Or, y'know, they're not space suits, as people have suggested before.
Space-suits are, for the moment, bulky because there's a lot of shit in them. They have to be pressurized, they have to maintain temperature, etc. If the planet they're on doesn't have too dense or thin an atmosphere and has reasonable temperatures, all they need to do is protect themselves from the elements and supply their own air.
Think above-ground SCUBA suits, not space suits.
This is absolutely correct. With NASA EVA space suits (operating in pure vacuum) the internal suit atmospheric pressure is around 5 psiA (normal sea level pressure is 14.7 psiA), and 100% pure oxygen. Whatever the movement of the "suit", the aim is to maintain a constan volume. If the external pressure is NOT vaccuum, but rather has some type of atmosphere (not necessarily breathable), then there will be some external pressure...and the EVA space suit does not necessarily have to be pressurized to the same relative pressure. In onther words, it the planet has an atmosphere, and the space suit is not working against a "pure" vacuum, then the internal pressure of the suit can remain higher.
What that means, is that the EVA suit (in such an evironment), does not necessarily have to be supplied with an "artificial" positive pressure. This makes it much easier for the EVA suit to be designed around a "mechanical pressure" rather than using internal pressurization. The "Sci-Fi" nomenclature for such a suit is a "skin suit", which uses some type of elastic pressure to maintain a more or less "constant" pressure against the astronuats skin. There are prototypes that are actually in development by NASA at this very moment, but research is still ongoing, and is not by all means practical in true vacuum, at this current point in time...although, who knows what the future will bring. Nevertheless, it is is quite apparent from existing data and research, that a "skin-suit" spacesuit using mechanical pressure would definitely be feasible, in an enviornment that has some type of atmosphere (exerting a given pressure).
This is a rather long explanation of how the current Prometheus screenshots showing a "skin-suit" type of environmental "EVA" suit is absolutley scientifically plausible, presuming the fact
that that they are on a planet/planetoid/moon that has a substantial atomsphere (although not necessarily breathable). Note that this explanation only deals with the partial pressure requirements for humans to function...I am "assuming" that any advance "skin-suit" will have the necessary automatic temperature regulation and built-in radiation attenuation to keep the astronaut protected.