Behind The Black – Robert Zimmerman

The graph comes from this paper [pdf], and shows the shift of Mars’
rotational tilt, or obliquity, for the past 80 million years.

Scientists analyzing data obtained by China’s Zhurong rover during its first four months on Mars have concluded that the Martian atmosphere in the northern lowland plains of Utopia Planitia has been more active hydrologically much more recently than previously believed.

You can read the paper here. From its conclusions:

Unlike the weak and friable thin veneer or crusty clods at previous landing sites for Mars Exploration Rovers and InSight lander, these rocks are more akin to the fractured duricrust observed at the Viking Lander 1 site. The thin and brittle layer of duricrust has been proposed to form by salt cementation via water vapor diffusion from the atmosphere. In contrast, duricrusts at Zhurong landing site appear to be more resistant to erosion, forming cliffs perched through loose soils in the surroundings, which require a substantial amount of liquid water rather than water vapor.

…The morphology and spatial extent of platy and bright-toned rocks investigated by the Zhurong rover argue for in situ formation and degradation of these rocks in the Amazonian-aged geological unit in southern Utopia Planitia. These observations suggest that aqueous activities may have persisted much longer than previously thought. Periodical climate cycles on Mars driven by obliquity oscillations are expected to result in a latitude-dependent distribution of ground ice over geologic history. Higher obliquities (>45°) could mobilize polar ice and form glaciers and water ice sheets at midlatitudes and stabilize ground ice at Zhurong landing site for extended periods of tens of thousand years when the obliquity exceeded 29° to 33°. The hydrated minerals and widespread salt cementations imply the presence of briny liquid water in the subsurface, which may have been generated by melting the ground ice during temporary climate perturbations (e.g., volcanism and impacts).

Obliquity refers to the planet’s rotational tilt, which fluctuates from 11 to 65 degrees, and is presently tilted at 25 degrees, only slightly different than Earth’s 23 degree tilt. Since Mars’ obliquity has exceed 29 degrees numerous times in the past 80 million years, as shown by the graph above, that means it was possible that this now dry equatorial region on Mars had seen plenty of surface water or ice as recently as a few million years ago, far more recently than the three billion years previously estimated.

For liquid water to have existed on the surface is still difficult, because Mars’ atmosphere would have been too thin and cold. However, if ice sheets had existed here, there is the possibility that the ice would have become liquid at the sheet’s base, and interacted with the ground in some manner to produce the duricrust now observed.