Astro 10-Lecture 7: Comparative Planetology: The cycles that shape planets How did planets become the way they are? What explains the differences?

Astro 10 – Giant Impacts What is the evidence for earlier impacts in the Solar System? How recently have impacts occurred? What is the likelihood of a life-threatening impact with Earth? What would happen? Last week you talked about the formation of the Solar System. Soon you’ll discuss the planets in more detail, and talk about the “debris” between the planets. But what happens when “debris” hits planets?

EVIDENCE FOR IMPACTS: CRATERS Craters are plentiful on many solid surfaces throughout the SS Earth Moon Moon of Saturn Venus

CRATERS (ctd)

EVIDENCE FOR IMPACTS: CRATERS (2) Objects with “old” surfaces (no modifications due to volcanism, plate tectonics, etc.) show many craters, indicating that cratering was common during the late stages of the solar system’s formation. “Younger” surfaces also have some craters, but far fewer.

FURTHER EVIDENCE FOR IMPACTS IN THE INNER SOLAR SYSTEM Mercury: –Orbit somewhat tilted Venus’ Rotation: –Doesn’t match the general trend of the solar system – goes “backwards” Earth-Moon: –Earth’s rotation axis is tilted relative to its orbit (and plane of solar system) –Moon believed to have formed from the impact of a Mars- sized body with the Earth, early in the Solar System’s history

FURTHER EVIDENCE FOR IMPACTS IN THE OUTER SOLAR SYSTEM ORBITS/ROTATION: –URANUS: rotates on its side, with all of its moons/rings (98 degree tilt relative to orbit) –NEPTUNE: 2 moons have odd orbits Moon Nereid has a large elliptical orbit Moon Triton orbits backwards –PLUTO/CHARON: elliptical and tilted orbit about the Sun

Outer Solar System Orbits

FURTHER EVIDENCE FOR IMPACTS IN THE OUTER SOLAR SYSTEM (2) RINGS: –Most Giant planets have rings, but they can’t have survived since the beginning of the solar system. –Therefore they are likely debris from impacts: Jupiter: Sunlight/solar wind would have blown them away if formed at time of SS formation Saturn: Too bright to be very old, should have been darkened by meteorites Saturn: Also too icy to have survived heat of SS formation.

RINGS

RECENT IMPACTS! 1994: Twenty ½-km fragments of Comet Shoemaker- Levy hit Jupiter Expect every years for Jupiter

COMET S-L IMPACTS JUPITER IR

GIANT IMPACTS ON EARTH: “RECENT” and FUTURE If in 1994 Jupiter was hit by a comet, could the Earth be hit by a comet or asteroid? Craters *do* exist on Earth, like this one in Arizona! This 50,000 yr-old, ~1-mile wide crater is from an asteroid ~1/2 size of football field

IMPACT BASICS Craters much bigger than the impact object (OH 112) Often energy released is measured as equivalent in Mtons (= million tons) of TNT explosive Barringer (Arizona) crater produced by a 50- ton or 50-m rock.

OTHER EARTH CRATERS

OTHER EARTH CRATERS (2) Quebec lake Manicouagan Henbury Ghana

Map of Earth Craters

“Debris” of the Solar System Asteroid Comet

“Debris” of the Solar System ASTEROIDS: –Rocky bodies between 10m and 1000km in size –~20,000 of them, ~1000 come into inner solar system –Concentrated in the plane of the solar system between Mars and Jupiter (OH 121) –~200 are > 100 km diameter, ~2000 are > 10 km diameter COMETS: –Icy bodies between 10m and 1000 km in size –Billions (?) of them in the distant Oort cloud (OH 108,119) –NOT concentrated in plane of solar system –Occasionally pass into the inner solar system METEOROIDS (OH): –Tiny bits of rock and metal, falling to Earth, heated by atmospheric friction until they glow

IMPACTS: How Often? Estimate by counting craters of a given size in the “younger” lunar surfaces (formed after heavy bombardment phase of solar system formation). Adjust for fact that Earth is bigger. For craters > 50 km in size, expect one impact every ~10 million years! Statistics: If you do a traffic survey, and count 6 cars every 60 seconds, expect 1 car every 10 seconds on average. BUT you wouldn’t let a car pass and step into traffic confident you have 10 seconds until the next car! Expect smaller ones more frequently (OH table)

Encounters in Human History Atmosphere protects us some, but – even objects ~1m in size can reach the ground –Meteorite damages a building every few years –Meteorites > few m, < m explode in atmosphere –Asteroid explodes in atmosphere with force of ~ 1 Hiroshima bomb each year October 9, 1992: –Meteorite smashed through rear end of car in Peekskill, NY –No one hurt, but Chevy Malibu wasn’t as lucky –Peekskill fireballPeekskill fireball Odds of being hit during your lifetime: –Without protection, a crude calculation implies odds of being hit by an object like this in a lifetime are roughly 1 in 1 billion

