Asteroids and comets: close up and personal

Alan Longstaff

SPA Preston MontfordNov 2015

Asteroids

JAXA Hayabusa mission to ItokawaLaunch: 2003Sample return: June 2010;Woomera Test Range,South Australia

Itokawa

• 1500 particles recovered• ~ 10 μm• Exposed at surface for ~ 8 Myr• Minerals: olivine, pyroxene, plagioclase, FeS• S-type asteroid matching LL chondrite

Dawn

Dawn spacecraft

4 Vesta

• a = 2.4 AU• P = 3.63 yr• e = 0.088• i = 7.1°• Mean diameter = 525 km• ρ = 3456 kg m-3

• T = 85 K to 270 K (-188°C to -3°C)

• V type asteroid

Core, 226 km: Fe-NiMantle: olivineCrust: 10 km: basalt;source of HED meteorites

Geological map of Vesta

Brown colors represent the oldest, most heavily cratered surface. Purple colors in the north and light blue represent terrains modified by the Veneneia and Rheasilvia impacts, respectively. Light purples and dark blue colors below the equator represent the interior of the Rheasilvia and Veneneia basins. Greens and yellows represent relatively young landslides or other downhill movement and crater impact materials, respectively.

1 Ceres

• a = 2.8 AU (main belt)• P = 4.6 yr• e = 0.075• i = 10.6°• obliquity = 4 °• R = 473 km (largest asteroid)• ρ = 2160 kg m-3 (must have rock core)• Max T = 235 K (-38°C)• C type asteroid

Discovered by GuiseppePiazzi, Academy of Palermo,1801 Originally classified as a planet as it seemed to satisfy the Titus Bode Law

Ceres HST 2004

Ceres: Dawn 47,000 km

Ceres: Topography

Ceres:Occator crater

Ice or salt?Haze impliesoutgassing orsublimation

Water on Ceres• Far-IR ground state water absorption line (556.936

GHz) detected by ESAs Herschel Space Telescope 2011 - 2013

• Strength of line variable on hourly timescale: several sources as Ceres rotates

• 2 sources identified as darker regions. Source A has a bright centre and appears to correspond to Occator crater

• Cryovolcanism or comet-like sublimation of near surface ice• Cryovolcanism independent of orbital position sublimation more likely at perihelion (but low

eccentricity)

Hypothesis

• Snowline ~ 4-5 AU• Water ice and organics on surface of 24 Themis and

65 Cybele• Main belt comets• Ceres has icy mantle, hydrated minerals and possibly

sublimation of water ice

• Is there a continuum of composition and ice content between asteroids and comets?

Comets

Stardust: tracks of cometary particles in aerogel

Aerogel is a low density silica designed to capture cometary particles with minimum of impact heating. The shapes of the tracks provide information about the particles

Stardust• Stardust went through coma of Comet 81P/Wild 2 on January 2,

2004 and collected cometary dust particles in aerogel.• Sample return canister landed in January, 2006.• Crystalline silicates; olivine, diopside, anorthite. These have

been formed from amorphous silicates in the presolar nebula by heating > 1000°C, Must have originated in inner solar system and been transported out by radiation pressure where they were combined with ices to form comets.

- i.e. early solar system experienced radial mixing• Mineral evidence for liquid water.• Wide range of organic molecules including methylamine,

ethylamine and glycine (first detection of an amino acid in a comet).

• Interstellar dust grains isolated from Stardust samples in 2014

The impact was imaged by the Deep Impact spacecraft, HST, Chandra, Spitzer, XMM Newton, Rosetta and many ground-based telescopes

• NASA mission to slam a 350 kg copper projectile into Comet 9P/Tempel 1 and take spectra of light from impact.• Spacecraft and impactor separated on July 3. • Impact on July 4, 2005

DeepImpact

Impact energy = 4.8 metric tons of TNT

Deep Impact• Crater subsequently imaged by Stardust: 150 m

across• Water ice in ejecta was from one metre depth, below

devolatilized crust • Dust very fine; amorphous carbon, crystalline silicates

plus carbonates and smectite clays – evidence of water action

• Polycyclic aromatic hydrocarbons derived from UV cookery of simple organic molecules

• Spacecraft subsequently made flyby of C/103P Hartley 2 as extended EPOXI mission

• Observed C/2012 S1 ISON in 2013• Contact with Deep impact lost August 2013

67P/Churyumov-Gerasimenko

• q = 1.2 AU• P = 6.45 yr• e = 0.64• i = 7°• Rotation period = 12.4 h• 2 lobes: 4.1 x 3.3 x 1.8 km; 2.6 x 2.3 x 1.8

km• ρ = 470 kg m-3 (hence high porosity ices)

Orbital insertion:6 Aug, 2014Philae descent:12 Nov, 2014

Philae has landed!

