Nov. 1st - Asteroids & Meteorites

Question 1

Comparing asteroids, comets, and dwarf planets

What’s the difference between an asteroid, a comet, and a dwarf planet?

Simple definitions

Answer 1

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

Question 2

Comparing asteroids, comets, and dwarf planets

Asteroids

Answer 2

• 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

Question 3

Comparing asteroids, comets, and dwarf planets

First discoveries of asteroids:

1801 - “Minor Planets”

Answer 3

• 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.”

Question 4

Comparing asteroids, comets, and dwarf planets

How are asteroids named?

Answer 4

• 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.

Question 5

Comparing asteroids, comets, and dwarf planets

Few ancient cultures made any attempt to explain comets in astronomical terms

1577 - Tycho

Answer 5

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.

Question 6

Comparing asteroids, comets, and dwarf planets

Few ancient cultures made any attempt to explain comets in astronomical terms

How did Newton go against Tycho?

Answer 6

A century later, Newton correctly deduced that comets orbit the Sun.

Question 7

Comparing asteroids, comets, and dwarf planets

Few ancient cultures made any attempt to explain comets in astronomical terms

1705 - Halley

Answer 7

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

Question 8

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…

Answer 8

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.

Question 9

Comparing asteroids, comets, and dwarf planets

Dwarf Planets

PLuto

Answer 9

• 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.

Question 10

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?

Answer 10

• 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.

Question 11

How does the Kuiper Belt challenge the current definition of a planet?

Answer 11

• 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

Question 12

What are meteors and meteorites?

Meteor

Answer 12

(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.

Question 13

What are meteors and meteorites?

Meteorite

Answer 13

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.

Question 14

Meteorite falls:

Meteorite Theories: Anaxagoras

Answer 14

• 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.

Question 15

Unless you actually see a meteorite fall, it can be difficult to distinguish a meteorite from an Earth rock.

3 clues can help:

Answer 15

• 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

Question 16

Meteorite Evidence:

Answer 16

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

Question 17

Sizes & Shapes of Asteroids:

Answer 17

• 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.

Question 18

How can asteroid size be estimated?

Answer 18

• 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.

Question 19

The most direct way to measure a distant object’s mass is to…

Answer 19

…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.”

Question 20

How is it beneficial to know the masses of some asteroids?

(in finding the masses of others)

Answer 20

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).

Question 21

How can density allow insights into an asteroid’s origins and make-up?

Answer 21

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

Question 22

Asteroids are made mostly of…

Answer 22

…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

Question 23

Asteroids up close:

Dawn

Answer 23

ADD

Question 24

Asteroids up close:

Vesta

Answer 24

• 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.

Question 25

Asteroids up close:

Ceres

Answer 25

• As the largest asteroid, Ceres contains nearly as much mass as the rest of the asteroid belt put together. Its low density and location in the outer part of the asteroid belt suggest that water ice makes up a significant fraction of its composition.
• Dawn images show bright spots on some craters that appear to be made of salts and other highly reflective mineral deposits.
• Another surprise is the presence of a single tall mountain; no one yet knows how this mountain formed. The rest of the surface looks more as expected for a dwarf planet: abundant craters and some tectonic fracturing.

Question 26

Meteorites tell us about asteroids and the early solar system

Primitive meteorites

Answer 26

• Primitive meteorites are “primitive” in the sense of being remnants from the birth of our solar system, essentially unchanged since they first accreted in the solar nebula.
• Radiometric dating shows them to be the oldest rocks in the solar system.

Question 27

Meteorites tell us about asteroids and the early solar system

Primitive meteorites - 2 subtypes

Answer 27

• Stony primitive meteorites are composed of rocky minerals with a small but noticeable fraction of pure metallic flakes mixed in.
• Carbon-rich primitive meteorites are also rocky but contain substantial amounts of carbon compounds and, sometimes, a small amount of water bound to the rock.

Question 28

Meteorites tell us about asteroids and the early solar system

Processed meteorites

Answer 28

• Processed meteorites apparently once were part of a larger object that “processed” the original material of the solar nebula into another form.
• Radiometric dating confirms that processed meteorites are slightly younger than primitive meteorites, just as we would expect.

