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 AS )

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.

Evaporation 320,000 km 3 Precipitation 285,000 km 3 Precipitation 95,000 km 3 Run-off: 35,000 km 3 Ocean Storage 1,370,000,000 km 3 Sediments 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 km 3 Hydrological Cycle and Global Warming ─ the Science of Climate Change

H and O, the two elements that comprise the water molecule, are quite common Atomic Number Abundance Relative to Si = 10 6

Mean Distance from Sun Mass Diameter Length of Day Length of Year Surface Gravity million km AU 12,104 km 243 Earth days 225 Earth days 0.91 AU million km 1 AU (5.974x10 24 kg) 12,756 km 24 hours days 1 AU 228 million km AU 6,794 km 24.6 Earth hours 687 Earth days AU Venus EarthMars Venus and Mars are Earth’s immediate neighbors and compositionally similar to the Earth.

Earth is farther from the Sun compared to Venus, and Mars is still farther.

Mean Surface Temperature Mean Surface Pressure Mean density Major Gases 15° C 60° F 1 bar 5.52 gm/cm 3 77% Nitrogen 21% Oxygen 0.93% Argon ~ 1% water (varies) Earth -55° C -67° F bars gm/cm 3 95% Carbon Dioxide 2.7% Nitrogen 1.6% Argon 1.3% Oxygen Mars 457° C 855° F 90 bars gm/cm 3 96% Carbon Dioxide 3.5% Nitrogen Venus The Atmospheres of Venus, Earth and Mars

Mars Thin atmosphere (almost all CO 2 in ground). Average temperature: -50°C Earth 0.03% of CO 2 in the atmosphere. Average temperature: 15°C Venus Thick atmosphere contains 96% CO 2. Average temperature: 420°C

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. Whole Earth density = 5.5 gm/cm 3 Density of the crust = 2.7 gm/cm 3 Outer core Mantle Core Crust Rocky Metallic Inner core Hydrogen Helium Oxygen Carbon Nitrogen Silicon Neon Magnesium Iron Sulphur Aluminum Calcium Nickel Sodium Argon Chromium Phosphorous Manganese Chlorine Potassium Other elements H He O C N Si Ne Mg Fe S Al Ca Ni Na Ar Cr P Mn Cl K Universe Whole Earth

SiO 2 MgO FeO Al 2 O 3 CaO Cr 2 O 3 Na 2 O MnO TiO 2 K 2 O Earth 45.1% 38.3% 7.8% 4.0% 3.5% 0.5% 0.3% 0.1% 0.2% 0.03% Mars 44.4% 30.2% 17.9% 3.0% 2.4% 0.8% 0.5% 0.1% 0.04% How about water on Mars? Mars is...  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. Mars has two moons, Phobos and Deimos. 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.

Some images of the Martian surface

Sunset on Mars An enhanced image of a Martian sunset as seen by the Sojourner rover in "Twin Peaks" on the horizon of the Mars Pathfinder landing site.

Evaporation 320,000 km 3 Ocean Storage 1,370,000,000 km 3 Precipitation 285,000 km 3 Precipitation 95,000 km 3 Evaporation 60,000 km 3 Run-off: 35,000 km 3 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?

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

The Wilson Cycle

Therefore, distance from Sun is not the reason why Earth has abundance of water and Venus and Mars lack water. 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. Venus Earth Mars 0.72 AU 1.00 AU 1.52 AU ~2500 W/m 2 ~1360 W/m 2 ~ 600 W/m 2 ~650 W/m 2 ~680 W/m 2 ~600 W/m 2 323°K 276°K 215°K 730°K 281°K 215°K Solar heat received at the planetary location... at the planetary surface Expected surface tempe- rature Observed surface tempe- rature Relative distance from Sun

Temperature profiles of the atmospheres of Venus and Earth

Major constituents of Seawater at 3.5% Salinity Constituent Water: Oxygen (O) Hydrogen (H) The most abundant ions Chloride (Cl - ) Sodium (Na + ) Sulfate (SO ) Magnesium (Mg 2+ ) Calcium (Ca 2+ ) Potassium (K + ) Bicarbonate (HCO 3 - ) 85.8% 10.7% 1.9% 1.1% 0.3% 0.1% 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

1, ,000 Quadrillion (10 15 ) Metric Tons Estimated existing quantity Annual volcanic output x Age of the Earth Water Sulfur Nitrogen Carbon Chlorine 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 Deuterium Total Hydrogen ratio Comet Halley Earth’s Oceans ppt 0.16 ppt Comets are >40% water. The deuterium/hydrogen ratio of comets and oceans overlap. During the initial 2 Ga of its history, Earth may well have received 2 x 10 8 to 1 x metric tons of cometary matter by way of bombardment episodes. Comets may well have contributed significantly, therefore, to the hydrospheric mass of x 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 billion gallons of water vapor into the atmosphere, and that Compare this to the total amount of water in the oceans = 1370x10 6 Km 3 (volume)  10 9 m 3 /Km 3  gallons/m 3 =362  gallons of water this rate has been constant through geological history (~4.5 billion years). This amounts to x gallons of water.