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 Europa Ganymede Enceladus

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 - CO 2 )

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 mv 2 )

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 CO 2 )  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 CO 2  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 (N 2 ), with some oxygen (O 2 : arises from photosynthetic life), carbon dioxide (CO 2 ), 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 (O 3 ), 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., H 2 O, CO 2, CH 4 - 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