life in the solar system

Life in the Solar System

!1

Announcements

• Quizzes • quiz 6: due at 1 pm on Sunday, Mar 2

!

• Midterm exam • marks available on OWL • will discuss problem questions in class next time

!2

Today’s Topics

• Review of last lecture • origins of life on Earth

!

• Evolution of life on Earth (Ch. 6.3, 6.5) • Impacts and extinctions (Ch. 6.4) • Life in the Solar System (Ch. 7)

!3

When did life begin?Three lines of evidence that life began between 3.85 Byr and 3.0 Byr ago. !

• stromatolites – rocks characterized by a distinctive layered structure: • evidence of life at least 3.5 Byr ago

• microfossils: • suggests life originated 3.5–3.0 Byr

ago • isotopes of carbon:

• enhanced carbon-12 to carbon-13 ratio in 3.85 Byr old rocks

!4

Implications for life elsewhere

!

• Life arose shortly after the end of Heavy Bombardment ~3.9 Byr ago !

• Life arouse rapidly on Earth, and could do so on another suitable world!

!5

Where did life begin?• Unlikely to have originated on land:

• no molecular oxygen (O2) in early atmosphere, so also no ozone (O3) to shield from UV

• Under-water or sub-surface environment more hospitable • water blocks UV • e.g., deep-sea volcanic vents also offer

chemical energy for metabolic reactions

!6

How did life begin?• Concoction of water vapour,

methane, and ammonia, with energy provided by electricity (lightning) can produce amino acids in a lab • Miller-Urey experiment !

• Other possible origins: • near deep-sea vents • organic material from space

(meteorites, comets)

!7

Summary of Origin of Life Hypothesis: RNA World

!8

RNA replication can be contained within spontaneously forming lipid membranes

• Keeping RNA molecules together increases likelihood of self-replication

• Isolation from outside preserves RNA and enzyme concentration: • speeds up reactions • natural selection-like

!9

Summary of Origin of Life Hypothesis: RNA World

!10

Today’s Topics


!

• Evolution of life on Earth (Ch. 6.3) • Impacts and extinctions (Ch. 6.4) • Life in the Solar System (Ch. 7)

!11

The Evolution of Life

Our goals for learning: • What major events have marked evolutionary history? • Why was the rise of oxygen so important to

evolution?

!12

What major events have marked evolutionary history?

• Early microbes: • simple organisms with a few enzymes and

rudimentary metabolism • resembling the simplest modern bacteria and archaea,

no nucleus • Oxygen-free atmosphere, so microbes were anaerobic • Microbes likely were chemoautotrophs

• photosynthesis and ability to digest other organisms must have arrived later

• Modern parallels are archaea in hot sulphur springs

!13

Early Microbial Evolution• With limited sets of enzymes, early DNA replication was buggy:

• high mutation rate: rapid evolution • e.g., photosynthesis is a complex metabolic process, but

already suggested in stromatolites and microfossils 3.5 Byr ago.

• Photosynthesis process also likely evolved: • first with development of light-absorbing pigments • then with utilization of a variety of products: e.g., hydrogem

sulphide (H2S), rather than water (H2O) • Oxygen build-up: from photosynthetic organisms

• 2.5–0.5 Byr ago

!14

The Evolution of Eukarya

• Oldest known fossils with clear cell nuclei date to 2.1 Byr ago. • Could have arisen earlier, but cell nuclei do not fossilize well.

!15

What major events have marked evolutionary history?

