Minor bodies I: Asteroids


Trans-Neptunian objects, centaurs, and trojans as of 2015 from Wikipedia
As we begin our survey of minor bodies of the Solar System, we need to remember that: For convenience, we recognize three major reservoirs of minor bodies: Some objects defy ready classification, including:

Objects that inhabit the inner Solar System are:


Asteroids


Asteroids to scale

Nomenclature: An asteroid's official name combines a name from Greco-Roman mythology, prefixed by a number representing its order in the discovery of asteroids. The first asteroid, 1 Ceres, was discovered by Giuseppe Piazzi in 1801, who regarded it as a new planet. The term "asteroid" referred to its telescopic appearance as a star-like point of light. The quick discovery of 2 Pallas (1802), 3 Juno (1904), and 4 Vesta (1807) made it clear that asteroids were not "normal" small planets but a new class of object.

The main asteroid belt lies between Mars and Jupiter. It consists of bodies that did not accrete into a larger planet, because of the gravitational influence of nearby Jupiter. There are >100,000 asteroids known, and it is expected that there are countless more that are too small to be detected. There are relatively few large asteroids. The largest body in the asteroid belt, 1 Ceres, is a dwarf planet with a diameter of 974 km. The total mass in the asteroid belt is only ~0.001 Earth masses.


Kirkwood gaps from Wikipedia
Origins: The proximity of the main asteroid belt to the orbit of Jupiter is no coincidence. Models indicate that as it neared its current size, Jupiter disrupted the accretion of planetesimals in its immediate vicinity by perturbing their orbits to prevent their achieving orbital stability. The result was frequent collisions. Some larger asteroids, including 1 Ceres and 4 Vesta, appear to be fully differentiated bodies with cores and mantles. Others are the fragmented remains of such bodies. Yet others are essentially undifferentiated chondrites.

Jupiter's continuing influence: The current distribution of asteroid orbits shows Jupiter's continuing influence. At some distances from the sun, an object's orbital period is a simple fraction of Jupiter's, creating an orbital resonance that tends to make the orbit unstable. The result is Kirkwood gaps - regions that are clear of asteroids.

These give us convenient reference points. For the purpose of this course:



Asteroid collision
Asteroids continue to collide into one another regularly, because their orbits are elliptical and somewhat inclined from the ecliptic plane. These collisions release many smaller chunks, some of which find their way to Earth as meteorites. This makes the asteroid belt one region of the Solar System that we can sample directly without having to go there.

What we know:

Without robot spacecraft, we would have two general sources of asteroid information:

Spectroscopy: Meteorites confirm what spectroscopic analysis tells up about the composition of asteroids - they are heterogeneous. Planetary scientists have developed a taxonomy of spectral types that includes:

Meteorites: Most meteorites originate in the main asteroid belt, and their composition generally confirms to the spectral types above. Prior to spacecraft missions, a consensus emerged that asteroids represented:

We now know at lease some asteroids have distinct crusts as a result of Day et al., 2009 report of a meteorite made of the intermediate igneous rock andesite. This was vindicated by data from the Dawn spacecraft at 4 Vesta revealing that it was fully differentiated, with a crust, mantle, and core (see Russell et al. 2013.)

Asteroid exploration:


Asteroids Ida and Dactyl from Wikipedia
Galileo on its way to Jupiter made the first up close asteroid observations as if flew past the main belt asteroids:


433 Eros from University of Wisconsin
NEAR Shoemaker rendezvoused with and orbited the near-Earth asteroid 433 Eros in 2000-2001. En route, the spacecraft flew past 253 Mathilde, also an S-type.


Surface of 433 Eros from University of Arizona
NEAR revealed a heavily cratered surface, and numerous boulders, including a house- sized boulder that has rolled down into the large crater. At the end of its mission, the NEAR vehicle, although not designed to return data from the surface, was "soft landed" on 433 Eros. This is one of the last images returned The image is 12 m high. The surface is rocky, with a heavy layer of regolith. The composition is similar to that of stony meteorites and other S-class asteroids.


25143 Itokawa from Wikipedia
Hayabusa (JAXA) launched in, 2003, rendezvoused with the tiny (0.535 km length) near-Earth S-type asteroid 25143 Itokawa in 2005, finally returning a sample capsule in 2010, returning roughly 1500 rocky grains of Itokawa material, making Itokawa the second (and last) Solar System object to have samples returned to Earth.

Itokawa was revealed to be:

The conclusion is that Itokawa is a rubble-pile of fragments held together by very weak gravity.


4 Vesta, 433 Eros, and 1 Ceres to scale from Wikipedia
Dawn launched on Sept. 27, 2007 and crossed the orbit of Mars to closely study two bodies in the main asteroid belt: the asteroid 4 Vesta (2011-2012) and the dwarf planet 1 Ceres (2015-present). 4 Vesta orbits at 2.36 AU, just inside the 2.5 AY Kirkwood gap, whereas 1 Ceres, at 2.76 AU inhabits the central region of the asteroid belt, and is distant enough that ices condensing directly form the solar nebula should be major compositional constituents.

Dawn is also a technological breakthrough:



S-type Asteroid 433 Eros from Rediff News

Asteroid Surface features:

Asteroids are mostly irregularly shaped, heavily cratered, and covered in a dusty regolith, indicating that they are geologically dead. On inspections, asteroids present certain distinct peculiarities (as in 433 Eros - right.):


4 Vesta from Wikipedia

Special Case 1: 4 Vesta

After two years of study by the Dawn mission, 4 Vesta begins to come into focus. 4 Vesta's specs:



Pitted terrain in crater floor on 4 Vesta from NASA
Surface features:


1 Ceres from Wikipedia

Special case 2: 1 Ceres

1 Ceres is the largest object in the main asteroid belt and contains a third of its mass. It is the only asteroid to qualify as an IAU "dwarf planet," being massive enough to pull itself into a sphere.

Orbit:

Composition: 1 Ceres' surface spectrum makes it a C-type asteroid (i.e. chondritic), but with evidence of hydrated minerals (minerals that incorporate water in their molecular structure) such as clay minerals. Where would the water for this come from?

1 Ceres' density of 2.161 kg/m3 (cf. 3.456 kg/m3 for 4 Vesta) indicates that water ice makes up a significant percentage of its volume. At 2.7875 AU, 1 Ceres is:

Current models of 1 Ceres' interior call for:


Occator crater from Wikipedia
Oddities and Puzzles:

Bright Spots: Visible even through fuzzy Hubble images, these turn out to be deposits of some kind of salt (possibly magnesium sulfate hexahydrite (hydrated epsom salt - MgSO4.6H2O) (Landau, 2015) or sodium carbonate (Na2CO3) (De Sanctis et al., 2016).

How did it get there? Possibly the evaporite crust left after freshly exposed ice or brines sublimated.



Ahuna Mons from Wikipedia
Ahuna Mons: A 5 km high mountain in the middle of nothing formed by - what? Possibly local tectonic response to overpressuring of freezing of subsurface water (Neveu and Desch, 2015).

Key concepts and vocabulary:
Additional reading: