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Prospect of Life in the Solar System. ASTR 1420 Lecture 11 Sections 7.1-7.3. Requirements for Life. If we want to identify habitable worlds in our solar system  need to know the range of environments acceptable for life.

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prospect of life in the solar system

Prospect of Life in the Solar System

ASTR 1420

Lecture 11

Sections 7.1-7.3

requirements for life
Requirements for Life
  • If we want to identify habitable worlds in our solar system

 need to know the range of environments acceptable for life.

We already touched upon this in some extent during previous lectures…

Let’s make a clear list of environmental requirements for life!

requirements for life1
Requirements for Life

“Metal” = Molecular Building Blocks

    • Earth life uses 25/98 chemical elements
    • CHON is 96% in mass.
    • Need the presence of all (or most) of these elements.
  • Rocky planets are made of “metal”
  • Smaller mass planets  hard to support life (e.g., Moon, Mercury…) : why?
    • Limit? : about 1/3 of Earth mass.
  • If the relative abundance of “metal” is lower than ~1/3 of the solar nebula, then it’s a problem! (B/c, it’ll be harder to form rocky planets).
  • Chemical composition of stars in our Galaxy

In astronomy, any chemical elements heavier than Helium are called “metal”…

our galaxy
Our Galaxy
  • Four major groups
    • Halo stars (metal poor, very old) : ~500 million stars (only about <10% of metal)
    • Thick disk stars (metal poor and old) : ~10% of all stars
    • Thin disk stars (more or less solar metallicity) : ~80% of stars
    • Bulge stars (old & metal rich) : good place to search for rocky planets.

But, dangerous place to be (stronger radiation, frequent super novae)

requirements for life2
Requirements for Life
  • Energy Sources
    • Sunlightgets weaker away from the Sun
      • At Saturn’s orbit (10 AU), sunlight is only 1/100 of that compared to Earth…
      • Far away from the Sun, it is hard to do photosynthesis
        • any photosynthesizing life form should be
          • very large in size to collect more light…
          • or very slow metabolism…
    • Chemical reaction energy: chemical reactions, in general, releases energy which can be utilized by life (e.g., CaCl2+ H2O  release heat!)
      • Chemical reaction requires reactants being in contact  needs for continual mixing.
      • Some mixable media : atmosphere or an ocean good places to look for life!

Inverse square law…

energy source
Energy source

Internal heat :

Original formation heat + heat generated by decay of radio active material.

  • Size of planet matters:
    • too small  no atmosphere as we’ve already seen and not enough internal heat
    • Smaller mass  lesser amount of radio active materials  lesser amount of energy creation
    • Smaller mass  larger ratio b/w surface area versus volume (ratio = 4πR2 / (4πR3/3) ~ 1/R)

 smaller planets radiate away internal heat faster than larger ones!

summary on the requirements of life
Summary on the Requirements of Life
  • Energy Source + Size
    • Planets in the habitable zone : sunlight, size of planets > 1/3 of Earth to have a protective atmosphere
    • Planets not in the habitable zone : chemical energy and/or internal energy, size still matters…
moon and mercury
Moon and Mercury
  • Similarity
    • Much smaller than Earth
      • Have lost internal heat already
      • No volcanism, no plate tectonics
      • No outgassing…
      • Airless
    • Lots of craters (preserved past scars) due to lack of erosion (no air) and tectonics.
    • Least likely habitable places in the solar system

 …due to no liquid water…

Small objects lose internal heat faster!

Mercury : 2450 km

Moon : 1740 km

Earth : 6400 km

  • Lacking heavy metals and radio active materials  lost internal heat faster
  • Smaller proportion of water and other volatiles. Why?
    • Due to the collisional creation
      • Volatiles were mostly vaporized and escaped during the collision
      • Heavy elements were lost to Earth during the collision

Collision b/w Earth & Theia

ice on the moon
Ice on the Moon?
  • Over the course of a lunar day (~29 Earth days), all regions of the Moon are exposed to direct sunlight.
  • Direct sunlight  395°K  ice will sublime into water vapor
  • Sunlight never reaches the bottom of some deep craters at both poles.
  • Outgassed ice in the past + newly added ices from impacts  deposited and preserved at the bottom of craters.

Evidence of water ice at the bottom of craters.

lunar ice good resource for colony
Lunar IceGood Resource for Colony!
  • Deposit of Ice at poles
    • Pristine cometary and asteroid material  more information on the early solar system formation
    • Lunar colony!!
      • Shipping water from Earth will be extremely expensive : $2,000 - $20,000 /kg.

= ~$80,000 on the Moon!!

