COM­MENT

Sky at Night Magazine - - BULLETIN - by Chris Lin­tott

As­ter­oids may well be the source of Earth’s wa­ter, yet I’m be­gin­ning to have sec­ond thoughts.

It is be­com­ing clear that the early So­lar Sys­tem’s small bod­ies have had a com­plex and in­ter­est­ing his­tory; and it is cer­tainly true that, back in the early his­tory of the So­lar Sys­tem, there was a pe­riod in which the sheer num­ber of as­ter­oids hit­ting Earth might have been high enough to make this ex­pla­na­tion fit. What I can’t see is how Earth’s wa­ter could have been de­liv­ered by as­ter­oids with­out be­ing di­luted by an equal in­flux of comets, al­ter­ing the deu­terium to hy­dro­gen ra­tio again.

In­stead, we should prob­a­bly look to­wards Earthly so­lu­tions. Ge­ol­o­gists have re­cently sug­gested that rocks deep in the Earth’s man­tle may have pro­vided a refuge for wa­ter, which would then be re­leased when plate tec­ton­ics car­ried them up to the sur­face. Rosetta may have trav­elled six bil­lion miles to tell us to look closer to home.

CHRIS LIN­TOTT co-presents

The Sky at Night

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