NSCI 314

LIFE IN THE COSMOS

8 – SEARCHING FOR LIFE IN OUR SOLAR SYSTEM

Dr. Karen KolehmainenDepartment of Physics

CSUSB

http://physics.csusb.edu/~karen/

HOW TO SEARCH FOR LIFE IN OUR SOLAR SYSTEM (AND OTHER SOLAR SYSTEMS)

OBSERVATIONS WITH TELESCOPES AND AUXILIARY EQUIPMENT

WHAT CAN A TELESCOPE DO?– GATHER MORE LIGHT THAN THE NAKED EYE (MAKE

FAINT OBJECTS BRIGHTER)– MAGNIFY OR ENLARGE IMAGE– PICK OUT DETAILS IN IMAGE (IMPROVE RESOLUTION)– CAN USE A VARIETY OF WAVELENGTHS (NOT JUST

VISIBLE LIGHT)– CAN DO SPECTROSCOPIC ANALYSIS

SPECTROSCOPIC ANALYSIS SPLIT LIGHT COMING FROM AN OBJECT INTO

DIFFERENT WAVELENGTHS (COLORS) USING A PRISM OR DIFFRACTION GRATING.

MEASURE HOW BRIGHTNESS VARIES WITH WAVELENGTH.

BY MEASURING WHICH WAVELENGTHS OF LIGHT ARE EMITTED OR ABSORBED (EMISSION AND ABSORPTION LINES), WE CAN DETERMINE THE COMPOSITION OF ANY MATTER THAT INTERACTS WITH THE LIGHT.– THIS IS HOW WE CAN FIND THE COMPOSITION

OF STARS.– WHEN LIGHT FROM A STAR PASSES THROUGH

THE ATMOSPHERE IS A PLANET, WE CAN ALSO DETERMINE THE COMPOSITION OF THE PLANET'S ATMOSPHERE.

HOW TO SEARCH FOR LIFE WITHIN OUR SOLAR SYSTEM

USE SPACECRAFT VISITS. PHOTOGRAPHS CAN BE TAKEN FROM CLOSER

DISTANCES, SHOWING GREATER DETAIL. OTHER MEASUREMENTS CAN BE MADE. WHAT'S BEEN DONE SO FAR:

– FLYBY (ALL PLANETS, PLUS MANY MOONS)– ORBITER (EARTH'S MOON, VENUS, MARS, JUPITER,

SATURN)– PROBE DESCENDS INTO ATMOSPHERE (JUPITER)– LANDER (EARTH'S MOON, VENUS, MARS, SATURN'S

MOON TITAN) – SAMPLE RETURN (EARTH'S MOON)– HUMAN VISITS (EARTH'S MOON)

TO ESTIMATE HOW EASY IT WOULD BE FOR US TO FIND LIFE (IF IT EXISTS) ON ANOTHER PLANET IN OUR SOLAR SYSTEM, TURN THE QUESTION AROUND: HOW COULD A HYPOTHETICAL ALIEN

CIVILIZATION (WITH TECHNOLOGY SIMILAR TO OURS) ON A NEARBY PLANET (IN OUR SOLAR SYSTEM) TELL THERE IS LIFE ON EARTH?– SPECTROSCOPIC DETECTION OF GASES IN THE

ATMOSPHERE THAT ARE PRODUCED BY LIVING ORGANISMS: OXYGEN AND MAYBE METHANE

– INFRARED OBSERVATIONS CAN DETECT SEASONAL CHANGES IN VEGETATION (DIFFICULT)

HOW COULD AN ALIEN ON A NEARBY PLANET TELL THERE IS A TECHNOLOGICALLY ADVANCED CIVILIZATION ON EARTH? LIGHTS FROM CITIES VISIBLE AT NIGHT

TV AND FM RADIO BROADCASTS ESCAPE INTO SPACE

SPECTROSCOPIC DETECTION OF GASES IN THE ATMOSPHERE THAT ARE PRODUCED BY TECHNOLOGY (CFCs, FOR EXAMPLE)