• Symbiotic relationship between eukarya and bacteria lead to complex cells, with mitochondria and/or chloroplasts

• confirmed by DNA sequencing of mitochondria and chloroplasts, which shows that they are from domain bacteria !16

The Cambrian ExplosionAnimals characterized by their “body plans” or phyla. • Mammals and reptiles are

of the phylum Chordata : with internal skeletons

• Modern animals comprise ~30 phyla

• All 30 phyla arise during a period of only 40 Myr: • <1% of Earth’s history • 542 Myr ago

!17

The geological time scale

!18

The Cambrian Explosion• Is the only major diversification

of phyla in the geological record • Possible reasons:

• oxygen reached a critical level for survival of larger life-forms

• a tipping point in the evolution of genetic complexity and diversification

• climate change: end of snowball Earth period

• absence of efficient predators

!19

The Colonization of Land

!20

The Colonization of Land• First by microbes • Then by plants – originating from algae in shallow

ponds • occasional drying up of ponds favours mutations with

thicker cell walls • 475 Myr ago

• Animal organisms aided by the build-up of a protective ozone layer. • by 400 Myr ago: amphibians and insects eating the

plants

!21

Why was the rise of oxygen so important to evolution?

• Oxygen can react strongly with organic molecules: • deadly to unadapted organisms • but much more efficient cellular energy production, with

ATP, compared to anaerobic organisms • Oxygen also reacts quickly with clays, rocks

• making them turn reddish • would last only a few Myr if not replenished

• So, early atmosphere must have been oxygen-free • cyanobacteria in the oceans created the atmospheric oxygen • starting 2.7 Byr ago.

!22

Why was the rise of oxygen so important to evolution?

!23

What have we learned?• What major events have marked evolutionary history?

• development of photosynthesis as an energy-producing reaction by 3.5 Byr ago

• the build-up of atmospheric oxygen by cyanobacteria (2.5–0.5 Byr ago)

• the Cambrian explosion of animal diversity 542 Myr ago • Why was the rise of oxygen so important to evolution?

• it offered a much more efficient energy production cycle than preceding anaerobic cycles.

!24

© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley

Human Evolution

Our goals for learning: • How did we evolve? • Are we still evolving?

How did we evolve?

• Not from chimpanzees or other modern apes

• Instead, from a common ancestor with them

!26

The Emergence of Humankind

!27


!28


!29

Are we still evolving?• Changes over the past 10,000–40,000 years have been

relatively small. • If we were to sequence the genome of a 40,000-old

human, it would be difficult to distinguish from a that of a person living today

• Most substantial change is in average height • better nutrition.

• Cultural and technological evolution are much faster • exponential

!30

Today’s Topics


!


!31


Impacts and ExtinctionsOur goals for learning: • Have we ever witnessed a major impact? • Did an impact kill the dinosaurs? • Is the impact threat a real danger or media hype?


• Comet Shoemaker-Levy 9 (SL9) was torn apart during an encounter with Jupiter in 1993.

• By early 1994, astronomers knew that it would collide with Jupiter later that year.

Image credit: NASA, Hubble Space Telescope


Comet SL9’ Crash into Jupiter


Jupiter Hit Again!

July 20, 2009; NASA/IRTF telescope


Did an impact kill the dinosaurs?


Mass Extinctions• Fossil record

shows occasional large dips in the diversity of species. !

• Most recent was 65 million years ago, ending the reign of the dinosaurs.


Evidence of an Impact

• Iridium is very rare in Earth surface rocks but often found in meteorites. • In 1978 Luis and Walter Alvarez found a worldwide layer

containing iridium, laid down 65 million years ago, probably by a meteorite impact.

• Same layer also contains: • shocked quartz—that requires the high temperatures and

pressures of an impact to form • spherical rock droplets—molten rock that solidified while

raining down • soot—from large-spread forest fires


Evidence of an Impact: Iridium Layer

Dinosaur fossils all lie in below this layer

No dinosaur fossils in upper rock layers

Thin layer, corresponding to the K-T boundary, contains the rare element iridium


Consequences of an Impact

• Meteorite 10 km in size would send large amounts of debris into atmosphere.

• Debris would reduce sunlight reaching Earth’s surface.

• Resulting climate change may have caused mass extinction: • 75% of all existing plant and animal species • 99% of all living plants and animals


Likely Impact Site: The Yucatan Peninsula in Mexico

• Geologists have found a 200 km-wide subsurface crater about 65 million years old in Mexico !

