Minor bodies I: Asteroids
Trans-Neptunian objects, centaurs, and trojans as of 2015 from Wikipedia
- They are gravitationally dominated by the larger planets, especially the jovian ones,
- Their current locations are the result of historical interactions with them.
- Over the time scale of the Solar System's history, they can jump from one reservoir to another.
- The main asteroid belt: Home to the predominantly silicate, metallic, and chondritic asteroids
- The Trojans of the major planets: These are co-orbital bodies that either lead or trail the giant planets (Mostly Jupiter and Neptune, but there are a few others) at a separation of 60 degrees.
- The trans-Neptunian region: Home to the mysterious, but at least partly icy Kuiper Belt objects and other more distant and more mysterious ones.
- Centaurs - Object whose orbits cross those of the outer planets.
Objects that inhabit the inner Solar System are:
- Asteroids if primarily rocky
- Comets, if they contain significant ices that sublimate to form an atmosphere as they near the Sun.
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
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:
- <1.52 AU (Mars' semimajor axis) is the realm of near-Earth asteroids
- From 1.52 AU to the Kirkwood gap at 2.5 AU is the inner asteroid belt
- >2.5 AU is the outer asteroid belt.
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:
- C-type: Dark, non-reflective chondritic material that has never undergone differentiation
- E-type: Reflective chondrites mostly made of enstatite (MgSiO3), a flavor of pyroxene, an ultramafic mineral.
- M-type: Metallic nickel and iron - probably the remains of the cores of differentiated bodies.
- S-type: Stony - probably the remains of the mantles of differentiated bodies.
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:
- Chondritic material mostly unaltered since the beginning of the Solar System (C-type and E-type)
- Remnants of the cores and mantles of differentiated bodies that have been shattered by collisions (M-type and S-type).
Milestones in Asteroid exploration:
Asteroids Ida and Dactyl from Wikipedia
- 951 Gaspra (1991), a main asteroid belt S-type object.
- 243 Ida. It discovered 243 Ida's moon Dactyl, the first known satellite of an asteroid. (Recall that Galileo the astronomer discovered the first satellites of another planet!)
433 Eros from University of Wisconsin
Surface of 433 Eros from NASA
25143 Itokawa from Wikipedia
Itokawa was revealed to be:
- Very low density: Whereas a sample of Itokawa material might be 2.9 kg/cm3, the asteroid over all was 1.8 kg/cm3
- Largely lacking impact craters
- With a surface of rough boulders
4 Vesta, 433 Eros, and 1 Ceres to scale from Wikipedia
Dawn is also a technological breakthrough:
- The first planetary exploration vehicle to use xenon ion thrusters, capable of sustaining low accelerations over long periods of time.
- It is the first spacecraft ever to enter orbit around a Solar System object, leave that orbit, and travel to enter orbit around another object.
- Ran out of fuel in October, 2018.
162173 Ryugu from Wikipedia
101955 Bennu from Wikipedia
Both 162173 Ryugu and 101955 Bennu are loosely consolidated "rubble-piles."
- They demonstrate the effect of centripetal force, causing boulders to accumulate at the equators of small bodies with very little gravity to hold them in place. (See also shepherd moons of Saturn.)
- Visible craters are few and subdued in profile, suggesting the unconsolidated asteroids are shaken by impacts.
- Rocks on the surface of 162173 Ryugu display varying compositions, indicating that they experienced different histories before coalescing.
- 101955 Bennu sometimes releases particles from its surface although the mechanism by which this happens is not completely clear. Could be associated with the thermal fracturing (Molaro et al., 2020) of rocks, cased by contrasts in daytime (up to 400K) and nighttime (as low as 200K) temperature contrasts. (Bennu's "day" is 4.3 hours long.)
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.):
- Craters lack ejecta blankets: Not surprising considering that asteroids typically lack the gravity to prevent ejecta from either covering large areas of the asteroid or escaping into space.
- There are few small craters: Perhaps shaking from larger impacts events causes them to lose cohesion.