REQUIREMENTS FOR LIFE ELEMENTS LIKE CARBON (OR SILICON?), HYDROGEN, OXYGEN,

NITROGEN – ENOUGH ALMOST EVERYWHERE

SIMPLE ORGANIC MOLECULES – PROBABLY ENOUGH IN MANY LOCATIONS

SOURCE OF ENERGY – SUNLIGHT (MORE AVAILABLE CLOSER TO SUN)– HEAT FROM INSIDE PLANET OR MOON– CHEMICAL SOURCES

PROTECTED ENVIRONMENT– ATMOSPHERE – OCEANS– UNDERGROUND

LIQUID SOLVENT (WATER OR SOMETHING ELSE?)– REQUIRES CORRECT TEMPERATURE RANGE

WHAT SHOULD A LIQUID SOLVENT DO?

MUST BE ABUNDANT MUST BE ABLE TO DISSOLVE A LOT OF DIFFERENT

MATERIALS IN IT SHOULD REMAIN LIQUID OVER A LARGE

TEMPERATURE RANGE TEMPERATURE RANGE OVER WHICH SOLVENT IS

LIQUID SHOULDN’T BE TOO HIGH OR TOO LOW– IF TEMPERATURE RANGE IS TOO LOW, CHEMICAL

INTERACTIONS PROCEED TOO SLOWLY– IF TEMPERATURE RANGE IS TOO HIGH, MOLECULES ARE

BROKEN APART TOO EASILY MIGHT BE BENEFICIAL IF IT HELPS ORGANISMS

REGULATE THEIR INTERNAL TEMPERATURE MIGHT BE BENEFICIAL FOR THE ORIGIN OF LIFE IF

IT CAN HELP CELLS FORM MORE EASILY

SPECIFIC AND LATENT HEAT SPECIFIC HEAT = THE AMOUNT OF ENERGY

THAT NEEDS TO BE ADDED TO 1 GRAM OF MATERIAL TO RAISE ITS TEMPERATURE BY 1oC

LATENT HEAT OF VAPORIZATION = THE AMOUNT OF HEAT THAT NEEDS TO BE ADDED TO BOIL ONE GRAM OF LIQUID MATERIAL (ASSUMING IT IS ALREADY AT THE APPROPRIATE TEMPERATURE, THE “BOILING POINT”).

SPECIFIC HEAT AND LATENT HEAT ARE DIFFERENT FOR DIFFERENT MATERIALS

SPECIFIC AND LATENT HEAT A MATERIAL WITH A LARGE SPECIFIC HEAT DOESN’T

UNDERGO EXTREME VARIATIONS IN TEMPERATURE. SINCE LARGE AMOUNTS OF ENERGY MUST BE ADDED (REMOVED) TO INCREASE (DECREASE) THE TEMPERATURE VERY MUCH, THE TEMPERATURE TENDS NOT TO CHANGE VERY MUCH.

IF A LIQUID WITH A LARGE LATENT HEAT

EVAPORATES, IT CARRIES A LOT OF HEAT AWAY WITH IT.

IF A LIQUID SOLVENT HAS A LARGE SPECIFIC HEAT AND A LARGE LATENT HEAT, ORGANISMS THAT USE IT WILL BE ABLE TO MAINTAIN A CONSTANT TEMPERATURE MORE EASILY.

ADVANTAGES OF WATER ABUNDANT (2ND MOST COMMON MOLECULE IN

INTERSTELLAR SPACE)

DISSOLVES MANY MATERIALS (BECAUSE H2O MOLECULE IS “POLAR”)

STAYS LIQUID OVER A LARGE TEMPERATURE RANGE

VERY LARGE SPECIFIC HEAT AND LATENT HEAT – KEEPS TEMPERATURE FROM VARYING TOO MUCH

LARGE SURFACE TENSION – DROPS FORM EASILY, MAY BE IMPORTANT FOR THE ORIGIN OF LIFE (FORMATION OF CELLS)

