If the year-round average surface temperature at the equator is - 40 C, and at the poles it is - 140C the why wouldn't you need to go MUCH deeper at the poles to get the same temperature?

According to the reference I gave said the coldest the temperature on Mars gets is about -133 C. So average temperature at the poles isn't -140 C. It also said the warmest surface temperature was 27 C.

In another reference:
"However, the temperature of the surface at the winter polar caps drop to -225° F, (-143° C)"

And:
The temperatures on the two Viking landers, measured at 1.5 meters above the surface, range from + 1° F, ( -17.2° C) to -178° F (-107° C).
http://www-k12.atmos.washington.edu/k12/resources/mars_data-information/temperature_overview.html

Also:
"Mars has an extremely eccentric orbit around the Sun. The closest point in its orbit to the Sun is called perihelion; this is the same time that it's summer in the southern hemisphere. This means that the summer temperatures in the southern hemisphere can be 30 °C warmer than the summer temperatures in the north."
http://www.universetoday.com/guide-to-space/mars/temperature-of-mars/

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