Question: Comparing Mars' atmosphere to Titan's

[Mr. Milton Banana]

I'm puzzled by something.

Mars is 4, 194 miles in diameter, and Titan is 3,200 miles in diameter. Despite the 994 mile difference, the pressure on Titan is 1.6 bars, 60% greater than Earth's at sea level.

Yet the air pressure on the surface of Mars is less than 1% of Earth's (7 millibars).

What gives?? Why does Mars have a much thinner atmosphere than Titan-and why doesn't Titan's leak into space, despite the lesser gravity?

:-k

[Brady Yoon]

Good question... I don't know myself. I think it could be something to do with the temperatures and melting points of various gases... Or a catastrophe, perhaps a huge asteroid impact, in the early history of Mars could have destroyed much of the atmosphere. Help us someone!!

[Brady Yoon]

Wait, I think I got it. It's the same reason why terrestrial planets don't have thick atmospheres and Jovian planets do. In the warmer, closer areas such as Mars, it was too warm for the planet to keep its lighter, more abundant gases. It simply escaped because the molecular velocities were too high. So it only managed to hold on to carbon dioxide, nitrogen, and some water vapor. On the other hand, Titan, which is much farther away from the sun, is cooler, so the molecules moved more slowly on average. This allowed lighter gases such as methane, ammonia, hydrogen, and helium to remain, even though Titan's gravity is less than Earth's. The deciding factor was temperature, in this case.

[Mr. Milton Banana]

Wait, I think I got it. It's the same reason why terrestrial planets don't have thick atmospheres and Jovian planets do. In the warmer, closer areas such as Mars, it was too warm for the planet to keep its lighter, more abundant gases. It simply escaped because the molecular velocities were too high. So it only managed to hold on to carbon dioxide, nitrogen, and some water vapor. On the other hand, Titan, which is much farther away from the sun, is cooler, so the molecules moved more slowly on average. This allowed lighter gases such as methane, ammonia, hydrogen, and helium to remain, even though Titan's gravity is less than Earth's. The deciding factor was temperature, in this case.

Hmmm...

Interesting-but then what about Jovians that are orbiting close to nearby stars? Yeah, I know-can o' worms there.

[Brady Yoon]

That's a real bugger...

[HenrikOlsen]

The original question was about the different composition of atmospheres of similar sized objects such as Mars and Titan, where temperature explains the difference.

In the case of Jovians around other stars but in a closer orbit, they have a much higher mass and can therefore hold the gasses even at higher temperatures.

As mentioned in another thread, most of the planetarty mass coalesced before solar ignition, so by the time temperatures started rising the jovian sized planets where already massive enough to hold on to everything they got.

I would therefore expect their composition to be fairly close to our Jupiters.

[Mr. Milton Banana]

Thanks for the responses.

[kenneth rodman]

titan is about half water ice and half rocky material. It is probably differentiated into several layers with a 3400 km rocky center surrounded by several layers composed of different crystal forms of ice. Its interior may still be hot. Though similar in composition to Rhea and the rest of Saturn's moons, it is denser because it is so large that its gravity compresses its interior.

Alone of all the satellites in the solar system, Titan has a significant atmosphere. At the surface, its pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organics are formed as methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started.

Titan has no magnetic field and sometimes orbits outside Saturn's magnetosphere. It is therefore directly exposed to the solar wind. This may ionize and carry away some molecules from the top of the atmosphere.

At the surface, Titan's temperature is about 94 K (-290 F). At this temperature water ice does not sublimate and thus there is little water vapor in the atmosphere. Nevertheless, there appears to be a lot of chemistry going on; the end result seems to be a lot like a very thick smog.

There are scattered variable clouds in Titan's atmosphere in addition to the overall deep haze. These clouds are probably composed of methane, ethane or other simple organics. Other more complex chemicals in small quantities must be responsible for the orange color as seen from space.

It seems likely that the ethane clouds would produce a rain of liquid ethane onto the surface perhaps producing an "ocean" of ethane (or an ethane/methane mixture) up to 1000 meters deep. Recent ground-based radar observations have cast this into doubt, however.

mars
Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the day side during summer.
Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's plate tectonics, Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant greenhouse effect. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun.

Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 millibars (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and vast dust storms that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a greenhouse effect but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on Venus and Earth.

Mars has permanent ice caps at both poles composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely sublimes, leaving a residual layer of water ice. ESA's Mars Express has shown that a similar layer of water ice exists below the southern cap (left) as well. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites).


http://www.nineplanets.org/mars.html


link where i got the info at

[ToSeek]

A movie of Titan's hazy atmosphere