ADVANTAGES OF WATER EXPANSION UPON FREEZING

– COULD BE AN ADVANTAGE BECAUSE:• ICE FLOATS ON LIQUID WATER• THIS PREVENTS BODIES OF WATER FROM

FREEZING COMPLETELY THROUGH, WHICH WOULD KILL ORGANISMS THERE

– COULD BE A DISADVANTAGE BECAUSE:• CELLS RUPTURE WHEN THEY FREEZE

– COULD BE NEUTRAL IF TEMPERATURE NEVER GETS LOW ENOUGH FOR FREEZING

OTHER SOLVENTS? CONSIDER AMMONIA (NH3), METHANE (CH4),

AND ETHANE (C2H6):– ALL ARE RELATIVELY ABUNDANT

– NOT POLAR MOLECULES - CAN’T DISSOLVE AS MANY MATERIALS AS WATER

– LOW SURFACE TENSION - HARDER TO FORM DROPS

– ETHANE IS LIQUID OVER A LARGE TEMPERATURE RANGE, METHANE AND AMMONIA ARE NOT

– ALL ARE LIQUID AT LOW TEMPERATURES (MUCH LOWER THAN WATER) – METABOLIC PROCESSES WOULD BE SLOW

– AMMONIA HAS A REASONABLY LARGE SPECIFIC HEAT AND LATENT HEAT, METHANE AND ETHANE DO NOT

– NONE EXPAND UPON FREEZING (LIKE WATER DOES)

OTHER SOLVENTS? EXCEPT FOR WATER, AMMONIA,

METHANE, AND ETHANE, MOST OTHER POSSIBLE LIQUID SOLVENTS AREN’T VERY ABUNDANT.

WATER SEEMS LIKE THE BEST CHOICE AS A LIQUID SOLVENT FOR LIFE, BUT WE CAN'T EXCLUDE THE POSSIBILITY OF AMMONIA, METHANE, OR ETHANE.

INNER (TERRESTRIAL) PLANETS

MERCURY, VENUS, EARTH & MARS

WARMER, CLOSER TO SUN

RELATIVELY LOW MASS

DENSE, ROCKY, FEW LIGHT ELEMENTS

FEW OR NO MOONS

NO RINGS

OUTER (JOVIAN) PLANETS JUPITER, SATURN, URANUS, NEPTUNE

COLDER, FARTHER FROM SUN

HIGH MASS

LOW DENSITY, MOSTLY GASEOUS, LOTS OF LIGHT ELEMENTS

LOTS OF MOONS (LIKE MINIATURE SOLAR SYSTEMS)

ALL HAVE RINGS

CHARACTERISTICS OF PLANETS

Planet Diameter(Earth=1)

Mass(Earth=1)

AverageDensity

(gm/cm³)

OrbitalPeriod(Years)

DistanceFrom the Sun

(AU)

MercuryVenusEarthMars[Asteroids]JupiterSaturnUranusNeptune

0.380.951.000.53

-11.29.54.03.9

0.060.811.000.11

-318951517

5.45.35.54.0-

1.30.71.31.6

0.240.621.001.88

-122984

165

0.390.721.001.52

[Avg. = 2.77]5.29.51930

1. FLAT SHAPE (PLANETARY ORBITS ARE IN THE SAME PLANE)•EXPLAINED BY CONSERVATION OF ANGULAR MOMENTUM

•NEBULA FLATTENS AS IT CONTRACTS

2. COMMON DIRECTION OF PLANETARY ORBITAL MOTION, MOST PLANETS’ ROTATIONS, & ORBITS OF MOST MOONS•EXPLAINED BY CONSERVATION OF ANGULAR MOMENTUM

•DIRECTION OF ROTATION IS THE SAME AS THAT OF ORIGINAL NEBULA

3. ORIGIN OF COMETS AND ASTEROIDS•LEFTOVER PLANETESIMALS THAT DIDN'T GET INCORPORATED INTO PLANETS

1. SOLAR SYSTEM PROPERTIES AS EXPLAINED BY THE NEBULAR THEORY OF FORMATION

1. - OUTER PLANETS ARE MUCH LARGER AND MORE MASSIVE THAN INNER PLANETS.

