Nov. 1st - Asteroids & Meteorites Flashcards
(36 cards)
Comparing asteroids, comets, and dwarf planets
What’s the difference between an asteroid, a comet, and a dwarf planet?
Simple definitions
Today, we use relatively simple definitions of asteroids and comets:
* Both orbit the Sun and are too small to be considered planets
* But asteroids are rocky while comets are ice-rich
Comparing asteroids, comets, and dwarf planets
Asteroids
- Asteroid means “starlike,” but there’s really nothing starlike about asteroids; the name is an artifact from the time when all we knew about them was that, like stars, they appeared as points of light in telescopes
- Result of their small sizes - appear point-like to most telescopes
Comparing asteroids, comets, and dwarf planets
First discoveries of asteroids:
1801 - “Minor Planets”
- 1801, Ceres (originally called a planet)
- Pallas, Juno, Vesta discovered in next 7 years
- As astronomers realized how small these objects were compared to the other planets, they came to be called “minor planets.”
Comparing asteroids, comets, and dwarf planets
How are asteroids named?
- Newly discovered asteroids first get a provisional name based on the discovery year and month and order of discovery.
- For example, the first asteroid discovered in January 2027 would be called Asteroid 2027 AA. (When letters run out, numbers are added after the letters.)
- Once an asteroid has been tracked long enough for its orbit to be calculated from the law of gravity, its discoverer may give it a name, subject to approval by the International Astronomical Union.
Comparing asteroids, comets, and dwarf planets
Few ancient cultures made any attempt to explain comets in astronomical terms
1577 - Tycho
In fact, comets were generally thought to be within Earth’s atmosphere until 1577, when Tycho Brahe used observations made from different locations in Europe to prove that a comet lay far beyond the Moon.
Comparing asteroids, comets, and dwarf planets
Few ancient cultures made any attempt to explain comets in astronomical terms
How did Newton go against Tycho?
A century later, Newton correctly deduced that comets orbit the Sun.
Comparing asteroids, comets, and dwarf planets
Few ancient cultures made any attempt to explain comets in astronomical terms
1705 - Halley
Then in 1705, English scientist Edmond Halley (1656–1742) used Newton’s law of gravitation to calculate the orbit of a comet that had been seen in 168 years
Comparing asteroids, comets, and dwarf planets
We now know that the vast majority of comets do not have tails and never come anywhere close to Earth. Instead…
They remain in the outer reaches of our solar system, orbiting the Sun far beyond the orbit of Neptune in the two vast reservoirs we call the Kuiper belt and the Oort cloud
The comets that appear in the night sky are the rare ones that have had their orbits changed by the gravitational influences of planets, other comets, or stars passing by in the distance, causing them to venture into the inner solar system.
Most of these comets will not return to the inner solar system for thousands of years, if ever.
A few happen to pass near enough to a planet to have their orbits changed further, and some (like Halley’s) end up on elliptical orbits that periodically bring them close to the Sun.
Comparing asteroids, comets, and dwarf planets
Dwarf Planets
PLuto
- Pluto was recognized as a misfit among the planets because of its small size, ice-rich composition, and an orbit much more eccentric and more inclined to the ecliptic plane than that of any of the other planets
- Pluto began to seem more and more like an unusually large comet: comets coming from the region of the Kuiper belt, and Pluto orbits the Sun near the middle of this region.
Comparing asteroids, comets, and dwarf planets
In the 1990s, astronomers began to discover other Pluto-like objects in this region, such as Eris, with the only major difference being that these other objects were smaller than Pluto
How did this affect the definition of a planet?
- In 2006, the International Astronomical Union created the dwarf planet category to accommodate Pluto, Eris, and other “small bodies” that are large enough to be round
- However, because the definition depends on roundness and we do not always know the precise shape of a distant object, dozens of other objects may yet join the list.
How does the Kuiper Belt challenge the current definition of a planet?
- The Kuiper belt, where all the objects, from the smallest boulders to the largest dwarf planets, probably share the same basic composition of ice and rock.
- In other words, they are all essentially comets of different sizes.
- That is why we often refer to all of them as comets of the Kuiper belt
What are meteors and meteorites?
Meteor
(which means “a thing in the air”; note the similarity to meteorology, which is the study of weather) is only a flash of light caused by a particle of dust or rock entering our atmosphere at high speed, not the particle itself.
What are meteors and meteorites?
Meteorite
The vast majority of the particles that make meteors are no larger than peas and burn up completely before reaching the ground.
Only in rare cases is a meteor caused by a chunk of rock large enough to survive the plunge through our atmosphere and leave a meteorite
Those cases make unusually bright meteors, called fireballs.
Meteorite falls:
Meteorite Theories: Anaxagoras
- Stories of “fallen stars” led the ancient Greek philosopher Anaxagoras to conclude that meteorites fell from the heavens and to argue that planets and stars must be flaming rocks in the sky
- This also made him the first person in history known to believe that the heavens and Earth are made of the same materials, even though his guess about the nature of planets and stars was not quite correct
- Today we know that rocks really do fall from the heavens.
