Curiosity's centimetre mud cracks are the only ground truth on a planet mapped in metres

Start with the arithmetic between the two datasets. The polygons Curiosity measured run 4 to 8 centimetres across ^[2]. The polygons HiRISE has been resolving from orbit since 2006 begin at 15 metres and run past 350 ^[8][9]. The smallest feature the orbiter can see is therefore roughly 190 times wider than the widest one the rover has its instruments on ^[1], and end to end the two catalogues sit nearly four orders of magnitude apart ^[2]. A crack network at those two scales is not obliged to have the same origin.

That is why the mechanism matters more than the picture. A centimetre-scale network is what a thin mud layer does when it dries and rewets, which is the reading a 2023 Nature study applied to earlier Curiosity observations in Gale Crater, tying them to intense wet-dry cycles ^[4]. Polygons hundreds of metres across need a different mechanical story, and elsewhere on Mars the candidates on offer are freeze-thaw cycling or tectonic and volcanic stress ^[10]. Orbital polygon counts cannot be read as a map of wet-dry cycling until something on the ground tells you which population is which.

The hedging in the new work is worth reading closely. Ashwin Vasavada, a Curiosity mission scientist at NASA's Jet Propulsion Laboratory, says the team "measured their shapes and chemistry carefully" and is "hopeful there are clues in the data as to how these features formed" ^[3]. The researchers studying the field say more work is needed to understand how it formed ^[6]. The 2023 study's wet-dry conclusion was itself framed as a hypothesis, pinned to the Noachian-Hesperian transition about 3.8 to 3.6 billion years ago ^[5]. That is a window of roughly 200 million years ^[3], about 13 percent of the 4.5 to 3.0 billion year span covering Mars's Pre-Noachian, Noachian and possibly Hesperian periods ^[13][4].

So what the encounter actually buys is a method rather than a destination: chemistry measured alongside morphology, at a site where both can be tied to the same layer. Orbit supplies shape and little else. The basins where crater floor polygons are read as evidence of past standing water are Hellas Planitia, Noachis Terra and Margaritifer Terra ^[11], and Hellas is the deepest hole on the planet, about 2,300 kilometres across and just over 7,000 metres below the Mars datum ^[12], which is where water would have lingered longest. Nothing has ever measured a polygon there from the surface. Gale remains the only mud-crack terrain any rover has examined at ground level ^[7], which means the calibration curve for the whole planet currently has one point on it, at the small end.