Features and Planetary Processes
tease:
FOUR ROCKS NEAR THE SUN... MERCURY, VENUS, EARTH AND MARS.
ALL HAVE SOLID SURFACES BUT, APART FROM THAT, THEY'RE VERY DIFFERENT IN MANY WAYS.
chapter head: FEATURES OF THE TERRESTRIAL PLANETS
IN THIS VIDEO, WE'LL EXPLORE THE FEATURES OF THE TERRESTRIAL PLANETS...
chapter head: WHAT MAKES THEM DIFFERENT?
WE'LL TRACK THE PLANETARY PROCESSES THAT MADE THEM INTO VERY DIFFERENT WORLDS...
chapter head: COMPARATIVE PLANETOLOGY
...AND WE'LL FIND OUT THAT STUDYING OUR NEIGHBORS HAS SOME VERY "DOWN-TO-EARTH" BENEFITS!
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MAIN TITLES
PASSPORT TO THE SOLAR SYSTM
program 3
4 ROCKS NEAR THE SUN
Features and Planetary Processes
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name title:
CHRIS CHYBA, Carl Sagan Chair for the Study of Life in the Universe
SETI Institute & Stanford University
When we look at our solar system, we see a tremendous diversity of worlds. They all look so different from one another, and why, why should that be? A lot of it seems to have to do with how big the world is.
How big a world is when it forms depends on how many planetesimals it accretes, how many things that look like asteroids and comets collect together. And once a world starts accreting, it tends to run away and gather up everything in its vicinity.
So worlds that really ran away, like Jupiter, not only gathered up the planetesimals like asteroids and comets, but they got big enough fast enough to haul in a whole lot of hydrogen gas before that material was lost from the solar system.
Worlds like the Earth didn't get that big that fast and they're formed almost entirely of rock.
MERCURY, CLOSEST PLANET TO THE SUN, REVOLVES AROUND IT THE FASTEST, WITH THE SHORTEST YEAR IN OUR SOLAR SYSTEM-88 DAYS.
MERCURY, NAMED FOR THE MESSENGER OF THE GODS-WHERE SPEED IS OF THE ESSENCE!
ONLY ONE SPACECRAFT HAS EVER VISITED THIS PLANET, "MARINER 10" IN THE MID-70'S, THOUGH A NEW MISSION CALLED "MESSENGER" IS PLANNED FOR THE END OF THIS DECADE.
MERCURY'S DAY / NIGHT TEMPERATURE VARIATIONS ARE THE GREATEST IN THE SOLAR SYSTEM.
SECOND PLANET FROM THE SUN IS VENUS... NAMED FOR THE GODDESS OF LOVE AND BEAUTY, AND SEEN FROM EARTH AS THE 3RD BRIGHTEST OBJECT IN THE SKY, AFTER THE SUN AND MOON.
IN SOME WAYS, IT'S EARTH SISTER PLANET, JUST A LITTLE SMALLER IN DIAMETER.
IN TEMPERATURE, IT'S RATHER LIKE THE TRADITIONAL PICTURE OF HELL!
THROUGH A TELESCOPE, VENUS IS ALL CLOUDS.
EARLIER, THERE WERE SPECULATIONS ABOUT PLENTIFUL WATER, SWAMPS AND LIFE.
LATER WE THOUGHT THE CLOUDS MIGHT ALL BE DUST...
BUT NOW WE KNOW THEY'RE MADE OF SULPHURIC ACID AND CARBON DIOXIDE.
IN THE 1990'S THE AMERICAN "MAGELLAN" SPACECRAFT MAPPED THE PLANET WITH RADAR, PEERING THROUGH THE CLOUDS TO REVEAL DETAILS OF THE SURFACE FOR THE FIRST TIME.
EARTH, 3RD ROCK FROM THE SUN, 150 MILLION KILOMETERS-OR ONE ASTRONOMICAL UNIT-AWAY.
EARTH IS THE DENSEST BODY IN THE SOLAR SYSTEM, WITH A CORE OF ROCK AND IRON.
IT'S THE ONLY PLANET WITH PLENTIFUL LIQUID WATER, WHICH COVERS MORE THAN TWO THIRDS OF ITS SURFACE.
