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Unique from landing on Mars itself what could we learn from Phobos that we couldn't learn from our own moon or from landing on Mars? Seeing as Phobos orbits Mars faster than the planet itself rotates, could there be any benefit from attaching a static probe to it's surface? Why have no rovers probes been planned for landed expeditions on the most studied planetary orbital body in our solar system? Is it the immense dust layer? The complexity of orbital maneuvers within the range of Mars? Or simply because we haven't even scratched the surface of Mars, why try for one of it's moons?

I only ask because PADME and other articles lack details on what we stand to gain.

In addition to this, I've read that Phobos may be hollow in up to a third of its volume. This is the closest body that supports this "hollow asteroid" theory. Is studying a hollow moon of any scientific consequence? This is false as pointed out (citation needed).

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    $\begingroup$ The wikipedia article on Phobos addresses a few of these points. The orbital period is 7 hours plus (the 4 hours is the time between Phobos rising and setting as seen from the surface). Phobos does have a rather low density (1.88 g/cm^3) but the original "hollow Phobos" theories were due to systematic errors in the data. A rover on Phobos wouldn't be at all like a rover on Mars or the Moon, because Phobos gravity is do weak. More like a free-flying spacecraft that makes regular rendezvous with Phobos. $\endgroup$ – Steve Linton Mar 31 '18 at 11:28
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The Japanese Space Agency JAXA is preparing such a mission, called Martian Moons eXploration (MMX). This mission is approved and in the works, the launch is foreseen for 2024 and the return to Earth with a Phobos sample in 2029.

A sample return from Phobos could answer the question of it's origin: Giant impact, capture or something else?
Also as Phobos is much closer to Mars than Deimos is, the hope is that there would be ejecta from impacts into Mars' crust lying around on the surface. The current estimate is '3 martian grains in 10g of Phobos surface material' (source: A recent conference, but I can try to dig up some papers about that if you're really interested).

So if the Moon was created through a giant impact with Mars, then we'd have a huge sample of martian crust mixed with something else, that would be great.
If the moon was captured, we'd have a huge sample of something coming from the outer solar system (though we wouldn't know where to place it initially).

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There's no point in going to Phobos with the intention of looking at Mars. If you want to observe Mars from orbit, go to Mars orbit - there are already plenty of Mars orbiters, and they can do their science much better without having a great big rock in the way.

What we could gain from going to Phobos is learning about Phobos, as described in AtmosphericPrisonEscape's answer. Even if the scientific value of a Mars mission might be higher, there's a trade off as going to Phobos is easier in some ways at least.

There have been at least 3 Soviet/Russian missions sent to Phobos, most recently Fobos-Grunt, all of which failed. A sample return mission is quite attractive - not dropping so far into Mars's gravity well may make it an easier target than Mars, and would probably answer the question of Phobos' origin. ESA was quite seriously looking at Phobos Sample Return a few years ago, but more recently it seems to have been sidelined in favour of cooperation with NASA on a Mars Sample Return mission.

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    $\begingroup$ For humans in orbit about Mars, landing on a satellite provides radiation shielding from half the sky. $\endgroup$ – MBM Apr 3 '18 at 17:52
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    $\begingroup$ Agreed @MBM - indeed more than half if you burrow in to it. The question mentioned robotic missions rather than human ones, but you're right that this is another relevant point. $\endgroup$ – djr Apr 3 '18 at 20:33

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