Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars

Abstract

Abstract-Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km-long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and ⁄ or an icy landscape. Meteorites could have been encased upon impact and ⁄ or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10-80 m of deflation in the last 3-3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and ⁄ or water ice at and ⁄ or near the surface.

Weathering rinds as mirror images of palaeosols: examples from the Western Alps with correlation to Antarctica and Mars

“…c. 20 Myr), weighed against faster weathering (c. 15 kyr) in the middle-latitude alpine environment of the Alps, make it difficult to assess rind development as age indicators between the two environments, it is evident that there is clear palaeoenvironmental archival information contained in both databases. By extension, it is possible to infer that similar palaeoenvironmental archives may be present in Martian clasts (Fairén et al 2011), and these may also correlate with resident palaeosols, which would extend the rind weathering record into the Noachian (c. 4 Ga), far earlier than on Earth.…”

“…Kenya, a tropical Afroalpine environment, have shown that identifiable bacteria and fungi first appear in clasts older than the Last Glacial Maximum (LGM). Meteorites encountered by the Mars Exploration Rover (MER) Opportunity on Meridiani Planum (Fairén et al 2011) exhibit evidence of long-lived weathering (c. 1 Gyr), presumably in an aqueous palaeoenvironment during the Noachian Period (c. 4 Ga). Deep cavernous weathering of six meteorites encountered on Mars is similar to observations of weathered clasts in the Antarctic (Mahaney et al 2012b), and provides the first close-up study of Fe-rich clasts under a palaeo-atmosphere considered similar to that of early Earth.…”

Survivability of copper projectiles during hypervelocity impacts in porous ice: A laboratory investigation of the survivability of projectiles impacting comets or other bodies

“…That is they favoured the origin of the meteorites observed on the surface as being from the falls of small bodies which decelerated in a dense atmosphere. In this case, Fairen et al (2011) based their findings on observation of the surfaces of the particular bodies observed on the martian surface, consideration of their likely age (over 3 Gya), the setting in which they are found and so on. The survival of so many individual iron meteorites in a limited region could be caused by coincidence or accumulation over time.…”

“…If the crater is subsequently eroded and sub-surface material exhumed, then the material could be present on the surface as the metal may be more resistant to erosion than the polar ice (or rocky surface in other regions). If we were to consider a larger impactor, one of size 15 m would produce fragments of mean in the size range 120-11 cm (impact speed of 5-10 km s À1 ), which spans the size of iron meteorites (30-60 cm) reported on the martian surface by Fairen et al (2011). The fragments will also be reasonably localised on scales of a few 10s of metres and will be paired, in that they will have a common origin and thus composition and age.…”

“…We also note the observation of iron meteorites on Mars (e.g., see Arvidson et al, 2011 andFairen et al, 2011, andreferences therein). These are of sizes 30-60 cm and indicate the arrival at Mars of metallic impactors.…”