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We perform a comparative study of the ex--situ second--parameter pairglobular clusters (GCs) M30 and M92, having similar metallicities but differenthorizontal branch morphologies. We obtain the similar mean primordial carbonabundances for both clusters. However, M92 shows a large dispersion in carbondue to a more extended C--N anticorrelation, while M30 exhibits a higherprimordial nitrogen abundance, suggesting that they have different chemicalenrichment histories. Our new results confirm our previous result that M92 is ametal--complex GC showing a bimodal metallicity distribution. We also find thatthe metal--rich group of stars in M92 shows a helium enhancement as large as$\Delta Y$ $\sim$ 0.05 from the red giant branch bump (RGBB) $V$ magnitudes,which can also be supported by (i) a lack of bright RGB stars, (ii) syntheticevolutionary HB population models and (iii) the more extended spatialdistribution due to different degree of the diffusion process from their lowermasses. We reinterpret the [Eu/Fe] measurements by other, finding that the twometallicity groups of stars in M92 have significantly different [Eu/Fe]abundances with small scatters. This strongly suggests that they formedindependently out of well mixed interstellar media in different environments.We suggest that M92 is a more complex system than a normal GC, most likely amerger remnant of two GCs or a even more complex system. In Appendix, weaddress the problems with the recently developed color--temperature relationsand the usage of broadband photometry in the populational taggings.

A Comparative Study Between M30 and M92 : M92 Is A Merger Remnant With A Large Helium Enhancement