A “NEW” PARADIGM FOR ORGANIC CHEMISTRY IN

PROTOSTELLAR NEBULAE: Large-Scale Nebular

Transport. Joseph A. Nuth III

Astrochemistry Laboratory, Code 691

Solar System Exploration DivisionNASA’s Goddard Space Flight Center

Greenbelt MD 20771

The Minimum Criteria for Acceptance of New Paradigms has

been met.

Outline of Presentation What was the ‘old’ Paradigm? What evidence suggests that the old

paradigm is no longer adequate? What are major characteristics of the

new paradigm for organic synthesis in the Primitive Solar Nebula?

What are the major implications of the new paradigm?

What was the ‘old’ Paradigm?

Interstellar reactions formed pre-solar organic materials and ices on the surfaces of amorphous silicate grains.

Collapsing cloud materials fell onto the disk surface and were transported inward.

Fischer-Tropsch-type catalytic chemistry on grain surfaces may have produced the complex organic materials seen in meteorites.

During inward transport some materials accreted into larger bodies, while the rest of the dust and gas in the disk fell into the growing protosun.

Chemistry in the ‘old’ Paradigm All chemistry occurred during inward transport

through the disk. Fischer-Tropsch-type (FTT) chemistry required the

presence of small iron grains: these grain surfaces could be ‘poisoned’ by a variety of elements.

Only the products released to the gas phase were considered to be important in modeling organic synthesis via FTT reactions.

Gas phase reactions, FTT synthesis and pre-solar synthesis could not make the quantity or variety of organic molecules observed in comets.

Giant Gaseous Protoplanets were required to produce larger quantities of complex gases.

Comet Hale-Bopp Modeled by the

old paradigm it would contain aggregates of interstellar dust and ices formed in Dark Clouds.

Interstellar silicates are amorphous to all observable limits

New Evidence I Campins & Ryan (1991)

were first to propose the presence of crystalline forsterite in cometary dust based upon the mid-infrared spectrum of Comet Halley.

These same features were later confirmed to be present in the more complete comet spectra obtained by ISO.

Comet Halley

New Evidence II

Observed with the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO) Mission.

Strong IR evidence for the presence of crystalline dust.

Only the magnesium-rich end members of crystalline minerals appear in the ISO spectra of Comet Hale Bopp

Comet Hale-Bopp

New Evidence III Since there are no crystalline silicates in the

Interstellar Medium, crystalline grains must be produced “locally” in the Solar Nebula.

Two mechanisms can produce crystalline dust: Thermally Anneal dust grains inside ~1 A.U. or Shock Anneal dust grains out to ~10 A.U.

In either case, dust must be transported out to (~50 to 200 A.U.) where the comets begin to form [Weidenschilling, 1997] or crystalline dust would only be found on the outside of comets.

New Evidence IV Deep Impact IR Observations show

significant quantities of crystalline silicate only just after impact. Before & after spectra were ~featureless.

STARDUST samples contain a large fraction of crystalline dust.

Both comets (Temple & Wildt 2) are Kuiper Belt Objects that formed well outside of 10 A.U.

A New Physical Model for the Solar Nebula

Vigorous thermal convection as well as conservation of angular momentum lead to wide scale transport of dust and gas both in towards the sun as well as outward to the region of comet formation at radii from ~50–200 A.U.

A New Chemical Model for the Solar Nebula

Nebular Chemistry is no longer a one-way trip in to the sun, through the higher (T,P) inner nebula.

Material from any nebular zone can travel either inward (most) or outward (some) [Boss, 2004] from the innermost regions out to ~200 A.U.

Chemical products from the higher (P,T) inner nebula must be transported to great distances in the nebula in order to begin aggregating into comets along with the annealed crystalline dust.

Mixing of highly processed materials with pre-solar ices and dust easily occurs in this scenario.

A few words about Fischer-Tropsch-type reactions I

In nature, these are really surface mediated reactions that convert CO, N2 and H2 into a wide variety of solid and gaseous products.

