Class 10 - Earth, Venus, Mars - I

....

Reading:

  1. Atmospheres - Goody & Walker - Review = pp .4-17, for Thursday's class pp23-43 on PHOTOCHEMISTY
  2. New Solar System - Ch 13.

Basic Comparisons

  VENUS EARTH MARS
DISTANCE (AU)
0.723
1
1.524
PERIOD (yrs)
0.61
1.00
1.88
RADIUS (km)
6052
6378
3398
MASS (10^24 kg)
4.87
5.98
0.642
GRAVITY (m/s2)
8.87
9.81
3.71
ESCAPE SPEED (km/s)
10.35
11.18
5.02
SPIN (hrs)
5832
24
24.62
COMPOSITION
CO2
N2, O2,
CO2
MOLECULAR MASS (amu)
44
29
44
ALBEDO
0.59
0.39
0.15
TEMPERATURE (eq)
268
252
222
TEMPERATURE (actual)
740
288
223
SURFACE PRESSURE (bar)
92
1
0.006
SCALEHEIGHT (km)
16
8
11
RADIUS/H
387
763
301
COLUMN MASS (kg/m2)
1.0E+06
1.0E+04
1.6E+02
TOTAL MASS (kg)
4.8E+20
5.2E+18
2.3E+16
COLUMN No. DENSITY (/m2)
1.5E+31
2.2E+29
2.3E+27
SURFACE No. DENSITY (/m3)
9.01E+26
2.52E+25
1.95E+23
SURFACE DENSITY (kg/m3)
6.58E+01
1.21E+00
1.42E-02

Here is a plot of density of molecules per cc (n) vs. altitude (z) - Fig 1-4 - for Earth, Venus and Mars. Note the different average scaleheight (i.e. look at the slopes) and the change in H with altitude.

 

Composition

There are multiple different tables of composition - not surprisingly, our understanding of the composition of the atmospheres of Venus and Mars have developed since Goody & Walker wrote Atmospheres in 1972.

The variation in total mass of the atmosphere is huge - but just look at composition for a moment....

Now compare the major volatiles - water, carbon dioxide and nitrogen - as fraction of the mass of the planet. Below the total height of the column is the total inventory of these molecules. The shaded region is the amount in the atmosphere. If not in the atmosphere, were are these volatiles?

 

PUZZLER 1- If all of the CO2 that is not in the Earth's atmosphere is in a layer of limestone covering half of the globe - how thick a layer would it be?

PUZZLER 2 - If the 10-9 of the mass of the Earth that is currently in the atmosphere has a life time of 10 years, how much limestone deposition per year is this equal per year? Assume a uniform layer covering 1/2 of the globe.

 

a - some of these values are measured at an altitude of 22km because surface data are unreliable.

b - The volume ratio is the fraction by number of atoms or molecules present. Chemists often refer to this as the mole fraction. It is also called the volume mixing ratio by some atmospheric scientists. When mulitplied by the atmospheric pressure, it gives a quanitity called the partial pressure, when may be envisaged as the contribution of a component ot the total pressure.

c - The water abundance in the Earth's atmosphere is highly variable!

This is a simplistic picture for each atmosphere.... in reality, there are many more chemicals in the atmosphere...

Note the timescale on the left - varying from 20 million years for the main species to just few days for the very minor species. But even 20 million years is short on the timescale of geological processes. All of these atmospheres must have been produced relatively recently.

 

Atmospheric Sources

1 - Volcanic outgassing - if there are active volcanoes, this is MAJOR.

2 - Submlimation/evaporation - requires some source of volatiles to start with.

3 - Meteoritic, particle or photon bombardment - only a source for tenuous atmospheres where bombarding flux gets to surface.

For Earth, Venus, Mars - what is the volcanic history of these planets? Below is a plot of the age of the surface - an indication of the amount of recent resurfacing - mostly due ot volcanism. Very little on Moon and Mercury (covered with craters), some on Mars - up until about 2-3 B yrs ago. Venus - totally re-surfaced ~600 Myrs ago. Earth has ongoing volcanism.

Atmospheric Losses

 

Of particular importance when considering the atmospheres of Earth, Venus and Mars are chemical reactions with surface materials. Most specifically, the carbonate cycle - this is where the terrestrial CO2 is stored. On Mars - where's the limestone? Does not seem to be there. Suggests that most of the atmosphere were removed by other processes - probably the solar wind stripping (see above).

This is where most of the CO2 has gone on Earth. Note that it requires LIQUID water. Mars is too cold, Venus too hot and Earth is just right for liquid water. We shall return to this issue when looking at climate change later in the semester.

 

To understand the atmospheres of Earth, Venus and Mars we will need to understand their PHOTOCHEMISTRY.... this we will do on thursday - reading pp 23-43 by then will make a big difference.