Solar System/Terrestrial Bodies


 * 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 2018 season. The first part, a written test, covers the terrestrial planets (excluding Earth), the Earth's moon, Phobos, Deimos, and Io, the asteroid belt, and near-Earth asteroids. 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.

Mercury
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
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.

Earth
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
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.

Earth's Moon
Earth's Moon is known as the Moon. It is Earth’s only known permanent natural satellite. It takes 27 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
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.

Asteroids
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.

Meteoroids
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.

Comets
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)

Hands-on/Interpretive Tasks
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.

Past, Current, and Planned Missions
See also: Solar System/Missions

Kepler’s Laws
See Solar System for more detail.

Kepler's Laws are as follows:
 * 1) The orbit of every planet is an ellipse with the sun at a focus.
 * 2) A line joining a planet and the sun sweeps out equal areas during equal intervals of time.
 * 3) The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

See here for more info. Many of the pictures and diagrams used in this section are from here.

Law 1
The orbit of every planet is an ellipse with the sun at a focus.

To understand this law, you must first understand ellipses. You can think of an ellipse as a flatten circle, with two axes. There is the major axis, which is the longer one, and the minor axis, which is the shorter one. There are always two focuses, which are on the major axis. There is also a semi-major axis, which is half the major axis, and a semi-minor axis, or half the minor axis. The sum of the distance to both of the foci is constant.



What the law states is that the sun is at one of the foci, and the planet orbits around it in an ellipse. Most of the time the ellipse is close to a circle in shape, but is never a circle.

Law 2
A line joining a planet and the sun sweeps out equal areas during equal intervals of time. This one is harder to envision. So we've established that the orbit is elliptical, right? Two lines extending out of the sun will always have the same area, and the planet we are talking about will always travel this distance in equal time. Look at this picture:



Make sense now? The blue sections have the same area, and the Earth will travel the distance the blue area covers in the same time. So when the blue is wider, the Earth moves faster. The blue is wider closer to the sun, so the closer to the sun you are, the faster the planet will orbit around the sun.

Law 3
The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. This is purely math. [math]\frac{P_1^2}{P_2^2} = \frac{R_1^3}{R_2^3}[/math]

So what this means is that these two fractions are equal. Remember in the first law, we defined the major and minor axes? The semi-major axis is half of the major axis. So that shows that the minor axis defines the orbital period! You can use this law to find either the semi-major axis, which can then be used to find the major axis, or the orbital period. Since [math]p^2 = a^3[/math], we can use the formula [math]p = a^{3/2}[/math] to find the orbital period, or [math]a = p^{2/3}[/math] to find the semi-major axis.

Libration, Phases, and Eclipses
See also: Solar System

Libration
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.

Lunar Eclipses
A type of eclipse that occurs when the Earth passes directly between the moon and sun, which means that the moon is in Earth's shadow. Since Earth is in the middle of the moon and sun, it must always be a full moon for a lunar eclipse to occur. There are several types of lunar eclipses:

Penumbral Eclipse- The moon passes through Earth's penumbra, causing its surface to darken slightly. Total Penumbral Eclipse- The moon passes "exclusively" through Earth's penumbra. The area of the moon closest to the umbra can appear darker than the rest of it. Partial Lunar Eclipse- A portion of the moon passes through Earth's umbra.

Total Lunar Eclipse- The whole moon passes through Earth's umbra. Totality can last up to 107 minutes, depending on the distance of the moon (at apogee, the moon's speed is slower, meaning a longer eclipse). Selenehelion- Also known as a "horizontal eclipse", this is when the sun and the eclipsed moon can be seen at the same time. It can only occur right after sunrise or just before sunset. Technically, the moon and sun shouldn't be visible at the same time, but Earth's atmosphere refracts light and things near the horizon appear higher in the sky than they really are. The name is derived from the Greek goddess of the Moon (Selene) and their word for Sun, helios.



Solar Eclipses
The moon passes between the Earth and sun so that the sun's light is partially or completely blocked. Solar Eclipses can only occur during a new moon, when the moon is between the earth and the sun. However, since the moon's orbit around the earth is inclined at about 5°, solar eclipses can only happen when the moon's orbit crosses the ecliptic. There are four types of solar eclipses:

Total Eclipse- The sun is completely blocked by the moon. A total eclipse often happens near perigee because the moon is closer to the earth and its apparent size is larger. When earth is close to aphelion, total eclipses are also more likely to occur. The sun's disk is obscured and its corona is visible. Total eclipses are only visible from the path of totality in the moon's umbra. Annular Eclipse- The sun and moon are in line, but the moon's apparent size is smaller than the sun because the moon is close to apogee. Annular eclipses are more likely to occur during earth's perihelion. The sun appears as a bright ring around the moon's outline. Annular eclipses are only visible in the antumbra. Hybrid Eclipse- A hybrid eclipse is visible as a total eclipse from some places on earth and is visible as an annular eclipse from other places. This kind of eclipse is rare compared to the other kinds. Partial Eclipse- The moon only obscures part of the sun. Partial eclipses can be seen from "a large part of earth" (the moon's penumbra) outside the path of totality for a total or annular eclipse. Some eclipses are only visible as a partial eclipse because the umbra passes above the poles.

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