1. Chapter 24 Life in the Universe
The quote here is excerpted from her poem A Brave and Startling Truth… We like this quote because it is suggestive of the idea that we CAN learn something very important about ourselves and our universe…

2. Earliest Life Forms
Life probably arose on Earth more than 3.85 billion years ago, shortly after the end of heavy bombardment
Grand Canyon layers record 2 billion years of Earth’s history…

3. Evidence
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

4. 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.
DNA = Deoxyribonucleic acid. Most instructors won’t insist their students learn that term.

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

6. Necessities for Life Nutrient source
Energy (sunlight, chemical reactions, internal heat)
Liquid water (or possibly some other liquid)
Hardest to find on other planets
Applies to “life as we know it.”

7. Searches for Life on Mars
This photo shows the Opportunity panorama that appears as the opening photo for Ch. 6. Spend a couple minutes reviewing why Mars is a promising place for life: evidence of past water, evidence for subsurface ice today… You might wish to repeat some slides from Ch. 7 on Mars to show the evidence for past water from orbit.
Mars had liquid water in the distant past
Still has subsurface ice; possibly subsurface water near sources of volcanic heat.

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

9. The Martian Meteorite debate
composition indicates origin on Mars.
1984: meteorite ALH84001 found in Antarctica
13,000 years ago: fell to Earth in Antarctica
16 million years ago: blasted from surface of Mars
4.5 billion years ago: rock formed on Mars
If you wish to discuss the martian meteorite debate, it’s good to start with a brief history of the meteorite as reconstructed from its geology and dating of various components.

10. Does the meteorite contain fossil evidence of life on Mars?
These microscopic photos show structures with a fossil-like appearance, but they may also have been formed by non-biological processes. Bottom line: meteorite is intriguing, but certainly not definitive. We’ll need much more evidence to conclude that life ever existed on Mars.
Depending on level of coverage, you might wish to go into the pro and con debate in greater detail, as discussed in the text on p
… most scientists not yet convinced

11. Could there be life on Europa or other jovian moons?
Review evidence for liquid water ocean on Europa, in which case there might be undersea volcanoes. Artists conception of volcanic eruption temporarily disrupting surface ice.

12. 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

13. Titan Surface too cold for liquid water (but deep underground?)
If you are teaching after the Huygens descent, you’ll certainly want to update this slide with new information/images…
Surface too cold for liquid water (but deep underground?)
Liquid ethane/methane on surface

14. Are habitable planets likely?
Definition:
A habitable world contains the basic necessities for life as we know it, including liquid water.
It does not necessarily have life.
Be sure students understand that “habitable” refers to a potential for life, not actually for life itself. And emphasize that while we may soon be able to search for worlds with habitable surfaces around other stars, we will NOT soon be able to look for worlds like Europa --- even though these may well be more common.

15. 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.
First question in the search for habitable worlds is how many stars could be homes to life…
Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.

16. Even if we focus only on G-type stars like our Sun, there are billions in the Milky Way galaxy alone. And we can’t rule out habitable worlds around smaller stars. (But much more massive stars have lifetimes too short for evolution.)
The more massive the star, the larger the habitable zone — higher probability of a planet in this zone.

17. Are Earth-like planets rare or common?
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

18. 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

19. 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

20. 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

21. The Bottom Line
We don’t yet know how important or negligible these concerns are.
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

22. How many civilizations are out there?
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

23. 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.
It’s usually worth going through this equation slowly so that students can see how it leads to the number of civilizations that we can communicate with.

24. 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?

25. Are we “off the chart” smart?
Humans have comparatively large brains
Does that mean our level of intelligence is improbably high?
One interesting question on Fciv is whether it is likely or unlikely that a planet with life would ever get a species as smart as US. This is a fun slide to show, since it can be used to argue the question both ways. On the one hand, we have much larger brains in proportion to body mass than any other species that has ever lived on Earth. On the other hand, while we are a statistical outlier, statistically some outliers are to be expected…

26. How does SETI work?
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

27. SETI experiments look for deliberate signals from E.T.
Emphasize that current SETI efforts could not detect signals as weak as our own radio/TV broadcasts. For now, at least, we are looking for deliberately broadcast signals..
SETI experiments look for deliberate signals from E.T.

28. We’ve even sent a few signals ourselves…
Message sent from Arecibo in 1974… M13 distance approx. 21,000 light-years -- but offers a lot of potential targets with its high density of stars…
Earth to globular cluster M13: Hoping we’ll hear back in about 42,000 years!

29. Your computer can help! SETI @ Home: a screensaver with a purpose.
Optional:There’s a lot of data to sift though, and you can be a part of the effort… .
Your computer can help! Home: a screensaver with a purpose.

30. How difficult is interstellar travel?
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

31. Current Spacecraft
Current spacecraft travel at <1/10,000 c; 100,000 years to the nearest stars.
Our first interstellar emissaries: the Pioneer and Voyager spacecraft.
Pioneer plaque
Voyager record

32. Difficulties of Interstellar Travel
If you discuss the difficulty of interstellar travel in any detail, you might wish to discuss the implications to UFOs; see box on p. 483.
Far more efficient engines are needed
Energy requirements are enormous
Ordinary interstellar particles become like cosmic rays
Social complications of time dilation

33. Where are the aliens?
The text goes into a fair amount of detail on the rare Earth hypothesis, primarily because it has been so much in the news over the past few years. You may or may not want to go into the same level of detail in class.

34. 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?
This is one of our favorite topics, and if you have time it is worth using these slides as a capstone to your course…

35. 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…

36. 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…

37. Possible solutions to the paradox
There IS a galactic civilization…
… and some day we’ll meet them…
A truly incredible possibility to ponder…