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Organic Remains in Sedimentary Rocks on Mars
Rovers are put on Mars to conduct research, collect samples, and perform fieldwork. Humans can’t yet go to Mars to collect data, so we have to rely on Rovers to do it for us. Many different sites have had Rovers sent to them to search for evidence of organic matter preservation in the sedimentary rocks on mars. I have focused on Gusev Crater and Meridian Planum, Burns Cliff, Jezero Crater, and Gale Craters. Craters are important because they have naturally exposed beds. It is easiest to use beds that are already naturally exposed as starting points because we cannot yet dig our own trenches on Mars. While each site is very similar, the goal of the research and methods carried out were different. Amongst the other tools used were high resolution cameras to take high quality images of the sites, map out the paths of the Rovers, and to determine the topography of the sites. Mars and Earth are very similar in their structures and processes. I have described many of the way that they are similar in general, and also specific structures that were formed in similar ways. The White Sands Dunes Field was created in a way very similar to the way that the Burns Cliff was formed.
I have investigated three scientific papers that discuss the possibility of finding organic remains, or fossils, in the sedimentary rocks in craters on Mars. There are many deep craters on the surface of Mars that allow for easy access to outcrops of sedimentary rocks. The craters supply a natural trench that humans use rovers to investigate. Looking for sedimentary rocks is where most of the research starts because we know that on Earth, sedimentary rocks are where organic remains are found. I will be discussing how organic remains are preserved on Earth, what conditions are necessary, and why we use this knowledge to make predictions for Mars.
Gusev Crater and Meridian Planum
Two Mars Exploration Rovers (MERs), Spirit and Opportunity, were sent to craters on Mars to determine if the conditions there were ever suitable for life (Squyres and Knoll, 2005). The MERs spent over a year and a half on Mars at sites at Gusev crater and Meridiani Planum. The beds exposed at these craters give the first chance to be able to explore the ancient sedimentary rocks on Mars and also the stratigraphy and geochemistry of these rocks (Fig 1). Opportunity landed on January 24, 2004 and first stopped at Eagle crater, and then traveled 800 meters east to a larger crater, named Endurance (Squyres and Knoll, 2005). The rover entered Endurance crater on day 134, then ascended to a feature called “Burns Cliff” on day 276, and eventually left the crater on day 315 (Squyres and Knoll, 2005). This is where predictions were made about the past environmental conditions.
The Burns Cliff was divided into lower, middle, and upper units with a total thickness of 7 meters (Squyres and Knoll, 2005). Each unit was assumed to be sandstones that are rich in sulfate salts (Squyres and Knoll 2005). Using the Pancam and Mini-TES, it was determined that the lower unit was formed by migrating sand dunes. The middle unit was determined to be a sand sheet deposit that was deposited as impact ripples moved along the flat sand surface. The Mossbauer, APXS, Mini-TES, and Pancam showed that the matrix throughout this whole section is made up of three main components- silicate minerals, sulfate salts, and oxidized iron-bearing phases. This leads to the interpretation, by Squyres and Knoll (2005), that the whole Burns formation originated as olivine basalt in sulfuric acid, which then evaporated to produce the sulfate salts, which accumulated alongside of the fine grained silicates. As these materials were eroded and redeposited, they formed the beds of Burns Cliff. These findings show that at some point in time there was a source region, like a wet interdune depression or a playa lake, near this area. This mission discovered evidence for an ancient aqueous environment that could have been suitable for life on Mars (Squyres and Knoll 2005). There was no organic matter discovered in this mission, but Meridiani Planum would be a good site to return to in the future.
