Atmospheres of the solid worlds II: Structure and heat transport
The surface of Venus from The Daily Galaxy
From Effective Temperature to Actual Temperature:
Effective temperature: The ideal temperature at which a planet precisely re-radiates all of the energy it receives from the sun. For an idealized black-body, it is:
Te = (L / (16πσD2))1/4
- L = Solar luminosity = 3.846*1026 W m-2 K-4
- D = distance from Sun
- σ = the Stefan-Boltzman constant = 5.6704 * 10-8 W
Thus, in our solar System, the only real variable in computing effective temperature is distance from the Sun.
If we treated real planets like idealized black-bodies, we get these effective temperature:
- Venus: 327 K
- Earth: 279 K
- Mars: 225 K
Iapetus: High and low albedo on a single world
- Reflective icy worlds like Enceladus have high albedo.
- Dark worlds like Mercury have low albedo.
- Iapetus (right) offers a contrast of high and low albedo. (Which side do you think is warmer?)
Albedo is measured on a scale from 0.0 (no reflectivity - a physicist's ideal "black body") to 1.0 (all light reflected). Modest examples:
- Mercury - 0.068
- Moon - 0.136
- Mars - 0.250
- Earth - 0.306
- Venus - 0.900
- Enceladus - 0.99
Te = (L(1-a) / (16πσD2))1/4
Where a = albedo.
Incorporating albedo we get the following estimates of effective temperature:
- Venus: 184 K
- Earth: 255 K
- Mars: 210 K
Again, makes sense. Mars is farther from the sun than Earth, and Venus has much greater albedo. For Mars, it also approximates mean surface temperature well.
But for Venus and Earth, true mean surface temperature is:
- Venus: 733 K
- Earth: 288 K
Hot objects reradiate energy as infrared radiation, however many atmospheric gasses that are transparent to visible light (allowing the energy in) are opaque to infrared, preventing this energy from radiating back into space. The result is that the atmospheric gasses opaque to infrared are warmed up. These are greenhouse gasses. Foremost among them:
- H2O (water vapor)
- CO2 (carbon dioxide)
- CH4 (methane)
So, how is this heat distributed vertically in the atmosphere?
The troposphere is warmed by infrared radiation from the planet's surface (which has been directly illuminated by visible sunlight) With increasing elevation, heat from this source diminishes rapidly, causing rapid cooling.
With two notable exceptions, the atmospheres of the Solar System conform to this schematic:
Earth's atmosphere from Astrobiology Magazine
The converse of heat of compression is the cooling that results from decompression. If a location on the planet's surface is particularly warm:
- It emits infrared radiation that warms the air above it.
- This warm parcel of air then rises.
- As it rises into regions of lower atmospheric pressure, it expands to equalize its pressure with that of the air around it.
- As a result of its expansion, the air cools.
Because this cooling occurs even if there is no transfer of heat to surrounding air, it is termed adiabatic cooling. (Adiabatic means "without transfer.") This is the minimum rate at which rising air cools. In practice, there is usually some heat transfer to surrounding air, also.
Adiabatic lapse rate: The rate at which rising air cools adiabatically. As long as the surrounding air cools faster with increasing height than the adiabatic lapse rate, that parcel will continue to rise. Typically, the adiabatic lapse rate is slightly less than general cooling in the lower atmosphere. Once the adiabatic lapse rate exceeds the general rate of cooling, the parcel will no longer rise. This places an upper limit on convection and defines the top of the troposphere.
The Mesosphere: Here, the transfer of heat is relatively straightforward. There is no convection, only conduction - the absorption and re-radiation of energy.
The Thermosphere: In its thin outer reaches, an atmosphere mixes and interacts with the solar wind with the result that it is heated and ionized. (Regions of extensive ionization are called the ionosphere). Heated molecules re-radiate energy at different rates depending on their physical characteristics, such that some retain their heat longer than others.
Earth's atmosphere from Annenberg Learner
But sunlight does more than heat atmospheres, it causes them to circulate. Atmospheric circulation is driven by two things:
- The unequal distribution of solar energy
Latitudinal differential in incoming sunlight: The amount of solar energy received per unit area of Earth's surface is a function of the angle at which the light strikes. The most concentrated energy is delivered to equatorial regions whereas polar regions receive very little. Resulting circulation is, in part, the atmosphere's attempt to equalize that heat distribution.
