Question: How does Venus's atmosphere compare to Earth's?
About the article "Solar System", section "Venus".

Candidate passages:
The current Venusian atmosphere has only ~200 mg/kg H2O(g) in its atmosphere and the pressure and temperature regime makes water unstable on its surface.
Nevertheless, assuming that early Venus's H2O had a D/H ratio similar to Earth's Vienna Standard Mean Ocean Water (VSMOW) of 1.6x10-4, the current D/H isotopic ratio in the Venusian atmosphere of 1.9x10-2, at nearly x120 of Earth's, may indicate that Venus had a much larger H2O inventory.
While the large disparity between terrestrial and Venusian D/H ratios makes any estimation of Venus's geologically ancient water budget difficult, its mass may have been at least 0.3% of Earth's hydrosphere. 
Earth is too large to lose a significant proportion of its atmosphere through Jeans escape.
The current rate of loss is about three kilograms (3 kg) of hydrogen and 50 grams (50 g) of helium per second.
The exosphere is the high-altitude region where atmospheric density is sparse and Jeans escape occurs.
Jeans escape calculations assuming an exosphere temperature of 1,800 K show that to deplete O+ ions by a factor of "e" (2.718...) would take nearly a billion years.
1,800 K is higher than the actual observed exosphere temperature; at the actual average exosphere temperature, depletion of O+ ions would not occur even over a trillion years.
Furthermore, most oxygen on Earth is bound as O2, which is too massive to escape Earth by Jeans escape.
Earth's magnetic field protects it from solar winds and prevents escape of ions, except near the magnetic poles where charged particles stream towards the earth along magnetic field lines.
The gravitational attraction of Earth's mass prevents other non-thermal loss processes from appreciably depleting the atmosphere.
Yet Earth's atmosphere is two orders of magnitude less dense than that of Venus at the surface.
Because of the temperature regime of Earth, CO2 and H2O are sequestered in the hydrosphere and lithosphere.
H2O vapor is sequestered as liquid H2O in oceans, greatly decreasing the atmospheric density.
With liquid water running over the surface of Earth, CO2 can be drawn down from the atmosphere and sequestered in sedimentary rocks.
Some estimates indicate that nearly all carbon on Earth is contained in sedimentary rocks, with the atmospheric portion being approximately 1/250,000 of Earth's CO2 reservoir.
If both of the reservoirs were released to the atmosphere, Earth's atmosphere would be even denser than Venus's atmosphere.
Therefore, the dominant “loss” mechanism of Earth's atmosphere is not escape to space, but sequestration. 
The atmosphere of Mars is, like that of Venus, composed mostly of carbon dioxide though far thinner.
There has been renewed interest in its composition since the detection of traces of methane that may indicate life but may also be produced by a geochemical process, volcanic or hydrothermal activity.
The atmospheric pressure on the Martian surface averages , about 0.6% of Earth's mean sea level pressure of and only .0065% that of Venus's .
It ranges from a low of on Olympus Mons's peak to over in the depths of Hellas Planitia.
This pressure is well below the Armstrong limit for the unprotected human body.
Mars's atmospheric mass of 25 teratonnes compares to Earth's 5148 teratonnes with a scale height of about versus Earth's .
The Martian atmosphere is about 96% carbon dioxide, 2.1% argon, 1.9% nitrogen, and traces of free oxygen, carbon monoxide, water and methane, among other gases, for a mean molar mass of 43.34 g/mol.
The atmosphere is quite dusty, giving the Martian sky a light brown or orange color when seen from the surface; data from the Mars Exploration Rovers indicate that suspended dust particles within the atmosphere are roughly 1.5 micrometres across. 
  Warming of the Earth's atmosphere due to greenhous gases in the atmosphere that allow solar radiation (visible, ultraviolet) to reach the Earth's atmosphere but do not allow the emitted infrared radiation to pass back out of the Earth's atmosphere.  
The 2012 transit gave scientists a number of research opportunities.
These included: Measurement of dips in a star's brightness caused by a known planet transiting a known star (the Sun).
This will help astronomers when searching for exoplanets.
Unlike the 2004 Venus transit, the 2012 transit occurred during an active phase of the 11-year activity cycle of the Sun, and would have provided practice in detecting a planet's signal around a "spotty" variable star.
Measurement of the apparent diameter of Venus during the transit, and comparison with its known diameter.
This will have given information on how to estimate exoplanet sizes.
The number of locations documenting the event will provide much data via parallax that will generate more accurate measurements.
Observation of the atmosphere of Venus simultaneously from Earth-based telescopes and from the Venus Express spacecraft.
This gave a better opportunity to understand the intermediate level of Venus's atmosphere than is possible from either viewpoint alone, and should provide new information about the climate of the planet.
Spectrographic study of the atmosphere of Venus.
The results of analysis of the well-understood atmosphere of Venus will be compared with studies of exoplanets with atmospheres that are unknown.
The Hubble Space Telescope used the Moon as a mirror to study the light reflected from Venus to determine the makeup of its atmosphere.
This may provide another technique to study exoplanets.
Experimental reconstruction of Lomonosov's discovery of Venusian atmosphere (1761) with antique refractors.
The researchers observed the "Lomonosov's arc" and other aureole effects due to Venus's atmosphere and concluded that Lomonosov's telescope was fully adequate to the task of detecting the arc of light around Venus off the Sun's disc during ingress or egress if proper experimental techniques as described by Lomonosov in his 1761 paper are employed.