astro 4 Flashcards
did the terrestrial planets all look quite similar last time
The surfaces of all 5 terrestrial worlds (Mercury, Venus, Earth,
Moon, Mars) must have looked quite similar when they were
young, and all were subjected early on to the Heavy Bombardment
How did the terrestrial inner worlds ended up so different although
they were made similarly from metal and rock that had condensed in
the initial solar nebula?
Their current appearances must be the result of changes that
occurred after their formation & initial ‘childhood’ (i.e. due to
their planetary evolution).
do all the terrestrial worlds have the same type of distinct
layering
g due to (at some time in the past) differentiation
= process where gravity separates materials by density -> ALL terrestrial planets must once have been hot enough for
the interior rocks & metal to melt
& separate by density
The interiors of any terrestrial planet is divided into 3 major layers
Core: made of the highest density materials (e.g. Ni, Fe)
Mantle: made of rocky material of moderate density
(minerals containing mainly Si, Mg, O)
Crust: made of rocky material of lowest density, e.g. granite
& basalt (volcanic rocks)
what is granite
Granite is a course-grained rock
composed mostly of quartz and solidifies
underground
what is basalt
Basalt is a fine-grained rock rich in Mg &
Fe exposed at or very near the surface of a
rocky planet or a moon. More than 90% of
all volcanic rock on Earth is basalt
which is denser & heavier, basalt or granite
Basalt
what is the rigid lithosphere
crust and part of mantle
what are the layers of earth in order
rocky crust>rigid lithosphere > mantle > metal core
The outermost rigid part of a planet is called
lithosphere
what generally encompasses the crust and part of the mantle
lithosphere
what is The upper layer of the mantle right below the lithosphere
asthenosphere
asthenosphere
It has relatively low resistance to plastic flow as it is hotter
and more fluid and this is where convection is thought to occur
what thickness is closely related to planetary size!
Lithospheric thickness
Small worlds tend to have thicker
=lithospheres
Why are planets round?
Rocks deform & flow under high
p and/or T, and the gravitational field originating from the center acts
equally in all directions and pulls everything toward it, ultimately
smoothing out the shape into a sphere.
The weak gravity of a small object is
s unable to overcome the rigidity of its
solid material clustered together
Within 1b y gravity will make into a sphere
any object bigger than ~500 km in diam.
Geological activity
the process of ongoing changes in the surface & structure of a planet
Why were the planets hot? What is/was the source of this energy?
3 sources of energy for the internal heat of terrestrial worlds:
Heat of accretion
Differentiation
Radioactive decay (fission
Heat of accretion
Gravitational potential energy of a planetisimal is converted into kinetic energy
which, upon impact, is converted into heat
Differentiation
Release of additional heat as dense
materials sank to core and convert their
gravitational potential energy into heat
Radioactive decay (fission)
Radioactive isotopes (U, Th, K) convert
their mass-energy (E=mc
2) into heat
Declines in time —>much more significant
when planets are big and/or young
Still
supplies heat to the terrestrial interiors but
at a lower level than when the planets were young.
How interiors cool off
convection, conduction, radiation
e most important factor
in planetary cooling.
size
Convection:
heat transported upward as hot material expands &
rises, dissipates energy at the outermost surface, then cooler
material contracts & falls
Occurs if the heating source is below
Conduction:
heat transfer from a hot material/region to a cooler
one through contact
Due to microscopic collisions of their constituent particles (atoms/molecules)
Radiation:
Thermal radiation carries
energy away from object
Planck’s law: ANY blackbody emits radiation
characteristic of their temp. T
Because of their low T, planets emit in IR
Large planets retain
n internal heat much
longer than smaller ones!
Lithosphere grows thicker
as planet’s interior cools.
what is the primary driver of geological activity
Interior heat
most important heat transfer process of Earth
Convection thru convection cells within the mantle
Mantle convection primarily involves
solid rock(not molten rock)
how long does mantle convection take
is a very slow process (a few cm/year!, i.e. a full cycle takes ~500m y)
The mantle is made of solid rock because
after Earth’s
formation it cooled over
m of years. Water trapped inside
minerals erupted with lava, a process called “outgassing—>the mantle solidified.
Mantle convection stops at
the base of lithosphere.
why does Mantle convection stop at the base of lithosphere.
Heat dissipates upwards primarily through conduction,
then radiates away into space once it reach Earth’s surface
Primary driving force for the movement of the tectonic
plates = the pieces into which the lithosphere is broken
Lava erupting from volcanoes comes only from a narrow
region of partially molten material beneath the lithosphere.
Planetary size determines
the strength of mantle
convection & lithospheric thickness
Venus is probably similar to Earth in
n its internal
heat & nearly as geologically active as Earth.
what is the problem on venus lol
Problem: lack of water, which on Earth (incorporated in many minerals, e.g. olivine) acts as lubricant and eases
tectonic plate movements.
