The question is simple:
Can a magus cast pit of the gaping earth on a place that's 7 paces away if all of the ground is covered with snow?
a) Yes, because you can see that there is ground below the snow. If he was a knight in full plate, his armor wouldn't protect him from spells either.
b) A magus cannot affect what he cannot preceive.
There's another consideration:
is fallen snow that's on the ground governed by Aquam (it's water) or Terram (it's a kind of earth)? Or both?
It's solid(-ish), so it would be Terram, as far as I currently understand it.
It's an argument very similar to that of Lava in Projects.
My understanding of the OP's question is that he wants to PotGE the ground under the snow, not the snow itself, so the "is it Aquam?" question doesn't really come into play.
And I'd say go for it in most normal circumstances. The ground is clearly there under the snow and, depending on the snow's thickness, you may even be able to see small rocks or other surface irregularities sticking up through it.
If there's an underlying solid layer of ice or thick-packed snow, then I'd say that's likely to block the spell, but, at the degree I'm thinking about, it's more of an academic question anyhow - if the ice/snowpack is thick enough and solid enough, then it can support a man's weight even if the ground beneath it is removed. If it's even close to that point, then I'd say you can't target the ground below and need to target the ice/snow instead. (...at which point you need to resolve the Aquam question with your troupe.)
Couldn't the PotgE just first make a hole in the snow, then in the earth - and the remaining discussion be about the need of an Aq casting requisite?
Cheers
B) if he can not perceive the ground under the snow then it can not be targeted even though it is Voice ranged.
Technically if it is voiced range I would say the real question is whether his voice would penetrate the snow, since even though he cannot see the ground he does effectively perceive it as being there (ground being rather ubiquitous and continuous), in the same way a mage through reason can perceive a knight inside the plate mail armor. If the snow is thick enough his voice does not penetrate, then it cannot be cast.
Look at it that way: If a magus casts PotgE in summer in his garden, and can see the soil underneath the plants, but not the loam underneath the soil. Will the PotgE then only destroy the soil - perhaps a foot deep - and stop before destroying the loam which the caster couldn't see?
Or the dirt under tall grass...
THen you begins down the long slippery slope of how much snow and ice would stop the voice from penetrating. Is it fresh snow vs packed snow ( as they have different sound characteristics)? Ice and packed snow tend to reflect sound waves. What is the percentage of that 7 paces away from the mage?
I think you can make a problem of anything, if you want to.
A lot of things aren't nailed firmly in place, for the very reason that the game needs some wiggle room IMHO.
Now Pit of the Gaping Earth (or similar Terram effects) should not be blocked by trivial things on the ground. My take on it is that things normally found there, things that do not make the area of earth something else, don't interfere with this spell.
So, grass, snow, mulch, twigs or a picnic blanket won't matter to me. A paved road or a stream? I wouldn't complain if you wanted the fissure I the earth to appear across this, if the road or stream is just a small portion of the length. I mean, you can see the earth on both sides of this interrupted space.
But I wouldn't want it to work while standing in a castle cellar, because you are not on ground but inside a structure, and any earth below if firmly covered in stone slabs. Unless the cellar has an earthen floor, in which case you can see earth.
And I would want it to appear under a lake. Unless you stood on the lake floor below all the water.
And I don't think I like it to work on a paved market square, this for me is like standing on a mountain side. No earth surface only rock. But if the spell has +1 mag to affect stone...yes.
While I understand that "perceived" part of the ground but it would only work if you were familiar with the ground of the target without the snow. To know that there is 5 feet of snow or 1 foot of snow or 1 inch of snow. Snow has a lot more variables than grass or armor.
Yes and no. If your footprints are reaching through the snow where you stand, they will probably do so 7 paces away, unless there is a clear snow drift. The point here is a simple "isit a lot of snow?" If yes, the spell may be in trouble. If no, the spellshould work. There may be a place between the voice cannot penetrate and you know it is a couple inches thick that might require a finesse roll, at the storyguide's discretion...
Consider also that perceive does not mean see.
If you can feel the ground under your feet you can perceive it, even when there is snow above it.
If you can see the form (mounds, ditches ...) you can still perceive it, just as you can perseive an invisible men walking through flour dust.
If you can here the gravel crunching under the feet of your foe ... you guess, you can perceive it.
If you can taste it ... well forget it 
Widewitt
I would say no.
Can you PeCo someone inside an igloo (while you are outside, and cannot see him?)
I suppose enough grass would have the same effect.
Poor analogy. If I see an igloo, I have no way of knowing whether there's a person inside or not without looking. If I see a layer of snow on the ground, I can be quite certain (under most circumstances) that there is ground underneath it.
Knowing that there is ground underneath is (IIRC explicitly) not enough. You must perceive your target or use an AC.
So a sight ranged spell does not have to see the target?
You still run into the Limit of Arcane Connections which says that you must SEE the target. Even if you use a Intelligo spell and know the ground is there.
In my understanding you don't have to see the target itself, seeing it's outline is good enough for me. So it's rather seeing where it is for me.
In the current example I would rule, that if there is so much snow that you can no longer see the form of the ground you also can't cast the spell. But as long you can see all the ditches and form of rocks etc. you are fine. Just my point of view.
Btw.
I don't understand that sentence. Do you mean you can use AC only if you see the target?
Widewitt
R:Sight is the one time that I would agree that you must actually see (as opposed to perceiving in other ways) the target because seeing the target is the explicit definition of the spell's range.
It does?
"Unsensed" is not synonymous with "unseen". The target can be sensed in other ways than sight. Also note the end of the example, where it says the magus can't PeCo people behind a wall "...until the magus is aware of them" - which implies that, once the magus is aware of unseen people on the other side of a wall, then he can PeCo them.