ASTR 380

Our Solar System: A worked Example

Themes in Our Solar System Temperature TrendsEnergy for Life

Rocky, Gassy. IcyAtmospheresMoonsStabilitySpecial ConditionsWinning the Lottery

Temperature Trends in our Solar System

The Sun is the primary source of energy in the Solar System.As a first approximation,

energy in = energy out equilibrium Temperature

Planets further from Sun are colder.

Temperature unit:Kelvins

373 K = boiling water273 K = 32 F Freezing77 K = air freezes

Temperature Trends in our Solar System

Mercury : 670 K on the day side70 K on the night side

Venus: 740 K on surfacelittle variation in day-night

BUT should be 350 K except foratmosphere -- greenhouse effect

Earth: average – 288 Klow – 225 K high – 325 K

But should be 255 K average ifno atmosphere

Temperature Trends in our Solar System

Jupiter: average – 150 Kdepends on depth in

atmosphere

Mars: average -- 215 low – 135 high – 290

Saturn: average – 130 Kdepends on depth in

atmosphere

Temperature Trends in our Solar System

#1 effect is heating by Sun……but

day-night and seasonal changes makes for variations!

Atmospheres tend to warm planetsand smooth out variations

Venus740K Not 350K

There remains a clear trend: 1 – 3 AU is best for life…

Temperature Trends in our Solar System

Secondary Effects:

Internal heat within planet:on Earth Geothermal

Even if the Earth were coldGeothermal activity would continue

Arises from Earth’s molten core and mantle

The heat comes from radioactive decay ofelements in the mantle and core!

Temperature Trends in our Solar System We can see that geothermal activitywas present on Venus, Mars, andeven our Moon.

But for bodies significantly smallerthan the Earth, radioactive decaydoes not generate enough heat tokeep the core molten.

We know that the Moon is solid

We suspect that Mars is solid

We expect that Venus is still active!

Olympus Mons on Mars

Moon

Temperature Trends in our Solar System Tides can heat Moons:

Io, a moon of Jupiter is theprime example in ourSolar System

Has observed lava flows withhot-spot temperaturesof 1300-1600 K

Io is a rocky moon – the other big moonsof Jupiter are icy

Temperature Trends in our Solar System Tides can heat Moons:

Titan, a moon of Saturn may be a second interesting example….

Methane volcanoes indicate that itsinterior is kept warm. Compare toits surface temperature of 94 K!

Energy for Life Basic truth: Life requires energy!

Solar, geothermal, chemical…. Energy to enable the ordered activity that is life.

From Earth’s example, life exploits all available energy.

But life is limited by the availability of energy.

Fundamental unknown:Can life originate in isolated islands of energy?Or does it require abundant, widespread availability?

Tubeworms near a vent

Bacteria 400 metersBelow the seabed

Rocky, Gassy, Icy Theme: rocky, gassy, and icy planets and dwarf planets in our Solar System.

Rocky: close to Sun; within 4 AUGassy: middle distance from Sun; 5 – 30 AUIcy: far from Sun; 30 AU an beyond

Temperature and availability of material during formation cause this trend of planet type with distance.

Rocky, Gassy, Icy Rocky: formed in the inner Solar System after most gas gone

modest amount of rocky material so no big planets

Rocky planets too small to capture gas at birth

Atmosphere created from gases in rocks and impacts of comets.

Early heat on planet kept away ices and gas.

Rocky, Gassy, Icy Gassy: grew in places where rocky cores could attract gas

rocky cores at center

primarily hydrogen and Helium gas

lots of material in this region in birth nebula

Rocky, Gassy, Icy Icy Dwarf Planets:grew in places with rock and ices

little material so never grew big

could not hold into hydrogen and helium so no big atmospheres

Big cousins of comets and perhaps millionsof Kuiper-Belt objects

Artist’s concept of SednaPlot of known Kuiper Belt Objects

Atmospheres

Solar System Atmospheres have three origins:

1. Gas captured during birth of planet Gas Giant planets did this! Hydrogen, Helium dominant

2. Outgassing from molten rock on the surfaceterrestrial planetscarbon Dioxide, sulfur dioxide, water,

methane, ammoniavolcanic activity continues this process

3. Comet impactshappened on all planets but most important

for terrestrial planets

Jupiter’s Atmosphere

Venus Atmosphere

Atmospheres Solar System Atmospheres have three origins:

1. Gas captured during birth of planet

2. Outgassing from molten rock on the surface

3. Comet impacts

Atmospheres BUT…. Why are Venus, Earth and Mars so different?

Venus probably kept all of its atmosphere

Mars lost most of its atmosphere

Earth kept just the right amount due to the collision that made the Moon.

Atmospheres Mars size impactor completely destroyed

Earth surface all moltenAtmosphere ripped off

Moon reform in orbit fromdebris

Earth crust reforms

Moons

The terrestrial planets have rocky moons, and few of them

The gas giants and icy dwarf planets have rocky and icy moonssometimes many moons.

There are different stories for many of the planets:

Mercury – no moons – too close to SunVenus – no moons -- ?Earth – 1 big moon – created by a collisionMars – 2 small moons – captured asteriodsGas Giants – many moons – some formed in

a mini-solar system during birth, others captured

Moons of Uranus

Phobos

Demos

Stability

Earth life required time to form and to evolve.

Life began on Earth 200-400 Million years afterthe crust became cool. About 1 Billion yearsafter formation.

Around 1.6 Billion years cyanobacteria

Around 3.4 Billion years multicell organisms

Earth life required sufficient stability to maintainlife but enough change and variability to driveevolution.

Special Conditions

Earth life required:rocky surfaceliquid waterright amount of atmosphereright temperature rangemulti-billion year stability

It is even argued that life required our big Moon…to get rid of the excessive atmosphere

at the collisionto raise large tides to drive evolution

Winning the Lottery One could argue that life on Earth is a

One in a millionOne in a billionOr larger

Long shot.

But we are here so may be it had to be…..

Or maybe

we are the one winners of the lifelottery in the millions of planets in our Galaxyand the millions of galaxies in the Universe!