It is incorrect to consider FTT reactions as pure catalytic processes where specific active surfaces produce very specific products.

Anders and colleagues measured the gas phase products from FTT reactions and concluded that these could not have produced meteoritic organics because the products were isotopically light while meteoritic carbon was isotopically heavy.

A few words about Fischer-Tropsch-type reactions II For meteorites, the organics deposited on grain

surfaces are not poisons, they are the reaction products one would expect to find in primitive meteorites prior to the action of thermal or hydrous metamorphism.

If the gas phase reaction products are isotopically light (e.g., Anders), then the residue left behind on the grain surfaces must be isotopically heavy.

If we assume that the organic crud deposited on grain surfaces is not a poison, but is the precursor to meteoritic carbon, then the isotopic results of Anders support the FTT hypothesis.

Surface mediated reactions form carbon, complex organics & CO2

Any surface promotes these reactions.

Once coated with a layer of carbonaceous material, the “catalytic” behavior of almost any solid surface improves (sometimes dramatically).

Reactions occur all the way ‘down’ the nebular disk, including the high temperature, high pressure environment of the innermost regions.

The products of these reactions can be transported throughout the entire nebula.

Why Access to the Innermost Nebula is Good for Chemistry

A + B => C + D Rate ~r[A][B] or ~r[A][B][M] r ~ [Collision X-section]exp{-G/kT} Therefore the overall rate increases

with pressure and temperature. If either A or B are reaction products

then the effect multiplies greatly.

New Coupled Chemical & Dynamic Paradigm for the Solar Nebula

Vigorous convection

Conclusions Surface-mediated reactions produce a wide range of

complex organic materials: both volatile molecules as well as a macromolecular carbonaceous coating.

Any grain surface can mediate these reactions including macromolecular carbonaceous coatings.

Reactions occur faster at higher P & T; therefore, the innermost regions of the nebular disk should produce large quantities of complex organics.

These materials will be spread throughout the nebula by the same mechanisms that transport annealed silicate grains out to forming comets.

Implications

Every protostar is a chemical factory that produces copious quantities of organic materials.

Why “poisoning” is good! Carbonaceous crud

on grain surfaces is not a ‘poison” but is the basic organic material that is incorporated into meteorites.

Our experiments show that this crud is a better catalyst than most minerals or amorphous dust.

OH phenol

OHO

benzoic acid

Murchison

Catalyzed Fe-silicate

Oxy

gen

Bea

ring

A

rom

atic

s

Murchison

Catalyzed Fe-silicate

aliphatics

aliphatics

aromatics

Ali

phat

ics/

Aro

mat

ics

Murchison

Catalyzed Fe-silicate

aliphatics

aliphatics

aromatics

Ali

phat

ics/

Aro

mat

ics

Similar types of compounds are formed in our experiments, as are found in meteorites

Many natural surfaces promote the disproportionation of CO

Iron silicate promotes methane production, but so do many other silicates.

Time (hours)0 100 200 300

Arb

. Int

egra

tion

Uni

ts

0

10

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CH4 production at 400°C using different catalysts

Iron silicate

Bronzite

SiOx

SiO2

Mg-SiOx

T (hours)

0 100 200 300 400 500

Inte

gra

tio

n o

f m

ain

CO

fea

ture

mo

nit

ore

d in

IR (

arb

. un

its)

2

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Iron silicate

SiO2

Mg-SiOx

SiOx

Plot of CO decay as a function of time for

different catalysts at 400ºC

Bronzite

A wide range of gas-phase products are produced

Major Products include: CO2, H2O, CH4

Minor products were analyzed via GCMS after considerable concentration and include aliphatics -methyl butane & pentane; aromatics -benzene, toluene & ethyl benzene; oxygenated organics -acetone & benzoic acid; more complex organics - N-Methyl Methylene Imine (H3C-N=CH2)