2. - INNER PLANETS ARE MADE OF ROCKY MATERIALS.

3. - OUTER PLANETS ARE MADE MOSTLY OF GAS.

4. - MOONS OF OUTER PLANETS ARE MADE MOSTLY OF ICE.

•FAST-MOVING GAS MOLECULES ESCAPED THE GRAVITY OF PLANETS IN THE HOTTER INNER SOLAR NEBULA. SLOWER MOVING GAS PARTICLES WEREN'T ABLE TO ESCAPE THE GRAVITY OF PLANETS IN THE COOLER OUTER NEBULA. ICE PARTICLES CONDENSED IN THE COOL OUTER NEBULA, BUT IT WAS TOO HOT IN THE INNER NEBULA.

• - ALSO RECALL THAT LARGER PLANETS AND MOONS COOL MORE SLOWLY THAN SMALL ONES, AND THEREFORE ARE STILL HOTTER INSIDE.

• DIFFERENCES BETWEEN INNER AND OUTER PLANETS AS EXPLAINED BY THE NEBULAR THEORY OF FORMATION

LOOSE ENDS SEVERAL MOONS ARE PROBABLY CAPTURED

KUIPER BELT OBJECTS (COMETS)– TRITON (NEPTUNE) – RETROGRADE ORBIT

SEVERAL MOONS ARE PROBABLY CAPTURED ASTEROIDS– PHOBOS AND DEIMOS (MARS)

EVIDENCE OF COLLISIONS– ORIGIN OF EARTH'S MOON

EVIDENCE OF NEAR-COLLISIONS– VENUS HAS RETROGRADE ROTATION– URANUS’ ROTATION AXIS IS TIPPED SIDEWAYS

COMETS “DIRTY SNOWBALLS” – WATER ICE, CO2 ICE,

SIMPLE ORGANIC MOLECULES, DUST, GRAVEL

NUCLEUS IS A FEW KM TO A FEW HUNDRED KM ACROSS, IRREGULARLY SHAPED

MOST ORBITS ARE HIGHLY ELLIPTICAL – VERY CLOSE TO SUN AT ONE END & VERY FAR FROM SUN AT THE OTHER END

ORBITS CAN BE IN ANY PLANE

LEFTOVER ICY PLANETESIMALS FROM FORMATION OF SOLAR SYSTEM

SOURCES OF COMETS OORT CLOUD

– SPHERICAL RESERVOIR OF COMETS

– HUGE - RADIUS OF APPROXIMATELY 1 LY (60,000 AU)

– PLANETESIMALS THAT WERE EJECTED OUTWARD

– COMETS HERE CAN BE SHIFTED INTO NEW ORBITS BY PASSING STARS

KUIPER BELT – SMALLER DISK-SHAPED RESERVOIR OF COMETS

– RADIUS OF APPROXIMATELY 50 AU

– INCLUDES PLUTO AND ERIS AS LARGEST OBJECTS

– PLANETESIMALS THAT WERE NOT EJECTED OUTWARD

– COMETS HERE CAN BE SHIFTED INTO NEW ORBITS BY MASSIVE PLANETS

NEW ORBIT CAN BRING COMET INTO INNER SOLAR SYSTEM, WHERE WE SEE IT

COMETS WHEN NEAR THE SUN

ICE SUBLIMATES (SOLID GAS), RELEASING DUST GRAINS FROM NUCLEUS

RELEASED GAS & DUST FORM A CLOUD CALLED COMA (CAN BE LARGER THAN THE SUN)

GAS IS IONIZED BY SUNLIGHT AND PUSHED DIRECTLY AWAY FROM SUN BY PRESSURE OF PHOTONS, FORMING BLUE GAS TAIL

DUST LAGS BEHIND COMET IN ORBIT, FORMING YELLOWER DUST TAIL

ASTEROIDS ROCKY OBJECTS

UP TO FEW HUNDRED KM ACROSS

MOST ARE IRREGULARLY SHAPED, NOT SPHERICAL

ORBIT THE SUN, MOST BETWEEN THE ORBITS OF MARS AND JUPITER

LEFTOVER ROCKY PLANETESIMALS THAT DIDN’T FORM A PLANET (PROBABLY BECAUSE OF JUPITER’S GRAVITY)

