Science-fiction horror, like historical horror, is a tricky beast. You're already asking your audience to suspect disbelief on one axis (monsters exist), and at the same time you're asking them to suspend disbelief on a second (this is all occurring in a future yet to happen). If the viewer's too caught up by the implausibility of the setting, it makes it harder for them to invest in the story to a degree that they'll be scared when you want them to be. And, for that matter, I suspect that "aliens" occupy a different space in our head from "monsters" - they may be adjacent, but our expectations for how the protagonists interact with them may differ enough to make the experience a little confusing. The best way to approach it, then, is to try and minimize the degree to which the details establishing it as a science-fiction story make it feel fantastic (e.g., the truckers-in-space angle taken by Alien), and make your alien as indistinguishable from a monster as possible (e.g., the Lovecraftian nightmare at the center of The Thing).
(Speaking of, I re-watched the director's cut of Alien recently, and was watching some behind-the-scenes stuff, and hearing Ridley Scott describe it as his attempt to make "The Texas Chainsaw Massacre in space" was illuminating. I don't know that he succeeded at doing that, but I suspect that taking that approach to the film IS a big part of what made it successful.)
So on these two counts, The Last Days On Mars does things right. I wouldn't put it in the same league as Alien or The Thing, not by a long shot. But it's a well-constructed, almost meditative film about, essentially, the failure of humanity - both as a species and as essential nature.
Sometime in the distant-enough future, mankind has sent people to Mars. Not once, but twice. The second mission is coming to a close after six months. They're on short time, with 19 hours and change remaining until the relief crew shows up and they get to board a ship home. They're trying to wrap things up and two crew members - a paramedic named Lane and a technician named Campbell - are driving a rover out to one of the research sites to pick up Kim, who is busy collecting core samples. They're in a hurry because there's a massive sandstorm coming and everyone needs to be back at base for a debriefing. Lane and Campbell are grousing because they know Kim is going to be a pain in the ass about it. Nobody seems to like Kim too much. Sure enough, she stalls for time and complains about not finding anything. She's abrasive, but there's a layer of frustration underneath it. Her research is coming up snake eyes - she's been on another planet for six months, just spinning her wheels.
Their arrival back at base sharpens this - there's a second scientist, Petrovic, who ducks out of the briefing by feeding the base commander a line about needing to repair a sensor. As it transpires, he's really trying to get back out to a dig site to collect some more samples - samples he intends to backdate, scooping Kim and securing for himself sole credit for discovering evidence of bacterial life on Mars. He is cheerfully unapologetic about this - he thinks it's funny - and we wonder just how well he's fooled everyone else. It's no wonder Kim is so angry. In fact, the whole situation is really dysfunctional - Brunel, the base commander, is largely ineffectual at keeping his crew in line, and there's a lot of hostility and free-floating resentment in the air, punctuated by frequent power outages and other equipment failure. The vehicles are falling apart. The base is falling apart. They are falling apart. They're less than 20 hours away from leaving, and not a moment too soon.
And then, as you might expect, something goes very wrong at the dig site. Petrovic has just discovered something really important in a core sample, but before he can communicate what it is, the entire ground around the sample site collapses into a sinkhole, taking Petrovic with it. Back at base, Kim - in open defiance of regulations - has gone through Petrovic's work and has figured out what he's up to. Before anything can be done about it, the technician who accompanied Petrovic calls in a mayday. After assessing the situation at the site, it's concluded that Petrovic must be dead because there's no way he could have survived the fall with his suit remaining uncompromised. Base doctor Dalby volunteers to stay behind while everyone else goes back to retrieve the gear needed to pull the body out of the sinkhole. Only when they return there's nobody there. No Dalby, and no Petrovic. The hole is empty, except that it's teeming with some sort of mold or fungus.
Back at base, there's a knock on the airlock door. Petrovic and Dalby - or, rather, what's left of them - have returned.
This sense of restraint is extended to the characterization as well. We don't get a lot of depth on all of these people - there's eight of them - but most of what we do learn we learn economically, through their actions and reactions, not exposition. You can infer a lot based on how people talk about other people, and how they respond to events. Dalby and Petrovic probably have a thing based on the vehemence of her reaction to Kim's criticism of him and to news of his assumed death, but it's never stated outright. Petrovic seems to be better liked by the crew in general than Kim, even though Kim isn't the one lying and falsifying records to snag credit for a major find, and you get the sense that Kim's the only one who sees through his bullshit and it's made her even angrier. Lane and Campbell seem to have a bit of history, but it doesn't seem to be romantic, Brunel seems tired, worn down by six months in this (literally and figuratively) toxic environment. Complex relationships are sketched in quickly and efficiently through acting choices. And just as our initial understanding of these people is established, much of it gets upended when the other shoe drops. We know just enough to be surprised, but it never feels like a convenient or plot-driven reversal. As one of the characters puts it early on before things go bad, crises are how you find out who people really are. The answers aren't always the ones we expect here, for good or ill.
Where this movie works especially well is in tension - when things go bad, they go bad quickly and then do not let up for the rest of the film. The pacing is relentless, but splitting the crew up early means that cuts can be made from frenetic action to quieter suspense and back again so that we aren't fatigued by non-stop running and yelling, but the sense of threat never dissipates. Everything is at a premium - air, power, fuel - and that scarcity makes every decision count, and every setback that much worse. The crew is extremely vulnerable - they're at the mercy of the environment and the threat that's overtaking them, and one injury can mean certain death one way or another. Every success comes at a cost, every failure pays a high price and you're just sympathetic enough for the most part that you want to see these people survive.
The heart of this movie, then, is how these people let each other (and themselves) down - physical frailty in the face of the environment is matched by psychological frailty. These people are selfish and weak, almost to a person - just as the base keeps breaking down, so do they. The question then becomes to what degree their failures as people are or are not liabilities in the face of this larger outside threat. Some of the least sympathetic people turn out to be the strongest, and some of the most sympathetic turn out to be the weakest at a tremendous price. One of the symptoms that plagues people infected by the alien bacterium is the gradual stripping away of humanity, of memories and experiences, and the characters wonder out loud if the people their teammates were aren't still trapped inside their bodies. Humanity fails itself, and so humanity recedes. In the end, everything - air, water, food, fuel, power, humanity - is a scarce resource, and decisions have to be made about what needs to be conserved and what can be expended. Crises strip away humanity and lay bare what lies beneath it.
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