Class 17: Small bodies of the solar system: comets, asteroids, and dwarf planets

Summary

• Comets: Icy bodies that release gas or dust; they often develop tails when near the Sun.

• Asteroids: Rocky objects primarily found in the asteroid belt between Mars and Jupiter; they vary in size and composition.

• Dwarf Planets: Celestial bodies that orbit the Sun and are similar to planets but do not clear their orbital path; examples include Pluto and Eris.

Asteroids

• Composition: mainly rock and metal

• Orbits are elliptical: the time of an orbit (or the period) depends on its semi-major axis

• 25 million larger asteroids have been identified in our solar system, with a significant number exceeding 1 kilometer in diameter.

• 600 larger asteroids have been identified to have a diameter of 50km

Jupiter’s affect on the orbit of the asteroids

• orbits in exact resonance with Jupiter get displaced

Ceres

• is the largest object in the asteroid belt and is classified as a dwarf planet; looks like the moon, or Mercury

• Although it’s a dwarf planet, Ceres is massive enough to have achieved hydrostatic equilibrium— the strength of gravity is stronger than the strength of the rock/ice it’s made of— giving it a nearly round shape and distinguishing it from other smaller asteroids.

• There is a large identifiable white spot, which is salt that deposits from a cryo-volcano, which is still active after a meteor strike

• There’s also a 4km tall mountain called Ahuna Mons

• there’s also landslides on the crater wall, taken by the satellite Dawn

• This geological activity suggests that Ceres has a complex interior structure, possibly containing a subsurface ocean, which intrigues scientists studying the potential for life beyond Earth.

Ceres’ Interior

• It has a rocky core followed by a liquid brine underneath an icy crust— topped off with dark dust on its surface

• this explained why Ceres was so geologically active- it’s not just a rock

Vesta

• The largest asteroid, around half the size of Ceres

• Does not have a spherical shape, so it is not considered a Dwarf planet

Vesta’s Interior

• nickle/iron core followed by a silicate mantle and crust, which contribute to its unique geological features and surface composition.

• at one point, Vesta was hot enough to have a liquid interior, but its layers formed due to differentiation (making it not homogenous

Eros

• Gravity measurements indicate that it’s homogenous

• it has cratering, meaning its surface is old

OSIRIS-Rex Mission to Asteroid Bennu

• OSIRIS-Rex observed small objects flying off the asteroid

• Tracking these particles were used to identify Bennu’s interior- findings suggest that it is low-density, meaning it may be hollow

Kuiper’s Belt

• 20-200x more massive than the Asteroid Belt

• Includes several known dwarf planets, including Pluto

• Formed outside the frost line

Pluto and its Moons

• Pluto is very distant and quite unique: it has a cratered surface, nitrogen ice plains, and water ice mountains

• its orbital period (year) is 248 Earth years

• its orbit is inclined, meaning its not in the ecliptic plane

• it crosses Neptune’s orbit and is in a 3:2 resonance with Neptune, so they never collide

• Charon, Pluto’s moon, is very large for a moon orbiting a dwarf planet like Pluto; this moon has more craters than Pluto and its surface is mainly water ice

Comets

• Appear (although rarely) in the night sky

• Are visible for days

• have tails pointing roughly away from the sun

• often come back on a fixed period (decades to millenia)

  

Comet Tails

• Comet tails are formed by the sublimation of ices and the release of gas and dust as the comet gets closer to the sun (causing its composition to boil away from the heat giving it the tail)

• has “two” tails— its gas tail and its dust tail, both point roughly away from the sun

• gas/plasma tail is pushed directly away from the sun by the solar wind, creating a straight, narrow tail, while the dust tail, composed of larger particles, follows a more curved path that points slightly behind the gas tail due to the comet's trajectory and the influence of solar radiation pressure, as it is accelerated less.

Albedo

• refers to the reflectivity of a surface, indicating how much sunlight is reflected back into space versus how much is absorbed. A higher albedo means a surface reflects a greater proportion of sunlight, which can affect the thermal characteristics and visibility of celestial bodies such as comets and asteroids.

• some comets are almost perfectly pitch black

The Oort Cloud

• A huge spherical cloud of comets outside the Kuiper belt

• the comets in the Oort cloud were formed near Jupiter, but were kicked out by gravitational interactions

Instant Review Quiz: What is the main distinction between an asteroid and a comet? Answer: its composition

Meteors & Meteorites

• every night, millions of objects strike the Earth— most are as small as a grain of sand and burn up in the atmosphere

• sometimes, they do hit the earth, and are then called meteorites which can be large enough to be destructive

Cretaceous-Paleogene Extinction Event

• an asteroid or comet— around 30km across— struck the Earth, causing the extinction of most dinosaurs