The Moon has a very thick lithosphere & no
geological activity due to
its small size
Mercury also cooled quickly but
may still retain some heat in its very large core.
Mars, intermediate in size, has cooled
significantly but
probably retains enough internal heat for some reduced geological activity.
Why do some planetary interiors create a magnetic field?
Interior heat plays another crucial role: it can create a global magnetic field (MF) → creates a magnetosphere.
3 basic requirements to generate a global magnetic field (MF):
An interior region of electrically conducting fluid (gas/liquid, e.g. molten
metal)
Convection in that layer of fluid
At least moderately rapid rotation around its axis.
Earth = the only terrestrial world that meets ALL 3 BASIC requirements
Dynamo effect: Earth’s MF is due to
convection in the liquid outer core AND the Coriolis force due to Earth’s rotation
The Coriolis force tends to o
organize the flow within the outer core
into rolls aligned along the N-S polar axis.
None of the other terrestrial planets has
a MF as strong as Earth’s
Electrons
(e–) in molten metal move within the outer core ________________
similar to the current flowing thru an electromagnet but in a self-sustaining
process.
Why do some planetary interiors create a magnetic field? Moon:
core has long since cooled & stopped convecting no MF
Why do some planetary interiors create a magnetic field? Mars
: Its core (part of mantle too??) probably still retains some heat, but not enough to drive core convection, hence it lacks a MF.
Why do some planetary interiors create a magnetic field? Venus
: probably has a molten core similar to Earth’s, but either its convection or very slow rotation is/are too weak to generate a MF
Why do some planetary interiors create a magnetic field? Mercury
an enigma as it does have a MF despite its small size &
slow rotation → most probably due to a huge core that may still
be partially molten and convecting.
Why do some planetary interiors create a magnetic field? Jupiter
has a very strong MF due to a layer of convecting metallic hydrogen & its rapid rotation.
Why do some planetary interiors create a magnetic field? Jupiter
At same planetary size, a molten metal layer generates a stronger MF
than an ionic liquid
Why do some planetary interiors create a magnetic field? Sun:
its MF is due to convection of ionized gas (plasma) & its
rotation.
A planet’s MF creates a
magnetosphere = a region of space
surrounding an astronomical object in which charged particles
are affected by that object’s magnetic field.
A magnetosphere acts as a
protective bubble which deflects
most of the charged particles in the solar wind
If the (Earth’s) magnetosphere were not present (or weak),
highly energetic particles can strip away atmospheric gases & cause genetic damage to living organisms.
ALL surface features on a terrestrial planet (e.g. on
Earth) are caused by just 4 major processes
Impact cratering:
Volcanism:
Tectonics:
Erosion:
Impact cratering:
excavation of bowlshaped craters by asteroids or comets
crashing into the surface
Volcanism:
eruption of molten rock, or
lava, from its interior onto its surface
Some planets may have instead cryovolcanism, in
which volatiles/liquefied gases (e.g. H2O, NH3,
CH4) are ejected instead of lava
Tectonics:
building/reshaping of surface
features due to stretching, compression or
other forces acting on the lithosphere
Erosion:
wearing down or building up of
geological features by wind, water, ice &
other planetary weather
Cratering
An impact crater forms when an
asteroid/comet slams into a solid surface
Many impact craters on Earth & other worlds,
especially Moon & Mercury.
Earth bombarded by impacts when it was young,
but most ancient craters were erased by
volcanic/geological activity & erosion
Craters can provide very important clues:
An impact crater forms when an
asteroid/comet slams into a solid surface.
The released energy vaporizes rock and blasts out a
crater
= circular since material is ejected uniformly
in all directions (for normal incidence!)
Many impact craters on Earth & other worlds,
especially Moon & Mercury.
Most cratering happened soon after the Solar system
formed (2 episodes of Heavy Bombardment).
Small craters far outnumber large ones
many more
small objects than large ones in the initial solar nebula
Craters can provide very important clues:
The more craters, the older the surface.
The geological conditions existent at the time of impact
Volcanism
Volcanism occurs when underground molten rock (magma) finds a path to the surface through the lithosphere
Volcanism produces volcanic mountains & explains the
existence of our atmosphere & oceans.
Volcanism occurs when underground molten rock (magma)
finds a path to the surface through the lithosphere
Molten rock/magma = lower density rises with respect to surrounding high density materials The solid rock surrounding a magma chamber can squeeze it, driving it upward under pressure Magma often contains trapped gases that expand as it rises dramatic & violent explosive eruptions
Volcanism produces volcanic mountains & explains the
existence of our atmosphere & oceans.
Water/ices originated from icy planetesimals, the rock from the
asteroids & planetesimals whose merger made up the planet. The
subsequent heavy bombardments may also have contributed
significantly, especially with water/ice.
Volcanic eruptions release some of the initially incorporated
gases/liquids (outgassing) made Earth’s atmosphere & oceans.
