Today in Astronomy 111: Venus and Earthdmw/ast111/Lectures/Lect_07b.pdfrotation period Length of day 24.0000 hours Magnetic field 0.5 gauss Full Earth, seen from Clementine (US DoD)

22 September 2011 Astronomy 111, Fall 2011 1

Today in Astronomy 111: Venus and Earth

Earth as a planet Venus, Earth and atmospheric

circulation: Hadley cells. Venus, Earth and the greenhouse

effect: one dead planet, global warming, and ocean acidification.

Distinctive features of Earth• Life and its influence on

geology and the atmosphere• Plate tectonics • The core and the global

magnetic field

Earth and Venus, from the Galileo and Magellan missions, respectively (JPL/NASA). Click to spin the planets.

http://www2.jpl.nasa.gov/galileo/�http://www2.jpl.nasa.gov/magellan/�http://www2.jpl.nasa.gov/magellan/�

Tomorrow afternoon, at 1:04 PM EDT – very close to truenoon – the Sun crosses the celestial equator again: it reaches the autumnal equinox.

Within a few minutes: what will the sidereal time be, at truemidnight (1AM EDT) tomorrow night?A. 0h B. 3h56.4m C. 6h D. 1h04m E. 12h F. 23h22 September 2011 Astronomy 111, Fall 2011 2

Photo:Joe Orman

http://joeorman.shutterace.com/Gallery.html�http://joeorman.shutterace.com/Gallery.html�


Earth’s vital statistics

27

8

-3

2

13

Mass 5.9736 10 gm

Equatorial radius 6.3781 10 cm

Average density 5.515 gm cm

Moment of inertia 0.33Albedo 0.37 Effective temperature 254.3 K

1.49597887147 10 cmOrbital semimajor axis(1.00000011 AU)

Orb

MR

×

×

×

ital eccentricity 0.01671022Sidereal

365.256 daysrevolution periodSidereal

23.9345 hoursrotation periodLength of day 24.0000 hoursMagnetic field 0.5 gauss

Full Earth, seen from Clementine (US DoD)

http://antwrp.gsfc.nasa.gov/apod/ap991225.html�


Venus, Earth and atmospheric circulation

In 1735, George Hadley, who was interested in explaining the direction and steadiness of the trade winds, extended an earlier suggestion by Edmund Halley.Warm air is less dense than cold air; if one makes a warm

(cold) “bubble” of air somewhere in the atmosphere, it will rise (sink) through the rest of the atmosphere.

The ground, and the air adjacent to it, is warmer at the equator than at the poles, because of the different incidence angles of sunlight.

Therefore there should be N-S circulation, with warm air at high elevations flowing toward the poles, and cooler air at the surface flowing toward the equator.


Atmospheric circulation (contd.)

From Sun

Equator

N.pole

Warm air rising

Cool airsinking



So far, this is Halley’s idea, and doesn’t explain the direction of the trade winds, which tend to be easterly between the tropics.

But Hadley realized that the N-S flow must be modified, since the planet and atmosphere are rotating about the axis, at higher speeds closer to the equator, this speed given by:

• At the equator (λ = 0) Earth’s rotational speed is 0.46 km/sec, but at λ = ±45º, it’s down to 0.33, and at ±75º, 0.12 km/sec, decreasing to zero at the poles.

2 cosdayr

Rv π λ⊕= (λ = latitude)



The Earth rotates counterclockwise, as viewed from the North. As warm air is pushed north or south, it finds itself moving into slower air, and each bubble of it moves out somewhat ahead of the normally-rotating airmass.

Thus the warm flow turns toward the rotation direction as it goes. Underneath, the cooler return flow goes the other way.

Direction of rotation

Warm (high-altitude) flow

Cool (surface) flow



Between cooling and turning, the warm high-altitude flow only makes it to latitude ±30º before sinking, driving surface flow back the way the warm flow came. This circulation pattern is called a Hadley cell.

Taking advantage of the extreme difference between rotational speed and solar heating from the poles to latitudes just below, another Hadley cell, usually called the polar cell to distinguish it from the equatorial one, works the same way between latitudes ±60º and the poles.

In between (i.e. in the temperate zones) a circulation pattern is driven by the Hadley cells that has the opposite sense of these two, counter to rotation. This one’s called the Ferrel cell.



