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I've thought about the possibility of claiming the planet's density was increased using heavy metals found on one of its moons,

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The low gravity does not make Mars uninhabitable. There are a few effects that you would need to take into account, though.

Things would fall more slowly.

People's muscles would probably be under-developed, and that could lead to some health problems.

People brought up on Mars would probably have great difficulty if they ever tried to visit Earth.

You'd have to think through the implications of lower gravity, but there is nothing inherently unbelievable about people living in 1/3 gravity.

Kathleen, do you agree that 1/3 gravity is not really something I need to solve, other than adressing its effects.

My first instincts were that 1/3 gravity wasn't a barrier to the planet sustaining life, but I wanted to make sure it wasn't going to be a case of the reader going "as if" and closing the book.

There's a site here with some articles about the subject, from the perspective of health effects on short-term visitors to Mars (under the Mars Direct mission plan, where I believe they'd spend about a year on Mars):

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other than somehow speeding up the planets rotation, which just brought cartoon imagines of giant rocket engines afixed to the side of the planet to my mind.

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Well, this wouldn't improve things-- the reason rotating a space station creates the feeling of gravity is because the people are standing on the outer rim, their heads pointing towards the center, so the centrifugal force feels like gravity. On a planet they'd be standing the other way around, so it would actually decrease the apparent gravity. (Also, planets are BIG and full of intertia, and BIG rockets are hard to come by.)

Eric--

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People's muscles would probably be under-developed, and that could lead to some health problems.

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I never thought about this....you're right, but also consider something I've read as a possibility in some scifi books...that there is less pressure on the heart, thereby increasing lifespands and improving health.

My guess about why it doesn't have an atmosphere now, is that the core of Mars has cooled(It's still liquid, just not as hot as it used to be). The last volcanic eruption was between 20 and 200 million years ago. Volcanoes pump out huge amounts of gases, refilling the atmosphere. The gravity on Mars is to weak to hold those gases forever, so over a long period of time the atmosphere leaks into space. (The same thing happens in Earth's atmosphere with hydrogen, which is light enough to escape our gravity.)

Taking all that into account, you just need to come up with a system to constantly replenish the necessary gases. Sure, they'll still be leaking out into space, but not so fast as to alarm the inhabitants.

The biggest issue I see it actually terraforming it in the first place, once you do, it shouldn't be too hard to maintain.

You would probably need gas mining operations. They could dig up rocks (like carbonates) that would be processed to release CO2. Once you have the CO2, plants can convert it to oxygen. Take a look at this article:

http://www.biotech-info.net/metabolically.html

The rice that he's describing would give a high O2 yield from the atmospheric CO2 during photosynthesis. And, as a bonus, it is a food crop to feed your hungry colonists.

When I read your post, I assumed your major issue with gravity is that it is insufficient to hold gases like nitrogen and oxygen, which is why the Martian atmosphere is made of heavier gases like CO2, which its gravity can capture and hold. There are many stories with domed cities/enclosed living environments on Mars. You said you have adressed O2 creation (assume you figured out water too), and I'm thinking maybe you didn't want to have domes/enclosed living environments (thus your terraforming). If you speed up the rotation, you will lose the gravitational effect and just fling your atmosphere into space (back to Gen's comment)!

So if you want to avoid domes and have a breathable atmosphere, perhaps gravity is your issue after all. Which brings up another issue - unprotected humans would suffer from pressure issues too. That takes us back to the idea of pressurized suits and protected living spaces, in which case your breathable atmosphere is inside... now the issues are those which Eric brought up. Of course, with proper gravity to hold the atmosphere, you'd probably solve your pressure issues...

Which brings us to the biggest question - why Mars (you said you needed your story to be placed on Mars)? Can you take advantage of how Mars is different from Earth for your story, or must you make it like Earth? Specialized living arrangements might be your simplest solution.

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The gravity on Mars is to weak to hold those gases forever, so over a long period of time the atmosphere leaks into space. (The same thing happens in Earth's atmosphere with hydrogen, which is light enough to escape our gravity.)

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Where do the leaked gases go? Would spaceships be able to detect these gases in space either near or far from the planets?

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The gases just diffuse into space. Space isn't really a vacuum, but it is very, very empty matter-wise. The most common thing floating around out there is hydrogen. (The Bussard ramjet is a theoretical engine that makes use of the free floating hydrogen for fuel. Link: http://www.daviddarling.info/encyclopedia/I/interstellar_ramjet.html )

You would probably need extremely sensitive equipment on your spaceship to detect a difference in the matter content in space relative to the distance from planets, but it should be possible.

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that there is less pressure on the heart, thereby increasing lifespands and improving health

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Regarding escape of atmospheric gasses: If the terraforming technology is sufficient to build a breathable atmosphere, presumably it is advanced enough to continue generating atmosphere at a low level to make up for the slow loss.

It seems the general consensus is that, while I do need to consider the long term effects of a lower gravity planet, the gravity in itself is not a barrier to the idea of life on the planet. The reason for Mars is it's distance to Earth, the idea being that Earth has colonized Mars as an overflow planet for the increasing population. Note, Earth DOES NOT plan on relocating her entire population there, not by a long shot. Earth just wants somewhere to put the extra people so Earth does not become uninhabitable. The actual story takes place a couple of hundred years after the settlement was created, with the ecosystem well established, at least in this area of the planet.

One solution is to terraform under domes--but this isn't really a way to change the whole planet, just little parts of it.

One other solution is to just ignore the problems of terraforming and say you've done it anyway. Most SF readers will buy the simple assertion, just as they accept FTL travel and humanoid aliens. This isn't weakmindedness on the part of the readers, it's just that most people will accept a big lie to have the opportunity to enjoy the small ones.

Just as one more confirmation, 1/3 gravity is no problem at all for colonization. I think that all of everybody's comments on that subject were correct. (For what it's worth, the Moon's gravity is only 1/6, and there've been lots of scientifically accurate stories about colonization there.)

A point about the atmosphere, however: the biggest factor in the loss of Mars's atmosphere is not its low gravity (that's a factor, but if Earth's gravity was reduced to 1/3, it would still keep most of its atmosphere for a very long time). The additional factor is that Mars has no magnetic field to protect it from the solar wind, so that its atmosphere is literally blown away. This doesn't mean it would happen overnight, but it does mean that replacement atmosphere would have to be provided at a greater rate than otherwise.

The other aspect of this is that colonists in the open on Mars would receive a much higher dose of radiation than on Earth. Unfortunately, I can't tell you how much more, or what measures would be sufficient to protect from it. Does anybody else know, or know of a site link that might have the information?

From what I've read, it looks like the Martian magnetic field is pretty much from the remnant paleomagnetic bits from the old field-- once rock goes above the Curie point heatwise, it resets to the prevailing magnetic field, and stays that way. It's one of the things used for reconstructions of paleocontinents. Nice article about how these work on Mars here, although I haven't read anything about it since the recent stuff.

Nice pix of the paleomagnetic stripes on Mars here. Earth's got them too, of course-- it was a major piece of evidence for plate tectonics, and it's probably pointing the same way on Mars, although the planetary dynamo and the plate motions there are dead nowadays. Meaning no geologic carbon cycle over the long term, even after terraforming....

Everyone knows that a comet develops a tail when it enters the inner solar system because the "solar wind" pushes gas and dust from the comet's surface. But I never stopped to wonder why inner solar system planets like Venus, Earth, and Mars don't have tails. Until now.

Based on this discussion, Earth doesn't have an appreciable tail because its magnetic field deflects so much of the "solar wind" by a tiny bit. Mars probably used to have a tail, but with no magnetic field to protect it, the solar wind has blown off all it can and now the tail is gone.

But if this is the case, why doesn't Venus have a tail? Venus has lots of atmosphere, light gravity, and hardly any magnetic field. Or am I wrong about one of these admittedly top-of-my-head recollections about the second planet?

My question is, what actually makes the tail visible? Actually, there are two tails, the gas tail and the dust tail. The dust tail is dimmer. The gas tail is brighter. What makes the gas tail so bright? Obviously most gases are pretty transparent. I think that maybe the gas in the tail gets ionized by the sunlight, but I'm not sure.