Small Bodies of the Solar System

Pregunta	Respuesta
AGENDA Slide 2	During this presentation: Moons Rings Asteroids Comets Interplanetary Dust
WHAT ARE SMALL BODIES? Slide 3	We will define small bodies to be anything that orbits the Sun inside the orbit of Neptune, that is NOT a planet. This necessarily excludes several major parts of our Solar System: Pluto, which was considered for years to be one of the planets, is not included. The Kuiper Belt and the Oort Cloud are also not included. However, we do include those comets that enter the inner Solar System, even though they may have originated in the Oort Cloud. Our primary focus will be the various moons of the planets, the ring systems formed around the outer planets, asteroids, comets, and interplanetary dust.
SMALL PLANETARY SATELLITES (MOONS) Slide 4	Many of the planets have moons – only Mercury and Venus do not. But the rest have at least one moon. The Earth, of course, has only one moon that is familiar to all of us. Mars has two moons, which likely originated from the asteroid belt, which lies between Mars and Jupiter. So for the inner planets there are only 3 moons. But each of the outer planets (Jupiter, Saturn , Uranus and Neptune) have many even dozens of moons. Why this is so is a question we do not address here, but it is an interesting one to consider.
MOONS Slide 5	We are all familiar with the sight of the Earth’s moon. We all know the moon keeps the same face pointing toward the Earth as it orbits. But the relationship between the Earth and the moon is more complex. Most people talk about the moon orbiting the Earth, but the ratio of the moon’s mass to the Earth’s mass is significantly larger than that for any other planet-moon system. That means that it is more accurate to say that the Earth and the moon orbit around a common center of mass. Despite what is commonly believed, the moon does have an atmosphere, but it is so thin that it cannot support life. Also, the atmosphere that does exist on the moon does not consist of the same gases as the Earth. Scientists recently discovered evidence that the moon actually had a relatively thick atmosphere at one time, but the forces of the solar wind gradually stripped that away, leaving only the faint atmosphere that remains. Only twelve humans have ever walked on the surface of the moon, making it the only other body in our Solar System, besides the Earth, where humanity has trod.
MOONS (cont’d) Slide 6	Jupiter, as noted before, has dozens of moons. The first four that were discovered by Galileo Galilei are among the most interesting ones in the Solar System. Each is unique and contains clues that scientists and astronomers are using to try and understand how our Solar System was formed and evolved over the more than the 4.5 billion years since the universe began. These four moons, called the Galilean moons, are Io, Europa, Ganymede, and Callisto. Io – the most volcanically active body in the Solar System Europa – its surface of water ice possibly covers an ocean of water – maybe twice as much water as on Earth Ganymede – the largest moon in the Solar System and the only one to have an internally-generated magnetic field Callisto – its surface is heavily cratered, preserving a record of events from the early history of the Solar System.
MOONS (cont’d) Slide 7	Saturn, like Jupiter, has dozens of moons, as well as its famous rings, which we will discuss shortly. While we cannot address all of them, there are two that have special characteristics worthy of mention: Titan – Saturn’s largest moon and the second-largest in the Solar System; the only known world (other than Earth) where liquid collects on its surface Liquid methane and ethane rain from the sky and fill liquid lakes Enceladus – home to a subsurface liquid water ocean near its southern polar region; appears to have all the components necessary to support life – water, food and heat.
MOONS (CONT’D) Slide 8	As we move to the outer regions of the Solar System, the planets of Uranus and Neptune also have moons with special features and characteristics: The moons of Uranus Miranda – has a giant fault canyon 12 times as deep as the Grand Canyon Cordelia and Ophelia – shepherd moons that keep Uranus’ outermost ring well-defined The moons of Neptune Triton – the only large moon in the Solar System that orbits its planet in a direction opposite to the planet’s rotation, suggesting it may have been captured by Neptune’s gravity; Triton is also one of the coldest objects in the Solar System (~ -400° F) Proteus – a slightly non-spherical moon whose mass is just slightly less than that required for gravity to have pulled it into a sphere.
RINGS Slide 9	Not only do the outer planets have many more moons than the inner planets, but each also has one or more rings. These rings, which are composed of either ice or rocks, are likely leftover fragments dating back to the time when the Solar System was formed. Jupiter’s ring is basically a band of dust that is transient in nature and constantly renewed by grains eroded from its many moons The rings of Saturn are the most visible from Earth since they are mostly ice particles that reflect the Sun’s light better Saturn has four main groups of rings and three fainter, narrower ring groups. These groups are separated by gaps called divisions. Close up views of Saturn's rings by the Voyager spacecrafts, which flew by them in 1980 and 1981, showed that these seven ring groups are made up of thousands of smaller rings. The exact number is not known. The rings of Uranus and Neptune are black and were only discovered by observing occulted stars.
ASTEROIDS Slide 10	Rocky remnants left over from the early formation of our Solar System about 4.6 billion years ago. Previously named “minor planets” Most are irregularly shaped, and have craters left over from collisions with other asteroids As they orbit the Sun, asteroids also rotate and tumble
ASTEROIDS (cont’d) Slide 11	Most of the asteroids in our Solar System orbit the Sun in the main asteroid belt, between the orbits of Mars and Jupiter. There are literally millions of these asteroids, and it is likely that we will never know exactly how many there are. Like the rocks the form the rings around the outer planets, these asteroids are remnants of the material from which the Sun and planets were formed. Some scientists believe there was a planet between Mars and Jupiter, which broke apart due to gravitational forces to create the asteroid belt, but this cannot be proven. Some of the asteroids are large enough to exert a gravitational influence on smaller asteroids, which disturbs their orbits and sends them plummeting towards the Sun. Some of these become what are known as Near-Earth asteroids, and some become meteors, which we will discuss later.
