I didn't read about an idea I had, so, may be you'll be interested to share your comments or point to me some references talking about it.
Idea : Ice, in the Polar caps keep the history of the atmosphere of planets. I was thinking that, by comparing Polar caps ice from both Earth and Mars that formed at the same time, we could deduce the behavior of the Sun. Did it gave less energy or if did the solar system crossed a dust cloud....?? I was particulary thinking of a recent period, 14 & 15 century when northern (Earth) hemisphere get a mini glaciation. The results are not very deep inside the ice and can eventualy be probed on Mars in a resonnable future. I don't think Phoenix will do it since it's not going THAT North.
Does it make sense to you or am I totaly wrong ?

Maybe, the poles are only formed due to the axis inclination. Another point is that the Antartic is the polar which resembles mostly to Mars South pole. The Mars' South pole is mostly made of solid carbon dioxide (dry ice) than the Mars' North pole. Anyway, both poles have water ice underneath of its dry ice covering. However, there is pitted layer which is dry ice, but the material below, which makes up the floors of the pits and the bulk of the polar cap, is water ice.

On the other hand, that I know that the difference between the two poles is that the south pole dry-ice cover is slightly thicker, about eight meters, and does not disappear entirely during the summertime.

One interesting point is about how the atmospheres of the inner planets, Venus, Earth and Mars are compared to each other. At the beginning of the formation of solar system, it is assumed that Earth, Venus, and Mars had similar in the total carbon dioxide content, with Earth having most of its carbon dioxide locked up in marine carbonates and Venus's carbon dioxide being in the atmosphere and causing the runaway greenhouse effect. By contrast, the 8 meter layer on the south polar ice cap on Mars means the planet has only a small fraction of the carbon dioxide found on Earth and Venus.

That means that the theory that Mars had a warm and wet at the beginning is doubtful due to the fact there is not much carbone dioxide found in the Martian poles to float in the atmosphere to trap enough solar energy to warm the planet.

That has leaded me to think that the best phrase that Mars was a cold, wet planet. Then where will be the water? It could be that the subterranean plumbing would allow for liquid water to be sealed off underneath the surface.

Dr. Andrew Ingersoll is a known specialist of Mars Poles which I have read mostly.

A resume of its article:

"The North residual cap -- the part that survives the summer -- is made of water ice. The South residual cap seems to be made of frozen carbon dioxide, otherwise known as dry ice, but we don't know if this is a veneer a few meters thick or a solid block that extends down 2 or 3 kilometers. These images may help us decide."

About the selection Phoenix landing, you can find at the other topic which is related to Phoenix. There you can find it.

High Resolution Views Comparing the Martian North and South Polar Residual Caps

Rodolfo

The mini-ice age is well correlated* with the Maunder minimum period of reduced sunspot activity. No dust cloud required.

*correlation, of course, does not equal causation

Would be nice to know if Marsian ice can show measurable effects for that period. That's the meaning of this thread. A correlation between the two planets would may be not a scientific breakthrough but could mentaly change the way of thinking. I know that some people don't think that the Greenhouse effect is real. By comparing Mars and Earth we could sort out the "exterior" effect i.e. the Sun or whatever can be of some influence .

I think that's a very good question. The ice core record for each planet should contain two superimposed signals, one solar/environmental and one planet-specific. It should be possible - and well worth doing - to sort them out.