Lec 17 Flashcards
(21 cards)
What’s the difference between an
asteroid, a comet, and a dwarf planet?
Asteroids and comets:
-Both orbit the Sun and are too small to
be considered planets, but asteroids are rocky while comets are ice-rich
Dwarf planet:
-A dwarf planet is a celestial body that:
Orbits the Sun
Has enough mass for its gravity to make it nearly round (hydrostatic equilibrium)
Has not cleared its orbital neighborhood of other objects
Asteroids are rocky leftover planetesimals that orbit the Sun, comets are icy leftover planetesimals, and dwarf planets can be either
asteroids or comets that are large enough to be round.
meteor
a flash of light
caused by a particle of dust or rock entering our atmosphere
at high speed, not the particle itself. Meteors are sometimes
called shooting stars or falling stars, because some people
once thought they really were stars falling from the sky.
meteorites
a meteor caused by a
chunk of rock large enough to survive the plunge through
our atmosphere and leave a meteorite (which means “associated with meteors”) on the ground. Those cases make
unusually bright meteors, called fireballs
how can we determine an asteroid’s size from its position in its orbit?
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
to observe its
gravitational effect on another object, and to date this is possible only for the relatively few asteroids visited by spacecraft
and for those that have smaller asteroids as tiny orbiting
“moons.”
What can we learn from primitive meteorites?
Because primitive meteorites represent samples of early accretion, they hold key clues to when and how the solar system formed
What can we learn from processed meteorites?
Their compositions look similar to those of the cores, mantles, or crusts of the terrestrial worlds.
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 why we find two types of processed meteorites
why is there an asteroid belt?
The vast majority of the asteroids orbit the sun in the asteroid belt between the orbit 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 inclined to the ecliptic plane of those planets
Why are asteroids concentrated in the asteroid belt, and why didn’t a full-fledged planet form instead?
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 ½, ¼, or 2/5
In that case, the asteroid experiences repeated tugs from Jupiter that tend to nudge it out of that orbit.
orbital resonance
explains why no planet formed between Mars and Jupiter.
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.
comet composition
Comet composition
Chunks of ice mixed with rocky dust and more complex chemicals
For a comet plunging inward, the frozen center is called the nucleus
As it accelerates toward the Sun, its surface temp increases, and ices begin to vaporize into gas that easily escapes the comet’s weak gravity
Some of the escaping gas drags dust particles away from the nucleus, and the gas and dust, create a huge, dusty atmosphere called a coma
Coma grows as comet continues into the inner solar system, and some gas and dust is pushed away from sun, forming comet’s tails (point away from the Sun)
Comets have 2 tails:
Plasma tail: consists of gas that’s ionized by ultraviolet light from Sun and pushed outward by solar wind
Extends almost directly away from sun at all times
Dust tail: consists of dust-size particles that are unaffected by the solar wind and instead are pushed outward by weaker pressure of sunlight (radiation pressure)
How could a comet be prevented from growing a tail?
In close interaction with the sun, comet may shed a layer of material
Dust that’s too heavy to escape gathers on surface, this darkens comet and may block escape of interior gas, which would prevent a new tail
what was the source of Earth’s water?
Primitive meteorites that contain water have a deuteriumto-hydrogen ratio more similar to Earth’s, suggesting that
Earth’s water came from impacts of asteroids that formed
in the outer regions of the asteroid belt (because that is
where these meteorites came from)
when do meteor showers occur
recur at about the same time each year
because the orbiting Earth passes through a particular comet’s orbit at the same time each year
the meteors of a meteor shower generally appear to radiate from a particular direction
in the sky, for essentially the same reason that snow or heavy rain seems to come from a particular direction in front of a moving car
Because more meteors hit Earth from the front than from behind (just as more snow hits the
front windshield of a moving car), meteor showers are best observed in the predawn sky, when part of the sky faces in the direction of Earth’s motion.
where do comets come from?
These comets must come plunging sunward from the vast spherical region of space that we
call the Oort cloud.
-collection of many individual comets
The leftover planetesimals that cruised the spaces between Jupiter, Saturn, Uranus, and Neptune were doomed to suffer either a collision or a close gravitational encounter with one of the young jovian planets.
Recall that when a small object passes near a large planet, the planet is hardly affected but the small object may be flung off at high speed
The planetesimals that escaped being swallowed therefore tended to be flung off in all directions
-some may have been cast away at such high
speeds that they completely escaped the solar system
-the rest ended up on orbits with very large avg. distances from the Sun, becoming the comets of the Oort cloud
how was it believed that pluto’s moons were created?
astronomers hypothesize that Pluto’s moons were created by a giant impact similar to the one thought to have formed our Moon
A large comet crashing into Pluto may
have blasted away its low-density outer layers, which then formed a ring around Pluto and eventually re-accreted to
make Charon and the smaller moons.
Such an impact may also explain why Pluto rotates almost on its side.
what caused pluto and charon to rotate synchronously with each other?
The mutual tidal forces acting between Pluto and Charon long ago made them rotate synchronously with each other [Section 4.5], which means that Pluto’s “day” is the
same length as Charon’s “month” (orbital period) of 6.4 Earth days.
This synchronous rotation also means that
Charon is visible from only one side of Pluto
what does triton suggest about the kuiper belt?
First, Triton is larger than both Pluto and Eris
(about 15% larger in diameter), suggesting that the Kuiper belt may once have contained more and larger objects than it does today.
Second, Triton shows signs of significant past
or present geological activity, reinforcing the idea that distant, ice-rich worlds can be much more geologically active
than we might have guessed from their sizes alone.
composition of arrokoth
-made up of two disk-shaped icy objects stuck together
Scientists think that this type of structure may be common in the Kuiper belt because of the way accretion occurred in the early solar system.
Recall that small planetesimals should have
collided and sometimes stuck together during relatively early stages of accretion.
-in regions of the solar system closer to the Sun, this process continued and ultimately built the planets
But in the distant Kuiper belt, this process slowed and often stopped due to the scarcity of building blocks, making the growth of larger objects like Pluto relatively rare
-evidence of this comes from the small number of craters on Arrokoth’s smooth surface
–(tells us that objects in this region of the solar
system were too spread out to suffer many
collisions)
why is there a kuiper belt?
First, the models suggest that orbital resonances between the jovian planets may have played a role not only in shaping the Kuiper belt but also in the formation
of the planets themselves.
Second, and more intriguingly, the models suggest that some of the planets of our solar
system may have undergone migration—moving to closer or more distant orbits—after their initial formation.
In particular, the models suggest that neither Uranus nor Neptune could have formed in their current orbits, because the density of the solar nebula at that distance was too low for such a large planet to have accreted.
Instead, all four jovian planets probably were much closer together when they formed, and shared the solar system with many other
planet-size planetesimals.
