24. Life in the Universe 2015

7/21/2019 24. Life in the Universe 2015

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Life in the Universe



Life in the Solar System

Our goals for learning• How do we find/identify life?

• Could there be life in the Solar System and wherewould it be?



Finding life in the Universe

• Solar System -close enough to visit, activeexploration possible. – Look for signs of life in likely places

• Other star systems -too far away to visit, passiveexploration dependant on naturally received dataonly.

– Finding stars with planets (preferably Earth-like),identifying right materials for habitability, looking forsignals from intelligence.



What Does Life look Like?

• Only one known example- Earth!

• We use Earth’s fossil record and environmentalniches to gain an idea of where, what and how life

exists. (Terrestrial analogues)



Earliest Fossils

• The oldest fossilsshow that bacteria-like organisms were

present over 3.5

billion years ago.

• Carbon isotopeevidence dates the

origin of life to morethan 3.85 billionyears ago.



Tree of Life

• Mapping geneticrelationships 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



• 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.



Chemicals to Life?

Experiments show that the building-blocks oflife could form easily and spontaneously underthe conditions of early Earth.



Brief History of Life on Earth

• 4.4 billion years - early oceans form

• 3.5 billion years - cyanobacteria start releasing

oxygen.• 540-500 million years - Cambrian Explosion

– (Only the last 13% of Earth's history!)

• 225-65 million years - dinosaurs and small

mammals (only last 5% of Earth’s history)

• >6 million years - earliest hominids



Necessities for Life

• Nutrient source• Energy (sunlight, chemical reactions, internal heat)

• Liquid water (or possibly some other liquid)

• Stable environment for life to form (e.g. not devastated

by volcanoes or impacts every 5 minutes)

Hardest to find onother planets



Looking For Life in the Solar

System

• Look for water and the other necessities

• Examine environments that life can flourish (extremophiles)

• Examining fossils to recognize past life

(identifying fossil bacteria)



Life in Unexpected Places

(Extremophiles)Extreme cold

Extreme heat

Extreme pressure No sunlight

Low water content

Poison tolerant

Salt tolerant

Acid tolerantAlkali tolerant

Radiation tolerant

Lives in minerals

No sunlight

No oxygen



1. Searching for Life on Mars

• Mars had liquid water in the distant past, still hassubsurface ice; possibly subsurface water •Energy sources: solar, possible old volcanic vents•Carbon compounds from meteorites



In 2004, NASA Spirit and Opportunity Rovers sent home new

mineral evidence of past liquid water on Mars. Also looking for

evidence of micro-fossils



2. Jovian Ice Worlds

Evidence for globalsubsurface oceans on

Europa, Ganymede

and Callisto.

Ganymede and

Callisto seem too

inactive to have

many energy sources



3a. Saturn Ice worlds-Titan?

• Surface too cold for liquid water (but deep underground?)• Liquid ethane/methane on surface•Lots of hydrocarbons

•Relatively stable, energy sources could be a problem.



3b. Saturn Ice worlds-Enceladus?

• Ice fountains on Saturn's

moon Enceladus suggest that

it might have liquid waterbelow the surface



What have we learned?

• Identifying Life – The most common and longest life forms are single-

celled. They arose at least 3.85 billion years ago on Earth

and occur in a variety of environments that complex lifecannot endure.

• What are the necessities of life? – Nutrients, energy, and liquid water




• Could there be life In the Solar System? – Evidence for present or past liquid water occur on Mars,

– Any life on Mars would be simple and small though

– Ice moons are cold, but several show signs of sub-surfacewater and other liquids, plus they have hydrocarbons for

building-blocks and nutrients.



Looking for Life Around Other Stars

Our goals for learning

• Are habitable planets likely?

• Hunting for Planets

• Are Earth-like planets rare or common?



Are habitable planets likely?



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.



Possible Requirements for Habitability(based on Earth)

1. Proper distance from its star

2. Volcanism – to make and maintain an

atmosphere and oceans.

3. Plate tectonics – for climate regulation.

4. Planetary magnetic field to protect the atmosphere.



The more massive the star, the

larger the habitable zone — higher

probability of a planet in this zone.

http://habitable_zone.htm/



Finding them will be hard

• Looking for an Earth-like 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.



Planet Finding IS Possible

• A variety of techniques exist to detect planets

• Most result in the preferential detection of

high-mass planets.

• Direct measuring techniques can detect Earth-

sized planets.

• Number of planets is rising into the 1000s

– A number are in the habitable zone.



Is life rare or common?



Impacts and Habitability

• Some scientists argue

that Jupiter-like

planets are necessary

to reduce rate ofimpacts

• If so, then Earth-like

planets are restricted

to star systems with

Jupiter-like planets



Climate and Habitability

• Some scientists argue

that plate tectonics

and/or a large Moon

are necessary to keepthe climate of an

Earth-like planet

stable enough for life



The Bottom Line

We don’t yet know how important or

negligible these concerns are.




• Are habitable planets likely? – Billions stars have sizable habitable zones, but

we don’t yet know how many have terrestrial planets in those zones

• Is life rare or common? – We don’t yet know because we are still trying to

understand all the factors that make Earthsuitable for life



The Search for Extraterrestrial

IntelligenceOur goals for learning

• How many civilizations are out there?

• How does SETI work?



How many civilizations are out

there?



The Drake Equation

Number of civilizations with whom we could potentially

communicate

= N HP ! f

life ! f

civ ! f

now

N HP = total # of habitable planets in galaxy

f life = fraction of habitable planets with life

f civ = fraction of life-bearing planets w/ civilization at some time

f now = fraction of civilizations around now.

We have no idea about the values of any of these.

H d SETI k?



SETI experiments look for deliberate signals from E.T.

How does SETI work?



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

purpose.




• How many civilizations are out there? – We don’t know, but the Drake equation gives

us a framework for thinking about thequestion

• How does SETI work? – Some telescopes are looking for deliberate

communications from other worlds



Interstellar Travel and Its Implications

to CivilizationOur goals for learning

• How difficult is interstellar travel?

• Where are the aliens?



How difficult is interstellar

travel?



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

• 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?



Fermi’s Paradox

• Plausible arguments suggest that civilizationsshould be common, for example:

• Even if only 1 in 1 million stars gets a civilizationat some time " 100,000 civilizations

• So why we haven’t we detected them?



Possible solutions to the paradox1) We are alone: life/civilizations much rarer than

we might have guessed.• Our own planet/civilization looks all the more

precious…



1) Civilizations are common but interstellar travelis 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



1) There IS a galactic civilization… … and some day we’ll meet them…

Possible solutions to the paradox




• How difficult is interstellar travel? – Interstellar travel remains well beyond our

current capabilities and poses enormousdiffculties

• Where are the aliens? – Plausible arguments suggest that if interstellar

civilizations are common then at least one ofthem should have colonized the rest of the

galaxy – Are we alone? Has there been no

colonization? Are the colonists hiding?

Documents

24. Life in the Universe 2015