Solar System/Terrestrial Bodies

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For general information about this Division B event, see Solar System.

Terrestrial Bodies is the topic of the Division B event Solar System for the 2019 season. The first part, a written test, covers the dwarf planets, Charon, Mimas, Phoebe, 'Oumuamua, 2007 OR10, centaurs, trojans, and trans-Neptunian objects. The second part, a hands-on/interpretive task, covers the geologic characteristics and evolution of the rocky bodies. The specific focus of the event is the history, formation, and geology of these rocky bodies.

In 2018, the event focused on terrestrial planets (excluding Earth), the Earth's moon, Phobos, Deimos, and Io, the asteroid belt, and near-Earth asteroids. Although these objects are not part of the 2019 event, they are still included in this page due to the similarity in the topic's focus.



Mercury is the smallest planet in the solar system with a radius of about 2439 km. It is also the planet closest to the sun at an average distance of about 57.9 million kilometers, or about 0.39 AU. (Mercury has a highly elliptical orbit.) The length of a year on Mercury is 88 Earth days long, and a day on Mercury is 59 Earth days long. The surface gravity of Mercury is 1/3 of Earth's, so it can only hold an extremely thin atmosphere consisting of small amounts of hydrogen, helium, and oxygen. Mercury’s surface is scarred with the craters of impacted meteors. The surface temperatures at day and at night are very different, because of the thin atmosphere. The temperature during the day is 227°C and the temperature at night is -173°C. The most noticeable feature is the largest impact crater on its surface, the Caloris basin.


Venus is the third smallest planet in the solar system with a radius of 6051 km. It is also the second closest to the sun at a distance of 108.2 million kilometers. It is the only planet whose day is longer than its year. Its day is 243 Earth days, and its year is 225 Earth days. The extremely long rotation periods of Venus and Mercury is thought to be caused by mutual gravitation pull; Mercury is sometimes thought to be an escaped moon of Venus. Venus is often called Earth's sister planet because of its close proximity to Earth, and because of its similar diameter and mass. People even thought it could hold life, but sadly people discovered that the greenhouse effect on Venus raised the surface temperature to the highest in the Solar System.


NOTE: Earth is not included as a major part of the event for 2018; however, a basic knowledge of how Earth fits in the context of the Solar System will likely be of use.

Earth is the third planet from the Sun. It is also the fifth largest planet at a radius of 6378 km, ranking below the gas giant planets. It is the only planet in the universe currently known to support intelligent life. It is used as a basis for many measurements of planets and other things in the solar system, for example, the AU (Astronomical Unit), the average distance between the Earth and the Sun, 93,000,000 miles or 149,600,000 km. The year on Earth is equal to about 365.256 Earth days, and a corresponding day is equal to one 1 earth day, hence the definition.


Mars is the fourth planet from the Sun at a distance of about 227,392,000 km, with a highly elliptical orbit. It is often called the Red Planet due to the large quantities of iron oxide present in its soil. The Romans saw it as blood, so they named it for their god of war, Mars. It is the second smallest planet, with a radius of 3397 km. It has a day length of about 25 hours, and a year equal to 687 Earth days. It has been suggested that Mars may hold life, but it has not been proven. Mars contains two polar ice caps consisting of frozen ice and carbon dioxide located on the southern and northern poles.

30% of the polar ice caps are made up of dry ice or CO2. The dry ice then sublimates creating large gusts sweeping across the polar ice caps creating cirrus clouds in the atmosphere. Mars has two tiny moons, Phobos and Deimos, believe to be captured asteroids.

Dwarf Planets

The definition of a Dwarf Planet is a planet with enough of a gravitational pull to keep a spherical shape, but not strong enough to "clear the neighborhood", which means that any object that comes close to the planet, it either "pushes away" or "pulls into an orbit". In addition to that it cannot be a satellite of a non-stellar body.


The largest object in the Asteroid Belt, containing 30% of its mass. When it was discovered in the early 1800s, Ceres was considered a planet, but was reclassified as an asteroid 50 years later. Since 2006, it has been considered a dwarf planet. Ceres orbits the Sun once every 4.6 Earth years and its day is about 9 hours.