• Estimate that the impactor was a 10 km asteroid or comet


Is the impact threat a real danger or media hype?

Chelyabinsk Impact

• Slide and video on Chelyabinsk meteorite. • 10 m impactor?

!46

10m-sized Crater in a Lake

!47


Tunguska, Siberia: June 30, 1908 A ~40 meter object disintegrated and exploded in the atmosphere


Meteor Crater, Arizona: 50,000 years ago (50 meter object)

Image: LSTS-9 Crew/NASA/GSFC

Manicouagan Crater, Eastern Canada 200 Myr, among oldest known 20 km across

Cosmic Impacts Are a Certainty!

• Approximately 150 craters known on Earth !

• None older than ~200 million years !

• Older craters erased by tectonics, erosion !

• 71% of Earth’s surface is water: • small impacts leave no mark! • evidence of large impacts >200 Myr ago recycled

with seafloor

Frequency of Impacts

• Small impacts happen almost daily. !

• Impacts large enough to cause mass extinctions are many millions of years apart

Arizona Yucatan


Facts about Impacts

• Asteroids and comets have hit the Earth. !

• A major impact is only a matter of time: – not IF but WHEN. !

• Major impacts are very rare: • Extinction level events ~ millions of years. • Major damage ~ tens to hundreds of years.

NASA Near-Earth Object Program• Aims to detect potentially hazardous asteroids • Uses a network of telescopes to search for such asteroids nightly • http://www.jpl.nasa.gov/asteroidwatch/ !

• Some terms used in next video from NASA: • PHA – “potentially hazardous asteroid” • Palermo Technical Impact Hazard Scale – used to assess

danger from an impact • Aten – a “family” of near-Earth asteroids that cross Earth’s

orbit (and so are PHA’s)

!56

http://www.jpl.nasa.gov/asteroidwatch/

The Asteroid with Our Name on It

• We haven’t seen it yet. !

• Deflection is more probable with years of advance warning. !

• Control is critical – breaking a big asteroid into a bunch of little asteroids

is unlikely to help. !

• We get less advance warning of a killer comet…

What have we learned?• Have we ever witnessed a major impact?

• Yes, on the planet Jupiter in 1994 and 2009 • Did an impact kill the dinosaurs?

• There is strong evidence to support this hypothesis: • world-wide layer of iridium at the K-T boundary: a rare element

deposited globally at the time the dinosaurs went extinct (65 Myr ago) • dinosaur fossils found below, not above K-T boundary layer • large crater on Yucatan peninsula dates to 65 Myr ago.

• Is the impact threat a real danger or media hype? • Dangerous impacts, albeit very rare, are a real danger • Cratering evidence on Earth and the Moon show that they have occurred

with regularity in the past

!59

Other Possible Reasons for Mass Extinctions

• Episodes of active volcanism → climate change • Rapid acceleration of mutation rates

• e.g., because of thinning of ozone layer and increased UV radiation

• weakening of Earth’s magnetic field and increased penetration of solar wind

• Nearby supernova explosions: • also increased irradiation by high-energy particles

(cosmic rays) • large influx of gamma ray photons can destroy ozone

layer → increased UV radiation

!60

Other Possible Reasons for Mass Extinctions: Us?

• Human activity may drive half of species to extinction within a few centuries.

• On a geological time scale, this is a another mass extinction

• Potentially unpredictable consequences on global environment.

!61

Break: 5 min

!62

Today’s Topics


!


!63


Environmental Requirements for Life

Our goals for learning: • Where can we expect to find the building blocks

of life? • Where can we expect energy for life? • Does life need liquid water? • What are the environmental requirements for

habitability?

Where can we find the building blocks of life?

• Chemical elements needed for life likely occur in all planetary systems • consequence of how

planets form from their parent proto-planetary nebulae

• Amino acids and complex organic molecules require a liquid or gas to form and move about • need either an

atmosphere or an ocean

!65

Where can we find energy for life?