The Mars Reconnaissance Orbiter (MRO) investigated Jezero crater in search of clay-rich sediments that could have organic matter preservation (Ehlman el al., 2008). Since clay rich sediments are where most organic matter is preserved, it is a key discovery to find organic matter on Mars. The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) discovered phyllosilicates in three lake basins with fans or deltas (Ehlman el at., 2008). This crater is an open lake basin that has sedimentary deposits of hydrogen-rich minerals that came from a smectite drainage area in the Nili Fossae region (Ehlman et al., 2008). Smectite is a type of clay known for its ability to trap organic material, therefore the deposits in the crater could be ideal for organic preservation. The Mars Reconnaissance Orbiter and Compact Reconnaissance Imaging Spectrometer (MRO-CRISM) system provides hyperspectral images and a High Resolution Imaging Science Experiment (HiRISE) instrument. They also paired these with Mars Orbiter Laser Altimeter (MOLA) topographic data (Ehlman et al., 2008). This is to be able to see the geomorphology and mineralogy of Jezero Crater (Fig 2). The crater is in an area of basalt and iron-bearing igneous minerals like olivine and low-calcium pyroxene (Ehlman et al., 2008). A tool called OMEGA was used to determine that the watershed at Jezero crater has many outcrops of iron-magnesium smectite-bearing rock. MOLA was able to show that 58 km3 of sediment was the clay-rich section of the Noachian plateau were eroded and transported into Jezero crater (Ehlman et al., 2008). This indicates that clay-rich minerals are what mostly comprise the crater (Ehlman et al., 2008). This is unique among Martian paleolakes, and these clay deposits record two periods of history when the surface of Mars was habitable (Ehlman et al., 2008). Any organic matter that was transported to the Jezero crater was mostly likely buried very rapidly within the smectite clay (Ehlman et al., 2008). Since smectite clay has a great potential to trap organic material, this crater is an ideal site for future exploration.
The Mars rover Curiosity was used to examine samples of sedimentary rocks from Yellowknife Bay within Gale crater (Ming et al., 2013). Curiosity was put on Mars on August 6, 2012 and obtained two samples consisting of mudstone. They chose this site because it was believed that the deposits were fluvio-lacustrine, produced by rivers and lakes (Ming et al., 2013). These types of systems are thought to preserve organic material. Curiosity began a drilling campaign in order to collect powder samples from mudstones at the base of the exposed strata (Ming et al., 2013). The two samples were named John Klein (JK) and Cumberland (CB). These samples were delivered to the Sample Analysis at Mars (SAM) and Chemistry and Mineralogy (CheMin) instruments (Ming et al., 2013). When put under extreme heat the samples gave off gases such as H2O, CO2, and many others. The JK sample intersected thing Ca-sulfate-rich veins, and the CB sample was collected out of an area that had many nodules and was poor in Ca-sulfate-rich veins (Ming et al., 2013). Both of the powders were gray, which suggests a relatively unoxidized material. They are going to describe the volatile and organic C content of the mudstone to determine the potential for preservation of organic C (Ming et al., 2013). The volatile bearing phases are indicators of the past environmental condition (Ming et al., 2013). They can show whether or not the environment was habitable. The compositions were determined by the SAM oven’s evolved gas analysis (EGA), gas chromatography-mass spectrometry (GCMS), and tunable laser spectroscopy (TLS) experiments (Ming et al., 2013). It was unclear if there were organics present in these mudstones. If organics remained throughout burial, they could have been altered by other mechanisms. The surface of Mars is subjected to ionizing radiation, which can alter organic molecules (Ming et al., 2013). The ability to detect and identify organic compounds in the samples is difficult due to the reactions that happen within the SAM oven (Ming et al., 2013). Since there is no definitive detection of organic compounds, this could mean that they were present. However, alteration and destruction of the organic material is the most challenging aspect of the search (Ming et al., 2013). The fluvio-lacustrine environment makes this a good site for organic material to become concentrated during sedimentary processes.
Fieldwork on Mars is mainly carried out by Mars Exploration Rovers and other vehicles and instruments. These rovers are solar-powered, six-wheeled robotic vehicles that can make long explorations on the surface of Mars (Squyres and Knoll, 2005). They each carry a copy of the Athena Science Payload, which is all of the scientific tools and instruments that are needed in order to carry out the mission. The elements needed for the payload at Gusev crater were a panoramic camera, a miniature thermal emission spectrometer, a microscopic imager, an alpha particle x-ray spectrometer, a Mossbauer spectrometer, and a rock abrasion tool (Squyres and Knoll, 2005). These instruments provide high-resolution multispectral imaging, remote sensing over a wavelength range from 5-29 μm, close up images of an area 3×3 cm, determines abundances of major and minor elements, determines abundances of iron-bearing phases, and a tool used to expose subsurface materials (Squyres and Knoll, 2005).
