Astr115 ch24 Dec12

Chapter 24Life in the Universe: 

Chapter 24 Life in the Universe

24.1 Life on Earth: 

24.1 Life on Earth Our goals for learning When did life arise on Earth? How did life arise on Earth? What are the necessities of life?

When did life arise on Earth?: 

When did life arise on Earth?

Earliest Life Forms: 

Earliest Life Forms Life probably arose on Earth more than 3.85 billion years ago, shortly after the end of heavy bombardment Evidence comes from fossils, carbon isotopes.

Fossils in Sedimentary Rock : 

Fossils in Sedimentary Rock relative ages: deeper layers formed earlier. absolute ages: radiometric dating

Fossils in Sedimentary Rock : 

Fossils in Sedimentary Rock Rock layers of Grand Canyon record 2 billion years of Earth’s history

Earliest Fossils: 

Earliest Fossils Oldest fossils show that bacteria-like organisms were present over 3.5 billion years ago Carbon isotope evidence pushes origin of life to more than 3.85 billion years ago

The Geological Time Scale: 

The Geological Time Scale

How did life arise on Earth?: 

How did life arise on Earth?

Origin of Life on Earth: 

Origin of Life on Earth Life evolves through time. All life on Earth shares a common ancestry. We may never know exactly how the first organism arose, but laboratory experiments suggest plausible scenarios.

The Theory of Evolution: 

The Theory of Evolution The fossil record shows that evolution has occurred through time. Darwin’s theory tells us HOW evolution occurs: through natural selection. Theory supported by discovery of DNA: evolution proceeds through mutations.

Tree of Life: 

Tree of Life Mapping genetic relationships has led biologists to discover this new “tree of life.” Plants and animals are a small part of the tree. Suggests likely characteristics of common ancestor.

Slide13: 

These genetic studies suggest that the earliest life on Earth may have resembled the bacteria today found near deep ocean volcanic vents (black smokers) and geothermal hot springs .

Laboratory Experiments: 

Laboratory Experiments Miller-Urey experiment (and more recent experiments) show that building blocks of life form easily and spontaneously under conditions of early Earth.

Slide15: 

Microscopic, enclosed membranes or “pre-cells” have been created in the lab.

Chemicals to Life?: 

Chemicals to Life?

Could life have migrated to Earth?: 

Could life have migrated to Earth? Venus, Earth, Mars have exchanged tons of rock (blasted into orbit by impacts) Some microbes can survive years in space...

Brief History of Life: 

Brief History of Life 4.4 billion years - early oceans form 3.5 billion years - cyanobacteria start releasing oxygen. 2.0 billion years - oxygen begins building up in atmosphere 540-500 million years - Cambrian Explosion 225-65 million years - dinosaurs and small mammals (dinosaurs ruled) Few million years - earliest hominids

Slide19: 

Thought Question You have a time machine with a dial that you can spin to send you randomly to any time in Earth’s history. If you spin the dial, travel through time, and walk out, what is most likely to happen to you? You’ll be eaten by dinosaurs. You’ll suffocate because you’ll be unable to breathe the air. You’ll be consumed by toxic bacteria. Nothing: you’ll probably be just fine.

Slide20: 

Thought Question You have a time machine with a dial that you can spin to send you randomly to any time in Earth’s history. If you spin the dial, travel through time, and walk out, what is most likely to happen to you? You’ll be eaten by dinosaurs. You’ll suffocate because you’ll be unable to breathe the air. You’ll be consumed by toxic bacteria. Nothing: you’ll probably be just fine.

Origin of Oxygen: 

Origin of Oxygen Cyanobacteria paved the way for more complicated life forms by releasing oxygen into atmosphere via photosynthesis

What are the necessities of life?: 

What are the necessities of life?

Necessities for Life: 

Necessities for Life Nutrient source Energy (sunlight, chemical reactions, internal heat) Liquid water (or possibly some other liquid)

What have we learned?: 

What have we learned? When did life arise on Earth? Life arose at least 3.85 billion years ago, shortly after end of heavy bombardment How did life arise on Earth? Life evolved from a common organism through natural selection, but we do not yet know the origin of the first organism What are the necessities of life? Nutrients, energy, and liquid water

24.2 Life in the Solar System: 

24.2 Life in the Solar System Our goals for learning Could there be life on Mars? Could there be life on Europa or other jovian moons?

Could there be life on Mars?: 

Could there be life on Mars?

Searches for Life on Mars: 

Searches for Life on Mars Mars had liquid water in the distant past Still has subsurface ice; possibly subsurface water near sources of volcanic heat.

Slide28: 

In 2004, NASA Spirit and Opportunity Rovers sent home new mineral evidence of past liquid water on Mars.

Could there be life on Europa or other jovian moons?: 

Could there be life on Europa or other jovian moons?

Slide30: 

Ganymede, Callisto also show some evidence for subsurface oceans. Relatively little energy available for life, but still… Intriguing prospect of THREE potential homes for life around Jupiter alone… Ganymede Callisto

Titan: 

Titan Surface too cold for liquid water (but deep underground?) Liquid ethane/methane on surface

24.3 Life Around Other Stars: 

24.3 Life Around Other Stars Our goals for learning Are habitable planets likely? Are Earth-like planets rare or common?

Are habitable planets likely?: 

Are habitable planets likely?