Regolith ponds on 433 Eros from NEAR Field Geology
- Ponds of regolith accumulate in depressions. What force moves it there? Possibly impacts, or as a result of levitation of dust from the accumulation of electrostatic charges from the solar wind, as observed directly by Apollo astronauts.
- On the daytime side of an object with no atmosphere or magnetic field, incoming solar radiation strips electrons from atoms, transforming them into positive ions.
- Electrons in the solar wind tend to adhere to atoms on the nighttime side, imparting negative charges.
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:
- Mass: 28% that of 1 Ceres and 0.00434% that of Earth.
- Density: 3.456 kg/m3 (surprisingly dense - between density of the moon and Io.)
- Semimajor axis: 2.36 AU
Pitted terrain in crater floor on 4 Vesta from NASA
- 4 Vesta's flattened southern region is a large impact structure the crater Rheasilvia. Its central peak is one of the tallest known mountains in the Solar System.
- Based on impact geochronology, the southern regions seems to be no more than 2 gy old, much younger than the north.
- A series of troughs encircle 4 Vesta's tropics. Possible connected to compression during the Rheasilvia impact.
- Water features: At 2.36 AU, 4 Vesta is a little too close to the Sun to contain ices as major components, however some craters have features related to water melted or vaporized by impacts.
- The big news: 4 Vesta is differentiated, with a distinct
- Basaltic crust (some of which comes to Earth as HED meteorites - a topic for later)
- Ultramafic mantle
- Metallic core.
Because of its differentiation, 4 Vesta is sometimes called "the littlest terrestrial planet," but in fact it more resembles the planetary embryos that coalesced in the early Solar System to form the planets.
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.
- Semimajor axis: 2.7675 AU (beyond the 2.5 AU Kirkwood gap)
- Orbital period: 4.60 years.
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:
- Far enough from the Sun to allow ices to be a big part of its mass
- Yet close enough at periapsis that they would be unstable at the surface and sublimate. (Indeed, there are numerous spectroscopic observations of the outgassing of water vapor (A'Hearn and Feldman, 1992).)
- roughly half of its volume to be a rocky core (possibly with a small metallic inner core Neumann et al., 2015)
- roughly half to be an icy mantle (perhaps with some residual liquid water (McCord and Sotin, 2005)). In fact, Marchi et al., 2016 note the improbably small number of large impact craters on Ceres, concluding that larger ancient impacts have been concealed by the viscous flowing of this ice.
- On top of this: a thin dusty crust of chondritic material and hydrated minerals. (Thomas et al., 2005)
Occator crater from Wikipedia
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
Key concepts and vocabulary:
- Main asteroid belt
- Trojan asteroids
- Hypothesis of origin through disruption of planetary accretion by Jupiter's gravity.
- Kirkwood gaps
- Orbital resonance
- Asteroid spectral types:
- C-type - chondritic
- E-type - enstatite
- M-type - metallic
- S-type - stony
- Asteroid exploration:
- Galileo, 951 Gaspra - 1991, 243 Ida - 1993
- NEAR Shoemaker, 433 Eros - 2000-2001
- Hayabusa, 25143 Itokawa - 2005
- Dawn, 4 Vesta - 2011-2012, 1 Ceres - 2015
- Hayabusa 2,- 2014 - 2020 - 162173 Ryugu
- OSIRIS REx - 2016-2023 - 101955 Bennu
- Asteroid surface characteristics:
- Craters without obvious ejecta
- Regolith ponds
- 4 Vesta:
- Flattened spheroid with huge impact structures in south
- Orbits in inner asteroid belt - ice not a major constituent
- Some features related to sublimation of ice
- Differentiated like a terrestrial planet
- 1 Ceres:
- Spherical "dwarf planet"
- Orbits in main asteroid belt
- Low density - ice is a major constituent
- Bright spots
- Lonely mountain - Ahuna Mons
- Michael F. A'Hearn and Paul D. Feldman. 1992. Water vaporization on Ceres. Icarus
- James M. D. Day, Richard D. Ash, Yang Liu, Jeremy J. Bellucci, Douglas Rumble III, William F. McDonough, Richard J. Walker, and Lawrence A. Taylor. 2009. Early formation of evolved asteroidal crust. Nature 457, 179-182.