Encounters in Human History (ctd) August 10, 1972: –Fireball over Montana/Wyoming in daylight. –Object entered atmosphere at 33,000 mph –Remained in atmosphere for 101 s (covering 1000 miles) –33 to 260 feet across –Skipped off atmosphere (entry angle) Tunguska, Siberia, 1908: –50-m object caused a 20-Mton explosion in the atmosphere (1000 x Hiroshima) –2000 square km of flat trees –Even 8km from ground zero devastation dramatic (next)

Tunguska

IMPACTS: Consequences 0.2-1km asteroid (bigger than Arizona crater maker): –Expect 1 per 10,000 to 100,000 years –Broil creatures within eyesight of atmospheric fireball –Fiery debris blasted into space, plummets back to Earth triggering huge fires –Skies darkened by soot/dust => block sunlight => winter lasting months –Formation of poisonous nitrous oxides => acid rain –CO2 released from vaporized rock => long-term global warming –Seismic shock => huge earthquakes –If hits water instead, raise 35-km high splash in 40 seconds; tsunamis over area size of Pacific Ocean

IMPACTS: Consequences (ctd) 0.2-1km asteroid (bigger than Arizona crater maker): –Several thousand Solar System objects are this size –NOT all have been identified! –But remember how BIG space is from our scale model –Probability of impact by undiscovered asteroid ~1/100,000 each year (one estimate, possibly conservative).

IMPACTS: How Big? (ctd)

IMPACTS: How Often? (ctd)

Mass extinction? We predict mass extinction due to certain impacts, and we expect such impacts to occasionally occur Evidence for previous mass extinctions? –There’s an Iridium layer in sediments 65 million yrs ago. Iridium is rare on Earth but more common in asteroids. Is this debris from an asteroid impact? –Believe an impact like this would raise temps by 10 C worldwide. –Soot in sediment layers from same era; from the fires/soot that are predicted for such an impact? –Where’s the crater?

Mass extinction: Evidence? Chicxulub: This crater in the Yucatan is mostly underwater, ~ 300 km across, buried deep. Coincidentally, the dino fossils vanish at ~ the same time (none above Iridium layer, many below). Global climate changes due to impact could have caused this OR the correlation could be a coincidence!

Your Chances of Dying Chance of dying in globally catastrophic impact is ~ 1 in 500,000 per year. –Same as commercial aircraft accident –Catastrophic impacts RARER, but kill more people –Technically, your chance of dying is ~ 1 in 20,000 over your lifetime. Would we know it was coming? Not likely! –We believe there are several thousand asteroids capable of causing catastrophic impacts, but have catalogued only a hundred or so.

CLOSE CALLS Have we had close calls? YES –Near miss Feb 24, m asteroid comes within 400,000 km. –Near miss in March 2002: 70-m asteroid < 461,000 km (288,000 miles) from Earth –Dec 1994, an asteroid passed within 100,000 km. –March 23, 1989: asteroid bigger than aircraft carrier (300 m) passed through Earth’s orbit < 700,000 km away. Earth had been at that spot 6 hours earlier. IF it had hit, the energy released would have been ~ Mtons of TNT. NOTE: –Distance to moon is 384,000 km. –Tunguska object ~50m in size –Neither seen until AFTER since they came from the direction of the sun

WHAT can we DO? Several teams are trying to catalogue the asteroids > 1km – smaller ones also dangerous, but they’re starting with the biggest IF detected soon enough, MIGHT try to get an international plan to nudge it away –Easier to nudge (rockets? Explosions?) EARLY (just like only a small turn is required to avoid a tree if you start soon enough)

Why do some planets have more craters than others? The Mercury has very many craters The Earth has very few What is the difference? Time! The surface of the Earth is very young compared with the surface of Mercury

Cycles that shape the planets. The forces that shape the surfaces and atmospheres of planets are predominantly driven by energy transport. Convection is one of the primary methods of energy transport.

Convection How convection works. –When matter is heated it becomes less dense and tends to rise relative to the neighboring material. –When matter is cooled it becomes more dense and tends to sink relative to the neighboring material. –This transports heat energy from bottom to top (assuming the heat source is at the bottom.) –Click HereClick Here Click

Applications of Convection: Plate Tectonics The interiors of planets tend to be hot. Heavy radioactive elements like uranium have sunk to the core where they generate heat There is also heat left over from the formation of the planets Convection transports heat from the core to the surface, pulling the crust along with it.

Applications of Convection: Plate Tectonics

Applications of Convection Volcanism It appears that water is a necessary ingredient for plate tectonics to function. Without it the mantle is too solid to convect. The heat still needs to rise and get out... –On Mars, mantle hot spots appear to have resulted in volcanoes in the past. (Olympus Mons) –On Venus, it appears that the heat builds up for hundreds of millions of years, then is released by intense volcanism. The entire surface of the planet is replaced in a few million years. –Mercury and the moon have cooled enough that heat conduction is sufficient to transport heat from the core to the surface.

