# Planets/Geology

Planetary geology is the geology of astronomical objects apparently in orbit around one or more stellar objects within a few light years.
Planetary geology may also be thought of as the geology of an astronomical object that is due to its planetary nature:
* approximate hydrostatic equilibrium (apparently spherical shape),
* a cleared orbital path, and
* an orbit around one or more stellar objects.

## Planetary astronomy

Planetary geology, astrogeology or exogeology, are planetary sciences concerned with the geology of the celestial bodies such as the planets and their moons, asteroids, comets, and meteorites. It includes determining the internal structure of the terrestrial planets, planetary volcanism and surface processes such as impact craters, fluvial and aeolian processes.

## Geology

Def. the intellectual and practical activity encompassing the systematic study through observation and experiment of the Earth's physical structure and substance, its history and origin, and the processes that act on it, especially by examination of its rocks, is called geology.

## Planets

Def. "a celestial body that
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit" is called a planet.
Def. a wanderer that is a moving light in the sky is called a planet.
This is the original description meant by the word "planet".
Def. a celestial body "formed by accumulation of a rocky core, on a much longer timescale, ≳ 10$^7$ yr, with subsequent acquisition of a gaseous envelope if the circumstances allow this, and with an initially fractionated elemental composition" is called a planet.

## Theoretical planetary geology

Def. a nearly round or spherically shaped, astronomical object that appears to have cleared its orbit around one or more stellar objects less than a few light years away is called a planet, or planet-like object.
Def. the geology of matching up objects on or in planetary, or planet-like, objects to suggest or infer parallel evolution is called planetary geology.
Def. the astronomy of observing on or in astronomical objects so as to geologically match up likely parallel evolution is called astrogeology.
Planetary geology is focused on the effects to an astronomical object, source, or entity from being in an orbit around any astronomical object, source, or entity less than a few light years away.

## Entities

Def. any "considerable and connected part of a space or surface; specifically, a tract of land or sea of considerable but indefinite extent; a country; a district; in a broad sense, a place without special reference to location or extent but viewed as an entity for geographical, social or cultural reasons" is called a region.

## Sources

Oblique images such as the one at the top right are taken by astronauts looking out from the ISS at an angle, rather than looking straight downward toward the Earth (a perspective called a nadir view), as is common with most remotely sensed data from satellites. An oblique view gives the scene a more three-dimension quality, and provides a look at the vertical structure of the volcanic plume. While much of the island is covered in green vegetation, grey deposits that include pyroclastic flows and volcanic mud-flows (lahars) are visible extending from the volcano toward the coastline. When compared to its extent in earlier views, the volcanic debris has filled in more of the eastern coastline. Urban areas are visible in the northern and western portions of the island; they are recognizable by linear street patterns and the presence of bright building rooftops. The silver-grey appearance of the Caribbean Sea surface is due to sun-glint, which is the mirror-like reflection of sunlight off the water surface back towards the hand-held camera on-board the ISS. The sun-glint highlights surface wave patterns around the island.

## Objects

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. Foliation may not correspond to the original sedimentary layering, but instead is in planes perpendicular to the direction of metamorphic compression. Slate is frequently grey in color, especially when seen, en masse, covering roofs. However, slate occurs in a variety of colors even from a single locality; for example, slate from North Wales can be found in many shades of grey, from pale to dark, and may also be purple, green or cyan.

## Emissions

As the image at the right and those in the sources section shows, volcanoes are a source of emissions.

## Bands

In contrast to the Proterozoic, Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments and banded iron formations.
Greenstone belts are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic island arcs, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. In short, greenstone belts represent sutured protocontinents.
Greenstone belts are zones of variably metamorphosed mafic to ultramafic volcanic sequences with associated sedimentary rocks that occur within Archaean and Proterozoic cratons between granite and gneiss bodies.
The name comes from the green hue imparted by the colour of the metamorphic minerals within the mafic rocks. Chlorite, actinolite and other green amphiboles are the typical green minerals.
A greenstone belt is typically several dozens to several thousand kilometres long and although composed of a great variety of individual rock units, is considered a 'stratigraphic grouping' in its own right, at least on continental scales.
"Greenstone belts" are distributed throughout geological history from the Phanerozoic Franciscan belts of California where blueschist, whiteschist and greenschist facies are recognised, through to the Palaeozoic greenstone belts of the Lachlan Fold Belt, Eastern Australia, and a multitude of Proterozoic and Archaean examples.
Archaean greenstones are found in the Slave craton, northern Canada, Pilbara craton and Yilgarn Craton, Western Australia, Gawler Craton in South Australia, and in the Wyoming Craton in the US. Examples are found in South and Eastern Africa, namely the Kaapvaal craton and also in the cratonic core of Madagascar, as well as West Africa and Brazil, northern Scandinavia and the Kola Peninsula (see Baltic Shield).
Phanerozoic ophiolite belts and greenstone belts occur in the Franciscan Complex of south-western North America, within the Lachlan Fold Belt, the Gympie Terrane of Eastern Australia, the ophiolite belts of Oman and around the Guiana Shield.

## Meteors

"Spectral properties of certain palagonitic soils found on Mauna Kea, Hawaii are similar to the spectral properties measured by earth-based telescopes for Martian soils [1,2,3]. ... Three layers with distinctly different colors (upper red, middle black, lower yellow) were sampled from hydrothermally altered basaltic tephra just below the summit of Mauna Kea."
"The clay fractions (< 2 µm) of three palagonite samples-MK11 (red), MK12 (black), and MK13 (yellow) collected at an elevation of 4145 meters near the summit of Mauna Kea volcano in Hawaii ... The fine fractions of the black (MK12) and yellow (MK13) samples were similar to those of martian bright regions in terms of their overall shape."
"Geologic evidence suggests that large amounts of water have likely flowed on Earth for the past 3.8 billion years—most of its existence. [It is] [b]elieved to have initially arrived on the surface through the emissions of ancient volcanoes".