- B. W. Denevi, D. T. Blewett, D. L. Buczkowski, F. Capaccioni, M. T. Capria, M. C. De Sanctis, W. B. Garry, R. W. Gaskell, L. Le Corre, J.-Y. Li, S. Marchi, T. J. McCoy, A. Nathues, D. P. O'Brien, N. E. Petro, C. M. Pieters, F. Preusker, C. A. Raymond, V. Reddy, C. T. Russell, P. Schenk14, J. E. C. Scully, J. M. Sunshine, F. Tosi, D. A. Williams, D. Wyrick. 2012. Pitted Terrain on Vesta and Implications for the Presence of Volatiles. Science 338(6104) 246-249.
- M. C. De Sanctis, A. Raponi, E. Ammannito, M. Ciarniello, M. J. Toplis, H. Y. McSween, J. C. Castillo-Rogez, B. L. Ehlmann, F. G. Carrozzo, S. Marchi, F. Tosi, F. Zambon, F. Capaccioni, M. T. Capria, S. Fonte, M. Formisano, A. Frigeri, M. Giardino, A. Longobardo, G. Magni, E. Palomba, L. A. McFadden, C. M. Pieters, R. Jaumann, P. Schenk, R. Mugnuolo, C. A. Raymond, and C. T. Russell. 2016. Bright carbonate deposits as evidence of aqueous alteration on (1) Ceres. Nature preprint June 2016.
- S. Marchi, A.I. Ermakov, C.A. Raymond, R.R. Fu, D.P. O'Brien, M.T. Bland, E. Ammannito, M.C. De Sanctis, T. Bowling, P. Schenk, J.E.C. Scully, D.L. Buczkowski, D.A. Williams, H. Hiesinger, and C.T. Russell. 2016. The missing large impact craters on Ceres. Nature Communications preprint July 2016.
- J. L. Molaro, K. J. Walsh, E. R. Jawin, R.-L. Ballouz, C. A. Bennett, D. N. DellaGiustina, D. R. Golish, C. Drouet d’Aubigny, B. Rizk, S. R. Schwartz, R. D. Hanna, S. J. Martel, M. Pajola, H. Campins, A. J. Ryan, W. F. Bottke, and D. S. Lauretta. 2020. In situ evidence of thermally induced rock breakdown widespread on Bennu’s surface. Nature Communications 11: 2913
- Wladimir Neumann, Doris Breuer, and Tilman Spohn. 2015. Modelling the internal structure of Ceres: Coupling of accretion with compaction by creep and implications for the water-rock differentiation. Astronomy and Astrophysics 584, A117.
- C. T. Russel, C. A Raymond, R. Jaumann, H. Y. McSween, M. C De Sanctis, A. Nathues, T. H. Prettyman, E. Ammannito, V Reddy, F. Preusker, D. P. O'Brien, S. Marchi, B. W. Denevi, D. L. Buczkowski, C. M. Pieters, T. B. McCord, J.-Y. Li, D. W Mittlefehldt, J.-P. Combe, D. A, Williams, H. Heisinger, R. A. Yingst, C. A. Polanskey, and S. P Joy. 2013. Dawn completes its mission at 4 Vesta. Meteoritics & Planetary Science 48(11) 2076-2089.
- J. E. C. Scully, C. T. Russell, A. Yin , R. Jaumann, E. Carey, H. Y. McSween, J. Castillo-Rogez, C. A. Raymond, V. Reddy, L. Le Corre. 2014. Sub-curvilinear gullies interpreted as evidece for transient water flow on Vesta. 45th Lunar and Planetary Science Conference abstracts (1796).
- P. C. Thomas, J. Wm. Parker, L. A. McFadden, C. T. Russell, S. A. Stern, M. V. Sykes and E. F. Young. 2005. Sub-curvilinear gullies interpreted as evidece for transient water flow on Vesta. Nature 437, 224-226.