  • 60,000-120,000 m3 of ice (~ a small lake)

 Source of H2O, O2, and H2 (for rocket fuel)

South pole region of the Moon.

  • Nearest planet to the Sun (w/o air)

+ long day/night (1 Mercury day ~ 6 Earth months)

    • Daytime T=425°C, nighttime T=-175°C
  • Highest mean density (2.3% higher than the Earth) among solar system planets
    • Giant impact thought to be stripping off the lower density crust in the past.
    • Any deposited ice from icy planetesimals (e.g. Earth) would have been lost during the collision.
  • Permanently shadowed regions at poles may contain some ice deposits from smaller subsequent impacts…

No chance of liquid water  no life!

venus runaway greenhouse effect
VenusRunaway Greenhouse Effect
  • If with Earth-like atmosphere  Venus’ global average temperature would be 35°C
  • Mariner 2 spacecraft : surface temperature of 470°C (persistent throughout the planet, day or night).
  • 90 times higher atmospheric pressure than Earth  high enough to crash military submarines (maximum depth~1,000 m)
  • Contains sulfuric acid, hydrochloric acid, and other toxicants
  • No ocean no CO2 cycle  all outgassed CO2 accumulated in the atmosphere  runaway greenhouse effect
  • A possibility of moderate climate in the past : lesser amount of CO2 + fainter young Sun
  • Surface crater count  entire surface is younger than 1Gyr!  implying tectonics…
  • Any hint of ancient life in the clouds?
  • Venus Express (ESA) : orbiting since 2006…  will give us a clue to the life in the atmo…

Surface temperature

hot enough to melt


  • A certainty of past flowing water, warmer climate, and thicker atmosphere.
  • About 3 billion years ago, climate changed. Planet got frozen…
  • Current atmospheric pressure is too low for liquid water.
  • Past life?
  • Later lectures…
jovian planets
Jovian Planets
  • Decreasing sunlight intensity : temperature at the high altitude is very low
  • But, plenty of internal heat! : high internal temperature
  • No solid surface.

 mid-altitude atmosphere maybe warm enough to support life?

jupiter and saturn
Jupiter and Saturn
  • At ~100km depth from the outermost clouds, temperature is right for a liquid water (water droplet) + lightnings…  Miller Experiment?
  • Life could have been originated in there or introduced life forms can survive there?

 No! Strong vertical mixing at speed of >100m/sec will take it down to high T below!

Jupiter and Saturn are very similar.

 No life!

From the inside : rock+metal core  metallic hydrogen  liquid hydrogen  gaseous hydrogen + clouds!

optical flashes from Jupiter were photographed recently by the Galileo orbiter. Each of the circled dots indicates lightning. The numbers label lines of latitude. The size of the largest spot is about 500 kilometers across and might be high clouds illuminated by several bright lightning strokes.

jupiter and saturn1
Jupiter and Saturn

Strong vertical mixing at speed of >100m/sec will take it down to high T below!

Jupiter and Saturn are very similar.

 No life!

uranus and neptune
Uranus and Neptune
  • Upper layer clouds are much colder than Jupiter & Saturn b/c of greater distance from the Sun.
  • Similar vertical mixing (though slower)
  • Outer core = ocean of water, methane, and ammonia  possibility of life?

?what about the energy source?

  • Even if they exist, our current technology cannot detect them!
large moons
Large Moons
  • Good prospects : Europa, Ganymede, Callisto, and Titan
  • These are small compared to other planets.
  • If Mercury lost all its internal heat, then these large jovian moons should have lost theirs also!
  • Main difference : they contain a great deal of ice (beyond the “snowline”)
  • Ice melt at much lower temperature than rocks.  “ice geology” = internal convection of icy material similar to tectonics!
  • For some moons (Io & Europa), a totally different kind of energy source, “tidal heating”  detailed studies later…

150+ moons around Jovian planets.

smaller bodies
Smaller Bodies


  • Small-medium size moons, asteroids, comets, and large Kuiper belt objects.
  • Small size  no liquid medium  no life…
  • Evidence of organic material on asteroids and comets…
  • Occasional melting of ice through impacts and close sun-passage…  too short
  • No evidence of life on asteroid belts…  unlikely places for life.
in summary
In summary…

Important Concepts

Important Terms

metal poor stars

  • Environmental requirements for life
  • Prospects of life (present and past) in the Solar System.
  • Reasons for some worlds being rejected…
  • Inverse square law
  • Good places for life in our Solar System
  • Chapter/sections covered in this lecture : 7.1, 7.2, & 7.3
  • Life on Mars : next class