Fossa Sulci and Other Terms for Planetary Landforms

O n Earth the names for geological features often connote how they were formed and what they mean in terms of surface and planetary evolution. A caldera, for example, is a round depression formed by volcanic activity and generally encompassing volcanic vents. Though a round depression on another planet may remind a planetary geologist of a terrestrial caldera, it would be misleading to call that feature a caldera until its volcanic nature was proven. Images of other planets are not always clear and seldom include topography, so at times the details of the shape in question cannot be determined, making their definition even harder.

To avoid assigning causes to the shapes of landforms on other planets, scientists have resorted to creating a new series of names largely based on Latin, many of which are listed in the following table, that are used to describe planetary features. Some are used mainly on a single planet with unusual features, and others can be found throughout the solar system. Chaos terrain, for example, can be found on Mars, Mercury, and Jupiter's moon Europa. The Moon has a number of names for its exclusive use, including lacus, palus, rille, oceanus, and mare. New names for planetary objects must be submitted to and approved by the International Astronomical Union's (IAU) Working Group for Planetary System Nomenclature.

Feature astrum, astra catena, catenae chaos chasma, chasmata colles corona, coronae crater, craters dorsum, dorsa facula, faculae fluctus

Nomenclature for Planetary Features

Description radial-patterned features on Venus chains of craters distinctive area of broken terrain a deep, elongated, steep-sided valley or gorge small hills or knobs oval-shaped feature a circular depression not necessarily created by impact ridge bright spot flow terrain

Feature

Description

fossa, fossae

narrow, shallow, linear depression

labes

landslide

labyrinthus, labyrinthi

complex of intersecting valleys

lacus

small plain on the Moon; name means "lake"

lenticula, lenticulae

small dark spots on Europa (Latin for freckles); may be

domes or pits

linea, lineae

a dark or bright elongate marking, may be curved or straight

macula, maculae

dark spot, may be irregular

mare, maria

large circular plain on the Moon; name means "sea"

mensa, mensae

a flat-topped hill with cliff-like edges

mons, montes

mountain

oceanus

a very large dark plain on the Moon; name means "ocean"

palus, paludes

small plain on the Moon; name means "swamp"

patera, paterae

an irregular crater

planitia, planitiae

low plain

planum, plana

plateau or high plain

reticulum, reticula

reticular (netlike) pattern on Venus

rille

narrow valley

rima, rimae

fissure on the Moon

rupes

scarp

sinus

small rounded plain; name means "bay"

sulcus, sulci

subparallel furrows and ridges

terra, terrae

extensive land mass

tessera, tesserae

tile-like, polygonal terrain

tholus, tholi

small dome-shaped mountain or hill

undae

dunes

vallis, valles

valley

vastitas, vastitates

extensive plain

(continues)

Fossa, Sulci, and Other Terms for Planetary Landforms

(continued)

The IAU has designated categories of names from which to choose for each planetary body, and in some cases, for each type of feature on a given planetary body. On Mercury, craters are named for famous deceased artists of various stripes, while rupes are named for scientific expeditions. On Venus, craters larger than 12.4 miles (20 km) are named for famous women, and those smaller than 12.4 miles (20 km) are given common female first names. Colles are named for sea goddesses, dorsa are named for sky goddesses, fossae are named for goddesses of war, and fluctus are named for miscellaneous goddesses.

The gas giant planets do not have features permanent enough to merit a nomenclature of features, but some of their solid moons do. Io's features are named after characters from Dante's Inferno. Europa's features are named after characters from Celtic myth. Guidelines can become even more explicit: Features on the moon Mimas are named after people and places from Malory's Le Morte d'Arthur legends, Baines translation. A number of asteroids also have naming guidelines. Features on 253 Mathilde, for example, are named after the coalfields and basins of Earth.

enough material left over in the early solar system to continue bombarding and cratering the early planets.

