**Table 7:** Proposed schema of the formation and destruction of SO, SO₂, H₂S, SiO and H₂CO as function of time. The times are indicative and are meant to represent a sequence.
Time (yr)	Formation	Destruction
0^a	H₂S, H₂CO, SiH₂, SiH₄
10³	SO, SO₂, H₂S, SiO
$3\times 10^4$		SO₂, H₂CO
10⁵		SiO

^a Injection in the gas phase of the molecules sputtered from the grain mantles.

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