Astronomy 217P r o t e c t i n g E a r t h ’ s

S u r f a c e

When we see the Earth from space, or any other planet, the view is dominated by the parts outside of the core and mantle. ✦ Core ✦ Mantle ✦ Crust ✦ Hydrosphere ✦ Atmosphere ✦ Magnetosphere

The View from Above

The coalescing Earth managed to capture some gases from the gas-rich protoplanetary disk.

Thus the original or primeval atmosphere was rich in hydrogen and helium, but these rapidly escaped the Earth’s gravity. More complex molecules like methane and ammonia were photo-dissociated by sunlight.

Secondary atmosphere formed from outgassing, as gas was released from the heating and differentiation of the mantle and core.

This was predominantly carbon dioxide, water and nitrogen.

Reactions changed the atmosphere!

Composition

Present Composition

Gas by Number

by Mass

N2 75% 78%O2 23% 21%Ar 1.3% 0.9%

CO2 0.05% 0.04%H2O*

The composition of the atmosphere has undergone many changes driven by chemistry, both internal to the atmosphere and with the crust and ocean.

CO2 dissolved in the early ocean, then reacted with minerals to form carbonates, like CaCO3.

With the appearance of algae about three billion years ago, CO2 was further removed from the atmosphere, replaced by O2 which is continually being replenished.

In modern times, human production of CFCs (Chlorofluorocarbons) damaged the ozone layer, creating an ozone hole.

Atmospheric Chemistry

GreenhouseThe composition of the Earth’s atmosphere can have a large impact on the surface conditions.

Gases like carbon dioxide, water vapor & methane have high opacities in the infrared, but lower opacities in the optical.

Thus the solar photons with an characteristic temperature of 5800 K (λpeak ~ 500 nm) can pass through the atmosphere.

Thermal photons from the Earth, with a characteristic temperature of 290K (λpeak ~ 10 μm) can not escape as easily.

Global WarmingIn addition to O2, the other product of photosynthesis is hydrocarbons in the form of biomass.

A significant amount of this biomass was sequestered by geologic processes, producing fossil fuels.

Modern society, by burning fossil fuels, has reversed this sequestration, increasing CO2 levels in the atmosphere.

A corresponding increase in global average temperature has been observed, changing the climate.

Global ConsequencesClimatologists have modeled the consequences of increased greenhouse gas and the resulting global warming.

These consequences include ✦ Rise in sea level, due to polar ice melting ✦ More severe weather, due to increased atmospheric energy ✦ Crop failures due to climate zones changing. ✦ Expansion of equatorial deserts ✦ Spread of tropical diseases away from the tropics ✦ Enhanced habitability of the near polar regions

Hydrostatic EquilibriumAs with the structure of the Sun, the structure of the Earth’s atmosphere is determined by the balance of gravity by thermal pressure.

A

P

P +ΔP

Δr

Fgrav

Fpres

This yields a pressure difference as a function of Δr.

Setting Fpres = Fgrav yields

Taking the limit of Δr→0

Equation of Hydrostatic Equilibrium

Scale HeightFor planetary atmospheres, the gases obey the ideal gas law,

The Eq. of Hydrostatic Equilibrium can be written

Over a small range in radius, the gravitational acceleration is nearly constant, thus

where

or

The solution is

or

where

Scale Height

H⨁ ≈ 8 km

The Earth’s atmosphere is not isothermal, but reveals a complex temperature structure (blue curve) determined by heat transport and local heat deposition.

The temperature and density in the stratosphere favor the production of ozone (O3) which shields the surface from UV.

The upper part of the thermosphere, the ionosphere, is fully ionized by solar radiation.

Earth’s Atmosphere

Thermosphere

The dominant source of heat for the Earth’s atmosphere is the Earth’s surface. The Sun-warmed surface re-radiates infrared thermal radiation, which is absorbed by the atmosphere.

In the troposphere, this causes higher temperatures near the surface, leading to convective instability.

Convection is a key ingredient to cloud formation and other weather phenomenon.

The stable temperature structure of the stratosphere is caused by heating when UV light dissociates O3.