Beyond this theory, though, there is visible evidence on Mercury, the Moon, and Mars in the form of ancient surfaces that are far more heavily cratered than any fresher surface on the planet (Venus, on the other hand, has been resurfaced by volcanic activity, and plate tectonics and surface weathering have wiped out all record of early impacts on Earth).The giant basins on the Moon, filled with dark basalt and so visible to the eye from Earth, are left over from that early period of heavy impacts, called the Late Heavy Bombardment.

Crater density on the sheet of impact ejecta from the Mare Imbrium Basin (actually an immense impact crater, as are all the basins on the Moon; see the sidebar "Fossa, Sulci, and Other Terms for Planetary Landforms" on page 136) is six times greater than that on lava flows formed 600 million years later. This decrease can be modeled as a 50 percent decrease every 100 million years (the 50 percent per 100 million years model assumes that cratering rates dropped off evenly, rather than abruptly).

Dating rocks from the Moon using radioactive isotopes and carefully determining the age relationships of different crater's ejecta blankets indicates that the lunar Late Heavy Bombardment lasted until about 3.8 billion years ago. Some scientists believe that the Late Heavy Bombardment was a specific period of very heavy impact activity that lasted from about 4.2 to 3.8 billion years ago, after a pause in bombardment following initial planetary formation at about 4.56 billion years ago, while other scientists believe that the Late Heavy Bombardment was just the tail end of a continuously decreasing rate of bombardment that began at the beginning of the solar system. Cratering rate before about 4 billion years before the present cannot be known by today's methods because older rocks on the Moon have been so severely pulverized by impact.

In the continual bombardment model, the last giant impacts from 4.2 to 3.8 billion years ago simply erased the evidence of all the earlier bombardment. If, alternatively, the Late Heavy Bombardment was a discrete event, then some reason for the sudden invasion of the inner solar system by giant bolides must be discovered. Were they bodies perturbed from the outer solar system by the giant planets there? The bodies may have been planetesimals left over from Earth accretion, or they could have been asteroids perturbed out of the asteroid belt, or they could have been broken-up planetesimals from Uranus or Neptune's formation.

On Earth the age of a rock can often be determined exactly by measuring its radioactive isotopes and their daughter products, and thereby knowing how long the radioactive elements have been in the rock, decaying to form their daughters. Before the discovery of radioactivity and its application to determining the age of rocks, all of which happened in the 20th century, geologists spent a few centuries working out the relative ages of rocks, that is, which ones were formed first and in what order the others came. Between 1785 and 1800, James Hutton and William Smith introduced and labored over the idea of geologic time:That the rock record describes events that happened over a long time period.

Fossils were the best and easiest way to correlate between rocks that did not touch each other directly. Some species of fossil life can be found in many locations around the world, and so form important markers in the geologic record. Relative time was broken into sections divided by changes in the rock record, for example, times when many species apparently went extinct, since their fossils were no longer found in younger rocks. This is why, for example, the extinction of the dinosaurs lies directly on the Cretaceous-Tertiary boundary: The boundary was set to mark their loss. The largest sections of geologic history were further divided into small sections, and so on, from epochs, to eras, to periods. For centuries a debate raged in the scientific community over how much time was represented by these geologic divisions. With the development of radioactive dating methods, those relative time markers could be converted to absolute time; for example, that the oldest known rock on Earth is 3.96 billion years old, and the Cretaceous-Tertiary boundary lies at about 66.5 million years ago.

As discussed in the chapter "The Visible Planet" in Part One of this book, by using detailed images of the Moon scientists have worked out the relative ages of many of the crustal features. By carefully examining images researchers can determine "superposition," that is, which rock unit was formed first, and which later formed on top of it. Impact craters and canyons are very helpful in determining superposition. Through this sort of meticulous photogeology, scientists have developed relative timescales for other planets. (A rough set of geological epochs has been built up for the Moon and is shown in the figure on page 74.) From oldest to youngest, epochs on the Moon are called the Pre-Nectarian, Nectarian, Imbrian, and Copernican, named after craters. Events and objects on the Moon tend to be approximately located in time by these epochs. The Late Heavy Bombardment began in the Pre-Nectarian, bridged the Nectarian with its most intense activity, and ended in the Imbrian.

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