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 (82,000 km2 (32,000 sq mi)) 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
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.
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