AND EARTH IS-SO FAR-THE ONLY WORLD WHERE WE KNOW FOR SURE THAT LIFE BEGAN AND FLOURISHED.
4TH ROCK FROM THE SUN, AND LAST OF THE TERRESTRIAL PLANETS-MARS.
THOUGH MARS IS HALF THE SIZE OF EARTH, ITS COLD, BARREN SURFACE IS ABOUT THE AREA OF ALL EARTH'S DRY LAND-AND MANY THINK ITS MAJESTIC CANYONS AND GIANT VOLCANOES SHOULD BE HUMANITY'S NEXT PORT OF CALL...
CHRIS CHYBA
I would say that the way to think about a planet is ...it's a collection of rock and liquid and gas that's gotten big enough to be spherical, to be in the shape of a sphere.
If you're the size of a small asteroid, you don't have to look anything like a globe, like a sphere. You can look like a potato or a dumbbell or anything else, because the gravity's not strong enough to sort of smooth out the high spots, to pull the high spots in and compress you down into a globe.
chapter head: FEATURES OF THE TERRESTRIAL PLANETS
LET'S COMPARE AND CONTRAST THESE 4 NEIGHBORS AND DISCOVER HOW THEY'RE ALIKE AND DIFFERENT.
REMEMBER, THOUGH, THIS GRAPHIC DOES NOT SHOW THESE WORLDS TO SCALE...
MOONS
THAT'S AN EASY "COMPARE AND CONTRAST."
NO NATURAL SATELLITES ORBIT MERCURY OR VENUS...
EARTH HAS ONE, THE MOON...
AND MARS TWO-PHOBOS AND DEIMOS.
title: CRATERS
ALL THE TERRESTRIAL PLANETS HAVE CRATERS, RESULTING FROM IMPACTS BY CHUNKS OF ASTEROIDS AND COMETS.
BUT THEY VARY IN HOW MANY ARE LEFT ON THEIR SURFACES, WHICH HELPS US UNDERSTAND THE PLANET'S HISTORY AND EVOLUTION.
MERCURY IS COVERED WITH CRATERS, MUCH LIKE EARTH'S MOON.
MARS ALSO HAS CRATERS, BUT THERE ARE DIFFERENT NUMBERS IN ITS NORTHERN AND SOUTHERN HEMISPHERES... A CLUE THAT MARS HAS BEEN GEOLOGICALLY ACTIVE.
EARTH AND VENUS ALSO HAVE CRATERS... BUT FAR FEWER THAN MARS AND MERCURY.
ON THE WHOLE THEIR SURFACES ARE YOUNGER. BOTH HAVE ACTIVE AND ONGOING PROCESSES WHICH WIPE OUT CRATERS.
name title:
CLAUDIA ALEXANDER
U.S. Project Scientist, Rosetta comet mission
JPL, NASA / Caltech
When the Magellan mission went and did the mapping exercise that it did, it actually brought back the radar evidence that showed that there were just as few craters on the surface of Venus as there was on the surface of the Earth. And what that means is that somehow or other Venus is turning over its surface just like the Earth is, and that's a complete surprise because on Earth we think we know what causes the surface to turn over-there's "continental drift."
The plates that continents sit on move around, and that causes, there's earthquakes as the plates move past one another, and they build mountains and they build islands and so we see evidence around the Earth for what we call plate tectonics ...a "continental drift," and we know that that causes the surface to turn over. But on Venus there's nothing like that.
So somehow the surface of Venus is turning over in a way that's completely different from the way the Earth's surface turns over. And so that's very exciting because it means that somehow or other, Venus is continuing to be alive and to be active and to have dynamics...
VENUS HAS GIANT PEAKS LIKE GULA MONS, NEARLY 2 MILES HIGH.
TOWERING OVER MARS-OLYMPUS MONS... THE LARGEST VOLCANO IN THE SOLAR SYSTEM, 3 TIMES HIGHER THAN MOUNT EVEREST.
MERCURY SEEMS TO HAVE FEW IF ANY VOLCANOES-THOUGH THERE'S EVIDENCE OF PLENTIFUL LAVA FLOWS EARLY IN ITS HISTORY.