The Northern hemisphere prevailing-wind system (SPaRCE/EVAC/U. Oklahoma).

http://sparce.evac.ou.edu/q_and_a/air_circulation.htm�


Aside: Columbus actually knew some of this

As we’ve discussed in recitation, Columbus made very good time – considering the nature of his ships – and made nearly perfect use of the trade winds (outbound) and westerlies (return), indicating that he knew to expect these patterns to persist all the way around the globe. Voyage #1, for example:

30ºNWesterlies

Calsp

ace/

UC

San

Dieg

o

http://calspace.ucsd.edu/virtualmuseum/climatechange1/08_1.shtml�


Another aside: Hadley was on to something

Hadley turned out to be correct about the effects of the Earth’s rotation on circulation. About 100 years later, it was described theoretically in accurate detail by Gaspard Coriolis, and we’ve called it the Coriolis force ever since. The Coriolis force, like the more-familiar centrifugal force,

is the effect of inertia and motion within an accelerating reference frame. It’s given by

As such, the Coriolis force is fictitious: it’s an artifact of the acceleration of one’s surroundings.

( )Coriolis 2m= ×F v ω ω = angular velocity of reference framev = velocity within reference frame



All this makes for a famous distinction of Venus. It turns out that the faster a planet rotates, the more bands

of alternating Hadley and Ferrel cells are obtained in the atmosphere.

By the same token, a slow enough rotator would have only one, ideal-looking, Hadley cell per hemisphere, stretching all the way from the equator to the poles.

This is the situation of Venus, the slowest rotator among the planets. • Most of the air circulation on Venus is north-south: by

far the simplest atmospheric structure of the planets.• This is why the surface temperature on Venus is so

uniform –as hot at the poles as at the equator, and as hot at night as during the day.


Venus, Earth and the greenhouse effect

Venus is also famous for having a dense, very dry atmosphere that makes it the exemplar of the greenhouse effect, as was first realized by Rupert Wildt (1940) and first explained in detail by Carl Sagan (early-mid 1960s). Venus probably started off with the same ingredients as

Earth, meaning that it had water after the surface cooled. As we’ve mentioned, the water was mostly from asteroids

and comets, rather than its original ingredients, or the pre-solar nebula.

But the atmospheres of Earth and Mars also endow their planets with a substantial greenhouse effect: their surfaces are also warmer than they would be just from solar illumination and blackbody radiation.

http://adsabs.harvard.edu/abs/1940ApJ....91..266W�

What constituent of Earth’s atmosphere makes the largest contribution to the greenhouse effect?

A. Carbon dioxide B. Methane C. Ozone D. NitrogenE. Water F. Argon


What constituent of Earth’s atmosphere makes the largest contribution to the greenhouse effect?

A. Carbon dioxide B. Methane C. Ozone D. NitrogenE. Water F. Argon

The greenhouse effect (continued)

so the terrestrial planets emit most of their light at infrared wavelengths. They would all be

brightest near a wavelength of 10 µm.

Solar heating arrives mostly at visible wavelengths, where the atmosphere is transparent.


Created for Global Warming Art by Robert A. Rohde

[ ]

14

2278 K ,

16 AU

LT

r rπσ

= =

http://www.globalwarmingart.com/images/7/7c/Atmospheric_Transmission.png�http://www.globalwarmingart.com/wiki/Global_Warming_Art:About�http://www.globalwarmingart.com/wiki/User:Robert_A._Rohde�http://www.globalwarmingart.com/wiki/User:Robert_A._Rohde�


Infrared light is absorbed very strongly by molecules in the atmosphere, notably by water and CO2.

Light can only escape directly to outer space through “windows”, of which the most important lie at wavelengths 8-13, 4.4-5, 3-4.2, 2-2.5, 1.5-1.8, and 1-1.4 µm.





Hotter blackbodies shine more at shorter wavelengths, so if not enough light escapes at 3-5 and 8-13 µm, the surface heats up until enough of the emission leaks out in the shorter-wavelength windows.

This effect warms all three of the atmosphere-bearing planetary surfaces.





If there’s liquid water on the surface, the greenhouse effect can be self-stabilizing, as water droplets form clouds that reflect sunlight. (CO2 forms neither droplets nor clouds.)

If temperature rises, → more water evaporates into atmosphere → more clouds form → albedo increases → less sunlight reaches surface → temperature drops.And vice versa.

