Planetesimals

Asteroids

Ida, an asteroid

Rocky and metallic, asteroids are minor planets mostly found in the asteroid belt between the orbits of Mars and Jupiter. Apollo and Aten asteroids cross Earth’s orbit and Trojan asteroids in the Lagrangian points are with in 60 degrees of Jupiter’s orbit. If asteroids are far from the Sun, their compositions are similar to that of comets (carbonaceous ice). If asteroids are closer to the Sun, their compositions are molten ice and iron core. Asteroids that crash into other cosmic objects are called killer asteroids. The three major types of asteroids are the C-type (carbonaceous, 75% of known asteroids), the S-type (silicaceous, 17%), and the M-type (metallic, 8%). Asteroids and comets are now classified as “small solar-system bodies.”

Meteors, Meteorites, and Meteoroids

Meteoroid

Meteoroids are debris in space from comets or asteroids; meteors are shooting stars or fire balls in air; meteorites are meteoroids that invade Earth’s atmosphere and impact the ground; micrometeorites are perfect shiny spheres microscopic in size and the major cause of small-scale erosion on the moon. The three major types of meteoroids are stony, stony iron, and iron.

  • Several meteors can be seen per hour on any given night; when this number increases dramatically, these events are called “meteor showers” that occur annually or at regular intervals as the Earth passes through the trail of dusty debris left by a comet
  • The Perseids peak around August 12 every year; each Perseid meteor part of the comet Swift-Tuttle that swings around the Sun every 135 years
  • Other meteor showers and their associated comets: Leonids (Tempe-Tuttle), the Aquarids and Orionids (Halley), and the Taurids (Encke)
  • Comet dust in meteor showers burns up in the atmosphere before reaching the ground
  • Most meteorites no bigger than an average Earth rock
  • Large meteorites can cause extensive destruction: Barringer Meteor Crater in Arizona (1,000 meters, 50,000 years old), asteroid impact which created the 300 km Chicxulub crater on Yucatan Peninsula (65 million years ago)
  • Ann Hodges of Sylacauga, Alabama was severely bruised by a 3.6 kilogram stony meteorite that crashed through her roof in November, 1954
  • Meteorites have a “burned” exterior, formed as the meteorite is melted by friction as it passes through the atmosphere
  • Three types of meteorites: “irons,” “stones,” “stony-irons”
  • More than 30,000 meteorites found on Earth, 99.8% came from asteroids
  • Evidence for an asteroid origin includes: orbits calculated from photographic observations of meteorite falls project back to the asteroid belt, spectra of several classes of meteorites match those of some asteroid classes
  • All but rare lunar and Martian meteorites are very old, 4.5-4.6 million years
  • Only one group of meteorites can be traced to a specific asteroid; eucrite, diogenite, and howardite igneous meteorites traced to third largest asteroid Vesta
  • Meteorites and asteroids that fall on Earth are of the original diverse materials from which planets formed; tells the conditions and processes during the formation and earliest history of the solar system
  • Remaining 0.2% of meteorites split equally between meteorites from the Moon and Mars
  • 35 known Martian meteorites blasted off Mars by meteoroid impacts; all igneous rocks crystallized by magma
  • Controversy of whether structures in meteorite ALH84001 might be evidence of fossil Martian bacteria
  • 36 lunar meteorites similar in mineralogy and composition to Apollo Moon rocks, but come from other parts of the Moon

