Meridiani Planum (alternatively Terra Meridiani) is a large plain straddling the equator of Mars. The plain sits on top of an enormous body of sediments that contains bound water. The iron oxide in the spherules is crystalline (grey) hematite (Fe2O3).
The Meridiani Planum is one of the most thoroughly investigated regions of Mars. Many studies were carried out by the scientists involved with NASA's Mars Exploration Rover (MER) Opportunity. Two outstanding features found by these investigations are the actions of water flow and aqueous chemistry in this plain's geological history and, particularly specific to the plain, an abundance and ubiquity of small spherules composed mainly of grey-hematite that sit loosely on top of the plain's soils and underneath embedded inside its sediments. The loose surface spherules were eroded out of the sediments. They are informally called "blueberries". The plain's sediments have extremely high sulfur content (as sulfates) and high phosphate levels.
The boundaries of the Meridiani Planum are not firmly fixed and accepted by the community of Mars planetary scientists. However, the boundaries of the hematite-bearing plain were operationally defined in the late 1990s and early 2000s by the extent of the orbital detection of the plain's surface hematite by the thermal emission spectrometer (TES) on the satellite Mars Global Surveyor. The various names for this region (i.e., Terra Meridiani, Meridiani Planum) started to be used in the published literature in 2002/2003/2004. Each name reflects the coincidental (somewhat arbitrary) fact that the plain straddles the prime meridian for the system of longitude lines introduced for east/west Mars mapping.
The area covered by the detected surface hematite is around 150,000 km2, i.e., larger than Lake Superior () in North America. Except for transport by large meteor impact, loose surface spherules tend to remain within a few meters of their starting embedded location. The surface hematite spherules and sediments are coextensive in surface area. So, the area extent of the underlying sediments is at least as large as the area of detected surface hematite spherules but likely somewhat larger since, for example, a significant area of surface hematite was covered by ejecta from the Bopolu Crater impact. The typical depth of the underlying sediments is several hundred meters. The Meridiani plain's sediments overlay older geological formations that appear around the sediments' boundary. The plain's sediments and surface hematite spherules were formed in three geological epochs and by three different sets of geological processes (more below).
The MER Opportunity rover investigated the rim of Endeavour Crater from August 2011 until the rover's demise in 2018. The plain's sediments do not cover this crater rim and are geologically younger than this rim. As such, the rim of Endeavour Crater is distinct from the plain, although it is surrounded by the plain and its sediments.
19th Century maps The Meridiani Planum was first observed as part of a larger region that appeared as a distinct dark (low albedo) spot in small telescope images of Mars. Around 1830 the earliest Mars map-makers, Johann Heinrich von Mädler and Wilhelm Beer, chose to place the prime meridian for maps of Mars through this dark spot. In the late 1870s, Camille Flammarion called this dark region Sinus Meridiani ("Meridian Bay"). The Meridiani Planum covers the western part of the Sinus Meridiani.
Viking 1 and Viking 2: Smooth terrain, sediments, water The Viking 1 and Viking 2 missions successfully landed the first landers on Mars at locations far away from the Meridiani Planum. However, both missions also included satellites (operating between 1976 and 1982) that took many images of the surface of Mars from orbit. Viking 1 and Viking 2 satellite images of what is today called the Meridiani Planum (and its adjacent regions) were studied in three works in the 1980s Scott, D. H., and Tanaka, K. L., "Geologic map of the western equatorial region of Mars, scale 1:15,000,000," U.S. Geol. Surv. Misc. Inv. Map, I-1802-A, 1986. Greeley, R., and Guest, J. E., "Geologic map of the eastern equatorial region of Mars, scale 1:15,000,000," U.S. Geol. Surv. Misc. Inv. Map, I-I802-B, 1987. and again in two 1997 papers published in the months between the launch of the Mars Global Surveyor mission and its arrival at Mars. Edgett and Parker noted the smooth terrain of what we now call the Meridiani Planum and realized early that the plain was likely made of sediments and probably had a wet, watery past.
Strategy impacting the exploration of Meridiani Planum: Search for water and life In the 1990s, NASA officials, especially Daniel S. Goldin, wanted to delineate a framework for "faster, better, cheaper" exploration of Mars. In this context, the "Water Strategy" was outlined in 1995/1996. The "Water Strategy" was "to explore and study Mars in three areas: - Evidence of past or present life, - Climate (weather, processes, and history), - Resources (environment and utilization)." All three areas were seen as intimately connected to water. High priority goals for NASA in the mid-1990s were to gather some evidence for surface water using satellite surveys and to land robotic rovers on the surface to collect detailed local evidence of water and signs of life.
Global satellite surveys: Surface hematite and water Two NASA missions arrived at Mars in mid-1997: Mars Pathfinder and Mars Global Surveyor. Mars Pathfinder made the first successful Mars landing in over twenty years and the first-ever deployment of a Mars rover, the small, short-lived Sojourner. Mars Global Surveyor surveyed most of the surface of Mars to map its surface topography, some mineral distributions, and make some other measurements.