But on Venus, sunlight and the greenhouse effect was sufficient to evaporate all of the water, leaving no liquid bodies on the surface. 22 September 2011 Astronomy 111, Fall 2011 18



Liquid water dissolves carbon dioxide, both from the atmosphere and from rocks, creating carbonic acid:

From there the carbon can be incorporated in carbonate minerals that can form readily in liquid water. • These days, this is done most readily on Earth by

ocean-dwelling organisms, creating Thus if there is a lot of liquid water, carbon from CO2 will

eventually be locked up in carbonate minerals, rather than allowed to be present in the atmosphere. • This is the case, for example, on Earth. • On Venus, though, the lack of liquid water let the CO2

remain in the atmosphere.

+ -2 3 3 3H CO (in solution, H O HCO ).+

3CaCO .

Carbon, as currently imprisoned on Earth

Flows (arrows) in petagrams (1 Pgm = 1015 grams, about 1 billion US tons) of C per year.


Woods Hole Research CenterSee also Le Quéré et al. 2009.

Carbonates in rocks

http://www.whrc.org/carbon/images/GlobalCarbonCycleLG.gif�http://www.whrc.org/carbon/index.htm�http://www.nature.com/ngeo/journal/v2/n12/full/ngeo689.html�http://www.nature.com/ngeo/journal/v2/n12/full/ngeo689.html�http://www.nature.com/ngeo/journal/v2/n12/full/ngeo689.html�http://www.nature.com/ngeo/journal/v2/n12/full/ngeo689.html�



Under solar ultraviolet illumination, water molecules high in the atmosphere dissociate readily, producing atomic hydrogen and oxygen. • Oxygen goes on to react with other molecules;

hydrogen does not.Hydrogen is too light to be retained by Venus’s gravity,

so it escapes, relatively quickly. • Soon there’s no more water, or possibilities for making

any more water! A dead world. • And all the carbon and oxygen winds up in CO2, the

atmosphere pressurizes, and the greenhouse effect cranks up to 735 K. A sterilized world.

Whither Earth’s greenhouse effect?

As you know, the natural balance of Earth’s greenhouse effect is being disturbed by the large-scale burning of fossil fuels. CO2 currently enters the atmosphere by this means about

ten times faster than the oceans and rain can dissolve the increase (Revelle & Suess 1957) …

… and the CO2 that is dissolved accumulates in the oceans vastly faster than it can be converted to carbonates.

In concert, global temperatures have increased, and the oceans become more acidic. (See next pages.)

Climate models currently cannot explain the magnitude of the temperature increase, which is a way of saying that we do not know how stable the natural balance is.

More on this topic in Astronomy 142 and Astronomy 106…22 September 2011 Astronomy 111, Fall 2011 22

http://www.uscentrist.org/about/issues/environment/docs/Revelle-Suess1957.pdf�http://www.pas.rochester.edu/~dmw/ast142�http://www.pas.rochester.edu/~dmw/ast106�

Atmospheric CO2 and global temperature For at least 800 years

before the Industrial Revolution, atmospheric CO2 changed little, while global temperature went through small – but significant – fluctuations uncorrelated with CO2.

Since then, both CO2 and global temperature have increased sharply, and their statistical correlation is very strong.


-0.6

-0.4

-0.2

0

0.2

0.4

0.6

200

240

280

320

360

400

200 700 1200 1700

NH

mea

n oc

ean

tem

pera

ture

-16

C

Atm

osph

eric

CO2

conc

entr

atio

n, p

pmv

Year

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

260

280

300

320

340

360

380

1800 1850 1900 1950 2000

NH

mea

n oc

ean

tem

pera

ture

-16

C

Atm

osph

eric

CO2

conc

entr

atio

n, p

pmv

Year

Atmospheric CO2 and ocean acidification

About half of the CO2increase since 1800 has been dissolved in the oceans, and has already reduced their pHsignificantly.

In concert with the pH decrease has been a decline in many aquatic species – in particular many species, like corals, that are involved in turning carbonate ions into carbonate minerals.

