Water in the Deepest Crater of Mars

Abstract

Hellas crater is a potential area where episodes of liquid water may appear. The depth of the structure (7,152 m below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: (11.55 mbar). This is higher than the pressure at the topographical datum (6.1 mbar) and above the triple point of water. That is why PFS and OMEGA, infrared spectrometers installed on Mars Express probe already determined the presence of minerals normally associated with the presence of liquid water on Hellas. Frequency of 1,160 cm⁻¹ has been identified in the spectra of the PFS, that fits very well to the band of sulfates. The band responsible for the occurrence of clay minerals associated with the presence of water on Hellas has been also found in the spectra obtained with the OMEGA instrument.

Appendix: Martian Rock Age

The geological history of Mars has been divided into three periods, and in fact the three eras. From oldest to youngest, these are the Noachian, Hesperian, and Amazonian. These names come from Martian places: Noachian from Noachis Terra, Hesperian from Hesperia Planum and Amazonian from Amazonis Planitia. The duration of these eras is defined by the number of meteorite craters on the surface of Mars. The oldest Martian surface has more craters. The younger surface will have less impact craters. Real-time of eras cannot be determined using this method. In depth studies of isotopes of elements are needed that are used when testing the age of rocks on Earth.

Noachian era dates back to the origins of the planet, about 4.5 billion years ago, and its end is estimated somewhere between 3.8 and 3.5 billion years ago (depending on the adopted model). The surfaces of this age are heavily wounded by many large craters.

During Noachian, Martian atmosphere was thicker than it is now, and the climate is likely to be warm enough to allow large amounts of rainfall. Huge lakes and rivers were present in the southern hemisphere, and the ocean could cover the northern lowland plains. Extensive volcanism took place in the Tharsis region, building a huge mass of volcanic material (Tharsis dome) and released large amounts of gases into the atmosphere. Weathering of rocks on the surface and producing minerals associated with the aquatic environment (clay minerals, carbonates, sulfates, etc.) occurred on a large scale.

In the next, Hesperian era, that lasted somewhere between 3.55 and 1.8 billion years ago, volcanism has become the basic geological processes on Mars, producing massive basalt plains in the form of broad pillow lava structures (highland paterae). During this time, all large volcanoes on Mars, including Olympus Mons, were being formed. Volcanic eruptions produced large amounts of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) to the atmosphere, causing a change in the chemistry of weathering more acid, thereby forming a sulfuric acid.

At the beginning of the late Hesperian, the atmospheric density started to decrease, resulting in a gradual cooling of the planet. Water contained in the upper part of the crust began to freeze to form a cryosphere covering the deeper strata containing liquid water. Expiring volcanic and tectonic activity sometimes caused cryosphere melt, releasing large amounts of water deep underground to the surface and sculpting large drainage channels. Most of this water flowed into the northern hemisphere, where large lakes, partially covered with ice, were probably formed in the estuaries. Basically, Hesperian is treated as a transition period between the end of heavy bombardment and the onset of cold and dry period of Mars.

Finally, the era of the Amazonian covers a period from about 1.8 billion years ago to today. The surfaces of the era of the Amazonian have few meteorite craters, but are quite varied. After this, the “martiancreation” processes were slowly dying, and glacial aeolian processes have been taking their place, forming landslides in Valles Marineris (such as in Gangis Chasma) on the broad plains of sand dunes near the poles of Mars. Large parts of Mars started to be covered by ice caps. Climate changed to dry and cold, and that state continues to the present day (Nimmo and Tanaka 2005; Hartmann 2005).