Hematite, water, plain, life potential: A place to land a rover
An important survey carried out between 1997 and 2002 by the Mars Global Surveyor collected surface hematite levels with the satellite's thermal emissions spectrometer (TES). The TES hematite survey data was turned into the low-resolution map shown in Figure 1a. This map, covering all of Mars, has just one large spot covering a region with high hematite levels. This green, yellow, and red spot straddles the equator and the prime meridian in the middle of Figure 1a. A higher resolution image of the high-hematite region is shown in Figure 1b.
In early the 2000s, the hematite map of Figure 1b and the confirmation (from the topography mapping done by the Mars Global Surveyor) that this area is a flat plain and relatively easy to land on were the decisive pieces of evidence for choosing the Meridiani Planum as one of the landing sites for NASA's two bigger Mars Exploration Rovers (MERs), named Opportunity and Spirit. The decisiveness for NASA of the hematite map of Figure 1b for choosing the landing site for Opportunity was due to the fact that NASA was using high hematite levels as proxy evidence for large amounts of liquid water flowing in the region in the past (hematite only forms in the presence of liquid water in geological settings). In 2003, this high-hematite region was a high-priority place to start to search for signs of life on Mars.
Present day water at Meridiani Planum (satellite evidence)
Since 2001, evidence for water at the present-day Meridiani Planum was collected by the High Energy Neutron Detector (HEND) mounted on the Mars Odyssey orbiter. This neutron detector collects signals of "water-equivalent hydrogen" (WEH) over the entire planet. It gradually built up global maps of surface WEH.Feldman, W.C. (2003). "The global distribution of near-surface hydrogen on Mars." In Sixth Int. Conf. Mars, Lunar and Planet. Inst., Pasadena, Ca, (20–25). These maps show that polar and near-polar regions of Mars have the highest levels of surface WEH; although, the Meridiani Planum has relatively high WEH for a non-polar region. The WEH maps are likely to underestimate the present-day water resources at Meridiani Planum since (a) the HEND has a shallow (1 m) penetration depth, (b) the majority of the plain's surface is covered in dehydrated soils, and hematite spherules.
Starting missions: Opportunity rover and other landers
Starting with Daniel S. Goldin's strategies and NASA's engineering attention to detail, Mars Exploration Rover Opportunity successfully made the "hole-in-one" landing into Eagle Crater at Meridiani Planum on January 24 (PST), 2004. NASA named this landing site the Challenger Memorial Station to honor the final crew of the Space Shuttle Challenger, who died in 1986 when that shuttle broke up in flight.
The Meridiani Planum was also the target landing site for two other missions: Mars Surveyor 2001 Lander and Schiaparelli EDM. However, these other two lander missions were not successful. The Mars Surveyor 2001 Lander was canceled after the failures of the Mars Climate Orbiter and Mars Polar Lander missions, while the Schiaparelli EDM (Entry, Descent, and Landing Demonstrator Module) system lost control during the descent stage and terminally crash-landed on October 19, 2016. (Schiaparelli EDM was part of the ExoMars program of the European Space Agency.)
==Opportunity'''s traverse==
Opportunity traveled 28.06 miles (45.16 kilometers) across the Meridiani Planum and around the rim of Endeavour Crater between January 2004 and June 2018. Figure 2 highlights the rover's traverse route (yellow line). The route's position within the Meridiani Planum is shown by the blue line traverse route labeled "OT" in Figure 1b. The journey started on January 25, 2004, with the landing in Eagle Crater. The rover crossed the plain's sediments and soils and studied many small and medium-sized craters until August 2011, when it reached the rim of the enormous Endeavour Crater. Between August 2011 and June 10, 2018, it studied the rim of Endeavour (which has different geological features from the plain). The rover's last communication with NASA was on June 10, 2018. The Opportunity mission was declared ended on February 13, 2019.
==Opportunitys human-like (Pancam) view of Meridiani Planum==
The Opportunity rover had five cameras. One, the Pancam (panoramic cam), was mounted at the height of 1.5 m, i.e., a height similar to the eye height of most people. The Pancam was used to take scientific data, and it also took images that were approximate true color (ATC) photographs of the rover's surroundings;Pancam Instrument site. This site is an archive of processed images taken by the Pancam (Panoramic Camera) of the MER Opportunity and details on the operation of this camera. The Pancam Instrument site is available at http://pancam.sese.asu.edu . that is, the Pancam photographed images close to what people would see if standing in the rover's place. The following pictures, Figure 3 through Figure 10, provide a selection of images that cover the common and outstanding features a person would see standing at locations along Opportunity's traverse of the Meridiani Planum.
The wide view: A flat plain with smooth or ripple soils
Inside craters: Layered sediments, Burns Cliff
Iron on the ground: Spherules (Blueberries), meteorites
Crater rim overview (large and small)
The dominant visual impressions at eye level are that: Meridiani is a very flat plain. The plain is primarily covered in dark soils but with patches of lighter sediment outcrop. Vast numbers of small bluish-grey hematite spherules (mostly 1 mm - 6 mm in diameter) sit loosely on the soils and sediment outcrop (many more spherules are on soils than outcrop). At most locations, you are close to a visible small crater (5 to 30-meter diameters) - the visible small crater density is 65 to 127 craters per km2 (about 1 per hectare or about 1 per 2.5 acres). (There are plenty of tiny craters with diameters below 5 m.)