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There likely was quite a bit of liquid water on the surface of Mars less than 4 billion years ago. We know this because we found rocks that old that have been modified by the presence of water. For example :

  • If the martian blueberries, or hematite spherules found by Opportunity formed by slow evaporation in mineral-rich liquid water, they likely formed 3 billion years ago.
  • Serpentines have been detected by CRISM.

What is the most recent rock we know of on Mars that was modified by the presence of liquid water on the surface?

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If you have access, I would recommend you to read Carter et al. (2013). It is a compilation of all occurrences of hydrous minerals that have been detected on Mars by spectrometers OMEGA (Mars Express/ESA) and CRISM (Mars Reconnaissance Orbiter/NASA). This data was used to investigate the spatial distribution, composition, and age of hydrous minerals on Mars.

Regarding age, the most recent hydrous minerals are opal and zeolites of Amazonian age (< 3 Ga):

The exposure with the youngest unit age is a series of small cones and dykes or fractures which have been identified thanks to a dust‐free window in the Utopia Rupes region, in a unit of Amazonian age. Both opaline silica and zeolites (or alternatively sulfates) are found there as shown in Figure 7. Zeolites are found mainly on the butte that could constitute a small volcano, as suggested by the presence of radial patterns resembling lava flows, to the southeast especially. Silica is found on an elongated structure which is either a dyke or a structural horst. Of interest is the presence of two small cones exposed in the NE corner displaying opaline silica. Such small cones on Mars have been interpreted either as volcanic vents or mud volcanoes [e.g., Allen, 1979; Farrand et al., 2005]. In both cases, the observation of opaline silica pleads for a local fluid circulation inside a hydrothermal context consistent with volcano‐ice interactions as predicted by previous studies [Allen, 1979; McGill, 2002; Farrand et al., 2005].

However, it does not really answer your question as their presence is likely not due to "liquid water on the surface", but rather to impact-driven hydrothermal systems inside the crust. Exposures of their "horizontal deposits" (i.e. sedimentary) and "alluvial fans and deltas" categories, most likely linked to surface liquid water, are of Hesperian and Noachian age, although I couldn't find which is the youngest.

A limitation to keep in mind: some exposures of hydrous minerals may be older than they seem, as the terrain could have been excavated by an impact long after the alteration process (Noachian terrain exposed by Hesperian or Amazonian impacts).

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