Those polygonal patterns in the martian surface imaged by
the Phoenix spacecraft may indeed be caused by thawing and freezing
cycles within the martian permafrost.

But, the temperatures are much too cold...normally.
[-30C to -80C are the norm; and it doesn't get much warmer below the surface
for kilometers.]
So, if geological heating and Solar photonic heating are not the answer
for the thawing of the martian permafrost, what is?

I believe I have stumbled upon the answer: solar flares.

How many solar flare ejection-masses have struck the planet Mars directly
in the past 1000 years? ...Going by the earth's experience, statistically at least ONCE!

Remember, the ejection mass of a powerful solar flare travels at speed of millions
of kilometers per hour, and the mass of solar flare material ejected is measured
in billions of tons....That's energy comparable to 1,000's if not millions of H-bombs.

Only a tiny fraction of that solar flare ejection-mass will strike Mars itself.
The good news is, unless you're an astronaut on the surface, that without a planetary magnetic field in recent
history, and with an extremely thin martian atmosphere, MOST of the solar-flare material arriving at Mars will
reach the surface with MOST of its energy unabsorbed and thus will smash into and penetrate the martian surface to depths measured in centimeters or meters.

That high-energy solar-flare material will penetrate down to depths where permafrost exists, losing most of the solar-flare kinetic energy [converted to heat energy] in the martian regolith and near-surface ice.

It is possible [though I do not have the numbers immediately available to confirm] that when a very-powerful solar flare eruption sends a huge mass of high-energy particles smashing into the martian surface in the polar regions, the near surface rockhard permafrost ice may actually briefly experience "thawing" for a few seconds or minutes until it refreezes when the flare material
and bombardment abates.
Thus the thawing-freezing cycle as evidenced by the polygonal patterns.

Just to let you know, Nick, this hypothesis of mine is TESTABLE!

Ifpotheticallya solar-flare eruption sends high-energy material smashing directly into Mars [perhaps] next month, and if hypothetically the Phoenix were to survive and recover from the radiation it might be in position to confirm the hypothesis
however small the chances of such a flare event happening.