"then" was 4 billion years ago.

Start counting seconds. Count day and night, without pause. There are 31 million seconds in every year. It will take you 30 years to count ONE billion seconds. Now try counting years...

4 billion years is a distinctly long time.

Of course, early Mars was an interesting planet, with extensive volcanism, frequent (but probably not permanent) liquid water, numerous impacts and associated global warming, despite the feeble young Sun.

But that was THEN.

We already knew this stuff about Mars' crust. A paper in "Geology" by Ken Tanaka, Bruce Banerdt, and myself discussed the putative northern ocean of Mars. Ken showed that the Northern plains WERE infilled by a vast dump of sediment, I proposed non-aqueous mechanisms (though they were not REQUIRED - an aqueous model "could" have worked, and Bruce showed that despite this infill, the northern plains failed to respond isostatically to the load, showing that the crust was ALREADY cold and thick back then.

Gravity studies of Mons Olympus show this too - it formed, and sat on the rigid crust - it didn't sink in.

Mars crust is thick, cold, and rigid - everywhere.

Trouble is, the people who dream about water on Mars, don't think what the planet Mars is like - what their ideas of liquid water imply for the rest of the planet (or what we KNOW about the rest of the planet implies for liquid water, or the lack thereof).

Still it's good to have a "Flagship" paper that nails a spike in the ground and says "This is what Mars is like". Interestingly, all the good papers are coming from the Europeans these days. Either they built a better spaceship than NASA, or they have better scientists?

What IS interesting about this new paper is that they infer that the present icecaps are a mere 5 million years old. This is a very dynamic timescale for the icecap - I would have expected hundreds of millions of years. It does attest to a very dynamic cycle between ice caps, "glaciers", and atmospheric water vapour. But don't get too excited. The glaciers are cold-based and don't flow much, they just sit. And they DON'T EVER melt...