67P: General results

• Jupiter-family comet (JFC) from Kuiper Belt• 20 cm regolith over hard ice, active jets and sinkholes

from collapse of active areas; 19 distinct terrains – is all this diversity due to solar heating?

• No magnetic field• Electrons above comet produced by photoionization

of water by solar radiation

67P: water• D/H = 5.3 x 10-4 (Earth’s D/H = 1.6 x 10-4) - Earth’s water could not have come from such a

comet - 103P/Hartley 2 is a Jupiter-family comet with

identical D/H ratio to Earth’s oceans.• Diurnal cycle of water ice (Nature, 2015; 525: 500): - insolation sublimes water ice from surface - shadow causes surface to cool faster than

subsurface few cm - subsurface ice continues to sublime through pores

and freezes on surface - this fresh ice sublimes away as area rotates into

sunlight again.

67P: N2 and O2

• N2 detected: condensing N2 in ice needs low T. N2/CO = 0.57% implies T ≤30 K when comet formed. JFCs could not be the main source of Earth’s N2 (Science, 2015; 348: 232).

• Free O2 surrounding comet: mean O2/H2O = 3.80% - first detection of free oxygen in cometary coma - O2 from photolysis and radiolysis of H2O unlikely - outgassed, hence primordial - O2/H2O 10-fold higher than predicted by models of

chemistry in molecular clouds (Nature, 2015; 526: 678).

67P: COSAC organics• COSAC mass spectrometer on Philae lander• Sniffing mode; 25 minutes after first bounce at 150 m above the

surface, sampling cometary material excavated by first touchdown

• m/z < 62 because signals beyond this are too faint to be distinguished from noise; hence not possible to detect amino acids.

• NH3, HCHO, and CO2 were expected to be present but could not be distinguished from other compounds at same m/z; e.g. NH3 has m/z = 17, same as OH from water.

• 16 organic molecules: many N-containing, no S-containing. • acetamide, acetone, methyl isocyanate and proprionaldehyde

identified for first time on a comet: • primordial – formed at very low T by action of UV on ices

(Science, 2015; 347: 387 – 391).

Fig. 3 Possible formation pathways of COSAC compounds.

Fred Goesmann et al. Science 2015;349:aab0689

Published by AAAS

Species in red are not confidently identified;

species in green are reported for the first time in comets by COSAC.

67P: Ptolemy organics• Ptolemy mass spectrometer on Philae lander (Monica Grady’s

kit!)• Sniffing mode, sampled coma gases 20 minutes after first

bounce• Ptolemy can detect m/z from 13 to 136• No S-containing, little N-containing, and no aromatic (carbon

ring) compounds - no signal of benzene ion (C6H6+) at m/z = 78

• Mass spectra consistent with H2O/CO2/CO in 10: 2: >1 ratio• Mass spectra consistent with formaldehyde (HCHO) and

acetaldehyde (CH3CHO) • Mass spectra had good fit for polyoxymethylene, which has also

been identified in coma of 1P/Halley by Giotto, but other polymers (ice tholins) could be possible, cooked up by UV on water, methanol, carbon dioxide and ethane.

• Peak at m/z = 92 could be glycerol (glycerine, C3H8O3)• Other peaks could be the oxonium ion (CH3.OH2

+) and C3H5+

Water H2O 18 80.92 100

Methane CH4 16 0.70 0.5Methanenitrile (hydrogen cyanide)

HCN 27 1.06 0.9

Carbon monoxide CO 28 1.09 1.2

Methylamine CH3NH2 31 1.19 0.6Ethanenitrile (acetonitrile)

CH3CN 41 0.55 0.3

Isocyanic acid HNCO 43 0.47 0.3Ethanal (acetaldehyde)

CH3CHO 44 1.01 0.5

Methanamide (formamide)

HCONH2 45 3.73 1.8

Ethylamine C2H5NH2 45 0.72 0.3Isocyanomethane (methyl isocyanate)

CH3NCO 57 3.13 1.3

Propanone (acetone)

CH3COCH3 58 1.02 0.3

Propanal (propionaldehyde)

C2H5CHO 58 0.44 0.1

Ethanamide (acetamide)

CH3CONH2 59 2.20 0.7

2-Hydroxyethanal (glycolaldehyde)

CH2OHCHO 60 0.98 0.4

1,2-Ethanediol(ethylene glycol)

CH2(OH)CH2(OH) 62 0.79 0.2

Table 1 Organic compounds on 67P revealed by COSAC mass spectrometryScience, 2015; 349: 497

New Horizons rendezvous with KBO in 2019