EARTH, OF COURSE, HAS MANY VOLCANOES...
CLAUDIA ALEXANDER
What happens like with the Hawaiian Islands is that there is a hot spot and it's causing the volcano, the lava to come out, but the plate is moving, so the plate moves past that and the little islands that form move off with the plate and eventually they erode back down and the island disappears, so you have this island chain.
But if the plate weren't moving, the island would just keep growing and growing and getting bigger and bigger, and that's what we see on both Venus and Mars.
EARTH IS AN OCEAN PLANET... BLUE WITH LIQUID WATER... WHITE WITH WATER VAPOR ALOFT IN CLOUDS...
AND THE QUESTION OF WATER IS CRUCIAL IN ASSESSING THE POSSIBILITY OF LIFE ELSEWHERE IN THE SOLAR SYSTEM.
VENUS ONCE DID HAVE WATER, BUT IT BOILED AWAY TO SPACE AS THE PLANET GOT HOT ENOUGH TO MELT LEAD.
EVEN SEARING MERCURY SEEMS TO HAVE WATER ICE IN SHADED REGIONS OF ITS POLE.
AND MARS ONCE CERTAINLY HAD PLENTIFUL LIQUID WATER, REVEALED BY THE WINDING CHANNELS IMAGED BY VIKING.
IMAGES FROM THE GLOBAL SURVEYOR SPACECRAFT EVEN RAISE THE POSSIBILITY OF MUCH MORE RECENT FLOWS OF LIQUID WATER-THOUGH THE EVIDENCE IS, AS YET, UNPROVEN...
CLAUDIA ALEXANDER
The water may be gone from the surface but maybe it's not "gone" completely, maybe it is near the surface and maybe there's processes that we don't quite understand that's causing a water turnover, maybe underground in ways that we still haven't figured out.
And so the question of whether or not there's still a cycle, whether the water cycle is turning over in a completely different manner than it does on Earth... we don't know whether that's true or not, but it's certainly a very exciting time to be studying both Venus, Earth and Mars.
WHAT ARE THE FORCES WHICH MAKE THESE FOUR WORLDS SO DIFFERENT... IN TEMPERATURE CRATERS, LIQUID WATER... LIFE?
chapter head: WHAT MAKES THEM DIFFERENT?
FIRST UP, A PRETTY BASIC CONCEPT-THE SIZE OF THE PLANET. THAT TURNS OUT TO HAVE CONSEQUENCES FOR JUST ABOUT EVERYTHING...
CHRIS CHYBA
A world that's small cools down much faster than a world that's big, and we're all familiar with this if you heat up a big potato in your over and a small potato, the small potato is just going to cool faster.
So our moon, the Earth's moon, cooled really quickly.
Mars, which is bigger than the moon, but still much smaller than the Earth, cooled less quickly but also cooled off, and the Earth is still geologically active, it's still hot inside.
A PLANET'S DISTANCE FROM THE SUN MAY ALSO BE ENOUGH TO SEAL ITS FATE.
CHRIS CHYBA:
Mercury is so close to the Sun that it's simply uninhabitable. If it did have an atmosphere it would lose it... the solar wind from the sun, charged particles would strip Mercury's atmosphere quickly. It's just too close to the Sun and it's too small a world.
THE SIZE OF A PLANET HELPS DETERMINE ITS TEMPERATURE... AND ITS SIZE AND MASS ALSO AFFECT ITS GRAVITY, WHICH HELPS IT KEEP AN ATMOSPHERE, OR ALLOWS IT TO LEAK AWAY TO SPACE.
CHRIS CHYBA:
What does all that mean? One thing that means is that the Earth is still replenishing its atmosphere. Volcanoes on the Earth are still belching carbon dioxide into our atmosphere, so we actually have a carbon dioxide cycle on the Earth where the volcanoes put the CO2 into the atmosphere, rainfall washes it back out, and we kind of maintain a balance of the right amount of carbon dioxide, and incidentally the right amount of water vapor, to give us an amount of Greenhouse Effect that's just right for liquid water to exist.
