Habitability

Conditions on Mars during the first billion years were dramatically different from the present day. Whether or not the conditions were once habitable depends largely on the volatile content, specifically water (H₂0) and carbon dioxide (CO₂), of the surface. The greatest source of evidence for the presence of these volatiles comes from observations of surface morphologies.^[2] Previous observations of Gale Crater show that the strata exposed at Yellowknife Bay are most likely fan or down slope equivalents, such as lacustrine deposits. Curiosity used its ChemCam and Mastcam instruments to analyse the chemistry and layering of a geological outcrop designated as "Shaler".^[3] This geologic formation displayed cross-bedding features, clear indicators of past interactions with water flows. Yellowknife Bay was chosen by the Mars Science Laboratory team as the first major site for exploration because the exposed strata was inferred to be a shallow fluvial-lacustrine deposit. These aqueous environments are believed to preserve evidence of paleo-habitability and potentially, earth-like microorganisms capable of breaking down rocks and minerals for energy, known as chemolithoautotrophs.^[4]

Until Curiosity had arrived on Mars, all surface age dating of Mars had been through relative techniques using geomorphology and crater counting methods in order to determine an estimated age of the rock layering. The Mars Science Laboratory team used the rover to collect samples from the mudstone at Yellowknife bay then, using the mass spectrometer, from the Sample Analysis at Mars (SAM) instrument package

measured argon isotopes for an absolute radiometric date of the rock member, and an approximate age of its exposure to the surface.^[5] The age of the lake bed rock was dated to 4 billion years old, and exposed by wind erosion between 30 and 110 million years ago, giving us the first absolute age of a rock on another planet.^[6] Still, a more ideal location to search for evidence of life on Mars would have been a member exposed more recently, as few as a million years or less, so it could have been better preserved from the harsh surface radiation.^[7]

Yellowknife Bay Geology

The primary composition of most terrestrial bodies in our solar system is igneous rock, but it has long been speculated that sedimentary rock exists in great quantity on Mars, as it does on Earth.^[8] The Curiosity rover has confirmed the presence of sedimentary rock composed of fine-, medium-, and coarse-grained sandstone basalt. This exposure is about {{convert|5.2|m|ft|abbr=on}} thick and is divided into three unique strata. From bottom to top, these strata have been designated: the Sheepbed member ({{convert|1.5|m|ft|abbr=on}} thick), the Gillespie Lake member ({{convert|2.0|m|ft|abbr=on}} thick), and the Glenelg member ({{convert|1.7|m|ft|abbr=on}} thick); the assemblage of members is known as the Yellowknife Bay formation.^[13]

Active erosion, attributed to both aeolain and fluvial events, has caused the Gillespie Lake Member to weather away, revealing the underlying Sheepbed layer and creating a topographic step observable in HiRise images from the Mars Reconnaissance Orbiter. The Gillespie Lake bed appears massive and is composed of poorly sorted, angular to well-rounded grains, which make up sheetlike sandstone basalt. These characteristics further support a model of fluvial transport and deposition. In addition, sandstone beds associated with the Gillespie Lake Member seem similar to microbially induced sedimentary structures (MISS) found on Earth, according to one study.^[9]

It was from the Sheepbed layer that Curiosity took two drilling samples from the mudstone sedimentary rock. These drill samples were named John Klein and Cumberland and were, respectively, the second and third drilling samples Curiosity retrieved from the martian regolith.^[1] The first being an eolian deposit, named Rocknest, sampled from an area {{convert|60|m|ft|abbr=on}} to the west of Yellowknife bay. The two samples were drilled {{convert|3|m|ft|abbr=on}} apart and within {{convert|10|cm|in|abbr=on}} of the same stratigraphic level.

John Klein and Cumberland Samples

A number of different instruments were used by Curiosity in attempts to assess the mineralogy of the mudstone sampled from the Sheepbed strata. The CheMin XRD, Mastcam, Chemcam, alpha particle x-ray spectrometer (APXS), and the Mars Hand Lens Imager (MAHLI) were all used to obtain the most complete picture possible of the chemistry and mineralogy of the two samples, which were used to describe the region as a whole.^[16]

A large amount of phyllosilicates, clay minerals such as smectite, were found to be major constituents in the two samples.^[17] Clay minerals are hydrous aluminum phyllosillicates and form only in the presence of water, further supporting the claim that an ancient crater lake once existed in this region. Other silicates were detected as well, such as the Magnesium-rich end-member of olivine called forsterite, pigeonite, plagioclase, augite, clinopyroxene, and orthopyroxene. These detected minerals are all indicative of a potential mafic source for the origin of the deposition.^[16][17]

After Yellowknife Bay

With the Mars Science Laboratory's primary goal of establishing whether a habitable paleo-environment could have existed on Mars accomplished while within Yellowknife Bay, the team of NASA scientists then directed Curiosity out of the Yellowknife bay formation and toward the original destination of Mount Sharp, which rises {{convert|3.4|mi|km|disp=flip|abbr=on}} from the base of Gale Crater.^[10] On July 4, 2013, Curiosity drove away from the Glenelg member and began its {{convert|5|mi|km|disp=flip|abbr=on}} journey to Mount Sharp. NASA scientists estimated this drive would take the rover a year to complete.^[11] {{further|Aeolis Mons#Curiosity at Mount Sharp}}

References