Question 29

Meteorites tell us about asteroids and the early solar system

Processed meteorites - 2 subtypes

Answer 29

• Metal-rich processed meteorites are made mostly of high-density iron and nickel mixed with smaller amounts of other metals. That is, they resemble the terrestrial planet cores in composition.
• Rocky processed meteorites have lower densities and are made of rock with compositions resembling that of terrestrial mantles and crusts. A few have compositions remarkably close to that of the basalts [Section 9.2] that erupt from terrestrial volcanoes.

Question 30

Lessons from primitive meteorites:

The metal flakes in primitive meteorites may represent…

Answer 30

…the tiny particles that first condensed from the gas of the solar nebula.

Question 31

Lessons from primitive meteorites:

The small roundish features visible in the meteorite may be…

Answer 31

…solidified droplets splashed out from nearby small planetesimals as they accreted

Question 32

Why are some primitive meteorites stony in composition while others are carbon-rich, and why do some contain water?

Answer 32

• Both theory and observation indicate that the answers depend on where the meteorites formed in the solar nebula.
• With regard to stony versus carbon-rich primitive meteorites, the nebular theory tells us that most meteorites accreted inside the frost line and are therefore made of metal and rock.
• However, models tell us that, beyond about 2.5 AU from the Sun, temperatures in the solar nebula were low enough for condensation of carbon compounds.
• Asteroids that formed beyond the frost line contain a small but significant fraction of water (Ceres is an example)—enough to potentially make them the objects that brought the terrestrial planets the water, carbon-rich compounds, and other ingredients necessary for habitability and for life.
• These models suggest that carbon-rich primitive meteorites come from beyond that distance, while stony primitive meteorites come from closer in.

Question 33

Lessons from Processed Meteorites:

Answer 33

• We conclude that these meteorites are fragments of larger asteroids that underwent differentiation, in which their interiors melted so that metals sank to the center and rocks rose to the surface. This idea explains the two subtypes.
• The metal-rich meteorites must come from large asteroids that underwent differentiation but were subsequently shattered in collisions. In this sense, they represent pieces of a “dissected planet,” making them valuable both because we lack the technology to drill for core samples on Earth and because they provide direct proof that large worlds really do undergo differentiation, confirming what we infer from seismic studies of Earth
• Many of them—including the ones that appear to come from Vesta—are so close in composition to volcanic rocks on Earth that they must have been made by lava flows. We conclude that these meteorites are rocks from the surfaces of large asteroids that once were volcanically active, probably chipped off by collisions with smaller asteroids

Question 34

Why is there an asteroid belt?

Answer 34

• The vast majority of the asteroids orbit the Sun in the asteroid belt between the orbits of Mars and Jupiter
• These asteroids orbit the Sun in the same direction as the planets, though their orbits tend to be more elliptical and more highly inclined to the ecliptic plane (up to 20°–30°) than those of planets
• Aside from the main asteroid belt, the only other major asteroid groupings are the two sets of Trojan asteroids, which share Jupiter’s 12-year orbit around the Sun (with one group always staying 60° ahead of Jupiter in its orbit and the other always staying 60° behind).
• In addition, a relatively small number of asteroids have orbits that take them through the inner solar system, where they are probably “impacts waiting to happen”

Question 35

Why are asteroids concentrated in the asteroid belt, and why didn’t a full-fledged planet form instead?

Answer 35

• The answer lies with gravitational effects of Jupiter.
• Virtually all planetesimals that formed inside the orbit of Mars eventually accreted onto one of the inner planets.
• But planetesimals that formed between Mars and Jupiter were strongly influenced by orbital resonances with Jupiter, and only a small fraction of these planetesimals ended up with orbits that have allowed them to remain in the asteroid belt to this day.
• Recall that an orbital resonance occurs whenever two objects periodically line up with each other. In the asteroid belt, an orbital resonance occurs whenever an asteroid has an orbital period that is a simple fraction of Jupiter’s orbital period, such as 12,14, or 25.

Question 36

How do Orbital resonances also explain why no planet formed between Mars and Jupiter?

Answer 36

Early in the solar system’s history, this region probably contained more than enough rocky material to form another terrestrial planet.

However, resonances with the young Jupiter disrupted the orbits of planetesimals in this region, sometimes sending them crashing into each other and sometimes kicking them out of the region. Once kicked out, the planetesimals ultimately either crashed into a planet or moon or were flung out of the solar system or into the Sun.