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Several landforms reminiscent of shorelines on Earth have been detected from orbit along the boundary of the north–south global dichotomy and close to the foothills of the Tharsis volcanic bulge. Some of these possible shorelines can be traced for hundreds of kilometers, implying the past presence of large expanses of water, with an estimated average depth up to a kilometer. These are extraordinary claims, and if true, they would have major implications for the geological and biological history of Mars. There is no consensus among researchers as yet. In fact, along their length they vary vertically over a range of several hundreds of meters. Clearly, this would not be compatible with a continuous body of water, which would find its own uniform level. Or a more subtle process could explain the observation. The actual water level is controlled by several factors, including the orientation of the planet’s spin and its mass distribution. On Mars, the Tharsis volcanic bulge is large enough for its development to have been able to drive changes in the orientation of the planet’s axis of spin as well as altering the gravitational field. A model that takes these changes into account has shown that the variation of the water surface level is roughly compatible with the observed elevation changes of the supposed shorelines.4 This model also indicates that two distinct shoreline systems could have existed. The older one, dubbed the Arabia shoreline, would reflect water level at a time when the Tharsis bulge was just beginning to develop, perhaps as early as 4 billion years ago. 
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The Arabia ocean would have had an average depth of 1 km. The second system, dubbed the Deuteronilus shoreline, formed a few million years later, when the Tharsis bulge was 80 percent formed. At this time, the ocean would have retreated to an average depth of 0.5 km. The imaging system was composed of three cameras, capable of capturing the surface at an amazing resolution of up to about 1.5 m per pixel. These pictures revealed surprising details. As on Earth, rock outcrops offer the opportunity to study the vertical structure of the crust. On Mars, several outcrops composed of stacks of layers, with thicknesses ranging from a few meters to hundreds of meters, were discovered. The ordering or superposition relationships of these layers were clearly defined, so they could be traced for hundreds of kilometers, as in Terra Meridiani, a region close to the dichotomy boundary east of Valles Marineris. These layers are found over a large expanse of the planet, but are confined mostly within a latitude of about ±30º from the equator, and they appear to be predominantly associated with impact craters. The excavation of rock due to impacts is a primary means of exposing vertical stratigraphy on Mars. Seize the Day
The bulk of these observations provided strong evidence for a sedimentary origin for the layers, implying watery environments. The layered rocks were typically associated with relatively confined regions, suggesting the presence of lakes rather than widespread oceans. And the wide range in the thickness of the layers further indicates that the conditions for deposition were highly variable, possibly suggesting multiple episodes of wet and dry environments early in the history of Mars. At that time, however, it was not possible to entirely rule out other formation processes that did not require the presence of surface water. For instance, it was argued that some of the layering could be the result of drastic changes in the atmospheric pressure. The new wealth of data fueled the scientific debate, and it became clear that definitive answers to the many questions old and new could only be found with a detailed exploration of the Martian surface. The time was ripe to deploy the Martian rovers. The mission’s main goal was to test landing technologies on Mars, and show that this could be done relatively cheaply, at least as compared to the cost of the Viking missions in the 1970s. The payload landed in Ares Vallis, the bottom of an outflow channel. The rover traversed about 100 m, before an anomaly resulted in the end of the mission, seven months after landing. Several Martian missions followed, including three rovers. The interest in this crater stemmed from the fact that it is breached by an outflow channel, which cuts the crater’s southern rim. All Fired Up
It seemed likely that the Gusev Crater hosted a lake at one time, possibly filling it up with sediments to produce the flat appearance. Spirit landed in a basaltic terrain. Nevertheless, under the careful guidance of scientists and engineers, Spirit managed to drive several kilometers across a barren plain to reach a gently undulating terrain, named Columbia Hills. Here the rover found rocks with a clear signature of a watery past, although in small amounts. Spirit’s twin rover, Opportunity, had better luck. Hematite is a common mineral on Earth, the result of water alteration of rocks. This was a promising starting point for Opportunity. Its work showed that the smooth landscape was covered in dust, pretty much the same as most of the planet, and that this dust had signatures of the minerals olivine and pyroxene. The dust had probably originated elsewhere and been transported by winds. On a closer look, rocky outcrops emerged from the sand. Some of the rocks seen at close range hosted little rounded particles, nicknamed ‘.....’ for their appearance, about 0.5 cm across and composed almost entirely of hematite. These rounded particles were responsible for the spectral signature of hematite detected from orbit. It is thought that the blueberries are the result of water percolating through the crust, and they have been concentrated at the surface due to erosion of the host rock. At one spot, on its way to Endeavour Crater, Opportunity spotted a white band in the rock, which was identified as a vein made of the mineral gypsum. In analogy to terrestrial conditions, this vein would have been formed by running water which had trickled through a crack and deposited the gypsum. Overall, Meridiani Planum revealed a surface that was dry most of the time, with windblown sand building up expansive ancient dune fields, but groundwater flowing beneath the surface cemented the sediments into rocks and altered their mineralogy.