A response to yesterday's questions

Yesterday Sweetly Smart asked some questions about Mars. My answer got to be so long that I thought it would be better as a new post.

1) where did the ice come from in the first place?
2) is there a belief that there was once atmospheric pressure?
3) If there is such a low pressure on the surface how does the planet get plagued with dust storms?

I'm gonna do what I can. Some of the answers I get to in a rather roundabout way.

We're fairly sure that Mars once had a thicker atmosphere. What happened to it is a bigger question.

Mars would have once been a lot hotter and had volcanic activity. They [planets] pretty much form as a big ball of hot. The volcanic activity would provide some atmosphere. Venus still has lots of volcanic activity which gives it a really thick atmosphere and a heavy greenhouse effect.

Mars is smaller than Earth and Venus and further away from the sun. So it cooled earlier than the other two. The core is still probably molten but no longer large enough to cause the plates to shift, trigger volcanoes, or provide a magnetic field for the whole planet. I specify "the whole planet" because there are smaller regional magnetic fields.

Between Mars' lower gravity and the collapsed magnetic field the sun could have just blasted away the atmosphere. The solar radiation ionized the molecules so the solar wind could easily blow them away.

Mars and the Moon have all kinds of craters while Earth has very few. This isn't because their luck is worse so they take all the asteroid hits. It's because Earth has the weather and life to erode the craters away. As well as the atmosphere to deal with the smaller stuff.

In the last several years satellites orbiting Mars have taken pictures of erosion. They've shown signs of large quantities of water freely flowing over the surface long ago in the form of rivers and tributaries as well as deposits resembling river deltas. Certain minerals have been found that could only have formed in the presence of water. They've also seen continuing signs of erosion along the inside of crater walls.

It is now thought that if you could melt the southern ice cap you'd have enough water to cover the planet to 11 meters deep. With the terrain as it is you'd end up with a massive ocean in the northern hemisphere and a few massive lakes in the south. Even more is thought to be underground.
http://mars.google.com can give an idea of the surface elevation.

Clouds seen from the surface of Mars.
http://www.space.com/imageoftheday/image_of_day_041122.html

This image compares two photos taken three Earth years apart. It shows erosion is still happening.
http://www.fas.org/irp/imint/docs/rst/Sect19/pia04290-browse.jpg
Before that picture was taken we thought that sort of erosion would be impossible on Mars today. It was thought that liquid water couldn't exist on the surface that long. Some still say it could be wind erosion but there are some issues with that line of thinking.

Dust storms. Right. Mars gets some real doozies. It's a very dry and dusty place. There have been times where telescopes have seen dust storms that cover the whole planet. The rover Opportunity once had to hunker down for 6 weeks while a storm ravaged the planet. Luckily the winds were also good enough to sweep the solar panels clean so Opportunity could keep getting power.
The dust devils from the other day were just lesser versions of the dust storms.

The thin atmosphere means that a lot more sunlight can reach the surface and heat things up. So in effect it can have the storms BECAUSE the air is so thin.

I once heard it said that, ignoring the air pressure, you could wear sandals on your feet but you'd still need a parka on your body.

The hot, but thin, atmosphere rises fast in the cold air and draws in the cold air from the surrounding area. This results in winds reaching 100 km per hour. But since the atmosphere is so thin the wind would have much less impact on you than a similar speed wind would have on Earth. But even so, you don't want to be hit by dust moving that fast.
[addendum: the storms can cover the whole planet because the planet is so dry and the storms so fierce that the dust can be thrown unbelievably high and take a long time to settle again. That and there's really no non-desert biome to break things up.]

Where does the ice come from? I've been putting this one off. I really can't say. It could be freezing out of what little is in the atmosphere. It could be snow. I don't want to say it's seeping out of the ground because there's no sign of glacial movement which would be needed to get the ice caps as big as they get.
What bothers me more is how the frozen CO₂ is forming.

In the search for life on Mars the best bet is the southern ice cap. We still find ancient bacteria living in bubbles of liquid water inside our own ice caps. It's not inconceivable that if there was once microscopic life on Mars some of it may remain inside the southern ice cap where the ice would protect it from the solar radiation that sterilized everything else.