A noie on the loss of hydrogen and the supply 

o f hydrogen to the Earth 

as a direct or indirect effect of the solar wind 

JST. A . B A R R I C E L L I - R . M E T C A L F E ( * ) 

R i c e v u t o il 29 M a r z o 1969 

SUMMARY. — T h e rate at which h y d r o g e n is supplied t o the E a r t h 
by the solar w i n d , and the rate at which h y d r o g e n escapes f r o m the E a r t h 
by diffusion into space are c o m p a r e d 011 the basis of recent estimates. It 
is f o u n d that the hydrogen supplied by the solar w i n d is roughly compar-
able to or m a y even be larger than present h y d r o g e n losses. T h e E a r t h 
m a y h a v e lost most of its hydrogen at an early stage when, according to 
U r e y and Miller (1959), P o x (1960) and others, the E a r t h had a reducing, 
instead of an o x i d i z i n g atmosphere. H o w e v e r , no sufficient e v i d e n c e is 
y e t a v a i l a b l e to determine w h e t h e r the E a r t h is still losing more h y d r o g e n 
than it receives. T h e c o m m o n notion that the E a r t h is still in the process 
of losing its hydrogen is t h e r e f o r e in question. 

RIASSUNTO. — Sulla base di recenti stime, si confronta il tasso di 
idrogeno f o r n i t o alla T e r r a dai v e n t i solari in r a p p o r t o al tasso di f u g a del-
l ' i d r o g e n o dalla T e r r a in seguito a diffusione nello spazio. Si constata che 
l ' i d r o g e n o f o r n i t o dai v e n t i solari è pressoché pari, forse anzi superiore a 
quelle che sono le perdite attuali di idrogeno. E da presumere che la T e r r a 
abbia perso la m a g g i o r parte del proprio idrogeno in uno stadio primario, 
quando — secondo U r e y e Miller (1959), F o x (1960) ed altri — l ' a t m o s f e r a 
della T e r r a era del t i p o riducente, i n v e c e che ossidante. Comunque, non 
sono state finora p o r t a t e p r o v e concludenti per determinare se, t u t t o r a , 
la T e r r a continui a perdere idrogeno in quantità superiore a qnella che r i c e v e . 
L ' i p o t e s i g e n e r a l m e n t e accettata, secondo cui il processo di riduzione del 
contenuto di idrogeno della T e r r a sarebbe t u t t o r a in corso, è quindi posta 
in dubbio. 

( * ) U n i v e r s i t y of W a s h i n g t o n , Seattle, W a s h i n g t o n . 



1 0 6 N. A. B A R R I C E L L I R. M E T C A L F E 

1. - INTRODUCTION. 

After the discovery of the substantial intensity ol' the stream 
of protons and hydrogen atoms carried by the solar wind, the question 
lias arisen whether the rate at which hydrogen is supplied to the Earth 
by the solar wind might be comparable to or larger than the rate at 
which the Earth is losing hydrogen by diffusion into space. Only 
very rough estimates of the time rates are possible at present, and 
the main purpose of this note is to attract attention to the problems 
involved and their possible implications rather then bring a final 
solution. 

2. - E S T I M A T E OF H Y D R O G E N S U P P L I E D BY SOLAR W I N D . 

Only a high estimate of the rate of hydrogen supplied to the 
Earth by the solar wind — obtained by disregarding the affect of 
the Earth's magnetic field on charged particles — seems meaningful 
on the basis of present information. 

According to Jokipii ('), the flux of the solar wind is approxi-
mately 3 x 10* protons/cm2.sec in the vicinity of the Earth. Other 
recent estimates (Brigde, (2() differ from this by a factor 3 either 
way. Using the first estimate, the amount of hydrogen from the 
solar wind which would be intercepted by the Earth if the Earth did 
not have a magnetic field or if the solar wind contained a comparable 
amount of uncharged particles would be: 

/ protons\ / 1 \ / gm \ /3.(> x 103 sec\ 
3 x 10» 1 w 1 ' 1 

7TR2 . 

cm2 • sec/ \ protons I \ mole of HI \ hr 
6.02 x 1023 

mole oi I I 

Í876Ü 1 U 'U7? 2 == (l.57 x 10 8 K m \ 
\ vr / \ cm2- vr/ 

hr\ „ „ „ ^ „ „ gm 
yr 

(where R is the radius of the Earth). 