Applications of Convection Plate Tectonics Jupiter’s moon Europa may have something like plate tectonics. –In this case, heat is generated by tidal forces from Jupiter. –Icy plates may flow on top of a liquid water ocean.

Applications of Convection Volcanism Jupiter’s moon Io, on the other hand, appears to have stationary volcanoes to release the heat. –Io is less icy than the other moons, because it formed closer to the hot young Jupiter. –Without water to float on, the crust doesn’t move.

Applications of Convection: Winds Winds arise because the planetary pole is cooler than the equator. –Air at the equator rises, air at the pole sinks. –If the earth didn’t rotate, that would lead to a single circulation cell in each hemisphere. –Rotation complicates the picture

Applications of Convection: Winds Rotation divides the convection into cells and diverts winds east and west

Applications of Convection: Winds The faster the rotation, the more cells you have

Applications of Convection: Winds The slower the rotation, the fewer cells you have

ConcepTest Suppose you lived on a planet that rotated to keep one side always facing the sun. Which direction would you expect the surface winds to flow? A. From the light side to the dark side. B. From the dark side to the light side. C. From the North.

ConcepTest The greenhouse effect causes there to be very little surface temperature variation on Venus. T/F This should result in high surface winds on Venus.

Applications of Convection: Storms Convection also powers storms on several of the planets. Condensation heats the air. Evaporation cools the air On Earth: 1. Warm moist air rises and cools. 2. As the air cools, moisture condenses to form clouds. 3. This heats the air, causing it to rise more rapidly. 4. Surface winds draw more moist air toward the rising column of air 5. Eventually enough moisture condenses out to form rain.

Applications of Convection: Storms

Applications of Convection: Ocean Currents Cool water in the Arctic sinks, warm water in the tropics rises, driving a “Global Conveyor Belt” that transports heat toward the poles.

Convective processes shape the planets These processes affect atmospheres, oceans, and surfaces Plate tectonics and volcanism shape the surface by building mountains and transporting buried materials to the surface. –This can change the chemical composition of the surface. –It releases gasses into the atmosphere, changing its composition. Wind and water currents can cause erosion. –This shapes the surface, wearing down mountains. –It also changes the chemical composition of the surface and oceans.

Cycles that shape planets The water cycle Powered by heat –Sun heats ocean –Evaporation cools ocean –Condensation warms atmosphere –Some of the heat is transferred into work. (Moving rivers tearing down mountains)

ConcepTest T/F Because there is no liquid water on Mars, rates of erosion on Mars should be lower than those on earth.

Cycles that shape planets The carbonate-silicate cycle

This cycle is powered by the water cycle and plate tectonics –Water dissolves ions from silicates. CO 2 dissolves in water to make carbonic acid (H 2 CO 3 ) –Ions and Carbonic acid in the water are incorporated into the shells of plankton, shellfish, and corals (CaCO 3 ). Some solid carbonates can be made by other processes. –Plate tectonics brings the shells of dead organisms deep into the crust. –Volcanoes release CO 2 that is buried in the crust

Negative Feedback Suppose there is too much CO 2 in the atmosphere –The atmosphere heats up. –More water evaporates –Weathering of rock increases –Concentration of ions and carbonic acid increases. –More carbonate shells and minerals are made. –More carbon is deposited in the crust, reducing the amount of CO 2 in the air. The helps the Earth to maintain a temperate climate.

Positive Feedback There are limits to this negative feedback. –Water also acts as a greenhouse gas. Too much water in the atmosphere can cause the temperature to rise further. –If it gets hot enough, water stops condensing. No rain. –Without rain, there is no erosion to help remove CO 2 from the atmosphere. –CO 2 from volcanoes starts to build up, increasing temperatures further. –More water evaporates, increasing temperatures further. –Eventually the oceans evaporate entirely. –Water in the atmosphere is broken into hydrogen and oxygen. –The hydrogen escapes, oxygen combines with minerals, no water remains. This is what happened to Venus. Runaway greenhouse

Another Type of Positive Feedback Suppose the planet got too cold. –Ice reflects light, and therefore heat. If it gets colder, you get more ice, and more heat is reflected to space. –Since it’s colder there will be less evaporation. –Less evaporation means less water vapor in the atmosphere, reducing the greenhouse effect, which makes it colder. –If it gets cold enough CO 2 can freeze out further reducing the greenhouse effect. This would be permanent. In the distant past (a billion or more years ago) the Earth went through periods where all the oceans froze over (called snowball earth). Fortunately it didn’t get cold enough to freeze CO 2. Volcanism eventually released enough CO 2 to raise temperatures above freezing.