Earthrise over Smythii impact basin with Schubert impact
crater on horizon. Views like this during Apollo missions
made it clear that Earth is part of a planetary system rather
than an isolated sphere, subject to the same bombardment
that battered the surface of the Moon. (Apollo 11 AS11-446551)
Earth, the “Third Rock from Sun” is
also called the “Lonely Planet”
because, to our knowledge as
yet, earth is the only
planet with
evidence of
life and it is
water that
creates this
uniqueness.
This uniqueness
comes from two
cycles that define
the Earth ...
hydrological cycle;
and
plate tectonics.
A conceptual look at the hydrological
cycle. Notice its following peculiarities:
1. Evaporation exceeds precipitation over the oceans,
but precipitation exceeds evaporation on land.
2. Currently, atmosphere transfers by precipitation all
the water it receives by evaporation.
3. Run-off carries excess water as also rock materials
from land to to oceans.
Evaporation
60,000 km3
Evaporation
320,000 km3
Precipitation
285,000 km3
Ocean
Storage
1,370,000,000 km3
Precipitation
3
Run-off: 95,000 km
35,000
km3
Hydrological Cycle
and Global Warming ─ the
Science of Climate Change
Abundance Relative to Si = 106
H and O, the two elements that
comprise the water molecule, are quite
common.
12
10
108
104
100
10-4
0
20
40
60
Atomic Number
Venus and Mars are Earth’s
immediate neighbors and
compositionally similar to
the Earth.
Mean Distance from Sun
Mass
Diameter
Length of Day
Length of Year
Surface Gravity
Venus
Earth
Mars
108.2 million km
0.815 AU
12,104 km
243 Earth days
225 Earth days
0.91 AU
149.6 million km
1 AU (5.974x1024 kg)
12,756 km
24 hours
365.2422 days
1 AU
228 million km
0.107 AU
6,794 km
24.6 Earth hours
687 Earth days
0.377 AU
Earth is farther from the Sun
compared to Venus, and Mars is
still farther.
The Atmospheres of Venus, Earth and Mars
Venus
Earth
Mars
Mean Surface
Temperature
457° C
855° F
15° C
60° F
-55° C
-67° F
Mean Surface
Pressure
90 bars
1 bar
0.007 bars
Mean density
5.204 gm/cm3
5.52 gm/cm3
3.933 gm/cm3
Major Gases
77% Nitrogen
96% Carbon
21% Oxygen
Dioxide
3.5% Nitrogen 0.93% Argon
~ 1% water
(varies)
95% Carbon
Dioxide
2.7% Nitrogen
1.6% Argon
1.3% Oxygen
http://www.planetary.org/saturn/atmos_compare.html
Venus
Thick atmosphere
contains 96%
CO2. Average
temperature:
420°C
Earth
0.03% of CO2 in the atmosphere.
Average temperature: 15°C
Mars
Thin atmosphere
(almost all CO2 in
ground). Average
temperature: -50°C
Whole
Universe Earth
Hydrogen H 74.500
Helium He 23.840
Oxygen O
Carbon C
Nitrogen N
Silicon
Neon
Magnesium
Iron
Sulphur
Aluminum
Calcium
Nickel
Sodium
Argon
Chromium
Phosphorous
Manganese
Chlorine
Potassium
Other elements
Si
Ne
Mg
Fe
S
Al
Ca
Ni
Na
Ar
Cr
P
Mn
Cl
K
0.8200 29.8
0.3750
0.0910
0.0830
0.0550
0.0570
0.1040
0.0380
0.0066
0.0074
0.0092
0.0033
0.0030
0.0032
0.0009
0.0011
0.0006
0.0003
15.6
13.9
33.3
1.5
1.8
2.0
0.2
1.9
The whole earth is richer in
Fe, Mg and Ni, and poorer
in Si, K and Al, than what
is found on the earth’s
surface.
Crust
Mantle
Outer
core
Inner
core
Whole Earth density = 5.5 gm/cm3
Density of the crust = 2.7 gm/cm3
How about water
on Mars?
Mars is ...
Earth Mars
SiO2 45.1% 44.4%
MgO 38.3% 30.2%
FeO
7.8% 17.9%
Al2O3 4.0% 3.0%
CaO
3.5% 2.4%
Cr2O3 0.5% 0.8%
Na2O
0.3% 0.5%
MnO
TiO2
K2O
0.1% 0.5%
0.2% 0.1%
0.03% 0.04%
Mars has two
moons, Phobos
and Deimos.
 compositionally similar to Earth; and
 appears to have once had water.
The problem is that Mars ...
 lacks the atmosphere that would have
enabled it to retain water; and
 no longer has the plate tectonics that the
planet appears to have once had.
