NASA's Opportunity rover spent an impressive 14 years exploring the Meridiani Planum region of Mars, as it is believed to contain important clues about the planet's early geology and environment. Of particular interest to scientists is the Burns formation, a layer of sandstone containing hematite “blueberries” that suggests the presence of liquid water and a sulfur-rich composition similar to other regions of Mars. The Burns formation sits on top of a thin rock layer known as the Grasberg formation, and in a recent study published in the Journal of Geophysical Research: Planets, researchers propose a new hypothesis about their common origin.
The study is based on the recent discovery that the Burns and Grasberg rocks share a similar chemical composition, except for the fact that the former is enriched in magnesium and sulfate. Previous hypotheses suggested that the Burns formation's chemical composition was due to a combination of ancient silicates and sulfate salts that precipitated from evaporating groundwater or the transformation of basaltic ash by volcanic or atmospheric sulfuric acid. The authors of the new study propose a third possibility: that the Burns/Grasberg precursor material was deposited onto Meridiani Planum as dust or falling ash from nearby volcanic eruptions, which solidified and was eventually weathered into sand grains.
According to the authors, periodic flows of groundwater enriched the upper layers of the sand dunes with magnesium and sulfate, and water flow cemented the sediments to form the sandstones observed by Opportunity. The researchers used data collected by Opportunity's Alpha Particle X-ray Spectrometer to construct a mass balance model simulating this scenario, which they believe better matches Opportunity's observations than previously proposed mechanisms.
These findings could alter the way scientists understand the early environmental conditions on Mars and transform interpretations of sulfate-rich deposits found in other regions of the planet.