Habitable Planets: 

Habitable Planets Definition: A habitable world contains the basic necessities for life as we know it, including liquid water. It does not necessarily have life.

Slide35: 

Constraints on star systems: Old enough to allow time for evolution (rules out high-mass stars - 1%) Need to have stable orbits (might rule out binary/multiple star systems - 50%) Size of “habitable zone”: region in which a planet of the right size could have liquid water on its surface. Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.

Slide36: 

The more massive the star, the larger the habitable zone — higher probability of a planet in this zone.

Finding them will be hard: 

Finding them will be hard Recall our scale model solar system: Looking for an Earthlike planet around a nearby star is like standing on the East Coast of the United States and looking for a pinhead on the West Coast — with a VERY bright grapefruit nearby.

Slide38: 

Kepler (2007 launch) will monitor 100,000 stars for transit events for 4 years. Later: SIM (2009?), TPF (2015?): interferometers to obtain spectra and crude images of Earth-size planets.

Spectral Signatures of Life: 

Spectral Signatures of Life Earth Venus Mars oxygen/ozone

Are Earth-like planets rare or common?: 

Are Earth-like planets rare or common?

Elements and Habitability: 

Elements and Habitability Some scientists argue that proportions of heavy elements need to be just right for formation of habitable planets If so, then Earth-like planets are restricted to a galactic habitable zone

Impacts and Habitability: 

Impacts and Habitability Some scientists argue that Jupiter-like planets are necessary to reduce rate of impacts If so, then Earth-like planets are restricted to star systems with Jupiter-like planets

Climate and Habitability: 

Climate and Habitability Some scientists argue that plate tectonics and/or a large Moon are necessary to keep the climate of an Earth-like planet stable enough for life

24.4 The Search for Extraterrestrial Intelligence: 

24.4 The Search for Extraterrestrial Intelligence Our goals for learning How many civilizations are out there? How does SETI work?

How many civilizations are out there?: 

How many civilizations are out there?

The Drake Equation: 

The Drake Equation Number of civilizations with whom we could potentially communicate = NHP  flife  fciv  fnow NHP = total # of habitable planets in galaxy flife = fraction of habitable planets with life fciv = fraction of life-bearing planets w/ civilization at some time fnow = fraction of civilizations around now.

We do not know the values for the Drake Equation: 

We do not know the values for the Drake Equation NHP : probably billions. flife : ??? Hard to say (near 0 or near 1) fciv : ??? It took 4 billion years on Earth fnow : ??? Can civilizations survive long-term?

Are we “off the chart” smart?: 

Are we “off the chart” smart? Humans have comparatively large brains Does that mean our level of intelligence is improbably high?

How does SETI work?: 

How does SETI work?

Slide50: 

SETI experiments look for deliberate signals from E.T.

We’ve even sent a few signals ourselves… : 

We’ve even sent a few signals ourselves… Earth to globular cluster M13: Hoping we’ll hear back in about 42,000 years!

Slide52: 

Your computer can help! SETI @ Home: a screensaver with a purpose.

What have we learned?: 

What have we learned? How many civilizations are out there? We don’t know, but the Drake equation gives us a framework for thinking about the question How does SETI work? Some telescopes are looking for deliberate communications from other worlds

24.5 Interstellar Travel and Its Implications to Civilization: 

24.5 Interstellar Travel and Its Implications to Civilization Our goals for learning How difficult is interstellar travel? Where are the aliens?

How difficult is interstellar travel?: 

How difficult is interstellar travel?

Current Spacecraft: 

Current Spacecraft Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars. Pioneer plaque Voyager record

Difficulties of Interstellar Travel: 

Difficulties of Interstellar Travel Far more efficient engines are needed Energy requirements are enormous Ordinary interstellar particles become like cosmic rays Social complications of time dilation

Where are the aliens?: 

Where are the aliens?

Fermi’s Paradox: 

Fermi’s Paradox Plausible arguments suggest that civilizations should be common, for example: Even if only 1 in 1 million stars gets a civilization at some time  100,000 civilizations So why we haven’t we detected them?

Possible solutions to the paradox: 

Possible solutions to the paradox We are alone: life/civilizations much rarer than we might have guessed. Our own planet/civilization looks all the more precious…

Possible solutions to the paradox: 

Civilizations are common but interstellar travel is not. Perhaps because: Interstellar travel more difficult than we think. Desire to explore is rare. Civilizations destroy themselves before achieving interstellar travel These are all possibilities, but not very appealing… Possible solutions to the paradox

Possible solutions to the paradox: 

There IS a galactic civilization… … and some day we’ll meet them… Possible solutions to the paradox

What have we learned?: 

What have we learned? How difficult is interstellar travel? Interstellar travel remains well beyond our current capabilities and poses enormous diffculties Where are the aliens? Plausible arguments suggest that if interstellar civilizations are common then at least one of them should have colonized the rest of the galaxy Are we alone? Has there been no colonization? Are the colonists hiding?