What have we learned about small bodies and the Solar Nebula over the past 30 years?

1. What have we learned about small bodies and the Solar Nebula over the past 30 years? Joseph Nuth Senior Scientist for Primitive Bodies Solar System Exploration Division (690)

2. Comets (Dirty Snowballs) Comets still produce beautiful images with both a plasma tail and a dust tail. However, we thought comets were pristine collections of grains and ices formed in Molecular Cloud Cores, the outer regions of the Solar Nebula or even in the interstellar medium. Above: Image of Comet Hale Bopp taken in March 1997. Right: “Greenberg” Model for the accretion of a comet.

3. Asteroids (Failed or Disrupted Planets) Top, Far Left: Canyon Diablo Iron Meteorite Left: Gideon Iron Meteorite (etched) Above: Dora Pallasite (rock & molten metal) Below: Leoville Carbonaceous Chondrite

4. The Solar NebulaFrom Hot to Cold it was a One-Way Trip The inner nebula reached temperatures as high as 3000K, then slowly cooled down over millions of years. All pre-existing grains were completely vaporized. Thermodynamic and Chemical Equilibrium ruled throughout the Solar Nebula (Left). The common assumption was that planets or small bodies formed from the materials that were in their “zone.” Gas, Dust, Ice and Comets all fell into the inner regions of the Solar Nebula over time. Comets were kicked out of the system, or thrown into the Sun, by close encounters with Jupiter, Saturn, Uranus or Neptune. log 10 PRESSURE (bars) Barshay & Lewis ARAA 1976

5. The “Modern” Solar Nebula Gas and dust move outward as well as inward during nebular accretion bringing crystalline material out to comets (Nuth, American Scientist, 89, 228 [2001])

6. Planetesimals are no longer considered to be of only academic interest. Left & Above: Tunguska, 1908 Below: Mitigation & Mining

7. Comets are “geologically” active! In addition to craters, Comet Temple 1 also showed large smooth areas (a, b) and layering (above). At least one of the smooth areas (b) appears to be an outflow – possibly a low density glacier. Large-scale flow Patterns 1 Km

8. Comets look like Asteroids, but some asteroids have cometary tails! Above: Comet Halley Image obtained by the Giotto Mission showed that comet surfaces were far from uniform. Left: Planetesimal Family Portrait obtained from many individual planetary science missions over the last few decades.

9. The Comet - Asteroid Continuum • At about Jupiter’s orbit, the nebula was cold enough that all water was in the form of ice. • At Mercury’s orbit only water vapor exists. • Planetesimals form from nearby solids. • Planetesimals exhibit a continuous increase in their water/rock ratio at formation, starting at zero near the Sun, increasing slowly at first, then more rapidly approaching the Snow Line, then again more slowly: reaching a plateau at a ratio of about 3:1.

10. Size Does Matter Radioactive heating and the release of gravitational energy can modify large, icy bodies in the outer solar system to make large, dense icy bodies (e.g. Pluto). Large, low-ice bodies in the inner solar system can evolve through various stages of planet formation: internal heating and chemical equilibration, melting, differentiation, core formation, etc.

11. Small bodies remain pristine – except… for cratering, collisional disruption, evaporation of volatiles, solar wind sputtering, dust deposition, gravitational settling,….

12. The Next Frontier for Small Bodies Detailed Exploration of their surface, as done by Apollo (above) or the Mars Rovers (right) will reveal surprises . Sample Return missions will reveal details about their formation and history unobtainable by any other means as shown by the Apollo, Stardust (upper right) and Genesis (lower right) missions.

13. Why would we want to visit small solar system bodies? Two examples: Oxygen Isotopes & Asteroid Spectroscopy

14. Processing Dust in the Solar Nebula- Then Plot showing the 18O / 16O and 17O / 16O ratios in chondrules and CAIs in meteorites. These particles define a line with much steeper slope than the Earth line consistent with loss or addition of 16O. Note that the variations in oxygen isotopic ratios are much larger than those shown by rocks from the Earth, Mars, and Vesta.

15. Processing Dust in the Solar Nebula - Now Oxygen three-isotope plot showing representative compositions of major primary components of solar system matter, the solar wind (SW), and our preferred value for the Sun. All data fall predominantly on a single mixing line characterized by excesses (lower left) or depletions (upper right) of 16O relative to all samples of the Earth and Moon. Plotted are the most 16O-enriched solar system samples:). K D McKeegan et al. Science 2011;332:1528-1532

16. New Questions for Sample Returns How was the oxygen isotopic ratio of the dust modified in the inner regions of the nebula? With extensive modification of the silicate mineralogy – at least in the inner nebula – how were more fragile specimens of pre-solar materials preserved ? Over what range in the nebula did dust modification occur? What is the oxygen isotopic composition of Trojans, Centaurs, TNOs or even Kuiper Belt Objects?

17. How much do we know about primitive bodies throughout the solar system? For both scientific as well as for HSF exploration goals we need to map the distribution of small bodies throughout the solar system. This will be done using telescopic techniques.

18. Can we interpret asteroid spectra? Start with a meteorite type. Add Considerable abuse… Due to solar wind exposure, well known to reduce oxide to metals ; Due to micrometeorite bombardment – again producing a metallic surface. Due to organic photolysis and deposition of coatings on surfaces. Forest City (H5 ordinary chondrite) is covered by a fusion coating. One tip has been cut off, exposing the lighter gray, speckled interior. Fusion coatings are very thin. Does the process start over again each time a new surface is exposed?

19. Even smaller asteroids are not uniform Asteroid Itokowa, target of the Hayabusa Mission

20. Zooming in on Itokowa, we see very different types of local terrain

21. Variation persists even at the local level

22. How do we translate spectra into Geology ? • If humans go to an asteroid as the first step in a long-term plan to explore and exploit the solar system then we need to understand how to translate the spectral signatures of asteroids into knowledge of their geology. • From previous experience, we must visit primary examples of spectrally characterized asteroids (comets) with well instrumented spacecraft.

23. Summary • 30 years ago we thought that we knew a lot about asteroids, comets and the formation of the Solar System . • Stardust, Genesis & Hayabusa demonstrated that the chemistry and structure of both small bodies and the primitive solar nebula are much more complex than we once expected. • Exploration of many small bodies is required to answer basic science questions and prepare for future expansion of humanity into space.