Hesperian
Encyclopedia
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. The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the late heavy bombardment
and probably corresponds to the start of the lunar Late Imbrian period, 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, with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth’s early Archean
Eon.
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 basalt
s and broad volcanic constructs (highland paterae). By Hesperian times, all of the large shield volcano
es on Mars, including Olympus Mons
, had begun to form. 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
. Liquid water became more localized in extent and turned more acidic as it interacted with SO2 and H2S to form sulfuric acid
.
By the beginning of the Late Hesperian the atmosphere had probably thinned to its present density. 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. 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.
of surface units from spacecraft images
. A surface unit is a terrain with a distinct texture, color, albedo
, spectral
property, or set of landforms that distinguish it from other surface units and is large enough to be shown on a map. Mappers use a stratigraphic
approach pioneered in the early 1960s for photogeologic studies of the Moon
. Although based on surface characteristics, a surface unit is not the surface itself or group of landform
s. It is an inferred geologic unit (e.g., formation) representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface. A surface unit may be a crater ejecta deposit, lava flow, or any surface that can be represented in three dimensions as a discrete stratum
bound above or below by adjacent units (illustrated right). Using principles such as superpositioning
(illustrated left), cross-cutting relationships, and the relationship of impact crater density to age, geologists can place the units into an relative age
sequence from oldest to youngest. Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic
) units, called systems. For Mars, three systems are defined: the Noachian, Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designated Pre-Noachian. The geologic time (geochronologic
) equivalent of the Hesperian System is the Hesperian Period. Rock or surface units of the Hesperian System were formed or deposited during the Hesperian Period.
Martian Time Periods (Millions of Years Ago)
, 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 lat. 20°S, long. 115°E. 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 basalt
s) that erupted from fissures. The number-density of large impact craters is moderate, with about 125-200 craters greater than 5 km in diameter per million km2. Hesperian-aged ridged plains cover roughly 30% of the Martian surface; they are most prominent in Hesperia Planum, Syrtis Major Planum, Lunae Planum, Malea Planum, and the Syria-Solis-Sinai Plana in southern Tharsis
.
based on the physical rock record of a type area
(type section) correlated with rocks sections from many different locations planetwide. A system is bound above and below by strata
with distinctly different characteristics (on Earth, usually index fossil
s) that indicate dramatic (often abrupt) changes in the dominant fauna or environmental conditions. (See K-T Boundary as example.)
At any location, rock sections in a given system are apt to contain gaps (unconformities
) analogous to missing pages from a book. In some places, rocks from the system are absent entirely due to nondeposition or later erosion. For example, rocks of the Cretaceous
System are absent throughout much of the eastern central interior of the United States. However, the time interval of the Cretaceous (Cretaceous Period) still occurred there. Thus, a geologic period represents the time interval over which the strata
of a system were deposited, including any unknown amounts of time present in gaps. Periods are measured in years, determined by radioactive dating. On Mars, radiometric ages are not available except from Martian meteorites whose provenance
and stratigraphic context are unknown. Instead, absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models
of crater formation over time. Accordingly, the beginning and end dates for Martian periods are uncertain, especially for Hesperian/Amazonian boundary, which may be in error by a factor of 2 or 3.
is a vast, low-lying plain that covers much of the northern hemisphere of Mars. It is generally interpreted to consist of reworked sediments originating from the Late Hesperian outflow channels and may be the remnant of an ocean that covered the northern lowland basins. Another interpretation of the Vastitas Borealis Formation is that it consists of lava flows.
The Hesperian System is subdivided into two chronostratigraphic series
: Lower Hesperian and Upper Hesperian. The series are based on referents or locations on the planet where surface units indicate a distinctive geological episode, recognizable in time by cratering age and stratigraphic position. For example, Hesperia Planum is the referent location for the Lower Hesperian Series. The corresponding geologic time (geochronological) units of the two Hesperian series are the Early Hesperian and Late Hesperian Epochs
. Note that an epoch is a subdivision of a period; the two terms are not synonymous in formal stratigraphy. The age of the Early Hepserian/Late Hesperian boundary is uncertain, ranging from 3600 to 3200 million years ago based on crater counts. The average of the range is shown in the timeline below.
