.

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 –

Travel through the “body” of the Earth.

P-waves (pressure or primary) are compressional waves. Move fastest, ~ 6km/s

S-waves (shear or secondary) transverse waves, travel at ~2km/s

Surface waves –

Both are transverse

Rayleigh waves describe the vertical motion. Slowest waves.

Love waves describe the horizontal motion.

1B11 Seismic wave transport

seismometer

p-wave

s-wave

surface wave

grou

nd

mot

ion

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

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

%ag

e of

par

ent

isot

ope

rem

aini

ng

0 12

½ 2

5

50

1

00

If n0 is the original number of

parent atoms and n the number remaining at time t, then:

0.693t

n

n

0

exp

where is the half-life.

1B11 Cross-section through the Earth

The outer crust is about 30km thick.

inner core solid (?)

outer core liquid (Fe, Ni)

mantle silicates – slow convection

asthenosphere (partially molten)

6380km

lithosphere (includes

crust)

5000km 2900km 250km100km

7000K 5000K 3800K 1300K

1B11 The outer layers & upper mantle

250km

40km

0km

5kmoceanic crust, ~2.9g cm-3

continental crust

continental shelf

sea level

Lithosphere (“rigid”)

Asthenosphere (“plastic”)

continental root30km thick, ~2.7g cm-3

base of upper mantle 400km below

~3.3 g cm-3

1B11 Lithosphere as a “condensate”

melting temp

depth (km)

Tem

p (K

)

0 500 1000 1500 2000

0

10

00

2

000

3000

400

0

temp

lower mantlelithosphere

asthenosphere

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.

mid-oceanic ridge

(new crust)

volcanoes

continental crust

earthquakes

melting due to release

of pressure

oceanic lithosphere

Mantle convection

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. Continental shapes

2. Biological and fossil evidence

3. Earthquake and volcano distribution

4. Topography of the ocean floor

5. 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

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.

87Rb -> 87Sr 48 x 109 yrs

238U -> 206Pb 4.5 x 109 yrs40K -> 40Ar 1.3 x 109 yrs

235U -> 207Pb 0.71 x 109 yrs

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

Weathering

Biology

Meteorite impacts

Continental drift, fold mountains, volcanism and earthquakes

Wind, rain and ice form new sedimentary rocks

Some erosion and sedimentation processes. Atmospheric evolution.

More important in the past – evidence removed by weathering

1B11 Structure of the atmosphere

Temp (K)

150

100

50

15

Hei

ght

(km

)

200 240 280 320

thermosphere

mesosphere

stratosphere

troposphere

ozone layer (heating)

ionosphere - dissociation and heating by solar

UV and X-rays

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:

8kmh

0hPhP

e

where P(h0) = 1.01 x 105 Pa

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

1B11 History of life on Earth

Tim

e (x

109

yea

rs a

go) 1

2

3

4

Atm

osph

eric

O2 in

crea

ses Origin of multi-celled animals (600M)

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, multi-cellular 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.

1B11 Impact craters

Impact energies: Meteorite: D=5km, =3g cm-3 and v=20 km/s

KE = 4x1022J = 107 MT TNT => Crater 50-100 km across

secondary crater

ejecta blanket ce

ntra

l pea

k

flat

floor

slum

ping

rim

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.

1B11 Impact cratering rate

num

ber

of c

rate

rs

4 3 2 1 0Age of surface (x109 years)

orig

in o

f th

e M

oon

orig

in o

f ba

sins

flood

ing

of b

asin

s

Curve calibrated using dated Apollo rock samples

Heavy bombardment epoch

The flux of impacting meteorites decreased rapidly in the Moon’s early history.

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

iron-poor mantle (density ~ 2.9 g cm-3)

crust

zone of moonquakes (homogeneous material)

iron-rich core (density ~ 3.5 g cm-3)

mare

To Earth