In general, an opinion is a subjective belief, and is the result of emotion or interpretation of facts. An opinion may be supported by an argument, although people may draw opposing opinions from the same set of facts. Opinions rarely change without new arguments being presented. However, it can be reasoned that one opinion is better supported by the facts than another by analysing the supporting arguments. In casual use, the term opinion may be the result of a person's perspective, understanding, particular feelings, beliefs, and desires. It may refer to unsubstantiated information, in contrast to knowledge and fact-based beliefs.

Collective or professional opinions are defined as meeting a higher standard to substantiate the opinion. (see below)

In economics, other social sciences and philosophy, analysis based on opinion is referred to as normative analysis (what ought to be), as opposed to positive analysis, which is based on scientific observation (what materially is or is experimentally demonstrable).

Mars is the fourth planet from the Sun in the Solar System. Named after the Roman god of war, Mars, it is often described as the "Red Planet" as the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the highest known mountain within the Solar System, and of Valles Marineris, the largest canyon. The smooth Borealis basin in the northern hemisphere covers 40% of the planet and may be a giant impact feature.

Until the first successful flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquid water on the planet's surface. This was based on observed periodic variations in light and dark patches, particularly in the polar latitudes, which appeared to be seas and continents; long, dark striations were interpreted by some as irrigation channels for liquid water. These straight line features were later explained as optical illusions, though geological evidence gathered by unmanned missions suggest that Mars once had large-scale water coverage on its surface. In 2005, radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.