All of these studies were looking at the sedimentary and clay-rich rocks on Mars. This is because, on Earth, sedimentary rocks are the only place that organic matter is preserved in the form of fossils. We have to draw upon our knowledge from Earth to use on Mars, since it is the only place that we can do in person fieldwork. There is an area on Earth called the White Sands Dune Field in New Mexico. This area was formed in a very similar way to Burns Cliff on Mars. The gypsum sands supplied to these dunes are from Lake Otero, and formed during the Pleistocene (Langford, 2002). The last shoreline of Lake Otero was formed during the late Pleistocene at 1,216m and was followed by a period of extreme dryness, and the formation of the playa lakes and the dune field (Langford, 2002). The dunes are downwind of a basin that was formed by deflation (Langford, 2002). The deflation that created the basin has incised into the sediments left behind by Lake Otero (Langford, 2002). Another sediment source to the dunes are a playa lake called Lake Lucero (Langford, 2002). But, the sediment from Lake Otero are what is needed to show the history of the White Sands Dune Field (Langford, 2002). The beds of this lake show a low-relief Pleistocene lake floor (Langford, 2002). These means that the steep basin below Lake Otero was formed from erosion after the lake had dried up (Langford, 2002). Since the shorelines of these lakes are preserved, we know that deflation has made the basin deeper, but not wider (Langford, 2002). There is a gypsum layer in the alkali flat that is 9m above Lake Lucero, which is suggested, by Allmendinger (1972), to have produced most of the dunes through deflation. These dunes were created by the precipitation, erosion, and transportation of gypsum from a previously lacustrine environment. Much like Burns Cliff was formed by the erosion and redeposition of sulfate salts.
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Mars and Earth have very similar structures and compositions (Williams, 2018). They both have a general structure of a terrestrial planet, which is core, mantle, crust (Williams, 2018). Mars is believed to have a mantle that is made up of silicate rocks, minerals that can be compared to the crust, and has partial viscosity which creates convection currents (Williams, 2018). On the surface, both planets have varying terrains as you travel along their surfaces. Earth has mountain ranges, volcanoes, trenches, canyons, plateaus, and abyssal plains. Mars has mountain ranges, sandy plains, and the largest dunes that are found in our solar system (Williams, 2018). It also has to the largest mountain in the solar system, which is a shield volcano called Olympus Mons, and the longest, deepest chasm called Valles Marineris (Williams, 2018). Both Mars and Earth have had many asteroid impacts, but Mars’ are better preserved due to the low air pressure and lack of precipitation (Williams, 2018). These craters are what were investigated in the three papers I read, since they are so well preserved they provided easy access to beds of sedimentary rocks. Similar geological processes occurred on both planets which gave them the varied terrain that they both have (Williams, 2018).
Since it is inferred that similar processes have occurred on both planets, it is a good assumption that if there were to be organic preservation on Mars, it would be in areas similar to where there is organic preservation on Earth. Only a small number of the sedimentary environments on Mars have been explored, and only the ones that have been naturally exposed in the form of various craters. There have not been any preserved organic matter found as of today from these sites. I think that with progressing technology and with more trips back to these areas, there is a lot of potential for preserved organic matter to be found. Maybe we just haven’t explored the right crater as of today. Also, one day we might be able to dig deeper than what is naturally accessible to our Rovers to get more information about the past of sedimentology of Mars.
Figure 1: The path that the rover Opportunity took from Sol 1 through 324. The top image is in map view from the spacecraft’s Descent Image Motion Estimation System (DIMES) camera. The bottom image is in oblique view obtained from the Pancom camera (Squyres and Knoll, 2005).
Figure 2: Mineralogy and extent of the Jerzero watershed (Ehlmann et al., 2008).
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