Fossils in rocks frozen in time hint at ancient climate on Mars

Long ago, wind and water currents shaped Mars’ soft sands and sediments into dunes, ripples, and other landform patterns called bedforms. Over billions of years, some of these features hardened into rock, which scientists call paleo-bedforms. Frozen in time, change only comes in the form of slow erosion by dust-laden winds, burial by ancient lava flows, or the occasional meteorite impact.

A team led by Planetary Science Institute senior scientist Matthew Chojnacki mapped and characterized paleosurface landforms across Mars to better understand their diversity and the planet’s ancient climate. The study is published in the journal Geomorphology.

Since 2013, Chojnacki has worked on HiRISE, NASA’s Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment.

“I collected HiRISE images with strange features that looked like bedforms, but they were cratered and covered with rocks. They looked old and fossilized,” Chojnacki said. “We wanted to investigate further.”

Their work uncovered paleo-bedforms across a range of landforms of different ages, latitudes and geological contexts, including craters, canyons and basins. These can be categorized into paleo-dunes and groups called paleomegaripples, formed by wind, fluvial paleo-dunes, and dune-cast pits, which are paleo-dunes that have been eroded so far that only shallow depressions remain.

Paleobedforms have been found all over the globe, but most are concentrated in the Valles Marineris and Athabasca Valles near the equator, Noctis Labyrinthus west of Valles Marineris, Arcadia Planitia in the northern lowlands, Hellas Planitia in the southern hemisphere, and at the boundary between the highlands and lowlands between Arabia Terra and Mons Apollinaris.

“The most compelling and obvious ancient landforms are the sand dunes,” Chojnacki said. “Many of these ancient dunes look very similar to modern dunes, but they appear more degraded.”

The most widespread paleo-bedforms are paleomegaripples, which resemble vast plains of parallel ridges. These tiny bedforms develop when wind blows over abundant coarse sand.

“Ancient giant ripples are also plausible, but less so than sand dunes, because there are other geological processes that could have formed similar features,” Chojnacki said.

Based on what they know about modern megaripples, the team proposes a model for how these features evolved: wind first sculpted them, then stopped, causing the sand to harden into rock, which was preserved, and eventually deteriorated.

The rarest and most degraded paleo-bedforms were likely formed by ancient water – called fluvial paleo-bedforms – and the team found them only in places thought to be remnants of ancient floods.

Chojnacki said he was surprised that more of these river paleobeds had not been found.

“Mars has an abundance of dry river channels where fluvial bed formation may have occurred, but their small size and infilling of the channels likely did not help preserve them,” he said.

The team estimates that most of the paleo-bedforms were embedded in the geological record about 2 billion years ago or more recently. Most of the bedforms were probably formed and moved by volcanic activity such as lava flows or ash fall, and then buried until they reemerged by erosion, but others were embedded in the rock without being buried.

“In other cases, active dunes along the north polar cap are moving over older, ancient dunes, causing their erosion. Seasonal ice can also erode dunes,” Chojnacki said. “The diversity of these dunes speaks to the diversity of dynamics and conditions occurring in the solar system.”

Now that the survey has revealed a large sample of ancient Martian rocks, the team hopes to identify modern-day dune fields that may be trending in a similar direction.

“While many bedforms on Mars are actively moving around today, other fields are stationary and show evidence of some sort of stabilization process that may eventually lead to lithification,” Chojnacki said. “Understanding this continuum will help us better understand the changing climatic conditions on the Red Planet.”