Because of the Earth's magnetic field and the effect of the magne-
tosphere on charged particles, the actual amount of solar-wind-hydrogen 
which would reach the Earth's atmosphere would be only a fraction 
of this figure which should therefore be considered as a high estimate. 
Present theories and available information give discrepant estimates of 



A N O T E ON T I I E LOSS OK H Y D R O G E N A N D T H E S U P P L Y , E T C . 107 

tliis fraction and it has not been decided whether this fraction would 
be substantial or not. 

3. - E S T I M A T E OF HYDROGEN LOSS FROM E A R T H . 

According to Spitzer (3), (table 5, page 245) the time of escape 
of free hydrogen from the Earth is 3.5 x 104 years, assuming an upper 
atmosphere temperature of 1000°K. According to Farley et al. (4) the 
approximate temperature from 350 to 500 km is 1000°K; according 
to Evans (5) the ion-temperature at about 400 km is approximately 
!)00°K. The time of escape is defined as the time in which the 
amount of hydrogen lost by the Earth would be equal to the amount 
of hydrogen in its atmosphere on the assumption that the hydrogen 
concentration in the atmosphere were maintained constant by a con-
tinuous supply. 

The amount of hydrogen in the atmosphere per cm2 of the Earth's 
<r]i) 

surface is 3.5 X 10 5 * - (Hutchinson, (")). Hence the rate of hydro-

gen loss from the atmosphere is estimated to be: 

Comparing this with the high estimate given in part 2 of the 
hydrogen accumulation shows that the rate of hydrogen gain could 
be as large as or even larger than the rate of hydrogen loss. Recent 
work by Chamberlain and Campbell (7) indicates that the rate of 
hydrogen loss from the atmosphere may be slightly smaller then the 
value calculated above. 

4. - CONCLUSION. 

In spite of the considerable uncertenity both in the hydrogen loss 
and the hydrogen supply by the solar wind, the least one can say about 
the result is that: One cannot take it for granted that the Earth's present 

cm2 • yr/ 

(where F is the radius of the Earth). 



N . A . B A R R I C E L I . I R. M E T C A L F E 

hydrogen balance is negative, or in other words that the Earth is still 
losing more hydrogen than is supplied to it by the solar wind. 

This conclusion does not contradict the explanation commonly 
given to the fact that the Earth as well as the other three terrestrial 
planets (namely Mars, Venus and Mercury) have a low content of 
hydrogen compared with the larger outer planets and the cosmic 
abundance of this element. The explanation that the Earth's gravita-
tional field is incapable of retaining a large concentration of hydrogen 
in it s atmosphere remains valid. However, the main loss of hydrogen 
by the Earth must have occurred at the time when the Earth had 
a reducing instead of an oxydizing atmosphere and probably contained 
a substantial concentration of hydrogen in its atmosphere (Urey and 
Miller (s), Fox (!l)). 

The transformation from reducing to oxydizing atmosphere caused 
by photosynthesis in living systems may have influenced the Earth's 
hydrogen balance in two different ways: 

1 - By binding most atmospheric hydrogen into water, thus 
preventing it from reaching the upper regions of the atmosphere where 
it could be lost by diffusion into space. 

2 - By supplying a chemical reagent capable of binding a signi-
ficant portion of the hydrogen carried to the Earth by the solar wind. 

Whether at present the net result of these phenomena is still a 
reduction or a tight balance or even an increase of the Earth's hydrogen 
is an unresolved question. However the very fact that the two fi-
gures are so close to one another that there can be any doubt at all 
about this point suggests an even balance between hydrogen supplied 
by the solar wind and hydrogen lost from the Earth. Otherwise one 
would have expected to find a difference by several orders of magni-
tude between the two figures, and the facts presented here would be 
difficult to explain as pure coincidence. 

The Earth may he losing approximately the same amount of 
hydrogen which is being supplied to it by the solar wind, and the con-
centration of hydrogen, and perhaps even the concentration of water 
vapor in the upper stratosphere may basically depend on this hy-
drogen balance. 

ACKNOWLEDGMENT. 

Thanks are clue to Professor Kenneth C. Clark for valuable 
advice and encouragement. 



A N O T E ON T I I E LOSS OF H Y D R O G E N A N D T H E S U P P L Y , E T C . 10'J 

R E F E R E N C E S 

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(*) HUTCHINSON G. D . , The Biochemistry of the Terrestrial Atmosphere. " T h e 
E a r t h as a P l a n e t " ( G e r a l d P . K u i p e r , ed.), 372, T a b l e 1, T h e 
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