Similarly, the thermosphere is heated by ionization.

Heat Transport

Even when molecules are unable to absorb photons of a given wavelength, they can still change the direction or scatter the light.

The strength of the scattering depends on the relative size of the wavelength of the light (λ) to the size of the scatterer (L).

When L > λ, all wavelengths scatter equally.

In the atmosphere, scattering can occur on molecules (L~ 0.3 nm), dust particles (L ~ 1 μm), water droplets (L ~ 10 μm) and ice crystals (L ~ 0.1 mm).

Scattering

Sunlight passing through clear sky is scattered by dust and molecules. For visible light (λ =400-700 nm), Rayleigh scattering causes more scattering of shorter wavelengths.

Along the light of sight to the Sun, blue light is removed leaving red sunsets and sunrises.

The scattered blue light dominates along other lines of sight.

In contrast, larger water droplets scatter all wavelengths equally, thus appearing white.

Why Is the Sky Blue?

For light with wavelength λ >> L, the particle size, it is appropriate to consider light as an electromagnetic wave. This moving wave generates movement in the electrons present in matter that scatter or deflect the light.

The strength of the response to these movements (α, the polarizability) depends on the scattering molecule.

Shorter wavelengths (higher frequencies) are closer to the molecules’ natural frequencies, thus strength of the scattering response is stronger, e.g. 7004/4004= 9.4.

Intensity depends on scattering angle θ and distance r.

Rayleigh Scattering

rθ

Above the ionosphere and the exosphere lies a region protected from the solar wind by the Earth’s magnetic field.

Magnetosphere

On the sunward side, the magnetosphere is compacted by the solar wind, but on the downwind side it extends much further, forming a teardrop shape.

Magnetic DipoleThe Earth’s magnetic field resembles a bar magnet.

Like a bar magnet, closed field lines result in a magnetic field strength that declines more rapidly than conservation of magnetic flux would indicate.

Dipole Field

Deflecting the solar wind requires a magnetic energy density greater than the solar wind’s kinetic energy.

⇒

For B⨁ = 3.1 × 10−5 T and ρʋ2/2 ≈ 10−9 J m−3, the radius of the magnetopause is ≈ 8.5 R⨁ = 5.4 × 104 km.

Solar wind particles that do leak through the magnetopause can become trapped in the Earth’s magnetosphere.

The trapping occurs because the particles spiral around the magnetic field lines.

The 2 principle regions the particles become trapped are called the Van Allen belts.

These belts extend from the top of the Earth’s atmosphere to the magnetopause but are strongest at ~ 0.5 R⨁ and ~ 3 R⨁.

Trapped particles

Near the poles, the Van Allen belts intersect the atmosphere. The charged particles collide with molecules and atoms in the atmosphere, collisionally exciting them.

The resulting photo-deexcitation produces the glowing light called aurorae, borealis near the north pole and australis in the south.

Aurorae

Though the Earth’s magnetic poles lie close to its rotational poles, the alignment is not exact.

The magnetic poles move as much as 60 km /year.

The new crust created at rift zones preserves the magnetic field present at the time it solidified. From this, we can tell that the magnetic field reverses polarity periodically.

Recently, this has occurred about every 500,000 years.

Magnetic ChangesNorth Magnetic Pole Movement Based on Magnetic Field Models

1590 − 2010

Produced by NOAA’s National Geophysical Data Center, December 2005Pole Location Data Produced by UFM and IGRF−10 Magnetic Field Models

International Geomagnetic Reference Field Model (IGRF−10)International Association of Geomagnetism and Aeronomy (IAGA)

Susan Macmillan and Stefan MausWorking Group V−MOD

December 2004

1950−20101590−1940Uniform Flow Model (UFM)

Jeremy Bloxham and Andrew JacksonUses modified code by J. Bloxham, 11 Oct. 1983

225˚

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245˚ 250˚ 255˚260˚

265˚

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275˚65˚ 65˚

70˚ 70˚

75˚ 75˚

80˚ 80˚

85˚ 85˚

1590

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18001900

1950

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2010

Next TimeOur Peculiar Moon.