METEORS, METEOROIDS, AND METEORITES METEOROIDS: SMALL ROCKY OBJECTS

ORBITING THE SUN – SMALLER THAN ASTEROIDS

METEORS: METEOROIDS THAT HAVE ENTERED EARTH’S ATMOSPHERE, USUALLY VAPORIZE DUE TO FRICTIONAL HEATING, “SHOOTING STARS” OR “FALLING STARS”

METEORITES: METEORS THAT HAVE PARTIALLY SURVIVED ATMOSPHERIC PASSAGE AND HIT THE GROUND

METEORITES TYPES (BASED ON COMPOSITION):

– METALLIC (IRON, NICKEL, ETC.)– STONY-IRON– STONY

• CARBONACEOUS CHONDRITES

CARBONACEOUS CHONDRITES CONTAIN WATER AND ORGANIC COMPOUNDS, INCLUDING 74 AMINO ACIDS (50% L, 50% R), ALL 5 GENETIC BASES, FATTY ACIDS, ETC.

BEFORE WE EXAMINE EACH PLANET IN DETAIL...

WHAT IS A PLANET, ANYWAY?

YOU MAY HAVE HEARD THAT PLUTO IS NO LONGER CONSIDERED TO BE A PLANET.

WHY?

FIRST, LET'S LOOK AT SOME BASIC FACTS ABOUT PLUTO.

DIAMETER: 20% OF EARTH MASS: 0.2% OF EARTH DENSITY: 2.1 gm/cm³

SUNLIGHT: 0.06% OF EARTH TEMP: 45 K (-230 oC OR –380 oF) ATMOSPHERE: VERY THIN, METHANE (CH4) ICE

CRYSTALS SURFACE: ICE AND ROCK, METHANE (CH4) ICE MOON: CHARON - COMPOSITION SIMILAR TO

THAT OF PLUTO, HALF THE SIZE OF PLUTO WAY TOO COLD FOR LIFE, NO LIQUIDS PRESENT

PLUTO

EVEN BEFORE PLUTO WAS DOWNGRADED FROM BEING A PLANET, IT WAS KNOWN TO BE AN UNUSUAL PLANET BECAUSE:

EVEN THOUGH ITS POSITION IN THE SOLAR SYSTEM (FARTHEST FROM THE SUN) WOULD MAKE IT AN “OUTER PLANET,” ITS SIZE AND COMPOSITION ARE NOT LIKE THE OTHER OUTER PLANETS.– PLUTO IS SMALLER THAN ANY PLANET IN THE SOLAR

SYSTEM (ONLY 2/3 SIZE OF EARTH’S MOON), WHILE OUTER PLANETS ARE LARGE.

– COMPOSITION IS A MIXTURE OF ICE AND ROCK (NOT A GAS GIANT LIKE OUTER PLANETS) – MORE LIKE THE COMPOSITION OF THE MOONS OF OUTER PLANETS OR BIG KUIPER BELT OBJECTS (COMETS)

ORBIT IS WEIRD– NOT QUITE IN THE SAME PLANE AS ORBITS OF PLANETS– LESS CIRCULAR THAN ORBITS OF PLANETS (PLUTO'S

ORBIT ACTUALLY CROSSES NEPTUNE'S ORBIT)

DISCOVERED IN 2005 DIAMETER: 20 TO 25% OF EARTH (A LITTLE LARGER

THAN PLUTO) MASS: PROBABLY 0.2 TO 0.3% OF EARTH DENSITY: PROBABLY ABOUT 2 gm/cm³ SUNLIGHT: 0.01% OF EARTH TEMPERATURE: ABOUT 30 K (-245 oC OR –405 oF) ATMOSPHERE: ?? (NOT MASSIVE ENOUGH TO HOLD

ONTO MUCH OF ONE) SURFACE: ICE AND SOME ROCK, METHANE (CH4) ICE MOON: HAS AT LEAST ONE PROBLEM: IF PLUTO IS CONSIDERED A PLANET, ERIS

MUST BE A PLANET TOO. (IT IS LARGER THAN PLUTO.) THERE ARE ALSO SEVERAL OTHER RECENTLY DISCOVERED OBJECTS THAT ARE ALMOST AS LARGE AS PLUTO.