When it was still considered a planet, Pluto was the ninth planet from the Sun and the smallest planet. It became a dwarf planet in 2006. Pluto is the largest and second-most-massive (after Eris) known dwarf planet in the Solar System, and the ninth largest and tenth most massive known object directly orbiting the Sun. It is also the largest known trans-Neptunian object by volume but is less massive than Eris. Like other Kuiper belt objects, Pluto is mostly made of ice and rock. Its atmosphere consists nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto's surface. It has similarly sized moon, Charon (pronounced "karen"). Charon is so big that they are sometimes considered as a binary system because the barycenter of their orbits does not lie within either body. Pluto was discovered in 1930 by Clyde Tombaugh, and was the only planet discovered in the 20th century. It is a part of the Kuiper belt, and is one of many similar Kuiper Belt objects. Pluto’s orbit is both elliptical and tilted and takes 248 years to orbit the sun. The orbit can take it as far as 49.3 AU from the sun, and as close as 30 AU. It also has four smaller moons, Nix, Hydra, Kerberos, and Styx. In 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto.


Eris is in the scattered disc, a region beyond the Kuiper Belt. Since Eris is larger than Pluto, its discovery led the IAU (International Astronomical Union) to define "planet" and reclassify Pluto as a dwarf planet. Its only known satellite is Dysnomia.


An "egg-shaped" dwarf planet in the Kuiper belt. The odd shape is believed to come from a high rotational speed, which flattens the poles and creates a bulge around the equator. Haumea has a year of about 283 earth years. It also has two moons, Hi'iaka and Namaka. These are believed to be fragments of the KBO - early in its history, it was hit by something, breaking off Hi'iaka and Namaka.


Makemake has no moons, making it unique among the larger Kuiper Belt objects. It orbits the sun every 310 years. Like most KBOs, it has an highly eccentric orbit - it's perihelion (distance closest to sun), is 38.5 AU(5,760,000,000 km). It's aphelion (distance farthest from sun), is 53.1 AU (7,939,000,000 km). It has a mass of 3 × 10^21 kg.


Earth's Moon

Earth's Moon is known as the Moon. It is Earth’s only known permanent natural satellite. It takes 27.5 Earth days for the Moon to fully orbit the Earth. The moon is widely thought to have formed 4.51 million years ago, not too long after Earth, supposedly after a Mars-sized body collided with Earth. The moon is in synchronous motion with Earth, and always shows the same face, although about 59% of it can be seen over time due to libration.

The moon appears the same size in the sky as the sun, resulting in total solar eclipses, but the moon’s distance from Earth is slowly increasing.


Io is the third largest of Jupiter's four Galilean moons, and is the most volcanically active body in our solar system, with over 400 volcanoes, the result of tidal heating from the pull of Jupiter and its other moons. It is the innermost of the four Galilean moons, and has the highest density of all the moons.Io was discovered in 1610 by the Italian scientist Galileo. Its largest volcano, Loki Patera, has a diameter of 202 kilometers (126 mi), and is one of the few moons to contain an active lava lake. During an eruption, a wave of foundering crust spreads out across the patera at the rate of about 1 kilometer (0.6 mi) per day, until the crust of the lake has been resurfaced.

Phobos and Deimos

Phobos and Deimos are the only two known moons of Mars, discovered in August 1877 by astronomer Asaph Hall and named after Phobos and Deimos from Greek mythology. Phobos is the inner moon and seven times as massive as Deimos, the outer moon. Phobos has an average diameter of 22.2 km. It takes approximately 8 hours for Phobos to complete an orbit around Mars, and its surface gravity is only about 0.0057 m/s squared. Phobos orbits only about 9377 kilometers away from Mars, compared to Earth's moon, which orbits 384,400 kilometers away from Earth. Deimos is much smaller, with an average diameter of only 12.6 km. Both moons are usually believed to be captured asteroids, but their origin is still controversial. Phobos and Deimos resemble C-type asteroids, with albedos, densities, and spectra similar to those of C-type asteroids. Another theory is that Mars was surrounded by many such objects, but many were ejected, or that the moons may have been a result of a large collision between Mars and a smaller protoplanet.

Phobos, however, won't zip around Mars forever. The doomed moon is spiraling inward at a rate of 1.8 centimeters (seven-tenths of an inch) per year, or 1.8 meters (about 6 feet) each century. Within 50 million years, the moon will either collide with its parent planet or be torn into rubble and scattered as a ring around Mars. The most dominant feature on Phobos is large impact crater dominates the moon. Stretching nearly 9.5 km, Stickney Crater covers most of the surface.

It is possible for Mars to have moons smaller than 100 meters in diameter and a dust ring between Phobos and Deimos, but neither have been discovered so far.


Charon is the largest of Pluto's moons, discovered in 1978 by James Christy and Robert Harrington. It was named after a character in Greek mythology that transports souls across the River Styx. Charon's diameter is 1,212 km, which is approximately half of Pluto's diameter. Its mass is also approximately 12% of Pluto's. Charon and Pluto orbit each other every 6.387 Earth days. Similar to the Earth and the Moon, Charon and Pluto are gravitationally locked to each other. However, Charon and Pluto are linked, meaning they always show the same face to each other as opposed to one body rotating while the other does not. The average distance between Charon and Pluto is 19,570 km.