The Coriolis effect: The greatest solar heating occurs near the equator. Picture a rising parcel of air in Earth's equatorial troposphere. It reaches the top of the troposphere then circulates toward higher latitudes. At the equator, its motion equalled that of the Earth's equator. At it moves north, this momentum is conserved even though it is now moving over a surface that is not moving as fast. As a result, to conserve momentum, it seems to accelerate with respect to Earth's surface. Viewed from the surface, it appears to follow a curved path, as it is deflected toward the east.
- Hadley cells: form adjacent to the equator as strong sunlight warms equatorial air, which rises to the top of the troposphere in the intertropical convergence zone (ICZ) and spreads north and south. Because it carries moist air into the colder upper atmosphere, the ICZ is a place of frequent rainfall.
- Polar cells: form as very cold air descends near the poles and spreads southward.
- Ferrel cells: occur in the middle latitudes as a result of interactions between Hadley and polar circulation.
From Lyndon State College
Variations on other worlds:
- Venus: Because its rotation is slow, the Coriolis force is weak and surface winds are weak.
- Mars: Mars shows proper tropical Hadley cells, but at higher latitudes global circulation is overwhelmed by the effects of polar ice.
Mars dust storms from Space Today
Mars:As we have seen, Mars displays a normal atmospheric profile with vigorous convection in the troposphere on most days. Two circumstances sometimes disrupt this:
- Global dust storms: Mars' winds are sufficient to raise significant dust storms. When large dust clouds are raised, they tend to absorb heat, causing a temperature differential that increases wind speeds that pump more dust into the atmosphere, in a positive feedback loop. One or twice a year, Mars is enveloped in a global dust storm that:
- elevates the temperature of the upper troposphere by up to 45 K (in effect, the dust creates a stratosphere-like temperature inversion.)
- shades the lower troposphere, lowering surface temperatures.
- Night: Mars' thin air loses heat quickly, dropping 100 K from mid-day to midnight. When this happens convection slows and the troposphere shrinks.
Titan in visible light from Wikipedia
Titan:Stranger and stranger.
- Previously we said that two worlds lacked the typical troposphere-mesosphere-thermosphere stratification. One was earth, with its stratosphere. Fulchignone et al., 2005 revealed that Titan also has a stratosphere, caused by the absorption of visible light by the dense photochemical smog above its troposphere.
- Seasonal "rainy" and "dry" seasons: Titan orbits in Saturn's equatorial plane, which is slightly inclined to the ecliptic. Thus, Titan experiences seasons during its 30 year "year." Atmospheric models predict that bodies of liquid methane/ethane will be more common near the winter pole. Confirming this, most of the lakes found so far have been in the northern high latitudes - the winter hemisphere. Turtle, et al., 2011 reported a storm front that was observed in Titan's equatorial region the previous year. But the tropics are dry most of the time. The probable reason: The rising air of intertropical convergence zone causes most of Titans rainfall, but unlike Earth, where the ICZ is restricted to the tropics, Titan's is able to migrate into the summer polar regions, leaving the tropics high and dry much of the time.
- Greenhouse effect vs. antigreenhouse effect: Titan has a greenhouse effect caused by absorption of infrared by cold N2, H2,and CH4 (methane). (At higher temperatures these gasses are transparent to infrared.) Some numbers:
- Titan's modeled effective temperature (with albedo factored in): 82 K
- Titan's modeled temperature with greenhouse effect: 105 K
- Titan's observed mean surface temperature: 93.7 K
Remember that visible light images of Titan show a featureless orange smog atmosphere. McKay et al., 1991 indicated the reason: Titan's greenhouse effect was being partially cancelled out by an antigreenhouse effect. Recall, images showing surface features are in infrared. Thus, infrared wavelengths definitely penetrate Titan's atmosphere and visible ones don't. The opposite of the greenhouse effect here. Infrared can carry heat away from Titan's surface, but visible light is, largely, blocked from bringing it in.