ERIS

ERIS AND PLUTO ARE SIMILAR. BOTH ARE:•SMALLER THAN ALL 8 PLANETS (MERCURY THROUGH NEPTUNE)•MOSTLY ICE, A LITTLE ROCK•ORBITS AREN'T AS CIRCULAR AS THOSE OF THE 8 PLANETS•ORBITS ARE TIPPED SLIGHTLY RELATIVE TO THE DISK FORMED BY THE PLANE OF THE 8 PLANETS

SEEM MORE LIKE BIG KUIPER BELT OBJECTS •RECALL THAT THE KUIPER BELT IS THE INNER GROUP OF COMETS IN OUR SOLAR SYSTEM. (THE OORT CLOUD IS THE OTHER GROUP OF COMETS FARTHER FROM THE SUN.)

WHAT IS A PLANET?

THE DISCOVERY OF ERIS PROMPTED RECONSIDERATION OF THE DEFINITION OF A PLANET. •THERE MAY BE MANY LARGE KUIPER BELT OBJECTS THIS LARGE WAITING TO BE DISCOVERED. •DO WE WANT TO CALL THEM ALL PLANETS?•IF NOT, THEN TO BE CONSISTENT, PLUTO SHOULDN'T BE CALLED A PLANET EITHER.

OFFICIAL DECISION (2006):•PLUTO AND ERIS ARE “DWARF PLANETS,” NOT “PLANETS.”•DEFINITION OF A PLANET:

•ORBITS THE SUN (OR SOME OTHER STAR)•IS MASSIVE ENOUGH FOR ITS GRAVITY TO PULL IT INTO A NEARLY SPHERICAL SHAPE•HAS “CLEARED OUT” OTHER BODIES FROM THE NEIGHBORHOOD AROUND ITS ORBIT, FOR THE MOST PART•(PLUTO AND ERIS MEET CONDITIONS 1 & 2, BUT NOT 3.)

WHAT IS A PLANET?

ADVANTAGES OF PLANETS AND LARGE MOONS FOR LIFE

CONDUCIVE TO CHEMISTRY (ATOMS COMBINING AND RECOMBINING TO FORM MOLECULES)

MANY PROVIDE A RELATIVELY STABLE AND PROTECTED ENVIRONMENT

– ATMOSPHERE PROVIDES PROTECTION FROM UV AND OTHER HIGH ENERGY PHOTONS AND FROM COSMIC RAYS

SMALL VARIATIONS IN ENVIRONMENT

– TEMPERATURE VARIATIONS – POSSIBLE INTERFACES BETWEEN LAND,

OCEANS, AND ATMOSPHERE

LIFE ON SOLAR SYSTEM BODIES OTHER THAN PLANETS & LARGE MOONS?

SMALL MOONS, ASTEROIDS, COMETS, DWARF PLANETS, ETC.?– BIG PROBLEM: NO ATMOSPHERE

• BIG TEMPERATURE VARIATION BETWEEN DAY AND NIGHT

• NO LIQUIDS - THEY BOIL INTO SPACE• NO UV PROTECTION• MOST ARE VERY COLD – FAR FROM THE SUN

SUN (OR STARS IN GENERAL)?– TOO HOT FOR COMPLEX MOLECULES TO FORM

INNER (TERRESTRIAL) PLANETS

MERCURY, VENUS, EARTH & MARS WARMER, CLOSER TO SUN RELATIVELY LOW MASS DENSE, ROCKY, FEW LIGHT ELEMENTS FEW OR NO MOONS NO RINGS