This image is a map of
Martian magnetic fields in
the southern highlands. It
is where magnetic stripes
possibly resulting from
crustal movement are
most prominent.
http://science.nasa.gov/newhome/headlines/ast29apr99_1.htm
Some images
of the Martian
surface
"Twin Peaks" on the
horizon of the Mars
Pathfinder landing site.
Sunset on
Mars
An enhanced image of a
Martian sunset as seen
by the Sojourner rover in
1997.
Without the run-off from land, ocean may eventually
dry-up but that can occur only if water gets locked
up in the atmosphere. Shouldn’t that foggy
atmosphere then end up lowering
the evaporation rate?
Evaporation
320,000 km3
Evaporation
60,000 km3
Precipitation
95,000 km3
Precipitation
285,000 km3
Ocean Storage
1,370,000,000 km3
Run-off: 35,000 km3
The hydrological cycle is self-destructive
The run-off from land
also erodes the rocks
and deposits this
eroded material in the
oceans, at the rate of
~15 billion metric tons
per year. As the
calculations alongside
show, this should take
no more than ~200 Ma
to fill up the ocean
basins. The run-off
component of the
hydrological cycle
should thus eliminate
the hydrological cycle
in ~200 Ma.
Time run-off needs to fill the ocean basins
http://www.ngdc.noaa.gov/mgg/image/sedthick9.jpg
The Wilson
Cycle
Solar heat received at the surface of Venus
is about the same as that received on the
Earth’s surface and on the surface of Mars.
Expected Observed
surface surface
tempetemperature
rature
Relative
distance
from Sun
Solar heat received...
... at the
... at the
planetary
planetary
location
surface
Venus
0.72 AU
~2500 W/m2
~650 W/m2
323°K
730°K
Earth
1.00 AU
~1360 W/m2
~680 W/m2
276°K
281°K
Mars
1.52 AU
~ 600 W/m2
~600 W/m2
215°K
215°K
Therefore, distance from Sun is not the
reason why Earth has abundance of water
and Venus and Mars lack water.
Temperature profiles
of the atmospheres
of Venus
and Earth
Major constituents of Seawater at
3.5% Salinity
Constituent
Water:
Oxygen (O)
Hydrogen (H)
85.8%
10.7%
The most abundant ions
Chloride (Cl-)
Sodium (Na+)
Sulfate (SO42-)
Magnesium (Mg2+)
Calcium (Ca2+)
Potassium (K+)
Bicarbonate (HCO3-)
1.9%
1.1%
0.3%
0.1%
0.04%
0.04%
0.01%
only 2% of Cl in
seawater could
have come from
land sources
only 20% of sulfur
in seawater could
have come from
land sources
Annual volcanic output x Age of the Earth
Estimated existing quantity
10,000
Quadrillion (1015)
Metric Tons
Water
1,000
100
10
Sulfur
Chlorine
Carbon
Nitrogen
1
This comparison of the total quantities of selected substances in the
oceans and atmosphere with what could have come from volcanism
favors the volcanic origin of these substances.
Adapted from Robert Decker & Barbara Decker: VOLCANOES (W.H. Freeman, New York, 1996)
A trio of frames from
Polar's Visible Imaging
System (VIS), taken 6
seconds apart on
December 31, 1998,
captures an object
rapidly descending
toward northern
Europe. Because the
camera's filter isolates
emission from hydroxyl
(OH) radicals, the
incoming object must
have contained
abundant water.
The case for extraterrestrial origin of oceans
Comets are >40% water.
The deuterium/hydrogen
ratio of comets and oceans
overlap.
Deuterium
ratio
Total Hydrogen
Comet Halley 0.06-0.48 ppt
Earth’s Oceans
0.16 ppt
During the initial 2 Ga of its history, Earth may well have
received 2 x 108 to 1 x 1017 metric tons of cometary matter by
way of bombardment episodes.
Comets may well have contributed significantly, therefore, to
the hydrospheric mass of 1.4-1.7 x 1018 metric tons. Indeed, all
this water could have been produced by either ~10% of the
cometary mass or entirely by the asteroidal source if initial
bombardment was of carbonaceous chondrites.
Adapted from C.F. Chyba & C. Sagan in COMETS AND THE ORIGIN AND EVOLUTION OF LIFE
(Ed: P.J. Thomas, C.F. Chyba & C.P. McKay; Springer-Verlag, New York, 1997).
Suppose
• annual influx from outer space is 50100 billion gallons of water vapor into
the atmosphere,
Compare this to the
and that
• this rate has been
constant through
geological history
(~4.5 billion years).
This amounts to 225450 x 1018 gallons of
water.
total amount of
water in the oceans
= 1370x106 Km3
(volume)
 109 m3/Km3
 264.2 gallons/m3
= 362  1018 gallons
of water