In 2005, Robin Canup suggested Charon was formed by a collision, similar to how Earth's moon was formed. However, it is now commonly believed that Pluto and Charon have always been separate bodies that collided and then began to orbit each other. The collision would have been violent enough to do damage to both bodies, but not enough to destroy them.


Originally designated Saturn I, Mimas was discovered in 1789 by William Herschel. It was named after one of the sons of Gaia in Greek mythology. Its diameter is only 396 km, making it the smallest rounded astronomical body due to self-gravitation. Mimas has a large crater named Herschel, after the man who discovered the moon. Herschel is approximately 5 km high, with some parts of its floors being 10 km deep.


Phoebe is an irregular satellite of Saturn, discovered in 1899 by William Henry Pickering. It has a mean diameter of 213 km and is thought to be a centaur that originated from the Kuiper belt. It orbits opposite to Saturn's rotation and was previously Saturn's outermost moon before the discovery of several smaller moons in 2000. Phoebe is roughly spherical, and is about one sixteenth the size of the Moon. It completes a full orbit around Saturn approximately every eighteen months. It has craters up to 80 km across, and is very darkly colored. Its color led scientists to believe it was a captured asteroid, due to its high carbon content.

It is theorized that Phoebe formed in the Kuiper belt, within three million years of the formation of the solar system. It was warm enough to have liquid water for millions of years, though it does not have any in the present. It was photographed by the Cassini spacecraft in 2004, and was observed for a few hours by Voyager 2 in 1981.

Small Bodies


Asteroids, sometimes called minor planets, are leftover remnants from the formation of the solar system. There are three major types of asteroids, C-, S-, and M-types. The C-type is the most common, consisting of clay and silicate rocks, and are usually dark in appearance. S-type asteroids are made of silicates and nickel-iron. Finally, the M-type asteroids are metallic, and contain the largest amount of nickel-iron.

Most asteroids are found in the asteroid belt, a donut-shaped region between Mars and Jupiter. Asteroids have a wide variety of sizes, from the largest, Ceres, at about 946 kilometers in diameters to tiny bodies smaller than a house. Only the largest asteroids are not irregularly shaped. Also, asteroids can have highly elliptical orbits, and some even have companion moons. Jupiter's high gravity can often alter asteroids' orbits and knock them out of the main asteroid belt.

A particularly important category of asteroids are Near-Earth asteroids, or asteroids that approach Earth's orbital distance, and pose an impact threat.


A "sand- to boulder-size" piece of space debris. The official definition from the IAU is "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom". Traditionally, anything smaller than 10 meters across is considered a meteoroid, while anything larger than 10 meters is an asteroid. Once a meteoroid enters the atmosphere of Earth or another planet, it is considered a meteor. If it reaches the ground and stays (more or less) intact, it's called a meteorite. A method to remember this is meteoroid is in the void of space and a meteorite is right here.


A small solar system body that has a coma (the dust particles gathered around the comet's nucleus that give it an "atmosphere") and/or a tail. The nucleus itself is made up of water ice, dust, frozen gases and small rocky particles. The nuclei range from 100 meters across to more than 40 kilometers. As the comet approaches the sun, solar radiation cause the gases inside to vaporize and carry the dust with them. The gases also become excited by sunlight and emit electromagnetic radiation. Comets leave a trail of solid particles behind them, and if a comet crosses earth's path, there will most likely be meteor showers when earth passes through the debris field. For example, Halley's Comet causes the Orionid Showers and the Swift-Tuttle Comet causes the Perseid showers.

Short-Period Comets- Comets with an orbital period of less than 200 years. Their orbits are in the same direction as the planets, close to the ecliptic, and their aphelion is generally in the area of the outer planets. They are divided into the Jupiter family (orbital period less than 20 years) and the Halley family (orbital periods between 20 and 200 years).

Long-Period Comets- Comets with orbital periods of more than 200 years, sometimes even thousands or millions of years. Their orbits are very eccentric, often don't lie near the ecliptic, and their aphelion is far beyond the outer planets. However, all long-period comets are still gravitationally bound to the sun; comets that have been ejected from the solar system by the gravity of the outer planets are no longer considered to have an orbital period.

