To determine the chemical composition of members in the solar system, one may first use mass density as a rough guide. In situ measurement of samples has been achieved so far only for Venus, Mars, Eros, Itokawa, Ceres, Comet 67P, Moon and Titan (more to come). Spectroscopy observation is the most common way to employ.
[Composition of Titan's atmosphere can be probed from its spectrum. Not shown in this part of spectrum is N2 found in UV and compounds of C and H found in IR. For in situ measurement at Titan, check ESA Cassini-Huygens mission.]
[Europa's solid surface (frozen water!) inferred from its reflection spectrum. Check ESA JUICE for a mission to Jupiter and its satellite Europa.]
Escape velocity Ves=(2GM/R)1/2
Gas thermal velocity Vth=(3kT/m)1/2
[In this figure, solar system bodies are plotted with red dots based on their surface temperature and escape velocity. Curves for different gases are their thermal velocity (times 6) as a function of temperature.]
The solar-system abundances, whose major source is the Sun, are similar to the overall cosmic abundances determined from spectra of stars in the Galaxy: H is about 3/4 and He is about 1/4 in mass, with various amounts of heavier elements.
[The top 10 abundant elements in the solar photosphere. H and He make up 98% in mass.]
[The cosmic abundances of lighter elements compared with 1012 H atom. Those of elements heavier than zinc are less than 1000 atoms per 1012 H atom.]
In the early universe, only H and He, plus very little Li and Be, were produced.
Other elements are all produced in stars; check General Astronomy (I).
We are all made of "star dust", or "re-cycled stellar material".