METEORITE DATING

Scientists measure the age of meteorites with the decay of radioactive isotopes. What is an isotope? Isotopes are elements with the same number of protons but a different number of neutrons. For instance, carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Some isotopes are very unstable and tend to decay into lighter elements by alpha or beta particle decay. Scientists use the half-life of certain elements to date objects. First, scientists must determine to isotope to use by examining the elemental composition of the object. For meteorites, scientists generally use Rubidium-87/ Strontium-87 decay, which has a half-life of 49 billion years. Rubidium-87 decays into Strontium-87. So if the object has 50% Rubidium-87 and 50% Strontium-87  (only formed by decay process), then the object is 49 million years old. Since some Strontium-87 may have been present originally, scientists use Strontium-86, whose content remains the same, as a reference. Determining the ratio between Rubidium-87/ Strontium-86 and Strontium-87/ Strontium-86 via mass spectrometer (vaporizes a tiny portion of the meteorite to form ions; the ions are then separated by mass in a magnetic field), scientists can then calculate the amount of each isotope present in the meteorite, and thus the age of meteorites. Although radioactive dating is the best method for scientists to date meteorites, many factors, such as the amount of sunshine or heavy rain, can affect measurements.

TIMELINE

4.55 billion years ago: Formation age of most meteorites, age of the solar system

65 million years ago: Chicxulub impact leads to the extinction of dinosaurs and 75 percent of animals on Earth

50,000 years ago: Age of Barringer Meteor Crater in Arizona

1478 BC: First recorded observation of meteors

1794 AD: Ernst Friedrick Chladni publishes first book on meteorites

1908 (Tunguska), 1947 (Sikote Alin), 1969 (Allende and Muchison), 1976 (Jilin): Important 20th century meteor falls

1969: Discovery of meteorites in a small area of Antarctica leads to annual expeditions by US and Japanese teams

1982-1983: Meteorites from the Moon and Mars are identified in Antarctic collections

1996: A team of NASA scientists suggests that Martian meteorite ALH 84001 may contain evidence of microfossils from Mars

2005: NASA’s Mars Exploration Rover Opportunity finds an iron meteorite on Mars

Comets

Halley’s Comet

A comet has a head (nucleus and coma) and tails (dust, iron, and sodium). The nucleus is a few miles in diameter and composed of ice with dust mixed in, hence the term dirty snow ball. The coma is gases vaporized from the nucleus by sublimation (solid to gas). The two types of tails are dust tails (sunlight reflected off particles) and ion tails (sunlight emitted by ions blown back by solar wind). Comets either come from the Oort Cloud (50,000 to 150,000 AU in radius, a billion swarming comet nuclei) or the Kuiper Belt (30 to 100 AU beyond Neptune). Nuclei detached by gravity are caught by the Sun’s gravitational field and pulled into orbits. Short period comets are less than 30 AU away and long period comets are thousands of AU away. The most famous comet, of course, is Halley’s Comet, which is only visible every 75 or 76 years.

DWARF PLANETS

Pluto and Its Moons

PLUTO: Named for the Roman god of the Underworld, Pluto, originally a planet, is now classified by astronomers as a dwarf planet. With the most eccentric orbit and the greatest inclination to the ecliptic, Pluto has a revolution period of 249 years. For 20 of those 249 years, Pluto is closer to the Sun than Neptune. Pluto has density similar to satellites of outer planets, a rocky core with an icy surface of water and methane, and a tenuous atmosphere of methane. Pluto has four moons— Charon, Nix, Hydra, and P4; the latter three, however, are much smaller than the former.

THE DEBATE OVER PLUTOWhy Pluto IS a Planet— 1) Massive enough to be spherical; 2) Orbits of objects in Kuiper belt affected; 3) Mercury’s orbit is elliptical; 4) Asteroid hunters mistakenly discovered Neptune; 5) Each planet is unique; 6) Earth-moon system. Why Pluto IS NOT a Planet— 1) Smaller than other planets; 2) Gravity too weak to affect other planets; 3) Wildly elliptical orbit; 4) Nearly always mistaken for an asteroid in searches; 5) Properties do not follow the pattern; 6) Charon closely resembles Pluto and is large as moon

DEFINITION OF AND THREE CRITERIA OF PLANETS: 1) large enough that, when it is formed, condenses under its own gravity to be shaped like a sphere; 2) orbits a star directly and is not a moon of another planet; 3) clear its path of all debris in its neighborhood. To be a dwarf planet, the first two criteria must be met.

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