## Visuals

Def. a "portion of land or territory which the eye can comprehend in a single view, including all the objects it contains" is called a landscape.
Def. each continuous surface of a landscape that is observable in its entirety and has consistence of form or regular change of form is called a landform.
Def. "[t]he study of landforms, their classification, origin, development, and history" is called geomorphology.

## Reds

"A higher-reflectance [HR], relatively red material occurs [on Mercury] as a distinct class of smooth plains [P] that were likely emplaced volcanically; a lower-reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals, possibly more common at depth."
"The distinctively red smooth plains (HRP) appear to be large-scale volcanic deposits stratigraphically equivalent to the lunar maria (20), and their spectral properties (steeper spectral slope) are consistent with magma depleted in opaque materials. The large areal extent (>10$^6$ km$^2$) of the Caloris HRP is inconsistent with the hypothesis that volcanism was probably shallow and local (10); rather, such volcanism was likely a product of extensive partial melting of the upper mantle."
"Despite the dearth of ferrous iron in silicates, Mercury's surface nonetheless darkens and reddens with time like that of the Moon. This darkening and reddening has been interpreted to be the result of production of nanophase iron (e.g., Pieters et al., 2000; Hapke, 2001), which could be derived from an opaque phase in the crustal material or from delivery by micrometeorite impacts (Noble and Pieters, 2003). On the Moon, deposits that are brighter and redder than the average Moon spectrum appear to be lower in iron (e.g., highland material); deposits that are darker and redder than average are higher in iron (e.g., low-Ti mare material) (Lucey et al., 1995)."

## Rocky objects

Def. a solid, or rocky, surface of an astronomical rocky object is called land.
Def.
* a "single, distinctive rock formation",
* "an area having a preponderance of a particular rock or group of rocks", or
* an "area of land or the particular features of it"
is called a terrain.
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun. The terms are derived from Latin words for Earth (Terra and Tellus), so these planets are, in a certain way, "Earth-like". Terrestrial planets have solid planetary surface making them substantially different from gas giants, which are composed mostly of some combination of hydrogen, helium, and water existing in various physical states.
Petrology is a branch of geology that studies rocks, and the conditions in which rocks form. Lithology focuses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography deals with microscopic details. Petrology benefits from mineralogy, optical mineralogy, geochemistry, and geophysics. Three branches of petrology focus on the three major rock types: igneous petrology, metamorphic petrology, and sedimentary petrology.

## Materials

Def. "[t]he study of ice and its effect on the landscape, especially the study of glaciers" is called glaciology.

## Shelters

The location of Mt. Hibok-Hibok volcano imaged at the right is on the Camiguin Island in the Philippines. This eruption produced an ashflow, which killed about 500 people.
The eruption in 1951 produced pyroclastic material and lava on the December 4, 1951. This encased bodies in gray material. Most of the people were found as if they were just asleep.

## Earth

So far hominins have set foot on only two planet, or planet-like, astronomical objects. The Earth and the Moon which has an orbit around the Earth at much less than a few light-years.

## Moon

In the image at right, planetary geologist and NASA astronaut Harrison "Jack" Schmitt collects lunar samples during the Apollo 17 mission.

## Ganymede

"NASA's Galileo spacecraft took this image of dark terrain within Nicholson Regio, near the border with Harpagia Sulcus on Jupiter's moon Ganymede. The ancient, heavily cratered dark terrain is faulted by a series of scarps."
"The faulted blocks form a series of "stair-steps" like a tilted stack of books. On Earth, similar types of features form when tectonic faulting breaks the crust and the intervening blocks are pulled apart and rotate. This image supports the notion that the boundary between bright and dark terrain is created by that type of extensional faulting."
"North is to the right of the picture and the Sun illuminates the surface from the west (top). The image is centered at -14 degrees latitude and 320 degrees longitude, and covers an area approximately 16 by 15 kilometers (10 by 9 miles). The resolution is 20 meters (66 feet) per picture element. The image was taken on May 20, 2000, at a range of 2,090 kilometers (1,299 miles)."

## Geography

Def. the "physical structure of a particular region [or] terrain" is called geography.

## History

"Historical geology is a study of life forms represented in the fossil record, as well as the chronology of geologic processes."
The image at the right shows two geochronologists and one paleontologist collecting ash at the Cretaceous-Paleogene Boundary in Wyoming, USA.

## Prehistory

The prehistory period dates from around 7 x 10$^6$ b2k to about 7,000 b2k.
Archaeology "studies human cultures through the recovery, documentation and analysis of material remains and environmental data, including architecture, artifacts, ecofacts, human remains, and landscapes."

## Recent history

The recent history period dates from around 1,000 b2k to present.
Sedimentology encompasses the study of modern sediments such as sand, mud (silt), and clay, and the processes that result in their deposition.
Sedimentary rocks cover most of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between rock layers or strata.

## Geophysics

In the image at the right, geophysicists from the Department of Earth Science at Aarhus University perform electrical measurements (DC/IP) at Ulstrup in Denmark. Studerende fra Instituttet for Geologi ved Århus Universitet, der udfører geofysisk feltarbejde i Ulstrup nær Viborg.

## Sciences

The image at the right shows rock strata in Cafayate, Argentina, the subject of stratigraphy.
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories.

## Hypotheses

* The geologic effects on an apparently spherical astronomical object, source, or entity of being in an orbit around one or more stellar objects is distinct from other geologic effects.