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.

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

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

Requirements for Life “Metal” = Molecular Building Blockso Earth life uses 25/92 chemical elementso CHON is 96% in mass.o 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…)o Limit? : about 1/3 Earth mass.

• If the relative abundance of “metal” is lower than ~1/3 of 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

• Four major groupso Halo stars (metal poor, very old) : ~500 million stars (only about <10% of metal)o Thick disk stars (metal poor and old) : ~10% of all starso Thin disk stars (more or less solar) : ~80% of starso Bulge stars (old & metal rich) : good place for rocky planet formation.

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

Requirements for Life

• Energy SourcesSunlight photosynthesis

o Sunlight gets weaker away from the Sun At Saturn’s orbit (10AU), 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…

o Chemical reaction energy 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 distance law…

Energy sourceInternal heat :

Heat generated by decay of radio active material.and trapped heat from the planet creation

• Size of planets matters:o 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!Io : small planet that is still active!?!

small planets are bad : no atmo., lesser amount of heat to start, lose heat faster

Environmental Requirements

1. Must have a source of molecules2. Must have a source of energy3. Must have a liquid medium

Any worlds meet the #3 requirement, likely meet the first two requirements also. Why?

Conservatively…

A habitable world = only if it has a liquid water!

Moon and Mercury

• Similarityo Much smaller than Earth

Have lost internal heat alreadyNo volcanism, no plate tectonicsNo outgassing…Airless

o Lots of craters (preserved past scars) due to lack of erosion (no air) and tectonics.

o Least likely habitable places in the solar system …due to no liquid water…

Small objects lose internal heat faster!

Mercury : 2450 kmMoon : 1740 kmEarth : 6400 km

Moon

• Lacking heavy metals and radio active materials no radioactive heat!

• 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?

• Over the course of a lunar day (~29 Earth days), all regions of the Moon are exposed to direct sunlight.

• Direct sunlight 395K 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.

• Deposit of Ice at poleso Pristine cometary and asteroid material more

information on the early solar system formationo Lunar colony!!

Shipping water from Earth will be extremely expensive : $2,000 - $20,000 /kg.

= ~$5,000 on the Moon!!

Lunar IceGood Resource for Colony!

South pole region of the Moon.

60,000-120,000 m3 of ice (~ a small lake) Source of H2O, O2, and H2 (for rocket fuel)

Mercury

• Nearest planet to the Sun (w/o air)+ long day/night (1 Mercury day ~ 6 Earth months)Daytime T=425C, nighttime T=-175C

• Highest mean density (2.3% higher than the Earth) among solar system planetsGiant 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!

VenusRunaway Greenhouse Effect

• If it had Earth-like atmosphere Venus’ global average temperature would be 35C

• Mariner 2 spacecraft : surface temperature of 900F (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 lead!

Mars

• 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

• 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 are very similar. No life!

• At ~100km depth from the outermost clouds, temperature is right for a liquid water (water droplet) + lightnings…

• Life could have been originated in there..• Can introduced life forms survive there? No! Strong vertical mixing at speed of

>100m/sec will take it down to high T below!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 Saturn

Jupiter and Saturn are very similar. No life!

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

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

• 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” more details on later lecture…150+ moons around Jovian planets.

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.

Eros

In summary…

Important Concepts• Environmental requirements for life• Prospects of life (present and past)

in the Solar System.• Reasons for some worlds being

rejected…• Inverse-square distance law

• Good places for life in our Solar System

Important Terms• metal poor stars

Chapter/sections covered in this lecture : 7.1, 7.2, & 7.3Life on Mars : next class