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If my memories are accurate, then there's only one celestial body beyond Mars, on which any man-made device was landed. Pictures made there are, in fact, quite popular among space fans.

But why Titan?

One would assume Europa is much more of an object of interest, because of the presumed ice layer.

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    $\begingroup$ Voyager 1 had the option to either fly by Titan or continue to Pluto. The Titan choice became a disapointment since its thick uniform atmosphere did not allow for as much useful science observations as hoped for. I suppose Titan remained a high priority, and seemed lucrative after Voyager 1 taught us that a lander would be a good idea there, as it was on Venus. $\endgroup$
    – LocalFluff
    Jun 15, 2016 at 14:24

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Simple. It was the easiest to land on. Titan has an atmosphere, which makes landing there quite a bit easier than landing on Europa, which does not.

In addition, Europa has only been known as an object of interest since Galileo, which was the last mission that even had a chance of sending a lander there. It was suspected as an object in the Voyager flyby, but it was much less sure until recently. Cassini was under works, along with the mission to Titan.

Titan is as interesting of an object as Europa, and has been known to be an object of such interest even well before the space age, due to it's atmosphere of hydrocarbons. Titan, Europa, Enceladus, Triton, and slowly Pluto are emerging as the most interesting objects in the outer Solar System, from the perspective of potentially having life, or at least the building blocks. While Europa is now known to be the most promising candidate, it took scientists a while to realize that, and there hasn't been time to send a mission there.

Lastly, there is the issue of radiation. Europa, being so close to Jupiter, is the subject of powerful radiation, similar to the Van Allen belts. That makes it even more difficult to manage a Europa lander.

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  • $\begingroup$ Yes, I've heard an analogy that if Titan had oxygen, a single matchstick could blow the entire atmosphere. Not sure if right though $\endgroup$
    – Zoltán Schmidt
    Jun 15, 2016 at 13:52
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    $\begingroup$ Well, yeah, it'd happen. Of course, Oxygen on Titan would be considered the fuel, and the "oxidizer" would be methane, a strange thought indeed. For the same reason that methane is essentially unknown on Earth, so would Oxygen be unknown on Titan. $\endgroup$
    – PearsonArtPhoto ♦
    Jun 15, 2016 at 13:55
  • $\begingroup$ Europa got great attention already after the Voyager flyby's. Its craterless cracked surface was unexpected and its subsurface water ocean was hypothesized already then. $\endgroup$
    – LocalFluff
    Jun 15, 2016 at 14:10
  • $\begingroup$ True that it had some attention, but it wasn't known until Galileo. I've added more details. We knew something unusual was going on with Europa, but we didn't know what was happening until after Galileo. $\endgroup$
    – PearsonArtPhoto ♦
    Jun 15, 2016 at 14:18
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    $\begingroup$ Not to mention "ALL THESE WORLDS ARE YOURS—EXCEPT EUROPA". $\endgroup$
    – Aron
    Jun 16, 2016 at 5:55
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They landed on Titan because Titan is the only natural satellite in the Solar System that has a dense atmosphere. And it is the only object other than Earth where surface liquid has been found. Source: Wikipedia.

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    $\begingroup$ I don't think the surface liquid presence was known when the mission to Titan was decided. $\endgroup$
    – gerrit
    Jun 15, 2016 at 15:09
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    $\begingroup$ Surface liquid presence in Titan was suggested already in the book 'A Pale Blue dot' by Carl Sagan in 1994 (maybe other sources even before). Cassini-Huygens mission launch was in 1997, so the topic was in discussion some years before. $\endgroup$
    – user15753
    Jun 15, 2016 at 17:21
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    $\begingroup$ Discussed, yes. Known, no. The first photos of the surface of Titan came with Cassini/Huygens. $\endgroup$
    – PearsonArtPhoto ♦
    Jun 16, 2016 at 2:15
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It's possible to argue that the NEAR Shoemaker probe was the first landing beyond Mars. The probe landed on near-Earth asteroid Eros on 12 February 2001.

On first encounter in February 2000, Eros was 1.7 AU from the Sun, outside of Mars' orbit. However, by the time of landing Eros was only 1.43 AU from the Sun; while Mars was near aphelion, so Eros was no longer 'beyond Mars' in the sense of being further from the Sun.

The 'ease of landing' argument is much stronger for an asteroid than a moon; really the only trick to landing on an asteroid is not bouncing off :-)

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