The Four Elements: It’s all in the Record

The silicate bodies of the Solar System (Mercury, Venus, Earth, the Moon and Mars). Image courtesy NASA/JPL-Caltech.

This time I will talk about ter­res­tri­al plan­et inte­ri­ors and geo­log­i­cal activ­i­ty. It turned out anoth­er lengthy post, but there are a lot of things to men­tion.

Geo­log­i­cal activ­i­ty is defined as the expres­sion of the inter­nal and exter­nal process­es and events that affect a plan­e­tary body. It has been pro­posed that plate tec­ton­ics dur­ing a long peri­od of time, say, at least a few bil­lion years, is a nec­es­sary con­di­tion for life. For com­plex life to evolve, as in Earth’s exam­ple, more than 3 Gyrs are required. How­ev­er, once tec­ton­ics shuts down, that might not be the end of the sto­ry. Both Earth and Mars are geo­log­i­cal­ly active (and Mars now is thought to be at a prim­i­tive stage of plate tec­ton­ics), just oper­at­ing in dif­fer­ent con­vec­tion regimes. So Mars still can be a good choice, if even­tu­al­ly ter­raformed. It will require a lot of work and a lot of new awe­some tech­nol­o­gy to get things spin­ning, though.

Well, for a Mars-like body there is a sec­ond chance to pos­si­bly get plate tec­ton­ics and mag­net­ic shield­ing going if some­thing big strikes and melts it again, and adds con­sid­er­ably more mass. May­be your sys­tem had some insta­bil­i­ty and anoth­er small plan­et smashed into sim­i­lar half-dead fish. Chances of this hap­pen­ing in the more or less sta­ble sys­tem are very, very small, almost improb­a­ble. But if this had hap­pened to our Mars, the after­glow would have been a spec­tac­u­lar sight from Earth, and so is the fly­ing debris. When things have cooled down, the ter­raform­ing can begin. Of course, it will be a VERY lengthy process. How­ev­er, if you are inter­est­ed in a short-term and a quick­er project, Mars isn’t any bet­ter than any space sta­tion in terms of life sup­port. And with cur­rent inte­ri­or process­es it wouldn’t get any bet­ter with age.

The silicate bodies of the Solar System (Mercury, Venus, Earth, the Moon and Mars). Image courtesy NASA/JPL-Caltech.
The sil­i­cate bod­ies of the Solar Sys­tem (Mer­cury, Venus, Earth, the Moon and Mars). Image cour­tesy NASA/JPL-Caltech.

What exact­ly pow­ers a plan­et?

In case of Earth 75% comes from radioac­tive ele­ment decay and the rest is the rem­nant heat (the trapped poten­tial ener­gy) from accre­tion times. Then, there’s anoth­er ques­tion: How old can a ter­res­tri­al plan­et be? Accord­ing to BB the­o­ry, the uni­verse is about 14 bil­lion years old (well, 13.75 to be exact, but that is irrel­e­vant here), and most ele­ments are much younger. Even if galax­ies appeared ear­ly in Uni­verse his­to­ry, I doubt there would be a hab­it­able plan­et that old. Pri­mor­dial gas giants? Per­haps, plen­ty. Pri­mor­dial rocky objects? Not a chance.

After the Uni­verse became more and more enriched in ele­ments, the pos­si­bil­i­ty for rocky plan­ets to form even­tu­al­ly appeared. How­ev­er, a plan­et, say, 7 bil­lion years old (or old­er) would be pow­ered by a dif­fer­ent ratio of radioac­tive iso­topes than an earth-aged plan­et. The age of chem­i­cal ele­ments can be esti­mat­ed using radioac­tive decay to deter­mine how old a given mix­ture of atoms is.

The long-term ther­mal evo­lu­tion of rocky plan­ets depends on the abun­dance of the long-lived radioiso­topes Th-232, U-235, and U-238 at the time of plan­et for­ma­tion, and those are pro­duced only dur­ing explo­sive nucle­osyn­the­sis (r-process) in stars with at least 8 to 20 solar mass­es (Chen et al. 2006).

