Homework #5 due next Tuesday, 4:00 pm

Interactions between the surfaces of planets and moon and their interiors play a large role in determining their

habitability

Impact cratering – from collisions with asteroids and comets

Erosion – wearing down or building up geological feature by wind, water, ice, etc.

Volcanism – eruption of molten rocks & outgassing

Tectonics – disruption of a planet's surface by internal stresses (upper mantle and lithosphere)

Four major processes affect planetary surfaces:

The Lithosphere…

Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below

While interior rock is mostly solid, at high pressures stresses can cause rock to deform and flow (think of silly putty)

This is why we have spherical planets/moons

Stresses in the lithosphere lead to “geological activity” (e.g., volcanoes, mountains, earthquakes, rifts, …) and, through outgassing, leads to the formation and maintenance of atmospheres.

Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses

Convection is the main cooling process for planets with warm interiors.

Initially, accretion provided the dominant source of heating of the interiors of planets.

Very early in a terrestrial planet’s life, it is largely molten (differentiation takes place).

Today, the high temperatures inside the planets are due to residual heat of formation and radioactive decay heating.

Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e.g., boiling water).

Tectonics: refers to the action of internal forces and stresses on the lithosphere to create surface features, i.e., “geological processes”

Can only occur on planets or moons with convection in the mantle:

Earth & Venus EuropaGanymedeEnceladus

Tectonics…

•can move large segments of the lithosphere (plate tectonics)

Tectonics…

• raise mountains

Tectonics…

create huge valleys (rifts) and cliffs

Tectonics…

• create new crust

Tectonics…

generate volcanoes => maintain atmospheres!

The interiors of the terrestrial planets slowly cool as their heat escapes.

Interior cooling gradually makes the lithosphere thicker and moves molten rocks deeper.

Larger planets take longer to cool, and thus:

1) retain molten cores longer

2) have thinner (weaker) lithospheres

The stronger (thicker) the lithosphere, the less geological activity the planet exhibits.

Planets with cooler interiors have thicker lithospheres!

Geological activity is driven by the thermal energy of the interior of the planet/moon

Earth has lots of geological activity today, as does Venus. Mars, Mercury and the Moon have little to no geological activity (today)

Side effect of hot interiors - global planetary magnetic fields

Requirements:

• Interior region of electrically conducting fluid (e.g., molten iron, salty water)

• Convection in this fluid layer

• “rapid” rotation of planet/moon

Earth fits requirements

Venus rotates too slowly

Mercury, Mars & the Moon lack molten metallic cores

Sun has strong field

In the absence of a “magnetosphere”, the solar wind will slowly strip away an atmosphere and will bombard its surface with high energy particles from the solar wind.

A magnetic field creates “magnetosphere” that deflects away solar wind particles.

ATMOSPHERES

Atmospheric Basics

Layer of gas surrounding a planet.

Usually very thin for terrestrial planets (exception Venus).

Affects conditions on the planet.

We would like to understand how each of the terrestrial planets ended up having such different atmospheres.

Venus’s thick atmosphere

Interlude:

thermal emissions and interactions

All terrestrial planets probably had minimal atmospheres at some point after they formed: “primary” atmosphere of H,He

These original atmospheres were swept away from the terrestrial

planets early in their life.

Current atmospheres are “secondary” atmospheres,

formed primarily by outgassing

(mostly carbon dioxide - CO2)

Holding onto an atmosphere requires gravity

The strength of gravity determines the escape velocity from the planet.

The temperature and composition of an atmosphere determines the velocities of atoms and molecules in the atmosphere; lighter molecules will move faster. At a given temperature, H and He will have higher velocities than more massive elements or molecules (recall that K.E. = 1/2 mv2)

Holding onto an atmosphere requires gravity

The strength of gravity determines the escape velocity from the planet.

The temperature and composition of an atmosphere determines the velocities of atoms and molecules in the atmosphere.

If the constituents of an atmosphere are moving faster than escape velocity, then a planet or moon will be unable to hold onto an atmosphere.

Larger (stronger gravity),

cooler (slower moving molecules) planets

can hold onto atmospheres better than

smaller (weaker gravity),

hotter (faster moving molecules) planets

● Moon and Mercury are “Airless” worlds

gravity too weak to hold onto an atmosphere

“black sky”

The little atmosphere that exists consists of particles of the solar wind that are temporarily trapped.

● Mars Very little atmosphere today (mainly CO2)

Mars had standing and running water on its surface in the past.

Therefore, it must have had a more substantial atmosphere in the past

Does it have water today? Yes - frozen in polar ice caps and beneath its soil

● Venus

Densest atmosphere of all Terrestrials

Mostly CO2

Temperature at surface hot enough to melt lead

Pressure at the surface ~ 90 times that on Earth

Perpetual cloud cover, sulfuric acid rain

Weather forecast “awful” all the time.

● Earth

A moderate atmosphere today

Mostly nitrogen (N2), with some oxygen (O2: arises from photosynthetic life), carbon dioxide (CO2), etc.

Enough to enable liquid water to exist (temperature and pressure adequate)

Together the air & water produce erosion

The Jovian planets (high gravity, cool/cold atmospheres) have very substantial

atmospheres, primarily H & He.

We see the tops of clouds

LAYERING OF ATMOSPHERES

Structure is created

within an atmosphere

through interactions of

atmospheric gasses

with light

Exosphere

hottest layer, v. rarified

Thermosphere

absorbs X-rays, ionized, ionosphere, reflects some radio,

aurora

Mesosphere• weakly absorbs UV

Stratosphere• strongly absorbs UV, ozone

(O3), stratified (no convection), Earth only Terrestrial planet with one.

Troposphere• absorbs IR (greenhouse);

convective; weather

From the perspective of life, stratosphere & troposphere are the

most important

Stratosphere absorbs harmful UV

Troposphere provides greenhouse effect

The greenhouse effect

Planets heat up by absorbing the Sun’s visible light

Planets cool off by radiating infrared out to space

Greenhouse gasses trap infrared radiation in

troposphere (lowest level of atmosphere), thereby

heating the lower atmosphere.

● greenhouse gasses (e.g., H2O, CO2, CH4 -

methane) transmit visible light but absorb

infrared light

● Greenhouse effect raises temperature of lower atmosphere

● Greenhouse effect is critical to the existence of life on Earth – it raises temperatures to “habitable” level, permits liquid water