Carbonic acid on Mars?

Planet. Space Sci., Vol. 44, No. 11, pp. 1447-1450, 1996 Cotwright 0 1996 Elsevier Science Ltd Printed% GreatBritain. All rights reserved 0032-0633/96 $15.00+0.00

Pergamon

PII: SOO32-0633(96)00079-7

Carbonic acid on Mars? G. Strazzulla,

J. R. Brucato, G. Chino

and M. E. Palumbo

Osservatorio Astrofisico and Istituto di Astronomia,

Citt& Universitaria,

I-95125 Catania, Italy

Received 6 November 1995; revised 22 May 1996; accepted 22 May 1996

Only recently the IR spectrum of carbonic acid (H,CO,) has been obtained (Moore and Khanna, 1991; Hage et al., 1993 ; Brucato et al., 1996). Ab initio quantum mechanical calculations for the gas phase reaction : H,O(gas)

+ CO,(gas) + H&O,

(1)

indicate (JGonsson et al., 1977) an activation energy of N 50 kcal mol-‘. Thus H&O,, once synthesized, can be stabilized at temperatures low enough to prevent its dissociation, and so its IR spectrum was obtained. The synthesis of carbonic acid has been obtained by two different techniques. The first is a new cryogenic technique (Hage et al., 1993, 1995) in which layers of glassy solution of HCO; and of excess HCl dissolved in CH30H were deposited one by one onto each other at 78 K in the form

Correspondence to: G. Strazzulla

of droplets and their reaction was studied in vacua by FTIR spectroscopy from 78 to 300K. At N 140-160K, well above the solvent’s glass transition temperature of 103 K, a decrease in the solvent’s viscosity enabled the reaction of HCO; with H+ as demonstrated by the disappearance of the bicarbonate band at N 1630 cm- ‘. Simultaneously formation of a band centred at N 1725 cm-’ was observed and attributed to the C=O stretching vibration of H2C03. The second technique involves ion irradiation of a frozen (N 10 K) H,O :CO, (1 : 1) mixture. The experiments have been performed, in vacuum chambers faced with IR spectrographs enabling to obtain “in situ” spectra, using 700 keV protons (Moore and Khanna, 1991), 3 keV He ions and 1.5 keV protons (Brucato et al., 1996). The complex chemistry induced by energy loss of the incoming ions into ices proceeds through the rupture of molecular bonds. The recombination of fragments leads to the formation of both more volatile and less volatile species (Johnson, 1990; Strazzulla, 1996). In the case of ion irradiation of H,O :CO, (1: 1) mixtures, CO and H,CO, have been identified as the major newly produced species (Moore and Khanna, 1991; Brucato et al., 1996). It is also important to outline that even in the case of pure CO2 ice, bombarded with 1.5 keV protons, the formation of carbonic acid has been demonstrated (Brucato et al., 1996). This indicates the capability of energetic ions to form molecules that include the incoming particle. Peak position of the IR bands attributed to carbonic acid by the three different groups is reported in Table 1. In Fig. 1 (top panels) the spectrum of carbonic acid (after Brucato et al., 1996) is shown and compared with observations of Mars (Encrenaz and Bouchet, 1989 ; Pollack et al., 1990) in the range 2.8-4.2 pm (left panel; after Encrenaz and Bouchet, 1989) and in the range 5-l 1 pm (right panel ; after Pollack et aZ., 1990). Finding firm conclusions, from a comparison of a laboratory (transmittance) spectrum with astronomical reflectance or emission spectra of a complex object like Mars, is a very difficult task. The relative intensities of the bands, as well as their widths depend upon a large number

1448

G. Strazzulla

Table 1. Peak position (cm-’ and ,nm) of bands attributed to H,C03. Spectra from Moore and Khanna (1991) and Brucato et al. (1996) were taken at T = 250 K after warming up a frozen mixture (HZ0 : CO2 = 1 : 1) irradiated at T N 10 K by 700 and 1.5 keV protons, respectively. In Hage et al. (1993) peak positions are obtained from spectra taken at 200 and 220 K Moore and Brucato et al. Khanna (1991) (1996) 3030 2840 2614 1705 1501

(3.30) (3.52) (3.83) (5.87) (6.66)

1296 (7.72) 1034 (9.67)

3034 2859 2621 1726 1508

(3.30) (3.50) (3.82) (5.79) (6.63)

1299 (7.70)

Hage et al. (1993)

2694 2580 1730 1486

O-H str. O-H str. O-H str. C==O COH in-pl. bend C-O* as. str. C-O* sim. str.

1309 (7.64) 1083 (9.23)

of parameters (temperature, pressure, dimension of responsible dust, presence of an atmosphere, and many others). On Mars the formation process and the physical state of the carbonic acid particles or inclusions as well as their thermal history are the most important parameters

c

Carbonic

Mars,

Carbonic

acid

acid

south

t

Mars center/NE I

I,

I

3

I

I

I

I

I

I

3.5

Wavelength

/

I

I

I, 4

(pm)

I,,

acid on Mars?

