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Source: Official Guide Revised GRE 1st Ed. Part 8; Section 4; #21


According to the passage, Titan differs atmospherically

According to the passage, Titan differs atmospherically from Ganymede and Callisto because of a difference in Saturn’s giant moon Titan is the only planetary satellite with a significant atmosphere and the only body in the solar system other than Earth that has a thick atmosphere dominated by molecular nitrogen. For a long time, the big question about Titan’s line atmosphere was how it could be so thick, given that Jupiter’s moons Ganymede and 5 Callisto, which are the same size as Titan, have none. The conditions for acquiring and retaining a thick nitrogen atmosphere are now readily understood. The low temperature of the protosaturnian nebula enabled Titan to acquire the moderately volatile compounds methane and ammonia (later converted to nitrogen) in addition to water. The higher temperatures of Jupiter’s moons, which were closer to the Sun, prevented them 10 from acquiring such an atmosphere. rate of heat loss, proximity to the Sun, availability of methane and ammonia, distance from its planet, size

3 Explanations


Bhavika Khare

I think a very ignored point in every explanation of this question is that methane and ammonia are mentioned as having existed on Titan ONCE but now have converted into nitrogen. Thus, had "past availability of methane and ammonia" been an option, it would be correct.
However, the (assumed current) availability of methane and ammonia on Titan and other moons can not be compared.

Nov 6, 2019 • Comment


Hey Bhavika, I see your logic, and it is sound! However, "past availability of methane and ammonia" still wouldn't be correct.
First, the text mentions water as well, which could be the crucial catalyst. It's possible that water turns methane and ammonia into nitrogen. The other planets, being hotter and lacking water, would thus be stuck with methane and ammonia, but no nitrogen.
Similarly, we aren't told that methane and ammonia turn into nitrogen in all circumstances. This may be unique to Titan. The key idea is that a process that occurs in one place and time may not repeat in another place and time with different conditions.
Lastly, the text states directly that Jupiter's moons did not acquire a nitrogen atmosphere because they were closer to the sun. It does not state directly that they would have a nitrogen atmosphere if only they had access to methane and ammonia (but no water).
Hope that helps!

Nov 23, 2019 • Reply


Rachel Wisuri

Hey Mibin--this is a great question! The reason (C) is incorrect is because the availability of methane and ammonia IS the atmosphere, it's not the REASON for the difference in atmospheres between the planets. The passage states that the low temperature enabled Titan to acquire methane and ammonia, which is converted to nitrogen. Previously, we learned that Titan has a thick atmosphere dominated by molecular nitrogen (ie, methane and ammonia). So, the availability of methane and ammonia does not EXPLAIN the difference in atmospheres--it doesn't give us the reason for this. The reason, thus, is the planets' proximities to the sun--the low temperature of Titan (ie far from the sun) allowed the planet to acquire these compounds (methane and ammonia, which make up the atmosphere), while Jupiter's moons, which were closer to the sun, could not acquire such an atmosphere because of their high temperatures.

Apr 25, 2013 • Comment


Chris Lele

Oct 10, 2012 • Comment

Mibin Kuruvilla Joseph

Can you please explain why C doesnt work? it says in the passage that the lack of these compounds is the reason that they lack atmosphere in jupiter. so atmospherical difference should be C . isnt it? Thank you

Apr 25, 2013 • Reply

Asif Kabir

Titan differs from Ganymede and Callisto because Jupiters moons (Ganymede and
Callisto) is closer to the Sun, so they don't have the atmosphere like Titan. that's why B is the answer.

Nov 26, 2017 • Reply

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