Links to these data: ocean temperature, Mauna Loa atmo-spheric CO2, ice-core bubble CO2, ocean-water CO2 and pH.22 September 2011 Astronomy 111, Fall 2011 24

Doney et al. 2009

ftp://ftp.ncdc.noaa.gov/pub/data/anomalies/monthly.ocean.00N.90N.df_1901-2000mean.dat�ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_mm_mlo.txt�ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_mm_mlo.txt�ftp://ftp.cmdl.noaa.gov/ccg/co2/trends/co2_mm_mlo.txt�http://cdiac.ornl.gov/ftp/trends/co2/lawdome.combined.dat�http://cdiac.ornl.gov/ftp/trends/co2/lawdome.combined.dat�http://hahana.soest.hawaii.edu/hot/hot-dogs/bextraction.html�http://hahana.soest.hawaii.edu/hot/hot-dogs/bextraction.html�http://hahana.soest.hawaii.edu/hot/hot-dogs/bextraction.html�http://adsabs.harvard.edu/abs/2009ARMS....1..169D�http://adsabs.harvard.edu/abs/2009ARMS....1..169D�http://adsabs.harvard.edu/abs/2009ARMS....1..169D�


The effect of life

The most distinctive feature of Earth – unique in the Solar system, as far as we know – is that it supports life. The chemical activity of organisms has had global influences: Abundant carbonaceous minerals. In the ocean,

organisms such as corals and mollusks synthesize carbonaceous minerals from dissolved carbon dioxide, in such large quantities that they are the leading producers of such minerals.

Lots of molecular oxygen in the atmosphere. Plants synthesize oxygen out of carbon dioxide, and do it so well that the atmosphere, which probably started out with just a trace, is now about 20% oxygen.

This of course is very different from the CO2-dominated environment on Venus and, as it turns out, Mars.


Earth and plate tectonics

Like Venus and Mercury, Earth is a differentiated body with a liquid-iron core, “plastic” silicate asthenosphere (mantle), and less-dense silicate lithosphere (crust). And as you all know, the crust is broken into plates that

float on top of the mantle. Some of the plates are

denser (mafic minerals), are generated from mantle material at mid-ocean rises, and are returned to the mantle when subductedunder less-dense plates. They comprise the ocean floors. (US Geological Survey)

http://geology.er.usgs.gov/eastern/plates.html�


Earth and plate tectonics (continued)

And some of the plates are dominated by lower-density silicates (felsic minerals), which float on top of everything else. These of course comprise the continents.

Earth’s plate-tectonic system is unique in the Solar system. Only Venus shows evidence, long extinct, of such activity.

The last 750 million years of continental drift (By C.R. Scotese 2001/PALEOMAP project).

http://www.scotese.com/�http://www.scotese.com/�http://www.scotese.com/�


Earth’s core and magnetic field

Earth’s iron core is mostly liquid, apart from the innermost part, which is compressed into a solid phase. It also rotates differentially,

with the inner parts going a bit faster than the outer parts, the solid inner core spinning the fastest of all (“superrotation”).

Furthermore, it probably undergoes convection: radial circulation of warmer core material outward, replaced by cooler material sinking inward.

Magnetic field twisting by differential rotation (MSFC/NASA)

http://science.msfc.nasa.gov/ssl/pad/solar/dynamo.htm�


Earth’s core and magnetic field (continued)

Iron is a conductor, and conductors exhibit magnetic flux freezing: moving conductors drag lines of magnetic force along with them, and vice versa.

The result is that a slowly oscillating, self-regenerating global magnetic field accompanies the core’s rotation: a dynamo.

Venus rotates too slowly for this, but Earth and Mercury have dynamo-generated fields.

Magnetic field stretching by differential rotation (MSFC/NASA)

http://science.msfc.nasa.gov/ssl/pad/solar/dynamo.htm�

Today in Astronomy 111: Venus and EarthSlide Number 2Earth’s �vital statisticsVenus, Earth and atmospheric circulationAtmospheric circulation (contd.)Atmospheric circulation (contd.)Atmospheric circulation (contd.)Atmospheric circulation (contd.)Atmospheric circulation (contd.)Aside: Columbus actually knew some of thisAnother aside: Hadley was on to somethingAtmospheric circulation (contd.)Venus, Earth and the greenhouse effectSlide Number 14The greenhouse effect (continued)The greenhouse effect (continued)The greenhouse effect (continued)The greenhouse effect (continued)The greenhouse effect (continued)Carbon, as currently imprisoned on EarthThe greenhouse effect (continued)Whither Earth’s greenhouse effect?Atmospheric CO2 and global temperatureAtmospheric CO2 and ocean acidificationThe effect of lifeEarth and plate tectonicsEarth and plate tectonics (continued)Earth’s core and magnetic fieldEarth’s core and magnetic field (continued)

Documents

Today in Astronomy 111: Venus and Earthdmw/ast111/Lectures/Lect_07b.pdfrotation period Length of day 24.0000 hours Magnetic field 0.5 gauss Full Earth, seen from Clementine (US DoD)