that influence the IR bands. Perhaps the spectral parameter that is less affected by the radiative transfer is the peak position of the bands. From Fig. 1 we can see that, on the basis of the peak position of the carbonic acid bands, we cannot exclude the presence of solid carbonic acid on the surface and/or the atmosphere of the red planet. This also in view of the data presented in Table 1 from which it is clear that band positions are sensitive to several parameters as testified by the different positions they exhibit in different experiments. Let us discuss in some detail this statement : the region at 3-4 pm (left panel, bottom spectrum in Fig. 1) is dominated by the reflected solar component. Encrenaz and Bouchet (1989) attribute the strong and broad absorption around 3 ,um to the O-H bond in hydrated silicates (this band is not seen in the figure because the portion of the spectrum shown in Fig. 1 is a wing of the strong 0-H absorption). Weak absorption features (we can see them in the figure) are superimposed at around 3,3.4 and 3.8 pm and tentatively attributed as representative of the surface mineralogy and namely to CO, ice for the first band, and carbonates for the other two bands. However this is only an indication and, in any case we cannot distinguish

Assignment

(3.71) (3.88) (5.78) (6.73)

et al.: Carbonic

I

4.5

4

/

I

I 6

s

I

I

Wavelength

I,, 6

I

(pm)

I

10

Fig. 1. The IR spectrum (at 250 K) of carbonic acid left-over after irradiation (1.5 keV H ions) of a frozen (1OK) HZ0 :CO, = 1 :I mixture, is compared with those obtained for Mars (Encrenaz and Bouchet, 1989; Pollack et al., 1990). In the left panel the range 2.884.2pm is shown, the Martian observations (Encrenaz and Bouchet, 1989) have been recorded at the 1 m telescope of ES0 (La Silla, Chile) on Sept. 30, 1988, and refer to the South region. In the right panel, where the range 5-l 1 pm is shown, the Martian spectrum has been obtained by the Kuiper Airborne Observatory (Pollack et al., 1990) on Oct. 13, 1988. In Pollack et al. (1990) this spectrum is indicated as the ratio Mars centre/NE early

1449

G. Strazzulla et al.: Carbonic acid on Mars? Table 2. Features in Mars data (after Pollack et al., 1990)

Peak position (pm)

Assignment

Absorptions 6.7 7.3 8.0 8.7 9.2 9.8 10.3

CO, (gas) + CO:, or HCO; CO, (gas) SO: or HCO, COZ (gas) SO: or HCO; COZ (gas)

Emissions 6.1 7.8-S 9.69.8

H,O (molecular) Silicates Silicates

CO: or HCO,

among different carbonates. We suggest to add carbonic acid, possibly synthesized at the surface, to the list of possible components. The region at 5-10pm (right panel in Fig. 1) exhibits both emission and absorption features. Pollack et al. (1990) (see Table 2) attribute some of the absorption features to gaseous CO, in the Martian atmosphere. The remaining absorption features likely arise from minerals present in the airborne dust and the emission ones from surface species namely silicates and water. Also in this case a firm identification either of carbonates or of sulfates cannot be done. Simulations (Pollack et al., 1990) indicate that sulfate- and carbonate-bearing minerals are contained in the same particles of the airborne dust as the dominant silicate materials. Once again we claim that the comparison between the spectrum of carbonic acid and the astronomical one cannot rule out the presence of this kind of carbonate, even as a component of the silicate airborne dust. On Mars CO* ice, in intimate mixture or segregated from lower amounts of water ice, is the dominant constituent of the polar caps (Calvin and Martin, 1994) and of ice particles in the atmosphere. The thickness of the atmosphere (which corresponds to a pressure of the order of millibars) prevents the planet surface from being heavily irradiated by cosmic ions. In reality since Mars has not an intrinsic magnetic field strong enough to deflect ions, galactic cosmic rays and energetic flare particles can reach the surface. According to calculations (Simonsen and Nealy, 1993) the Martian surface receives a total dose of 8 x lo-*eV(molecule yr)-‘. This means that some 107yr are necessary for fast ions to deposit enough energy (N 1 eV molecule-‘) to produce, according to laboratory results (Brucato et al., 1996; Strazzulla, 1996), an appreciable quantity of carbonic acid. This time should be compared with the resurfacing time which is unknown to the present authors. If this latter is greater than or of the order of some 10 million years then ion irradiation could be effective in producing carbonic acid. Moreover, if silicate dust is present in the atmosphere, carbon dioxide and water could be freezed, or at least adsorbed, on them at altitudes where cosmic ions can more easily penetrate and synthesize carbonic acid.

It is also important to outline that, as for all of the irradiation experiments, their applicability to astrophysical scenarios is justified by their dependence on the total energy deposited on the target and by their independence from the ion fluence. The results we have presented are in any case relevant because, independently of the processes that have formed it, carbonic acid, now that its IR spectrum has been obtained, can be searched for. It is also interesting to note that sublimation of carbonic acid produ.ces HCO and H2C0 (no gaseous H&O, has been detected in laboratory mass spectra (Moore and Khanna, 1991)). Thus, we can argue that if carbonic acid is present on Mars its sublimation could account for the presence of formaldehyde. H&O has been tentatively identified in the gas phase during solar occultation observations of the Martian atmosphere near the limb of the planet by means of the Auguste IR spectrometer on board the Phobos spacecraft (Korablev et al., 1993). The search for carbonic acid would be a relevant task of the several space missions that are anticipated towards Mars, for the next years, by the major space agencies. During these missions remote observations, “in situ” analyses and collection of materials are planned. The confirmation of the presence of carbonic acid on the red planet would be of great relevance because of the important role that carbonic acid can play in both organic and inorganic chemical processes (Nguyen and Ha, 1984).

We are grateful to B. Schmitt and to an anonymous referee for criticisms and suggestions that have strongly improved this paper. This research has been supported by the Italian Space Agency (ASI). Acknowledgements.

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