- More than 1000 meteorite falls have been directly observed, and tens of thousands of meteorites have been found and cataloged. Meteorites are often blasted apart in their fiery descent through our atmosphere, scattering fragments over an area several kilometers across.
Unless you actually see a meteorite fall, it can be difficult to distinguish a meteorite from an Earth rock.
3 clues can help:
- Meteorites are usually covered with a dark, pitted crust resulting from their fiery passage through the atmosphere
- Some have an unusually high metal content, enough to attract a magnet hanging on a string
- The ultimate judge of extraterrestrial origin is laboratory analysis: Meteorites often contain elements such as iridium that are very rare in Earth rocks, and even common elements in meteorites tend to have different ratios among their isotopes than are found in rocks from Earth
Meteorite Evidence:
The most direct evidence comes from the relatively few meteorites whose trajectories have been observed or filmed as they fell to the ground.
In every case so far, these meteorites clearly originated in the asteroid belt.
In a few cases, scientists have identified meteorites with compositions that appear to match that of either the Moon or Mars, and careful analysis makes us very confident that these meteorites were indeed chipped off these worlds.
This makes sense: Moderately large impacts can blast surface material from terrestrial worlds into interplanetary space, where the rocks can orbit the Sun until they come crashing down on another world. Calculations show that it is not surprising that we should have found a few lunar meteorites (from the Moon) and martian meteorites (from Mars) in this way
Sizes & Shapes of Asteroids:
- An asteroid’s shape depends largely on the strength of its gravity.
- Only large asteroids have gravity strong enough to have molded them into somewhat spherical shapes, and only Ceres is round enough to be currently counted as a dwarf planet; however, the next two largest asteroids (Pallas and Vesta) are not too far from spherical.
- The gravity of smaller asteroids is too weak to have reshaped their rocky material, leaving them looking much like potatoes.
- In some cases, objects that appear to be single asteroids are probably two or more distinct objects held in contact by a weak gravitational attraction, while other small asteroids are little more than weakly bound piles of rubble.
How can asteroid size be estimated?
- Size can be estimated through careful measurements of an asteroid’s brightness, which depends on its size, distance, and reflectivity.
- For example, if two asteroids at the same distance have the same reflectivity, the one that appears brighter must be larger in size.
- We can determine an asteroid’s distance from its position in its orbit. Reflectivity can be measured by comparing the asteroid’s visible brightness, which comes from the sunlight it reflects, to its infrared brightness, which depends on the asteroid’s temperature and hence tells us how much sunlight it absorbs
- Astronomers can then use the reflectivity and distance to calculate the asteroid’s size.
The most direct way to measure a distant object’s mass is to…
…observe its gravitational effect on another object, and to date this possiblity only for the relatively few asteroids visited by spacecraft and for those that have smaller asteroids as tiny orbiting “moons.”
How is it beneficial to know the masses of some asteroids?
(in finding the masses of others)
Astronomers know precise masses for only a few dozen asteroids, but these cases are important because they allow us to estimate masses of other asteroids and to calculate average density for those with known masses (by dividing by the volumes known from size measurements).
How can density allow insights into an asteroid’s origins and make-up?
Densities give some insight into composition, and we can learn more from spectra; recall that spectra of distant objects contain spectral lines that are essentially “fingerprints” left by the objects’ chemical constituents
Asteroids are made mostly of…
…metal and rock, because they condensed within the frost line in the solar nebula.
- Those near the outskirts of the asteroid belt contain larger proportions of dark, carbon-rich material, because this material was able to condense at the relatively cool temperatures found in this region of the solar nebula but not in the regions closer to the Sun; some even contain small amounts of water, telling us that they formed close to the frost line
Asteroids up close:
Dawn
ADD
Asteroids up close:
Vesta
- Vesta’s south polar crater has a central mountain formed from the rebound after the impact that made the crater. The impact excavated so deeply into Vesta’s interior that it should have blasted out substantial amounts of rock; the rock that shows near the crater bottom has a composition matching that expected deep inside terrestrial worlds.
This fact has helped confirm suspicions that Vesta is massive enough to have undergone differentiation, giving it a metallic core, a low-density rocky crust, and a mantle in between
- The spectral signature of the crater-bottom rock also matches that of many small asteroids and of many meteorites that have been found on Earth, suggesting that these asteroids and meteorites are pieces of Vesta that were blasted away by the impact that formed the south polar crater.
- Vesta still presents numerous mysteries. For example, some regions of the surface show the spectral signature of volcanic rock, and this signature is also present in the asteroids and meteorites that appear to have come from Vesta. However, scientists have not yet identified any ancient volcanoes or lava flows in the images of Vesta.
