For the first time ever, scientists have been able to collect unequivocal evidence that water had once flowed on Mars some three billion years ago. Observations by NASA’s Mars rover Curiosity have revealed pebble-rich rock exposures that are so very characteristic of a riverbed. The round-shaped pebbles embedded in a matrix of coarse-grained sand are a telltale sign of fluvial transportation. Importantly, pebbles with a long axis of 10-40 mm can be rounded only when transported by river.

The pebbles have “smooth, rounded perimeter and an equant to tabular shape.” Based on the roundness indices, pebbles have been classified as “subrounded or rounded.” They were discovered between the north rim of the Gale Crater and the base of Mount Sharp, a mountain inside the crater. Multiple exposures of pebble-rich rock were observed and Curiosity took high-resolution images of these pebbles at three locations — Goulburn, Link and Hottah. The results were published on May 31 in Science.

Though evidences of fluvial transportation on Mars some three billion years ago have been well documented in the past, this is the first time ever that multiple in situ evidences, strongly indicative of sustained fluvial transportation for a few kilometres, have been obtained.

Scientists led by R.M.E. Williams from the Planetary Science Institute, Tuscon, have postulated that flowing water had a depth of 10 cm to less than one metre and had flowed at a rate of less than one metre per second. Since the pebbles would have been angular in the beginning and subsequently rounded by abrasion, the well-rounded pebbles seen on the outcrops strongly indicate several kilometres of transportation.

Considering the fact that Mars has reduced gravity, the pebbles would have been abraded at a slower rate. Hence the distance travelled by the pebbles to attain the well-rounded shape would have been much longer than what one would normally see on the Earth.

The particle size, roundness and sorting together strongly support fluvial transportation. The pebbles are 2mm to 40 mm in diameter and sand grains are coarse with one mm diameter. Based on the grain-size distribution, the scientists have classified the sediments to have been moderately sorted.

All these strongly suggest that ancient fluvial transportation on Mars closely resemble Earth’s.

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