• Recall energy sources: • sunlight • organic molecules • inorganic molecules

• Sunlight is everywhere in Solar System, although its intensity decreases with distance from the Sun

!

Intensity of light ∝ 1/distance2

!66

Does life need liquid water?

Properties of potential liquids for life (at 1 atmospheric pressure)

!67

Liquids are needed for: dissolution of chemicals, transport, metabolic reactions

Advantages of Water: I. Temperature Range of Liquidity

• wider than for other wide-spread fluids • at higher temperatures → faster chemical reactions

!68

Advantages of Water: II. Ice floats

!69

Advantages of Water: III. Charge separation

• Affects how water dissolves other substances • Allows hydrogen bonds in biochemical reactions

!70

Summary of environmental requirements for habitability

1. A source of molecules from which to build living cells

2. A source of energy to fuel metabolism

3. A liquid medium—most likely liquid water—for transporting the molecules of life.

!71

What have we learned?• Where can we expect to find the building blocks of life?

• the necessary chemical elements—on almost any planetary body • the complex organic molecules and amino acids—in liquid media

!

• Where can we expect energy for life? • anywhere there is sunlight or heat (e.g., volcanic vents) !

• Does life need liquid water? • it needs a liquid medium • water has many advantages over the other most common ones

!72


A Biological Tour of the Solar System

Our goals for learning: • Does life seem plausible on the Moon or Mercury? • Could life exist on Venus or Mars? • What are the prospects for life on jovian planets? • Could there be life on moons or other small

bodies?

Does life seem plausible on the Moon or Mercury?

• No surface liquids !

• No atmosphere !

• Verdict: negative.

!74

Water Ice on the Moon

• Does exist in some permanently shadowed craters

• Discovered in 2009 by • NASA’s LCROSS • India’s

Chandrayaan-1 • Deposited over

billions of years of impacts

!75

Could life exist on Venus?

• Enshrouded in a thick cloud cover

• Surface can not be seen from the Earth

• Without the atmospheric greenhouse effect, surface temperature would be ~35°C.

!76

Venus: an Inhospitable “Hell”• Russian landers in 1970’s and 1980’s

Venera-1 and Venera-2 revealed: • thick atmosphere:

• 90 atmospheric pressures at surface

• contains 96% CO2 • 470°C surface temperature: day

and night! • no liquid water • runaway greenhouse effect

• Verdict: • possibly only in the distant past. • will never know - evidence erased

by geological processes.!77

Could life exist on Mars?

• One of the best candidates for life beyond the Earth.

• The most explored planet after Earth.

• Will discuss in detail next time! !

• Verdict: possible.

!78

What are the prospects for life on jovian planets?

!79

Jupiter and Saturn• No surfaces • Very high densities and pressures in interior

!80

Jupiter and Saturn• Water clouds do exist in upper

atmosphere • But strong vertical

(convective) winds would continually circulate any life forms between very cold and very hot regions !

• Verdict: negative.

!81

Uranus and Neptune• Atmospheres much colder

than those of Jupiter and Saturn

• Also strong vertical winds • But… outer liquid cores of

water, methane, ammonia • still: very high pressures,

no clear energy extraction mechanism

• Would be extremely difficult to detect life that deep inside these planets

• Verdict: unlikely.!82

Could there be life on large moons?

• Some have liquid oceans underneath their icy surfaces.

• One (Titan) has a thick atmosphere.

!83

• Will explore in 2 weeks! • Verdict: possible.

Could there be life on other small bodies?

• No atmospheres, no liquids. • Verdict: negative.

!84

What have we learned?• Does life seem plausible on the Moon or Mercury?

• No. • Could life exist on Venus or Mars?

• Venus: possibly in the distant past. • Mars: possibly.

• What are the prospects for life on jovian planets? • Jupiter and Saturn: negative. • Uranus and Neptune: unlikely.

• Could there be life on moons or other small bodies? • large moons: possibly. • small bodies: no.

!85

Next time: Mars!

!86

life in the solar system

Documents