re: Proposal for landing site of the Phoenix Mars lander.
Here's a MOC image of a polar cone:
Polar Cone
MGS MOC Release No. MOC2-1519, 10 July 2006.
http://www.msss.com/mars_images/moc/2006/07/10/
It's located at 79.5°N, 57.0°W (303°E). If you look at the image I copied above of the meltwater channel, the channel seems to be headed towards the direction of this cone.
However, the caption to the image of the polar cone claims it is not a volcano, but does not provide the argument for why it is believed not to be.
I attached a copy of the polar cone image below. I rotated it 90° clockwise to make it clearer it's a raised feature.
Bob Clark