A GREENHOUSE ON THE SURFACE OF THE EARTH HELPS PLANTS GROW BY ALLOWING THE HEAT AND LIGHT OF THE SUN IN THROUGH A GLASS ROOF, BUT REDUCING THE AMOUNT OF ENERGY RE-RADIATED BACK TO SPACE. THAT'S WHY A GREENHOUSE IS WARMER IN WINTER THAN THE AIR OUTSIDE.
A PLANETARY GREENHOUSE EFFECT IS CAUSED WHEN GASES IN THE ATMOSPHERE DRIVE UP SURFACE TEMPERATURES BY ALLOWING IN SOLAR ENERGY, BUT CUTTING BACK ITS RE-EMISSION.
JUST AS IN A GREENHOUSE, THE BALANCE BETWEEN INCOMING AND OUTGOING HEAT IS CRITICAL FOR LIVING CREATURES.
IT'S POSSIBLE TO MAKE THINGS TOO HOT... OR TOO COLD.
CHRIS CHYBA:
If a planet cools off and stops being able to put carbon dioxide into the atmosphere because volcanism shuts down on that planet, then the planet can't maintain the greenhouse, and that's what went wrong with Mars. There used to be the biggest volcanoes in the solar system but they're all dead now... and now the world is freeze-dried. It's become a kind of barren desert at its surface.
Volcanism on Venus kept putting carbon dioxide into Venus' atmosphere. But what went wrong on Venus is that Venus was so close to the Sun that Venus lost its oceans. The oceans evaporated, they went up into the atmosphere. The sunlight breaks up the water molecules into hydrogen and oxygen, the hydrogen can leak out the top of the atmosphere, and once that happens the water's gone, that's it, end of story.
Once Venus lost its water, it was no longer able to remove the carbon dioxide from its atmosphere, so the greenhouse effect on Venus went haywire in the opposite direction that it went haywire on Mars.
VENUS HAS THE THICKEST ATMOSPHERE...WITH SURFACE PRESSURES 90 TIMES THAT OF EARTH.
EARTH'S ATMOSPHERE IS 77% NITROGEN AND 21% OXYGEN.
MUCH OF EARTH'S CARBON DIOXIDE IS NOW STORED IN CARBONATE ROCKS... BUT WHAT REMAINS IN THE ATMOSPHERE HELPS DRIVE A MODEST, NATURAL GREENHOUSE EFFECT WHICH KEEPS EARTH 35 DEGREES CENTIGRADE WARMER THAN IT WOULD OTHERWISE BE.
MARS HAS A THIN ATMOSPHERE, STRONG WINDS AND GREAT DUST STORMS WHICH SOMETIMES COVER THE ENTIRE PLANET... BUT ONLY A SMALL GREENHOUSE EFFECT, RESULTING IN AN AVERAGE TEMPERATURE OF MINUS 55 DEGREES CENTIGRADE...
THE WORLDS OF THE INNER SOLAR SYSTEM ARE FASCINATING IN THEMSELVES... BUT WHAT'S THE VALUE OF STUDYING THEM WITH TELESCOPES AND SPACECRAFT?
WHAT'S THE POINT OF THE NEW SCIENTIFIC DISCIPLINE CALLED "COMPARATIVE PLANETOLOGY"?
chapter head: COMPARATIVE PLANETOLOGY
CHRIS CHYBA:
There are two reasons why you need to explore the solar system if you want to understand the Earth. The first reason is very straightforward. If you want to understand a particular region of the Earth, you need to understand the surrounding region to understand its environment. If you want to understand a planet in our solar system, you have to understand the solar system. That's what sets the conditions for that planet.
The second reason we need to understand the other planets to understand the Earth is because we can't do experiments with the Earth. There's a sense in which we are doing a huge uncontrolled experiment with the Earth right now by pumping greenhouse gases into our atmosphere, but there's no control for that experiment. We don't know how it's going to come out. We can get insight into those questions by seeing how other worlds evolved.
We can look at Venus and see what happens when you have a runaway greenhouse effect.
We can look at Mars and see what happens when you have too little greenhouse effect.
Even though we can't do experiments with the Earth, we can do comparative planetology and learn about what might happen or what could happen on a world like the Earth by looking at other worlds where those things have already happened.
