Ancient Martian basins: Can they be a mirror of early life on Earth?
Have you ever heard of a hydrothermal vent? It is a vent at the marine seafloor, where many scientists believe to be a place for the origin of life on Earth. However, earliest samples from such a setting are highly restricted because the Earth had gone too intense metamorphism and metasomatism; so to say, the signs are ‘erased’. Thus, researchers had to shift their interests into another, though analogous, place. Recently, a group of researchers suggested the appropriate stage: Ancient Martian basin. Let’s find out how can it be so, and its significance.
Michalski and colleagues assert that huge seas once existed, more than 3 billion years ago, in Eridania, which is a basin on Mars. Morphological evidences strongly support that the basin was once filled with water; the Ma’adim Vallis outflow channel is positioned at the edge of the basin at ~1100m elevation.
Below the ancient sea, there exist thick deep-sea deposits. The researchers analyzed mineralogy of the deposits, and discussed how they could have formed. Using CRISM(Compact Reconnaissance Imaging Spectrometer for Mars), the researchers found out that the deposits are mostly composed of Mg-Fe rich clay minerals such as saponite, talc, serpentine, and sepiolite. The spectra of these minerals match well with those of terrestrial seafloor clay minerals. A mineral called jarosite, which is considered a proxy for the presence of sulfide-bearing ore deposits since it forms mainly due to oxidative chemical weathering of sulfide minerals, has also been detected sparsely along with the clay minerals. Also, using THEMIS(Thermal Emission Imaging System), some chloride minerals have been identified to exist along the rim of the basin, which indicates the evaporation of the shallow sea.
Until now, roughly three hypotheses about the formation of the deep-sea deposits have been proposed; evaporative origin, detrital origin and air fall origin. Here, however, the authors argue that the deposits are the products of the deep-water hydrothermal system, an idea supported by massive and thick clay deposits along with igneous rocks. The clay mineral assemblages of Eridania basin provide a good analogues for the terrestrial seafloor deposits. Presence of sulfide-bearing ore deposits, inferred by jarosites, also resembles massive sulfide deposits near terrestrial deep-sea hydrothermal vents.
Given the fact that the origin of life on Earth is thought to have aroused near the hydrothermal systems, this ancient deep-sea hydrothermal deposits are worthy of attention regarding astrobiological research. Furthermore, -rich clay minerals and Fe-sulfides in Eridania deposits indicate reductive environments of the ancient sea, which is similar to that of the early ocean on Earth. In other words, Eridania seafloor deposits can be a mirror of early life on Earth, and thus are important not only for Martian exploration, but also for the terrestrial investigation. Who knows? Martian deposits may help filling out the first page of the terrestrial history of life!
Figure 1. A reconstruction of Eridania basin on ancient Mars. Deep-sea deposits at the center of the basin formed through hydrothermal alteration are marked with greenish color. Some subsurface structures such as faultings might have helped lavas to ascend. Chloride deposits are found at the rim of the basin.