Since we are talk­ing about ter­res­tri­al plan­ets, we are look­ing for sil­i­cates. In con­trast to the above­men­tioned iso­topes, sil­i­con (Si) is pro­duced by the whole range of mas­sive stars. Tho­ri­um (Th) and ura­ni­um (U) are dif­fi­cult to detect, but there is anoth­er ele­ment, europi­um (Eu), which is pro­duced in the same reac­tion type (r-process) as Th and U, and can be mea­sured. All r-process ele­ments scale close­ly with solar val­ues (Fre­bel 2008). This means that the aver­age abun­dance of Th-232, U-235, and U-238 can be pre­dict­ed from the europi­um trend with metal­lic­i­ty, the age-metal­lic­i­ty rela­tion­ship and the star for­ma­tion his­to­ry of the Galaxy, and the half-life of each iso­tope. The aver­age abun­dance of Eu to Si decreas­es by a fac­tor of 0.63 as the metal­lic­i­ty increas­es by a fac­tor of 100 to a solar val­ue (Ces­cut­ti 2008).

An over­whelm­ing­ly large pro­por­tion of ura­ni­um on Earth is U-238. This makes it the heav­i­est atom com­mon­ly found in nature. U-238 and Th-232 have half-lives of 4.468 and 14.05 Gyrs respec­tive­ly, but the ura­ni­um is under­abun­dant in the Solar Sys­tem com­pared to the expect­ed pro­duc­tion ratio in super­novae. This is not sur­pris­ing since the U-238 has a short­er half-life.

Plan­ets form­ing ear­ly in the his­to­ry of the Galaxy would have 50% more U-238, but six times more U-235, than Earth. The high­er abun­dance is because the amount of radioiso­topes in the inter­stel­lar medi­um only reflects mas­sive star for­ma­tion over a few half-lives, where­as Si-28 and oth­er sta­ble iso­topes accu­mu­late over the his­to­ry of the Galaxy. There­fore, the­se sys­tems are sil­i­con-poor. How­ev­er, the high abun­dance of U-235 could have an impor­tant role in the ear­ly ther­mal his­to­ry of such plan­ets.

Heat trans­fer

Ther­mal ener­gy from the hot inte­ri­or of the plan­et flows out of the sur­face into space. When an object is at a dif­fer­ent tem­per­a­ture than its sur­round­ings, heat will trans­fer from the region of high­er tem­per­a­ture to the region of cold­er tem­per­a­ture to achieve ther­mal equi­lib­ri­um. The mantle trans­fers heat from the hot core at the planet’s cen­ter to the cold­er sur­face. When an upward heat flux pass­es through a flu­id, the ther­mal ener­gy is trans­port­ed in two ways: con­duc­tion and con­vec­tion. The con­trol para­me­ter that selects between the two regimes is the Rayleigh num­ber.

In con­duc­tion the ener­gy trans­fer occurs from hot vibrat­ing atoms and mol­e­cules to neigh­bor­ing atoms and mol­e­cules, while the flu­id stays put, as if it were a solid. In a grav­i­ta­tion­al field, if tem­per­a­ture gra­di­ents in the mantle are large enough, insta­bil­i­ty due to buoy­an­cy will cause con­vec­tion.

Con­vec­tion is heat trans­fer by the move­ment of flu­id. The increase in tem­per­a­ture pro­duces a reduc­tion in den­si­ty and the warm, buoy­ant mate­ri­al begins to rise, while cold­er, denser mate­ri­al near the sur­face is dis­placed and sinks. Con­vec­tion trans­ports heat more effi­cient­ly than con­duc­tion due to mass trans­port.

Mantles are pre­dom­i­nant­ly solid rocky shells (get­ting more solid with depth), yet over very long time scales (tens to hun­dreds of mil­lions of years) mantle rocks under extreme pres­sure and tem­per­a­ture slow­ly deform like a vis­cous flu­id, mov­ing in cir­cu­lar cur­rents called con­vec­tion cells.

Mantle con­vec­tion ulti­mate­ly dri­ves all geo­log­i­cal events, such as vol­ca­noes, earth­quakes, and plate tec­ton­ics. It moves con­ti­nents to new posi­tions. The con­ti­nents, in turn, mod­i­fy the flow inside the mantle due to “ther­mal blan­ket­ing”, act­ing like per­fect ther­mal insu­la­tors. Thus, con­ti­nen­tal growth strong­ly affects mantle cool­ing.

