Laboratory Work

For a better insight into the most complex and less volatile material, one can also turn to experimental laboratory work. The principle ofsuch experiments is the following: from observations of the most abundant species in comae and in the interstellar medium, one can infer the probable composition ofthe nucleus ices. A gaseous sample ofthe key species is deposited under vacuum on a cold substrate and irradiated during or after deposition by UV photons or charged particles. Condensed ices are sometimes simply warmed up slowly without irradiation. When the sample is warmed up for analysis a refractory organic residue remains on the substrate as the volatiles sublimate (Fig. 5.3). Mayo Greenberg, who conceived that kind of experiments, called this residue "Yellow Stuff".22

The diversity of organic compounds synthesized during such laboratory simulations is remarkable but their identification is seldom exhaustive.23 The nature ofthe complex molecules depends on the ice composition and the nature of the energy source. The three kinds of energetic processing used during the experiment (thermal cycle, UV photolysis, energetic particles irradiations) can occur to ice mixtures either on icy coated dust grains in interstellar clouds (potentially precometary ices), or within the Solar Nebula during the accretion of icy planetesimals, or in the outer layers of comet ices in the Solar system. Constraining the degrees to which different processes affect cosmic ices is a highly convoluted problem. Differences between the products synthesized during processing, according to the energy sources, could give information on the history of cometary matter and comets. Investigations are still in progress to address this question.

From an astrobiological point of view, it must be noted that a great number ofamino acids (such as glycine, alanine, sarcosine, valine, proline, serine etc...) are reported in residues obtained after UV irradiation ofice mixtures made ofH2O: NH3: CH3OH: HCN and

Table 5.1 Molecules detected in comets and some upper limits

Cometary Volatlles Category

Molecule

Hale-Bopp Abundance (H2O = 100)

Intercomet Variation

Detected Comets + Upper Limits

H

H2O

100

Water

h2o2

<0.03

Hydrogen peroxide

C,O

CO

23

<1.4-23

9 + 8

Carbon monoxide

CO2

6

2.5-12

4

Carbon dioxide

C,H

ch4

1.5

0.14-1.4

8

Methane

c2h6

0.6

0.1-0.7

8

Ethane

c2h2

0.2

<0.1-0.5

5

Acetylene

C4H2

0.05?

Butadiyne

ch3c2h

<0.045

Propyne

C,O,H

ch3oh

2.4

<0.9-6.2

25 + 2

Methanol

h2co

1.1

0.13-1.3

18 + 3

Formaldehyde

ch2ohch2oh

0.25

Ethylene glycol

HCOOH

0.09

<0.05-0.09

3 + 2

Formic acid

hcooch3

0.08

Methyl formate

ch3cho

0.025

Acetaldehyde

C,O,H upper limits

h2cco

<0.032

Ketene

c-C2H4O

<0.20

Oxirane

C2H5OH

<0.1

Ethanol

ch2ohcho

<0.04

Glycolaldehyde

ch3och3

<0.45

Dimethyl ether

ch3cooh

<0.06

Acetic acid

N

nh3

0.7

<0.2-1

4

Ammonia

HCN

0.25

0.08-0.25

32 + 0

Hydrogen cyanide

HNCO

0.1

0.02-0.1

4 + 2

Isocyanic acid

HNC

0.04

<0.003-0.035

12 + 3

Hydrogen isocyanide

ch3cn

0.02

0.013-0.035

9 + 2

Methyl cyanide

hc3n

0.02

<0.003-0.03

3 + 7

Cyanoacetylene

nh2cho

0.015

Formamide

N upper limits

nh2oh

<0.25

Hyd roxylamine

HCNO

<0.0016

Fulminic acid

ch2nh

<0.032

Methanimine

nh2cn

<0.004

Cyanamide

n2o

<0.23

Nitrous oxide

nh2-ch2-cooh

<0.15

Glycine

c2h5cn

<0.01

Cyanoethane

hc5n

<0.003

Cyanobutadiyne

S

h2s

1.5

0.13-1.5

15 + 5

Hydrogen sulfide

OCS

0.4

<0.09-0.4

2 + 5

Carbonyl sulfid

SO

0.3

<0.05-0.3

4 + 1

Sulfur monoxide

SO2

0.2

Sulfur dioxide

CS2

0.17

0.05-0.17

15 + 3

Carbon disulfide

h2cs

0.02

Thioformaldehyde

S2

0.005

0.001-0.005

5

Disulfur

ch3sh

<0.05

Methanethiol

NS

0.02

<0.02-0.02

1 + 1

Nitrogen sulfide

P

PH3

<0.16

Phosphine

Metals

NaOH

<0.0003

Sodium hydroxide

NaCI

<0.0008

Sodium chloride

Figure 5.3. A typical experimental setup allowing the photolysis of cometary ice analogs made by deposition of a gas mixture on a cold sample carrier cooled down to 10 K in a cryostat (the UV lamp can be replaced in some setups by an ion or an electron gun). The ice evolution can be analysed in situ by infrared spectroscopy. The room temperature residue can be collected for further analysis such as GC-MS (Gas Chromatography coupled to Mass Spectrometry), HPLC (High Performance Liquid Chromatography) and many others. Picture courtesy of Jan Hendrik Bredehoft, University of Bremen, [email protected].

Figure 5.3. A typical experimental setup allowing the photolysis of cometary ice analogs made by deposition of a gas mixture on a cold sample carrier cooled down to 10 K in a cryostat (the UV lamp can be replaced in some setups by an ion or an electron gun). The ice evolution can be analysed in situ by infrared spectroscopy. The room temperature residue can be collected for further analysis such as GC-MS (Gas Chromatography coupled to Mass Spectrometry), HPLC (High Performance Liquid Chromatography) and many others. Picture courtesy of Jan Hendrik Bredehoft, University of Bremen, [email protected].

H2O: CH3OH: NH3: CO: CO2. Unhydrolyzed residues (without any liquid water introduced to the analysis protocol) produce only a trace of glycine. The detection of the other amino acids requires an acid hydrolysis ofthe residue under very strong conditions (HCl > 6 M and T > 100 °C).24,25 Therefore it is not clear to date if (1) amino acids are present themselves in the laboratory residues and henceforth in cometary ices, or if "only" amino acids precursors are synthesized and (2) if the residues' processing (acid hydrolysis) is relevant to any chemistry which could have turned the amino acids' precursors imported by cometary impacts in the primitive oceans of the early Earth into actual amino acids.

The chirality issue has also been investigated through laboratory simulations. It has been reported that an asymmetric vacuum UV photolysis of a racemic mixture of leucine in the solid state results in the production of an enantiomeric excess of one form of the amino acid.26 However, such an enantiomeric excess has not been detected yet when amino acids are directly synthesised within an ice mixture under circularly polarized light.27 Further work on this topic is planned with the opening of the new French synchrotron SOLEIL in 2007 (beamline DESIRS).

Following remote sensing and in situ observations which can only probe the atmosphere of comets, sample return of a limited amount of cometary material and laboratory work on simulated cometary ices, an ambitious next step is the landing on a comet to study its composition. This will be achieved with the Rosetta mission from the European Space Agency (ESA).

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