LET'S CONSIDER THESE ONE AT A TIME,– ALSO INCLUDING EARTH'S MOON (ONLY LARGE MOON

OF AN INNER PLANET)

EARTH DIAMETER: 1 EARTH

MASS: 1 EARTH

DENSITY: 5.5 g/cm³

SUNLIGHT: 1 EARTH

TEMP: 210 to 320 K (-60 to 50 oC or –80 to 120 oF)

ATMOSPHERE: 78% N2, 21% O2, 1% Ar, 0.03% CO2; H20 CLOUDS

WATER: COVERS 71% OF SURFACE, BUT ONLY 0.02% OF TOTAL MASS, OCEANS 3.6 KM DEEP ON AVERAGE

SURFACE: ROCKY, WITH OCEANS, ICE CAPS, VOLCANOES, PLATE TECTONICS, LIFE TO ~5 KM DEPTH

EARTH’S MOON DIAMETER: 27% EARTH

MASS: 1.2% EARTH

DENSITY: 3.3 g/cm³

SUNLIGHT: 1 EARTH

TEMP: 160 to 400 K (-110 to 130 oC or –170 to 260 oF)

ATMOSPHERE: NONE

WATER: NO LIQUID WATER, BUT PERMANENT ICE IN A FEW CRATERS NEAR THE POLES THAT NEVER GET SUNLIGHT

SURFACE: ROCKY, HEAVILY CRATERED, SMOOTHER MARIA (FORMED VIA LAVA FLOODING BASINS AFTER LARGE IMPACTS), NO CURRENT GEOLOGICAL ACTIVITY (COOL SOLID INTERIOR)

TIDAL FORCES THINK ABOUT THE GRAVITATIONAL FORCE OF THE MOON

PULLING ON THE EARTH. THE SIDE OF THE EARTH THAT IS CLOSEST TO THE MOON

EXPERIENCES A STRONGER FORCE THAN DOES THE SIDE OF THE EARTH THAT IS FARTHEST FROM THE MOON.

THE DIFFERENCE BETWEEN THE FORCES ON DIFFERENT SIDES OF THE EARTH CAUSES TIDES. (OCEANS CAN ALTER SHAPE MORE EASILY THAN ROCK.)

EARTH ROTATES INSIDE TIDAL BULGES, PRODUCING TWO HIGH TIDES PER DAY.

FRICTION BETWEEN WATER AND OCEAN BOTTOM IS SLOWING DOWN EARTH’S ROTATION.

TIDAL FORCES EARTH ALSO EXERTS TIDAL FORCES ON THE MOON. THE MOON HAS NO OCEANS, BUT THE ROCK ITSELF IS

DISTORTED INTO AN ELONGATED SHAPE. WHEN THE MOON FIRST FORMED, IT WAS ROTATING

MORE QUICKLY. AS IT DID SO, THE TIDAL FORCES KEPT CHANGING THE MOON'S SHAPE.

FRICTION SLOWED DOWN THE MOON’S ORIGINAL ROTATION UNTIL IT NOW ROTATES ONLY ONCE PER ORBIT.

SYNCHRONOUS ROTATION: THE MOON NOW KEEPS ONE SIDE FACING THE EARTH.

THIS HAS ALSO HAPPENED WITH MOST MOONS OF OUTER PLANETS.

LIFE ON EARTH’S MOON? ALMOST CERTAINLY NOT PROBLEMS:

– NO ATMOSPHERE (MASS OF MOON IS SO LOW THAT ITS GRAVITY CAN'T HOLD ONTO LIGHT GAS MOLECULES)

• NO PROTECTION FROM UV BOMBARDMENT OR COSMIC RAYS

• NO LIQUID WATER (OR ANY OTHER LIQUID THAT COULD SERVE AS A SOLVENT)

• LARGE TEMPERATURE EXTREMES– IF THERE WERE AN ATMOSPHERE, IT WOULD

SPREAD HEAT AROUND AND PREVENT TEMPERATURE EXTREMES