Processes that alter atmospheric composition over time:
Atmospheric retention the distribution of atmospheres in the Solar System can still be puzzling:
- Why does Earth have an atmosphere and the moon not, even though they are roughly the same average temperature?
- Why does the icy moon Titan have a thick atmosphere while the physically similar icy moon Ganymede is essentially airless?
A planet's ability to retain its atmosphere is a function of two variables:
- Gravity: determines its escape velocity.
- Temperature: determines the speed with which gas molecules actually move.
Escape velocity from Hyperphysics
Vesc = (2 G M / R)1/2
- G = gravitational constant
- M = mass of planet
- R = radius at which molecule escapes (top of atmosphere)
Notice that the escape velocity depends only on the ratio M/R of the planet. It has nothing to do with characteristics of the object that may escape. The escape velocity is the same for a rocket and for a gas molecule.
Some escape velocities:
- Earth: 11.2 km/s
- Venus: 10.3 km/s
- Mars: 5.0 km/s
- Jupiter: 59.5 km/s
These are the speeds that a gas molecule must attain to escape from a planet's atmosphere. How do they attain those speeds?
Thermal energy: So, what is the velocity of a gas molecule? It depends on the temperature and the molecular mass of the gas. Hot gas molecules move at higher velocities than cold gases, and heavy molecules move more slowly than light ones. Note that this is an average velocity. Gas molecules do not all move at one speed; there is a wide distribution of speeds at any temperature.
With this in mind, we can see by pairwise comparison that:
- Differences in atmospheric retention between bodies that are roughly the same temperature, like:
- Earth and moon
- Titan and Rhea
- Differences in atmospheric retention between bodies with similar escape velocities may occur because of different temperatures. E.G. Ganymede (120 K) and Titan (89 K)
Artist's impression of the Moon, 3.5 Ga. From Phys.org
Misleading textbook figure with confused x-axis
For an atmosphere to remain bound to its planet over the age of the solar system (4.56 Ga), the average speed of gas molecules should be less than 1/6 of the escape velocity.
Note: The temperatures we use here are temperatures of the upper atmosphere - the region from which gas molecules actually have a clear shot at escaping. Nevertheless a glance at the x-axis figures of this graph from your text shows that something is horribly wrong.
Use this version based on information from The University of Nebraska Lincoln
Here is a correct version. To learn more, play with the applet from the University of Nebraka Lincoln
Key concepts and vocabulary:
- Atmospheric structure:
- Thermosphere (a.k.a. Exosphere)
- Heat of compression
- adiabatic cooling
- adiabatic lapse rate
- Ozone (O3)
- Effective temperature
- Greenhouse effect
- Major greenhouse gasses
- H2O - water
- CO2 - carbon dioxide
- CH24 - methane
- Inequality of solar heating
- Coriolis effect
- Earth's global convection cells:
- Hadley cells
- Ferrel cells
- Polar cells
- Be familiar with cloud characteristics for the four worlds with atmospheres
- Smog - photochemical haze
- Cumulus clouds
- Mars idiosyncrasies
- Global dust storms
- Titan idiosyncrasies
- Rainy and dry seasons
- Anti-greenhouse effect
- Gas retention
- Molecular mass
- Thermal energy
- Escape velocity
- Beware of the bad graph!
- M. Fulchignoni F. Ferri, F. Angrilli, A. J. Ball, A. Bar-Nun, M. A. Barucci, C. Bettanini, G. Bianchini, W. Borucki, G. Colombatti, M. Coradini, A. Coustenis, S. Debei, P. Falkner, G. Fanti, E. Flamini, V. Gaborit, R. Grard, M. Hamelin, A. M. Harri, B. Hathi, I. Jernej, M. R. Leese, A. Lehto, P. F. Lion Stoppato, J. J. Lopez-Moreno, T. Mäkinen, J. A. M. McDonnell, C. P. McKay, G. Molina-Cuberos, F. M. Neubauer, V. Pirronello, R. Rodrigo, B. Saggin, K. Schwingenschuh, A. Seiff, F. Simões, H. Svedhem, T. Tokano, M. C. Towner, R. Trautner, P. Withers and J. C. Zarnecki. 2005. In situ measurements of the physical characteristics of Titan's environment. Nature 438, 785-791.