This article is about the Mars geologic system and time period. For other senses, see Hesperia.
MOLA colorized relief map of Hesperia Planum, the type area for the Hesperian System. Note that Hesperia Planum has fewer large impact craters than the surrounding Noachian terrain, indicating a younger age. Colors indicate elevation, with red highest, yellow intermediate, and green/blue lowest.
The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today.[1] The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the late heavy bombardment[2] and probably corresponds to the start of the lunar Late Imbrian period,[3][4] around 3700 million years ago (Mya). The end of the Hesperian Period is much more uncertain and could range anywhere from 3200 to 2000 Mya,[5] with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth’s early Archean Eon.[2]
With the decline of heavy impacts at the end of the Noachian, volcanism became the primary geologic process on Mars, producing vast plains of flood basalts and broad volcanic constructs (highland paterae).[6] By Hesperian times, all of the large shield volcanoes on Mars, including Olympus Mons, had begun to form.[7] Volcanic outgassing released large amounts of sulfur dioxide (SO2) and hydrogen sulfide (H2S) into the atmosphere, causing a transition in the style of weathering from dominantly phyllosilicate (clay) to sulfate mineralogy.[8] Liquid water became more localized in extent and turned more acidic as it interacted with SO2 and H2S to form sulfuric acid.[9][10]
By the beginning of the Late Hesperian the atmosphere had probably thinned to its present density.[10] As the planet cooled, groundwater stored in the upper crust (megaregolith) began to freeze, forming a thick cryosphere overlying a deeper zone of liquid water.[11] Subsequent volcanic or tectonic activity occasionally fractured the cryosphere, releasing enormous quantities of deep groundwater to the surface and carving huge outflow channels. Much of this water flowed into the northern hemisphere where it probably pooled to form large transient lakes or an ice covered ocean.
Contents
1Description and name origin
2Hesperian chronology and stratigraphy
2.1System vs. period
2.2Boundaries and subdivisions
3Mars during the Hesperian Period
3.1Impact Cratering
4Notes and references
5Bibliography and recommended reading
Description and name origin[edit]
The Hesperian System and Period is named after Hesperia Planum, a moderately cratered highland region northeast of the Hellas basin. The type area of the Hesperian System is in the Mare Tyrrhenum quadrangle (MC-22) around 20°S245°W / 20°S 245°W / -20; -245. The region consists of rolling, wind-streaked plains with abundant wrinkle ridges resembling those on the lunar maria. These "ridged plains" are interpreted to be basaltic lava flows (flood basalts) that erupted from fissures.[12] The number-density of large impact craters is moderate, with about 125-200 craters greater than 5 km in diameter per million km2.[13][14] Hesperian-aged ridged plains cover roughly 30% of the Martian surface;[2] they are most prominent in Hesperia Planum, Syrtis Major Planum, Lunae Planum, Malea Planum, and the Syria-Solis-Sinai Plana in southernTharsis.[15][16]
The Dalles, Oregon From Wikipedia, the free encyclopedia Jump to navigation Jump to search This article is about the city in Oregon. For the nearby geological formation, see Celilo Falls. For other uses, see Dalles. Not to be confused with the town of Dallas, Oregon. City in Wasco County The Dalles City The Dalles and the Columbia River in November 2008 Flag Seal Motto(s): "Cognito timor Vincit" (Latin), "Knowledge Conquers Fear" (English) Location in Oregon Coordinates: 45°36′4″N 121°10′58″W / 45.60111°N 121.18278°W / 45.60111; -121.18278 Coordinates: 45°36′4″N 121°10′58″W / 45.60111°N 121.18278°W / 45.60111; -121.18278 County Wasco County Incorporated 1857 Government • Mayor Stephen Elliott Lawrence (D) [1] [2] Area [3] • Total 6.61 sq mi (17.12 km 2 ) • Land 6.35 sq mi (16.45 km 2 ) • Water 0.26 sq mi (0.67 km 2 ) Elevation 109 ft (33 m) Population (2010) [4] • Total 13,620 • Estimate (2013) [5]
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