The mission was led by Project Manager Peter Theisinger of NASA's Jet Propulsion Laboratory and Principal Investigator Steven Squyres, professor of astronomy at Cornell University.

Primary among the mission's scientific goals is to search for and characterize a wide range of rocks and soils that hold clues to past water activity on Mars.

In recognition of the vast amount of scientific information amassed by both rovers, two asteroids have been named in their honor: 37452 Spirit and 39382 Opportunity.

[1][2][3][4] The search for evidence of habitability, taphonomy (related to fossils), and organic carbon on the planet Mars is now a primary NASA objective.

[1] On March 2, 2004, NASA announced that "Opportunity has landed in an area of Mars where liquid water once drenched the surface".

Associate administrator Ed Weiler told reporters that the area "would have been good habitable environment", although no traces of life have been found.

This statement was made during a press conference, where mission scientists listed a number of observations that strongly support this view: Detailed analysis of environmental, chemical, and mineralogical data taken from the Opportunity rover led to the elimination of the competing hypotheses, and the confirmation of the conclusion that the spherules were formed in place as post-depositional sedimentary concretions from an aqueous source[5] On March 23, 2004, NASA announced that they believe that Opportunity had not landed in a location merely "drenched in water", but on what was once a coastal area.

The evidence for water was published in a series of scientific papers, with the initial results appearing in the journal Science[6] and then with a detailed discussion of the sedimentary geology of the landing site appearing in a special issue of the journal Earth and Planetary Science Letters[7] Early in the mission, mission scientists were able to prove that the abundant spherules at Eagle crater were the source of hematite in the area discovered from orbit.

Geologists were eager to reach a hematite-rich area (in the center of the picture at right) to closely examine the soil, which may reveal secrets about how the hematite got to this location.

"Grey hematite is a mineral indicator of past water," said Dr. Joy Crisp, JPL project scientist.

"We want to know if the grains of hematite appear to be rounded and cemented together by the action of liquid water or if they're crystals that grew from a volcanic melt," said Crisp.

The Mini-TES spectral data was analyzed in a way that the concentration of the mineral hematite was deduced and its level coded in color.

The next picture shows a hematite abundance "index map" that helps geologists choose hematite-rich locations to visit around Opportunity's landing site.

During Sol 23 (February 16) Opportunity successfully trenched the soil at Hematite Slope and started to investigate the details of the layering.

When Opportunity dug her first trench (Sol 23), pictures of the lower layers showed similar round spherules.

"They appear shiny or polished," said Albert Yen, science team member, during a press conference on February 19.

This supports the notion that they grew in place, since if their origin was related to volcanic or meteoric episodes one would expect layers of spherules as a "record in time" for each event.

This observation was added to the list of evidence for liquid water being present at this rock site, where it is thought the spherules formed.

They contain the minerals olivine, pyroxene, plagioclase, and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes (geologists would say they have vesicles and vugs).

[8] Plain's rocks have been very slightly altered, probably by thin films of water because they are softer and contain veins of light colored material that may be bromine compounds, as well as coatings or rinds.

One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron.

[24] Also, MB found that much of the iron in Paso Robles soil was of the oxidized, Fe+++ form, which would happen if water had been present.

[25] After Spirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich rocks, which means that regions of the planet may have once harbored water.

During a press conference on March 11, 2004, mission scientists presented the first temperature profile of the martian atmosphere ever measured.

Atmospheric science from the MER rovers has been published in a series of scientific papers in Science[28][29] and Journal of Geophysical Research[30] Opportunity observed the eclipse, or transits of Phobos and transits of Deimos across the Sun,[31] and photographed the Earth, which appeared as a bright celestial object in the Martian sky.