**2. Study area**

It must be emphasized that Mars was formed through an accretion process from volatile-poor planetesimals in the inner solar nebula. The relatively small size of Mars indicates possible Jovian interference during the accretion process. The accretion process and the decay of radioisotopes released gravitational potential energy that melted the Martian protoplanet. Both events led to differentiation into a core–

It is known from Earth analog studies that most of the tungsten, iron, and other metals sink to the core of a planet in its molten state early in its formation. From radioisotope ratio studies of the Martian meteorites, evidence supports the hypothesis that Mars formed in 2 to 4 million years. However, recent research suggests that tungsten and platinum on the planetesimals themselves could have altered the Martian crust ratios of tungsten, iron, and other metals. The provenance of the two hundred meteorites found on Earth does not provide conclusive evidence of the Martian surface's composition. The latest literature suggests that it is most likely that the small sample of meteorites comes from a few impacts. Those few impacts render the sample size too small to draw accurate conclusions of Martian formation. An alternate hypothesis states that large projectiles with formed cores and mantles bombarded the red planet [4]. According to new modeling, the planetesimals' composition varied enough in ferrophilic element content to provide the tungsten variance, resulting in a heterogeneous Martian mantle. This alternate hypothesis

On the other hand, and in relation to the Mars atmosphere, the Mariner mission

Regarding the geology of Mars, it is interesting to comment that this planet has two vastly different hemispheres (**Figure 1**), that is, the older heavily cratered highlands of the Southern Hemisphere and the Northern Hemisphere's younger lowland plains. Mars presents extreme topography when compared to Earth and even the Moon. A marked 30-kilometer elevation difference exists between the summit of Olympus Mons and the floor of the Hellas Basin. The hemispheres' difference can be explained by possible volcanic eruptions or seas that smoothed out the Northern Hemisphere. In addition to the highland terrains and the lowland

offers an estimate of a Martian formation period of up to 20 Mya [5].

revealed a fine Martian atmosphere that varies greatly. Because it is thin, the atmosphere of this planet expands rapidly in the warmer months and contracts rapidly in the colder months [6]. In general, it can be said that the atmosphere of the red planet is similar in composition to that of Venus but much thinner. Carbon dioxide comprises about 95% of the atmosphere, with the rest being nitrogen (3%) and argon (1%). The carbon dioxide freezes over the poles, and frozen water lays

mantle–crust planet structure [2, 3].

*Solar System Planets and Exoplanets*

underneath the frozen carbon dioxide layer.

**Figure 1.**

**78**

*MOLA global image of Mars surface [7].*

As is known, Opportunity (Mars Exploration Rover-B) landed in the Eagle crater, located in the Meridiani Planum (0,2° N; 357,5° E in planetocentric coordinates), on January 25, 2004.

The reason why Opportunity was sent to Meridiani Planum was because the Thermal Emission Spectrometer (TES), of the Mars Global Surveyor mission, found, from its orbit, crystalline gray hematite on the surface of Mars in an amount around 20%. Hematite is an iron oxide that, usually its gray crystalline variety, is formed in association with liquid water on Earth.

According to [8], at Opportunity's landing site, gray hematite within a kind of spherules was found in outcrops of soft and stratified sandstone rocks. It should be emphasized that for these structures to form, the acidic aqueous alteration of basalt rocks rich in goethite (a mineral that contains iron) was necessary. Subsequently, the alteration of the goethite gave rise to hematite, which formed spherules in the rocks and, as these were worn away by the action of acidic water, they accumulated on the surface.

In relation to what was previously specified, and based on [9, 10], it is interesting to say that the soil of Meridiani Planum is composed of fine grains of basalt sand, in addition to a surface of spherules, with a high content of hematite, and other granules. According to [9], the erosion by action of the wind is visible, as well as small impacts of craters and layers of sedimentary rocks, finely laminated, rich in sulfides [10] and sulphated salts. Regarding cross lamination, it is known that on small scales it provides evidence that liquid water flowed through the study area. These rocks were probably a mixture of siliciclastic and chemical sediments formed in an episode of shallow water flooding followed by evaporation, exposure, and settling, similar to what occurs in a salt-marsh on Earth [11].