Plate tec­ton­ics

Plate tec­ton­ics is a mod­el in which the out­er shell of a plan­et is bro­ken into a num­ber of thin rigid plates that move with respect to one anoth­er. The rel­a­tive veloc­i­ties of plates are of the order of a few tens of mil­lime­ters per year. Vol­can­ism and tec­ton­ism are con­cen­trat­ed at plate bound­aries. Much of Earth’s inter­nal heat is relieved through this process and many of Earth’s large struc­tural and topo­graph­ic fea­tures are con­se­quent­ly formed.

Con­ti­nen­tal drift is cyclic and the main rea­son for this is the move­ment in con­vec­tion cells. Ocean basins open and close, form­ing super­con­ti­nents, which lat­er break apart again dur­ing anoth­er open­ing of the ocean; this process is known as the Wilson cycle.

Con­ti­nen­tal col­li­sion is one of the pri­ma­ry mech­a­nisms for the cre­ation of moun­tains in the con­ti­nents; the oth­er is sub­duc­tion, the process when one tec­ton­ic plate moves under anoth­er, sink­ing into the mantle, as the plates con­verge. The Himalayas and the Alps are exam­ples of moun­tain belts caused by con­ti­nen­tal col­li­sions, and the Andes are asso­ci­at­ed with sub­duc­tion. That’s the awe­some pow­er of Earth’s con­vec­tion mode and grav­i­ty com­bined. In con­trast, the extreme height of the Mar­tian vol­ca­noes can be attrib­ut­ed to the low sur­face grav­i­ty and the lack of rel­a­tive motion between the lithos­phere and the mag­ma source.

Impli­ca­tions to cli­mate

Con­ti­nents are not just a fan­cy dec­o­ra­tion for your maps. Con­ti­nent cycle is a major play­er in the glob­al geo­chem­istry and cli­mate of a plan­et.

Dur­ing con­ti­nen­tal dis­per­sal the sea lev­el is high, and warm and humid mar­itime cli­mate is dom­i­nant. The lev­el of ocean floor spread­ing is high and rel­a­tive­ly large amounts of car­bon diox­ide are pro­duced at ocean­ic rift­ing zones. Seafloor spread­ing cen­ters cycle sea­wa­ter through hydrother­mal vents, reduc­ing the ratio of mag­ne­sium to cal­ci­um in the sea­wa­ter through meta­mor­phism of cal­ci­um-rich min­er­als in basalt to mag­ne­sium-rich clays (Wilkin­son and Given, 1986; Lowen­stein et al., 2001). This reduc­tion in the Mg/Ca ratio favors the pre­cip­i­ta­tion of cal­cite over arag­o­nite, thus the sea­wa­ter chem­istry is that of a cal­cite sea.

Dur­ing con­ti­nen­tal aggre­ga­tion the ocean lev­el drops due to lack of seafloor pro­duc­tion. The cool­er and arid con­ti­nen­tal cli­mate dom­i­nates, cor­re­spond­ing with sev­ere desert envi­ron­ments and fre­quent con­ti­nen­tal glacia­tions. The sea­wa­ter chem­istry is that of an arag­o­nite sea, with high mag­ne­sium con­tent.

The con­ti­nen­tal shelf has a very low slope and a small increase in sea lev­el will result in a large change in the amount of flood­ed land. With aver­age­ly young world ocean the seafloor will be rel­a­tive­ly shal­low, mak­ing the sea lev­el high with more land­mass flood­ed. The old world ocean is rel­a­tive­ly deep and more land will be exposed due to low sea lev­el.

When con­ti­nents are dis­persed, the plate tec­ton­ic flux is high and Andean-type vol­can­ism is exten­sive. With a sin­gle super­con­ti­nent and a low plate flux (the huge plate is a “blan­ket”) the mantle heats up due to the decay of radioac­tive iso­topes. The increase in mantle tem­per­a­ture and the warm­ing near the core–mantle bound­ary leads to an increase in the plume flux and the breakup of the super­con­ti­nent.

Impli­ca­tions to life

Con­ti­nen­tal spreads stim­u­late life diver­si­ty on evo­lu­tion­ary scale. Not only that, con­ti­nen­tal cycle influ­ences the size of the species. Accord­ing to Bergmann’s rule most mam­mals tend to be larg­er in cold cli­mates and small­er in hot ones. The study by Smith et al.(2010) also shows that the cold­er the envi­ron­ment and the big­ger the land sur­face, the big­ger the large mam­mals become.