Cryovolcanism: v
volcanism that may take place on cold worlds covered by ices (provided that an internal heat source is present).
Molten rock/magma is called lava after it reaches the surface.
Lava can shape 3 different types of volcanic features, depending on its viscosity:
Lava can shape 3 different types of volcanic features,
depending on its viscosity:
Runny lava (i.e. with the lowest viscosity) makes flat lava plains.
Slightly thicker (i.e. more viscous) lava makes broad shield volcanoes.
The thickest (i.e. most viscous) lavas solidify very quickly and make
steep stratovolcanoes
Tectonics
building/disruption of surface features by internal stresses caused (directly or indirectly) by convection in the underlying mantle.
Tectonics =
Compression forces where adjacent convection cells push rock together —> mountain ranges
Cracks & valleys form where adjacent convection cells pull the crust apart
The stresses due to underlying mantle convection
fractured Earth’s
lithosphere into more than a dozen pieces called plates
plates
Plates move over, under & around each other → plate tectonics
Only Earth has
plate tectonics
what generally occur together
Tectonics & volcanism generally occur together—> heat
Erosion
refers to weather-driven processes that break down or
transport rock through action of ice, liquid, or gas.
Erosion
Shaping of valleys by the flow of ice bodies (glaciers).
Carving of canyons by the flow of liquid water bodies (rivers).
Shifting of sand dunes by winds (gas).
Erosion can also
build up geological features
Sand dunes, river deltas, lake bed deposits, accumulation of sediments into
layers on ocean floors -> sedimentary rocks.
Atmosphere
In most cases it is a surprisingly thin layer (compared to the size of the world)
Can be a mixture of many different gases that may consist of individual atoms or of molecules.
The collisions of individual atoms/molecules in an atmosphere create pressure
Ideal Gas Law equation: pV = NRT, where p & V = the pressure &
the volume of the gas, N = the number of moles of gas, R = the universal gas
constant, and T = the absolute temperature.
The gas in an atmosphere is held down by gravity.
The collision-resulting pressure acts uniformly, pushing in all directions,
including upwards, making the atmosphere expand
Planetary
atmospheres exist in balance between the downward weight of the gases &
upward push of their gases.
p↓ with↑ altitude H→ because air density ρ↓: p=ρTR/M, with M = molar mass.
An atmosphere can have several key effects onto its planet:
Its pressure (& temperature) determines if liquid (water) can exist on the
surface.
Absorbs and scatters light.
Creates wind & weather.
Aurora is produced when the solar wind particles trapped in the
magnetosphere make it through & collide with atoms/molecules in the
atmosphere.
Can make planetary surfaces warmer via the greenhouse effect.
The greenhouse effect occurs only when
an atmosphere contains
gases that can absorb IR light (e.g., H2O vapor, CO2, CH4)
radiated back from the planet surface.
The energy of an absorbed IR photon is not retained for long but
re-emitted as another IR photon, which is again absorbed by another molecule, etc., etc.
greenhouse gases significantly slow down the escape of IR radiation
from a lower atmosphere, while the molecular motions of the latter’s
molecules heat it up (and also the planet’s surface)
The greenhouse effect does NOT alter a planet’s overall energy balance
Earth’s atmosphere consists of
77% N2, 21% O2 & small
amounts of other gases
Earth’s atmosphere consists of 77% N2, 21% O2 & small
amounts of other gases.and what does that cause
Enables presence of liquid water = explains extensive erosion on Earth
Sustains Life: without it, Earth’s surface would be lifeless due to the
dangerous solar radiation & so cold that all water were frozen
Earth’s atmosphere is
~480 km thick but ⅔ of our atmospheric
air lies within 10 km of surface (90% below 16 km)!
Still other higher layers have vital roles (O 3 layer to block UV, absorption of X-rays)
Earth’s atmosphere has 5 basic
layers/regions:
Troposphere Stratosphere Mesosphere Thermosphere Exosphere
Troposphere =
= the lowest layer; here T↓ with↑ altitude H;
Stratosphere=
composed of
stratified temperature layers; it
begins where T↑ with↑H, due to UV absorption by ozone (O3);
Mesosphere =
= the region where T↓ again with↑ altitude H:
The stratosphere and the mesosphere are sometimes collectively
referred to as the “middle atmosphere”
Thermosphere =
= begins where T again↑ with↑ altitude H;
Exosphere =
the uppermost region where the atmosphere gradually fades into space.
The layering is shaped by the way each region interacts with various
wavelengths of light
Exosphere
Heated by solar UV & X-rays
Fast-moving gas molecules
can escape to space
Thermosphere
X-rays heat & ionize gases
Ionosphere
Made of ions created by energetic particles from solar wind and outer space Reflects radio waves and solar wind Comprises 4 layers (D, E, F1, F2)
Stratosphere
Heated by UV, layered, no convection
Troposphere
Greenhouse gases trap IR
radiation from the ground
Convection important