Hesperian Epochs (Millions of Years Ago)
Stratigraphic terms are typically confusing to geologists and non-geologists alike. One way to sort through the difficulty is by the following example: You could easily go to Cincinnati, Ohio
and visit a rock outcrop
in the Upper Ordovician
Series of the Ordovician System. You could even collect a fossil trilobite
there. However, you could not visit the Late Ordovician Epoch in the Ordovician Period and collect an actual trilobite (unless you have a time machine
).
The Earth-based scheme of rigid stratigraphic nomenclature has been successfully applied to Mars for several decades now but has numerous flaws. The scheme will no doubt become refined or replaced as more and better data become available. (See mineralogical timeline below as example of alternative.) Obtaining radiometric ages on samples from identified surface units is clearly necessary for a more complete understanding of Martian chronology.
features on Mars formed at this time. The weight of the immense Tharsis Bulge
stressed the crust to produce a vast network of extensional fractures (fossae
) and compressive deformational features (wrinkle ridges) throughout the western hemisphere. The huge equatorial canyon system of Valles Marineris
formed during the Hesperian as a result of these stresses. Sulfuric-acid weathering at the surface produced an abundance of sulfate minerals that precipitated in evaporitic environments
, which became widespread as the planet grew increasingly arid. The Hesperian Period was also a time when the earliest evidence of glacial activity and ice-related processes appears in the Martian geologic record.
. The Hesperian was thus a time period of rapidly declining impact cratering rates. However, the timing and rate of the decline are uncertain. The lunar cratering record suggests that the rate of impacts in the inner Solar System
during the Noachian
(4000 million years ago) was 500 times higher than today. Planetary scientists still debate whether these high rates represent the tail end of planetary accretion
or a late cataclysmic pulse that followed a more quiescent period of impact activity. Nevertheless, at the beginning of the Hesperian, the impact rate had probably declined to about 80 times greater than present rates, and by the end of the Hesperian, some 700 million years later, the rate began to resemble that seen today.
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is 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...
characterized by widespread volcanic activity
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
and catastrophic flooding that carved immense outflow channels
Outflow channels
Outflow channels is the term used to describe extremely long, wide swathes of scoured ground on Mars, commonly containing the streamlined remnants of pre-existing topography and other linear erosive features indicating sculpting by fluids moving downslope...
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. The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the late heavy bombardment
Late Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
and probably corresponds to the start of the lunar Late Imbrian period, 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, with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth’s early Archean
Archean
The Archean , also spelled Archeozoic or Archæozoic) is a geologic eon before the Paleoproterozoic Era of the Proterozoic Eon, before 2.5 Ga ago. Instead of being based on stratigraphy, this date is defined chronometrically...
Eon.
With the decline of heavy impacts at the end of the Noachian, volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....
became the primary geologic process on Mars, producing vast plains of flood basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s and broad volcanic constructs (highland paterae). By Hesperian times, all of the large shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
es on Mars, including Olympus Mons
Olympus Mons
Olympus Mons is a large volcanic mountain on the planet Mars. At a height of almost , it is one of the tallest mountains in the Solar System, three times as tall as Mount Everest and more than twice the height of Mauna Kea the tallest mountain on Earth. Olympus Mons is the youngest of the large...
, had begun to form. Volcanic outgassing released large amounts of sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
(SO2) and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
(H2S) into the atmosphere, causing a transition in the style of weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
from dominantly phyllosilicate (clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
) to sulfate
Sulfate
In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
. Liquid water became more localized in extent and turned more acidic as it interacted with SO2 and H2S to form sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
.
By the beginning of the Late Hesperian the atmosphere had probably thinned to its present density. As the planet cooled, groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
stored in the upper crust (megaregolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
) began to freeze, forming a thick cryosphere
Cryosphere
The cryosphere is the term which collectively describes the portions of the Earth’s surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps and ice sheets, and frozen ground . Thus there is a wide overlap with the hydrosphere...
overlying a deeper zone of liquid water. Subsequent volcanic or tectonic activity occasionally fractured the cryosphere, releasing enormous quantities of deep groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
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.
Hesperian chronology and stratigraphy
Martian time periods are based on geologic mappingGeologic map
A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...
of surface units from spacecraft images
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
. A surface unit is a terrain with a distinct texture, color, albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
, spectral
Thermal infrared spectroscopy
Thermal infrared spectroscopy is the subset of infrared spectroscopy that deals with radiation emitted in the infrared part of the electromagnetic spectrum. The emitted infrared radiation, though similar to blackbody radiation, is different in that the radiation is banded at characteristic...
property, or set of landforms that distinguish it from other surface units and is large enough to be shown on a map. Mappers use a stratigraphic
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
approach pioneered in the early 1960s for photogeologic studies of the Moon
Geology of the Moon
The geology of the Moon is quite different from that of the Earth...
. Although based on surface characteristics, a surface unit is not the surface itself or group of landform
Landform
A landform or physical feature in the earth sciences and geology sub-fields, comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography...
s. It is an inferred geologic unit (e.g., formation) representing a sheetlike, wedgelike, or tabular body of rock that underlies the surface. A surface unit may be a crater ejecta deposit, lava flow, or any surface that can be represented in three dimensions as a discrete stratum
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
bound above or below by adjacent units (illustrated right). Using principles such as superpositioning
Law of superposition
The law of superposition is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences:...
(illustrated left), cross-cutting relationships, and the relationship of impact crater density to age, geologists can place the units into an relative age
Relative dating
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...
sequence from oldest to youngest. Units of similar age are grouped globally into larger, time-stratigraphic (chronostratigraphic
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of...
) units, called systems. For Mars, three systems are defined: the Noachian, Hesperian, and Amazonian. Geologic units lying below (older than) the Noachian are informally designated Pre-Noachian. The geologic time (geochronologic
Geochronology
Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating methods are used by geologists to achieve this, and schemes of classification and terminology have been proposed...
) equivalent of the Hesperian System is the Hesperian Period. Rock or surface units of the Hesperian System were formed or deposited during the Hesperian Period.
Description and name origin
The Hesperian System is named after Hesperia PlanumHesperia Planum
Hesperia Planum is a broad lava plain in the southern highlands of the planet Mars. The plain is notable for its moderate number of impact craters and abundant wrinkle ridges. It is also the location of the ancient volcano Tyrrhena Mons...
, a moderately cratered highland region northeast of the Hellas
Hellas Planitia
Hellas Planitia, also known as the Hellas Impact Basin, is a huge, roughly circular impact basin located in the southern hemisphere of the planet Mars. It is the second or third largest impact crater and the largest visible impact crater known in the Solar System...
basin. The type area
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
of the Hesperian System is in the Mare Tyrrhenum quadrangle
Mare Tyrrhenum quadrangle
The Mare Tyrrhenum quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey Astrogeology Research Program. The Mare Tyrrhenum quadrangle is also referred to as MC-22 ....
(MC-22) around lat. 20°S, long. 115°E. 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 basalt
Flood basalt
A flood basalt or trap basalt is the result of a giant volcanic eruption or series of eruptions that coats large stretches of land or the ocean floor with basalt lava. Flood basalts have occurred on continental scales in prehistory, creating great plateaus and mountain ranges...
s) that erupted from fissures. The number-density of large impact craters is moderate, with about 125-200 craters greater than 5 km in diameter per million km2. Hesperian-aged ridged plains cover roughly 30% of the Martian surface; they are most prominent in Hesperia Planum, Syrtis Major Planum, Lunae Planum, Malea Planum, and the Syria-Solis-Sinai Plana in southern Tharsis
Tharsis
The Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as...