Tec­ton­ic envi­ron­ments and humans

Tec­ton­ic activ­i­ty, as earth­quakes and vol­ca­noes, has a great influ­ence on the course of devel­op­ment of civ­i­liza­tions and oth­er com­plex cul­tures. Beside the obvi­ous destruc­tion, tec­ton­ism appar­ent­ly accel­er­at­ed cul­tur­al devel­op­ment. How this hap­pened (and hap­pens) exact­ly, is dis­cussed in great detail in Tec­ton­ic Envi­ron­ments of Ancient Cul­tures blog by Eric Force.

A mat­ter of mass

The mass of a plan­et is prob­a­bly one of the most impor­tant prop­er­ties regard­ing con­vec­tion mode. Adding more mass can add trou­bles sim­i­lar to that of reduc­ing it (Venus, Mars). Earth might be a neat bor­der­line case here, when things oper­ate smooth­ly. Well, no one real­ly knows for sure yet if plate tec­ton­ics will oper­ate on mas­sive plan­ets, and regard­ing that there is a dis­agree­ment in the sci­en­tific com­mu­ni­ty. Sev­er­al stud­ies were car­ried out and the results are quite inter­est­ing in all cas­es.

For my per­son­al designs I have cho­sen a con­di­tion that plate tec­ton­ics does indeed oper­ate on larg­er worlds (but not extreme­ly large, though, because of sev­er­al oth­er fac­tors). How­ev­er, I take things from the oth­er mod­els into account as well.

There is also anoth­er thing worth men­tion­ing here, a paper by Mar­tyn Fogg (I have men­tioned it in one of my pre­vi­ous posts). There, in sec­tion 3.2, page 7, is a for­mu­la (14) for esti­mat­ing the dura­tion of viable volcanic/tectonic recy­cling of volatiles on a given plan­et – basi­cal­ly, the timescale for plate tec­ton­ics. As the con­ti­nent cycle goes on, increas­ing con­ti­nen­tal area would even­tu­al­ly ini­ti­ate a tran­si­tion between plate tec­ton­ics and a stag­nant lid mode of mantle con­vec­tion.

Plate tec­ton­ics is the pri­ma­ry mech­a­nism through which Earth (and any plan­et with the same regime) los­es its heat. Lenardic et al.(2005) sug­gests a poten­tial con­straint on con­ti­nen­tal sur­face area at which con­ti­nen­tal growth will stop. This crit­i­cal point is pre­dict­ed as a func­tion of mantle heat flow. For the Earth’s cur­rent glob­al heat flux, the crit­i­cal con­ti­nen­tal sur­face area is esti­mat­ed to be 35–50% to allow plate tec­ton­ics to ini­ti­ate, and present day Earth’s con­ti­nent crust area is about 40%.

So, how would things work on big­ger plan­ets?

There are sev­er­al con­di­tions nec­es­sary for plate tec­ton­ics to oper­ate (Mar­t­in et al.,2008). First of all, the plan­et must have cooled enough so that it is too cold to sus­tain mag­ma ocean. Sec­ond, its inte­ri­ors must be hot enough to main­tain con­vec­tion with­in the upper lay­ers of the body to pre­vent the exis­tence of stag­nant lid. Third, its lithos­phere needs to be cool enough, strong enough, dense enough and thin enough to allow sub­duc­tion. Fourth, liq­uid water on the sur­face is prob­a­bly the most vital ingre­di­ent for suc­cess­ful plate tec­ton­ics.

Mag­net­ic field life­time is also tight­ly relat­ed to the ther­mal evo­lu­tion of a plan­et. To dri­ve dynamo action, a liq­uid metal­lic core must be in an active con­vec­tion state.

Valen­cia et al. (2006), (2007) used plan­e­tary para­me­ter scal­ing to argue that high­er grav­i­ty favors sub­duc­tion and plate tec­ton­ics are inevitable on larg­er ter­res­tri­al worlds. Near­ly at the same time O’Neill and Lenardic (2007) pro­posed that super-sized Earths are like­ly to be in an episod­ic or stag­nant lid regime.

