Origins of?Earth‘s Salty?Oceans

Scientists have?long?been?interested?in discovering?the?origin of?Earth‘s?water and?establishing?why?Earth’s oceans are?so?salty?There?has?been speculation that earliest?Earth was so?hot that no?liquid water existed,?and?all?light?elements?(such as hydrogen and oxygen)?were?rapidly?stripped away?from?Earth?by?the?solar?wind (a?stream?of?charged?particles emitted by the Sun).?If?this?were?true,?then the elements needed to form?water on?Earth?would not?have?been freely?available.?As?a?consequence,?it?was proposed that collisions with icy?comets?or?similar?gas–and water–rich?materials brought water to?Earth after?the planet?had?sufficiently?cooled to retain it. This?concept?was supported?by?comparisons of the gas compositions?of meteorites?with?those of?rocks?from beneath?Earth’s?surface, notably?using?krypton and?xenon, nonmetallic?gases that?do.?not react?with other?materials.?There certainly?is?enough ice in space?to have supplied our?water?(and?atmosphere)in this?manner.

?

?

In?July?2015,?the?space?probe Philae.?which landed?on comet?Churi,?discovered not?only?ice and?dust,?but also?16?types?of organic?compounds,?present?not?in a?loose?distribution?but?in discrete?clumps.?Suddenly,?the idea?gained?lots of traction that?comets?brought?not only water, but also the?ingredients?for life,?even?in?ready-made clumps.?Intriguingly,?in?October?2015?it was?reported. that-as this comet?slowly?thaws-molecular oxygen(02)?escapes in a?constant and?high?proportion(1%?to?10%)?relative?to?water?which?suggests?that the comet also?contains a?surprising amount of?primordial?(ancient)?oxygen, which was?incorporated?during the?comet‘s?formation.

?

?

Other work?favors an?alternative explanation.?This work?found?that?the?hydrogen isotope?ratio(the?proportion of?different?forms?of?hydrogen)?of?ice in?comets?may?be different from that of?water?on?Earth.?It instead emphasizes?that the?chemical?composition?of water on?Earth resembles that?of?the small?percentage of water contained?within?rocky?meteorites, and thus?in asteroids,?which?are essentially?very?large?meteorites. Thus,?a theory?was developed?that?the asteroids?planetesimals.?and?protoplanets?that?clumped?together?to form?Earth?had?carried?enough water?in?their?rock minerals?to explain our?oceans.?It?would?have escaped from the?planet‘s interior as steam,?which in turn?would?have condensed into water at the surface and in the early?atmosphere. Calculations indicate that this?mechanism?can?also provide?plenty of?water?to?explain?Earth‘s?observed?water content.

?

?

We have a?more?complete?understanding of?the origin of?salt in?our oceans. It represents an accumulation?of?dissolved?minerals over?tens of millions to?hundreds of?millions of years.?These?minerals were broken up?and?dissolved?during?chemical?weathering?We?are all?familiar?with?this?process from?limestone?buildings that?become?pitted?or smoothed?by?the action of?water, wind,?and weather;?this?is?where the term?weathering?comes?from.?The?key process at?work?is one?of?chemical?reactions between?the rock?and?the?water, with?an important?role?for?gases that?are dissolved?in water,?such?as?carbon dioxide or?sulfur dioxide,?since these?make the?water?more corrosive.?The?chemical?weathering?reactions?break?up rock?minerals?into?charged?atoms?or?molecules,?called ions,?which are removed?in?solution?by?river?water and?groundwater.?This is exactly?what?happens?when?you dissolve?table salt?in?water:?the?mineral salt?breaks?down?into?sodium?and?chloride ions that?are held?in?a?solution.

?

?

The?early?atmosphere contained?high?levels of carbon?dioxide,?or?CO2.?This gas is?easily dissolved in water,?forming a?mildly?acid solution. In?the?Co2-rich?early atmosphere, this resulted in?a?corrosive acid rain that?was?highly?effective?at chemically?weathering?rocks,?and?fresh?volcanic?rocks?are especially easily?weathered.The intense?weathering released?dissolved?minerals in?the form of ions into river?water?and?groundwater.?From?early?times onward, river?and?groundwater?flow?has transported the dissolved?minerals to their final?collection?point,?the?ocean basins.?Given?the extremely?slow input and?removal?of salts,?it becomes clear that?the?oceans‘?vast?store?of?salt has accumulated because the oceans?have?for ages?been the end station for salt transport. Meanwhile?water?itself?continually?evaporates?from?the oceans-concentrating-?its salts-and–?the evaporated?fresh?water continues the weathering?cycle.?