ASTEROIDS (cont’d) Slide 12	Most Near-Earth asteroids do not pose a threat, either because they are too small or their orbits do not intersect the Earth as it orbits the Sun. However, there are roughly 1400 that could potentially strike the Earth and cause severe or even catastrophic damage. At present, there is no known way to prevent this from happening. If you are still listening to me, then it probably has not happened yet.
ASTEROIDS (cont’d) Slide 13	Asteroids can have three different compositions The C-type (chondrite) - most common, probably consist of clay and silicate rocks, and are dark in appearance. The most ancient objects in the solar system. The S-types ("stony") - made up of silicate materials and nickel-iron. The M-types are metallic (nickel-iron). The asteroids' compositional differences are related to how far from the sun they formed. Experienced high temperatures after they formed and partly melted, with iron sinking to the center and forcing basaltic (volcanic) lava to the surface.
METEOROIDS, METEORS, AND METEORITES Slide 14	So, some of the Near-Earth asteroids do intersect the Earth as it orbits the Sun. Before they enter the Earth’s atmosphere, these Near-Earth asteroids are called “meteoroids” – as noted earlier, most of these are small, even when compared to other asteroids. Once one of these enters the Earth’s atmosphere, the speed of entry combined with the thickening atmosphere causes frictional heating – the phenomenon commonly referred to as a “shooting star”. If the meteor is large enough or its composition is such that it does not completely burn up as it descends through the atmosphere, it is known as a meteorite. Meteor showers happen when the Earth passes through the tail of a comet and fragments that have broken off from the comet enter the Earth’s atmosphere.
NOTABLE ASTEROIDS Slide 15	The Tunguska Event The largest asteroid impact in recorded history On June 30, 1908, a meteoroid (considered by some to be a comet) estimated to be 60-190 meters in size exploded over Russia The force of the explosion is estimated to be roughly 1,000 times greater than that produced by the Hiroshima, Japan atomic bomb explosion 80 million trees were estimated to have been knocked down, but there were no casualties due to the remote location Of local interest, the Chesapeake Bay is likely the result of an asteroid impact roughly 35 million years ago.
NOTABLE ASTEROIDS (cont’d) Slide 16	Asteroid 951 Gaspra was the first asteroid ever to be closely approached by a probe from Earth – the Galileo spacecraft in 1991. Orbits near the inner edge of main asteroid belt between Mars and Jupiter Size: 18x11x9 km Discovered and named by the Russian astronomer G. N. Neujmin in 1916. S-type asteroid, composed of mixture of rocky and metallic mineral.
COMETS Slide 17	Comets are known as “dirty snowballs” since they are largely a mixture of ice and dust. Most comets originate from either the Kuiper Belt or the Oort Cloud, where gravitational influences occasionally send them into orbits approaching the Sun (as with Near-Earth asteroids). As a comet gets nearer to the Sun, the forces of the solar wind and radiation create the commonly observed tail, which always is directed away from the Sun.
COMETS Slide 18	Comets originate from two regions surrounding our Solar System - the Kuiper Belt and the Oort Cloud. Kuiper Belt - Comets in this region have short orbital periods and orbit the Sun in roughly the same plane as do the planets. Oort Cloud - Comets in this region have longer orbital periods than those of the Kuiper Belt. A Great Comet is one which is bright enough to be visible from Earth without the need for a telescope. Great comets happen approximately every 10 years. More than 3,500 comets have been discovered
PARTS OF COMETS Slide 19	Read about Nucleus and Coma. The hydrogen cloud has only been observed relatively recently. It is created by interactions between the comet and ultraviolet light from the Sun, so it is not observable from the Earth. It was detected by satellites specifically designed to observe in the ultraviolet spectrum. It’s size much larger than the extent of the tail, which can extend for 100 million kilometers. The hydrogen cloud surrounds the comet in all directions, creating an envelope. A comet has two tails – the dust tail and the ion tail created due to the effects of radiation from the Sun. The dust tail is smaller (up to 10 million kilometers) – as its name suggests, it is composed of dust emitted as the ice melts. It often shows a slight curve indicative of the speed of the comet in its orbit. The ion tail, which can extend up to 100 million kilometers, consists of ionized particles created through interaction with the solar wind. This tail does not curve like the dust tail, but always points straight away from the Sun.
COMET 67P/CHURYUMOV– GERASIMENKO Slide 20	Observed by the Rosetta spacecraft in 2014 – not a round nucleus as typically represented Rosetta was the first spacecraft to accompany a comet as it entered the inner solar system and the first to attempt a landing on a comet While the lander (“Philae”) touched down, it failed to land in a position such that it’s instruments could perform long-term analysis of the comet However, it did capture the first images from a comet's surface, and several of the instruments were able to send back data regarding the composition of the surface.
HALLEY’S COMET Slide 21	Arguably the most famous short-period comet, which is observable from Earth every 75-76 years Recorded by astronomers since 240 BCE with clear reference made by Chinese, Babylonians, and medieval European astronomers Named by English astronomer Edmond Halley in 1705, who used concepts proposed by Isaac Newton in his Philosophiæ Naturalis Principia Mathematica to prove the gravitational fields of Jupiter and Saturn made it a periodic comet Last approach was in 1986, and the next appearance will be in 2061.
INTERPLANETARY DUST Slide 22	Interplanetary dust in our solar system comes from two main sources: - Comets, as they approach the Sun - The collision of asteroids in the main asteroid belt Roughly in the nano and micrometer size range (one-billionth and one-millionth of a meter, respectively) Some is also interstellar dust Its presence likely played a critical role in the formation of our solar system Scientific examination may reveal clues about that early phase.

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