.
System vs. period
System and period are not interchangeable terms in formal stratigraphic nomenclature, although they are frequently confused in popular literature. A system is an idealized stratigraphic columnGeologic record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...
based on the physical rock record of a type area
Type locality (geology)
Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....
(type section) correlated with rocks sections from many different locations planetwide. A system is bound above and below by strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
with distinctly different characteristics (on Earth, usually index fossil
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...
s) that indicate dramatic (often abrupt) changes in the dominant fauna or environmental conditions. (See K-T Boundary as example.)
At any location, rock sections in a given system are apt to contain gaps (unconformities
Unconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
) analogous to missing pages from a book. In some places, rocks from the system are absent entirely due to nondeposition or later erosion. For example, rocks of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
System are absent throughout much of the eastern central interior of the United States. However, the time interval of the Cretaceous (Cretaceous Period) still occurred there. Thus, a geologic period represents the time interval over which the strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
of a system were deposited, including any unknown amounts of time present in gaps. Periods are measured in years, determined by radioactive dating. On Mars, radiometric ages are not available except from Martian meteorites whose provenance
Provenance
Provenance, from the French provenir, "to come from", refers to the chronology of the ownership or location of an historical object. The term was originally mostly used for works of art, but is now used in similar senses in a wide range of fields, including science and computing...
and stratigraphic context are unknown. Instead, absolute ages on Mars are determined by impact crater density, which is heavily dependent upon models
Scientific modelling
Scientific modelling is the process of generating abstract, conceptual, graphical and/or mathematical models. Science offers a growing collection of methods, techniques and theory about all kinds of specialized scientific modelling...
of crater formation over time. Accordingly, the beginning and end dates for Martian periods are uncertain, especially for Hesperian/Amazonian boundary, which may be in error by a factor of 2 or 3.
Boundaries and subdivisions
The lower boundary of the Hesperian System is defined as the base of the ridged plains, which are typified by Hesperia Planum and cover about a third of the planet’s surface. In eastern Hesperia Planum, the ridged plains overlie early to mid Noachian aged cratered plateau materials (pictured left). The Hesperian’s upper boundary is more complex and has been redefined several times based on increasingly detailed geologic mapping. Currently, the stratigraphic boundary of the Hesperian with the younger Amazonian System is defined as the base of the Vastitas Borealis Formation (pictured right). The Vastitas BorealisVastitas Borealis
Vastitas Borealis is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the Northern plains or Northern lowlands of Mars. The plains lie 4–5 km below the mean radius of...
is a vast, low-lying plain that covers much of the northern hemisphere of Mars. It is generally interpreted to consist of reworked sediments originating from the Late Hesperian outflow channels and may be the remnant of an ocean that covered the northern lowland basins. Another interpretation of the Vastitas Borealis Formation is that it consists of lava flows.
The Hesperian System is subdivided into two chronostratigraphic series
Series (stratigraphy)
Series are subdivisions of rock layers made based on the age of the rock and corresponding to the dating system unit called an epoch, both being formally defined international conventions of the geological timescale. A series is therefore a sequence of rock depositions defining a...
: Lower Hesperian and Upper Hesperian. The series are based on referents or locations on the planet where surface units indicate a distinctive geological episode, recognizable in time by cratering age and stratigraphic position. For example, Hesperia Planum is the referent location for the Lower Hesperian Series. The corresponding geologic time (geochronological) units of the two Hesperian series are the Early Hesperian and Late Hesperian Epochs
Epoch (geology)
An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...
