Dust Devils, Ancient Habitable Environments & More
NASA's Spirit and Opportunity rovers are exploration legends. The two solar-powered robots landed a few weeks apart in January 2004, then embarked on 90-day missions to search for signs of past water activity on the Red Planet. Both rovers far outlasted their warranties; Spirit was declared dead in 2011, and Opportunity kept rolling until a monster dust storm cut off its sunlight in June 2018. And they both made big discoveries that have fundamentally reshaped scientists' understanding of Mars and its environmental history.
Rover deputy principal investigator Ray Arvidson, of Washington University in St. Louis, submitted a Spirit and Opportunity Top 10 list to Space.com to mark the mission's 10 years of Mars exploration in 2014. To be clear, these are Arvidson's personal opinions, not a consensus of the mission team.
Spirit and Opportunity: Using argon as a tracer of atmospheric dynamics. Both rovers employed their APXS instrument to measure the atmosphere of Mars and detect argon.
Martian Dust Devils
Modern Water or Frost
Modern Waters, Part 2
Spirit: Ferric sulfates were moved down the soil column by modern waters at Troy, Husband Hill, Gusev Crater. [Related: Trapped Mars Rover Finds Signs of Buried Martian Water in Recent Past]
Water Interacting with Hot Rock
Spirit: Iron-magnesium carbonates at the Comanche outcrop, Husband Hill, Gusev Crater, showing that water interacted with magma.
An Ancient Hydrothermal System
Spirit: Opaline silica at Home Plate, Gusev Crater, formed in volcanic fumaroles and/or hydrothermal vents. Again, showing that water was interacting with magma. [Related: Habitable Hotspots on Mars? Volcano Vents May Be Signs]
Opportunity: Wind-blown ripples on the Meridiani plains are relict from previous wind regime, probably when Mars' spin axis tilt was different than today’s value.
A Habitable Environment Long Ago
Opportunity: The rover has found evidence that ancient Mars may have been habitable for millions of years. At the rim of Endeavour Crater, Cape York; the rover discovered ferric and aluminous smectite clays in finely layered Matijevic formation rocks that pre-exist the Endeavour impact event. Alteration in moderately acidic and reducing waters, perhaps mildly oxidizing for ferric smectites. These are the oldest rocks examined by Opportunity (roughly 4 billion years old), and they were exposed to waters much more habitable than the waters that led to the Burns formation. [Video: Mars Could Have Supported Life, NASA Finds]
Ancient Acidic Lakes
Opportunity: The Meridiani plains' Burns formation as sulfate-rich sandstones with hematitic concretions formed in ancient acidic and oxidizing shallow lakes and reworked into sand dunes and cemented by rising groundwaters.
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