A Review of the "Black Beauty" Martian Regolith Breccia and Its Martian Habitability Record

Zircon trace element evidence for early hydrothermal activity on Mars

Finding direct evidence for hydrous fluids on early Mars is of interest for understanding the origin of water on rocky planets, surface processes, and conditions essential for habitability, but it is challenging to obtain from martian meteorites. Micro- to nanoscale microscopy of a unique impact-shocked zircon from the regolith breccia meteorite NWA7034 reveals textural and chemical indicators of hydrothermal conditions on Mars during crystallization 4.45 billion years ago. Element distribution maps show sharp alternating zoning defined by marked enrichments of non-formula elements, such as Fe, Al, and Na, and ubiquitous nanoscale magnetite inclusions. The zoning and inclusions are similar to those reported in terrestrial zircon crystallizing in the presence of aqueous fluid and are here interpreted as primary features recording zircon growth from exsolved hydrous fluids at ~4.45 billion years. The unique record of crustal processes preserved in this grain survived early impact bombardment and provides previously unidentified petrological evidence for a wet pre-Noachian martian crust.

Sustained wet-dry cycling on early Mars

The presence of perennially wet surface environments on early Mars is well documented1,2, but little is known about short-term episodicity in the early hydroclimate3. Post-depositional processes driven by such short-term fluctuations may produce distinct structures, yet these are rarely preserved in the sedimentary record4. Incomplete geological constraints have led global models of the early Mars water cycle and climate to produce diverging results5,6. Here we report observations by the Curiosity rover at Gale Crater indicating that high-frequency wet-dry cycling occurred in early Martian surface environments. We observe exhumed centimetric polygonal ridges with sulfate enrichments, joined at Y-junctions, that record cracks formed in fresh mud owing to repeated wet-dry cycles of regular intensity. Instead of sporadic hydrological activity induced by impacts or volcanoes5, our findings point to a sustained, cyclic, possibly seasonal, climate on early Mars. Furthermore, as wet-dry cycling can promote prebiotic polymerization7,8, the Gale evaporitic basin may have been particularly conducive to these processes. The observed polygonal patterns are physically and temporally associated with the transition from smectite clays to sulfate-bearing strata, a globally distributed mineral transition1. This indicates that the Noachian-Hesperian transition (3.8-3.6 billion years ago) may have sustained an Earth-like climate regime and surface environments favourable to prebiotic evolution.

Early crustal processes revealed by the ejection site of the oldest martian meteorite

The formation and differentiation of the crust of Mars in the first tens of millions of years after its accretion can only be deciphered from incredibly limited records. The martian breccia NWA 7034 and its paired stones is one of them. This meteorite contains the oldest martian igneous material ever dated: ~4.5 Ga old. However, its source and geological context have so far remained unknown. Here, we show that the meteorite was ejected 5-10 Ma ago from the north-east of the Terra Cimmeria-Sirenum province, in the southern hemisphere of Mars. More specifically, the breccia belongs to the ejecta deposits of the Khujirt crater formed 1.5 Ga ago, and it was ejected as a result of the formation of the Karratha crater 5-10 Ma ago. Our findings demonstrate that the Terra Cimmeria-Sirenum province is a relic of the differentiated primordial martian crust, formed shortly after the accretion of the planet, and that it constitutes a unique record of early crustal processes. This province is an ideal landing site for future missions aiming to unravel the first tens of millions of years of the history of Mars and, by extension, of all terrestrial planets, including the Earth.