Sungrazing Comets- Comets that have a parabolic or hyperbolic trajectory, i.e. their trajectories only let them enter the solar system once (hence the name). Other than that, they are very similar to long-period comets. The large sungrazers often break up into chunks while smaller ones can disintegrate (e.g. comet ISON)


Centaurs are small bodies, sharing traits from planets, comets, and asteroids. They are close in size to asteroids, with the largest being around 250 km in diameter. The first centaur was discovered in 1920, but the group wasn't named until 1970 with the observation of 2060 Chiron (not to be confused with Charon). Many centaurs are too small to be observed, though centaurs are designated as both minor planets and asteroids. They are found between Jupiter and Neptune, and it is estimated that there are around 44,000 centaurs in the solar system with a diameter greater than 1 km.

Their orbits are typically unstable, due to the fact that they often intersect with the orbits of giant planets. Three centaurs have also been discovered that feature comas similar to comets - 2060 Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are classified as both asteroids and comets.


Trans-Neptunian Objects

Geologic Characteristics

The second part of the competition will ask competitors to complete one or more hands-on or interpretive tasks from these topics:

History and Formation of Rocky Bodies

The leading theory for the formation of the moon is from a collision of a Mars-sized body with the Earth. Known as the Giant-Impact Hypothesis, it states that Theia, the aforementioned body, collided directly with the Earth, and the resulting debris formed the Moon. Alternate hypotheses include the capture of the Moon, fission (Earth spun so fast that material was ejected), and finally accretion.

Surface Dating- In order to approximate the age of a surface, many would use the amount of creating or weathering on the planet. The more craters a planet has, the older it probably is. Another way to look at this is based on whether or not it is still geologically active. However, there are some surfaces proven to be geologically young by radioactive dating, that look very old based on this method. An example of this is Venus. It is puzzling that it is geologically young, because it has no evidence of plate tectonics or other forms of surface recycling. Radioactive dating is often used on earth to identify fossils or prehistoric remains, but if samples of dirt come back, like from the moon, radioactive dating can be used to get an exact date on the surface age. Planetary surfaces can also be dated relatively, for example, with any set of overlapping craters, the most prominent crater will be the youngest and so on.

Compositions of Rocky Bodies

Differentiation- Differentiation is the process of separating out different parts of the interior of a body as a consequence of their physical or chemical characteristics. The densest materials sink to the center of the planet, as can be seen in the iron-nickel core of the Earth. Dwarf planets, some natural satellites, and asteroid belt objects like Vesta and Ceres have differentiated interiors.

Constituents- The terrestrial bodies in the Solar System are classified as Silicate Planets. They have silicon-based mantles and iron cores. Ceres consists of a rocky inner core followed by an icy mantle. Some of these silicates include olivine and pyroxene. and Additionally, solar radiation fluctuations early in Ceres' history points to volatile materials in its core as well. Pluto's internal structure is also hypothesized to be differentiated, including a rock core, a water ice mantle, and a layer of frozen nitrogen near the surface. Haumea appears to have a rocky core, then a thin layer of ice, but no ice mantle, wich suggests that the impact that created the Haumean system destroyed the layer.

Geographic Features

Regolith- Regolith is a layer of unconsolidated material on top of solid rock, like moon dust. On Earth, this is soil.

Volcanism, Weathering, and Cratering

Extraterrestrial volcanos form differently on other planets than they would on earth, as the other terrestrial planets are not large enough to sustain plate tectonics, Earth just barely making the cut. Planetary volcanism is not always hot. Although most volcanos on earth are 800-1200°C Silicate Magma, extraterrestrial volcanism can be of mud, ice, carbonate, and more. Whether or not there is volcanism on a planet can depend on planet size, core size, and internal temperature. Really Great Resource for Extraterrestrial Volcanism

Planetary weathering by meteorites, micrometeorites, cosmic winds, and solar rays is directly responsible for the formation of regolith. Galactic cosmic rays and solar cosmic rays may speed up the weathering of rocks on airless bodies. Irradiation is basically indirect radiation or the process of being exposed to radiation. Sputtering is when particles excreted from the sun hit celestial bodies. The other thing that it may come from is the bombardment of meteorites and micrometeorites.In-Depth Article On It

Remote Sensing and Satellite Imagery

Motions of Terrestrial Bodies

Rotation and Revolution

Libration, Phases, and Eclipses

See also: Solar System#Eclipses


Libration is the apparent occilation of an orbiting body, most notably the perceived movement of the Moon as seen from Earth. Lunar libration causes slight changes in the Moon's apparent size, and as a result, observers on Earth can see about 59% of the Moon over time, despite tidal locking.

Kepler’s Laws

See Solar System#Kepler's Laws of Planetary Motion for more detail.

Gravitational Effects of the Moon

Missions to Terrestrial Bodies

This section focuses on the missions highlighted in the 2019 rules. For a more complete list of all missions, please see Solar System/Missions.