As plan­et mass increas­es, it pos­es an increas­ing­ly sev­ere prob­lem for plate tec­ton­ics. Mid­dle-aged super-Earths may suf­fer from con­ti­nen­tal spread, which could choke off plate tec­ton­ics, plac­ing a plan­et in stag­nant lid mode. Pro­vid­ed that crustal flow lim­its con­ti­nen­tal thick­ness, it was shown that con­ti­nents will spread out to coat the sur­face of a ter­res­tri­al plan­et with more than three Earth mass­es in much less time than the age of the Earth (Kite et al., 2009).

More recent stud­ies show that there might be anoth­er sev­ere prob­lem to plate tec­ton­ics on super-sized “Earths”. The con­vec­tive pat­tern and the heat trans­port in a ter­res­tri­al plan­et depends on the vis­cos­i­ty of the mantle mate­ri­al. The vis­cos­i­ty, on the oth­er hand, depends strong­ly on tem­per­a­ture and pres­sure, i.e., the vis­cos­i­ty decreas­es with increas­ing tem­per­a­ture but increas­es with increas­ing pres­sure. Thus, the larg­er the plan­et the stronger is the influ­ence of the pres­sure on the vis­cos­i­ty and flowa­bil­i­ty of mate­ri­al. This depen­dence becomes an impor­tant fac­tor for the mantle con­vec­tion of plan­ets with mass­es larg­er than one Earth mass. With more mass, plan­ets are sub­ject to slug­gish con­vec­tion regime in the low­er mantle and for­ma­tion of a con­duc­tive lid over the core-mantle bound­ary. Even­tu­al­ly con­vec­tion stops and heat is trans­port­ed only due con­duc­tion. Ther­mal­ly induced mag­net­ic field gen­er­a­tion is sup­pressed as well. Com­po­si­tion­al dynamos might also become sup­pressed due to small cool­ing rate of the inner core; such plan­ets will end up with small sur­face mag­net­ic fields, high­er radi­a­tion envi­ron­ment and stronger atmos­pher­ic loss (Sta­menkovic et al., 2010).

The results of anoth­er study using 2D spher­i­cal mantle con­vec­tion mod­el (Noack & Breuer, 2011) show that the propen­si­ty of plate tec­ton­ics has a peak at a speci­fic mass: assum­ing vis­cos­i­ty increas­es with pres­sure, the peak occurs between one and five Earth mass­es. How­ev­er, the vari­a­tion of vis­cos­i­ty with pres­sure is strong­ly debat­ed and might as well decrease with pres­sure (Kara­to, 2011, Icarus; Armann et al., 2010, Nature).

Tidal­ly locked plan­ets

This is anoth­er inter­est­ing case. If a plan­et lacks an atmos­phere thick enough to bal­ance the sur­face tem­per­a­ture, it will be in a stag­nant lid con­vec­tion regime at the night side and in a mobile regime at the day side. The rea­son for such dif­fer­ence is the upper con­vec­tive mantle, which is sen­si­tive to tem­per­a­ture vari­a­tions on the sur­face (Noack et al., 2010).

The hab­it­abil­i­ty of plan­ets in syn­chro­nous rota­tion about their star may lie well out­side the Hab­it­able Zone (Gel­man et.al 2011). Such world may still sup­port liq­uid water on its sur­face, or shal­low sub­sur­face, in cer­tain regions of the plan­et. Thus, tidal­ly locked mantle and cli­mate pat­terns must be com­bined and assessed to deter­mine the sur­face envi­ron­ment, keep­ing in mind this may vary great­ly from the sub­stel­lar to anti­s­tel­lar regions.

# Well, this is it for now; may­be some­thing will be added lat­er.

Jeno Marz
JENO MARZ is a science fiction writer from Latvia, Northern Europe, with background in electronics engineering and computer science. She is the author of two serial novels, Falaha’s Journey: A Spacegirl’s Account in Three Movements and Falaha’s Journey into Pleasure. Marz is current at work on a new SF trilogy. All her fiction is aimed at an adult audience.

2 Comments

  1. Great arti­cle. Brought back a lot of stuff I’d for­got­ten in col­lege, and the new research about Mars is kin­da cool. 🙂 At least I know if I ever need resources for a SF, I can come here. ^_^

    1. In all hon­esty, I for­got myself what I have writ­ten in my world­build­ing arti­cles. It was so long ago. Keep in mind this is from ear­ly 2012, so sci­ence has more in store about Mars by now. I’m not exact­ly fol­low­ing the research. 🙂

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