. Note that an epoch is a subdivision of a period; the two terms are not synonymous in formal stratigraphy. The age of the Early Hepserian/Late Hesperian boundary is uncertain, ranging from 3600 to 3200 million years ago based on crater counts. The average of the range is shown in the timeline below.
Stratigraphic terms are typically confusing to geologists and non-geologists alike. One way to sort through the difficulty is by the following example: You could easily go to Cincinnati, Ohio
Cincinnati, Ohio
Cincinnati is a city in the U.S. state of Ohio. Cincinnati is the county seat of Hamilton County. Settled in 1788, the city is located to north of the Ohio River at the Ohio-Kentucky border, near Indiana. The population within city limits is 296,943 according to the 2010 census, making it Ohio's...
and visit a rock outcrop
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
in the Upper Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
Series of the Ordovician System. You could even collect a fossil trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
there. However, you could not visit the Late Ordovician Epoch in the Ordovician Period and collect an actual trilobite (unless you have a time machine
Time Machine
A time machine is a fictional/hypothetical device used to achieve time travel. The term may also refer to:-Novels and films:* The Time Machine, an 1895 novel by H. G...
).
The Earth-based scheme of rigid stratigraphic nomenclature has been successfully applied to Mars for several decades now but has numerous flaws. The scheme will no doubt become refined or replaced as more and better data become available. (See mineralogical timeline below as example of alternative.) Obtaining radiometric ages on samples from identified surface units is clearly necessary for a more complete understanding of Martian chronology.
Mars during the Hesperian Period
The Hesperian was a time of declining rates of impact cratering, intense and widespread volcanic activity, and catastrophic flooding. Many of the major tectonicTectonics
Tectonics is a field of study within geology concerned generally with the structures within the lithosphere of the Earth and particularly with the forces and movements that have operated in a region to create these structures.Tectonics is concerned with the orogenies and tectonic development of...
features on Mars formed at this time. The weight of the immense Tharsis Bulge
Tharsis
The Tharsis region on Mars is a vast volcanic plateau centered near the equator in Mars’ western hemisphere. The region is home to the largest volcanoes in the Solar System, including the three enormous shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons, which are collectively known as...
stressed the crust to produce a vast network of extensional fractures (fossae
Fossa (geology)
Fossa is a term used in planetary geology to describe a long, narrow depression on the surface of an extraterrestrial body, such as a planet or moon...
) and compressive deformational features (wrinkle ridges) throughout the western hemisphere. The huge equatorial canyon system of Valles Marineris
Valles Marineris
Valles Marineris is a system of canyons that runs along the Martian surface east of the Tharsis region...
formed during the Hesperian as a result of these stresses. Sulfuric-acid weathering at the surface produced an abundance of sulfate minerals that precipitated in evaporitic environments
Evaporite
Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...
, which became widespread as the planet grew increasingly arid. The Hesperian Period was also a time when the earliest evidence of glacial activity and ice-related processes appears in the Martian geologic record.
Impact Cratering
As originally conceived, the Hesperian System referred to the oldest surfaces on Mars that postdate the end of heavy bombardmentLate Heavy Bombardment
The Late Heavy Bombardment is a period of time approximately 4.1 to 3.8 billion years ago during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, Mercury, Venus, and Mars as well...
. The Hesperian was thus a time period of rapidly declining impact cratering rates. However, the timing and rate of the decline are uncertain. The lunar cratering record suggests that the rate of impacts in the inner Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
during the Noachian
Noachian
The Noachian is a geologic system and early time period on the planet Mars characterized by high rates of meteorite and asteroid impacts and the presence of abundant surface water...
(4000 million years ago) was 500 times higher than today. Planetary scientists still debate whether these high rates represent the tail end of planetary accretion
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
or a late cataclysmic pulse that followed a more quiescent period of impact activity. Nevertheless, at the beginning of the Hesperian, the impact rate had probably declined to about 80 times greater than present rates, and by the end of the Hesperian, some 700 million years later, the rate began to resemble that seen today.