Christopher P. McKay
Mail Stop 245-3
NASA Ames Research Center
Moffett Field CA 94035 USA
Exploration of the Solar System by spacecraft (now complete except for
Pluto) indicates that Earth is the only planet with life. However,
there is one intriguing aspect to this otherwise lifeless story: Mars
appears to have been habitable early in its history. Photographs of
the Martian surface from orbit show ancient river features, now dry
and covered with craters, that are testimony of this early clement
environment. The fact that liquid water existed on the surface of
Mars at about the same period of time that life appeared on Earth
(over 3.5 thousand million years ago) is arguably the most interesting
biological fact we know about the other planets. So the search for
fossils on Mars has begun and aspires to be the main motivation --- or
scientific rationalization --- for human exploration of Mars. The
observation that Mars was once a habitable world leads us to ponder
how Mars became the cold harsh desert it is today and to speculate
under what conditions, natural or artificial, it could be restored to
a habitable state.
Humanity has now demonstrated that it is capable of inadvertent
modification of environments on planetary scales. Furthermore, there
are now suggestions that we undertake purposeful global engineering
efforts to redress, or at least mitigate, the untoward effects of our
past actions, in particular with respect to the planetary greenhouse
and the ozone layer (1,2). Couple this development with the recent
call for human exploration of Mars --- and the suggestion of a past
habitable state on that planet --- and the question of `terraforming'
Mars is not so out of place.
In a recent paper my colleagues and I attempted to address
terraforming Mars in a rigorous scientific manner, limiting our
discussion to current technologies (3). Under this restriction it is
not possible to alter the basic physical parameters of a planet; its
mass, distance from the sun, rotation rate, etc.. It is equally
impossible to import materials from which to build a planetary
atmosphere or hydrosphere. For example, the transport to Mars of an
atmosphere with a surface pressure equal to sea level on Earth would
require 10^15 tons. Compare this to the lifting capability to low
Earth orbit of about 100 tons for the biggest rocket currently
available, the Soviet Energiya. The only thing way to engender a
habitable state on Mars is to affect the distribution of elements and
compounds already present and thereby alter the climate and chemical
state of the planet --- which is what we are doing on Earth.
As we considered the question of terraforming Mars, it became clear
that there are two potentially habitable states for that planet. One,
suitable for plants and microorganisms, would have a thick atmosphere
composed primarily of carbon dioxide with small amounts of nitrogen
and oxygen as needed for fixation and plant respiration, respectively.
Such an atmosphere would be similar to what we believe existed on Mars
during its early history and would also be similar to the conditions
thought to prevail on the early Earth until the rise of oxygen
sometime near the end of the Precambrian, about a thousand million
years ago. Such an atmosphere would be warm and would provide enough
pressure that humans would not need a space suit, but it would not be
breathable.
The other possible habitable state would be one with an Earth-like
mixture of gases in the atmosphere with carbon dioxide set to the
upper limit of long term human tolerance, about 1% (3,4). Without the
thick carbon dioxide greenhouse this atmosphere cools to --55 degc,
much too cold to support life. Warming the planet would require
augmentation of the carbon dioxide greenhouse effect. A practical way
to do this was proposed in 1984 by James Lovelock (5), famous for his
Gaia hypothesis of life on Earth, in a science fiction novel in which
Earth exported its chlorofluorocarbons compounds to Mars on ballistic
missiles rendered obsolete by the end of the Cold War. While Lovelock
was prescient in anticipating that these missiles would soon be
`available,' his scheme for transporting materials from Earth to Mars
fails from the sheer mass of matter that must be carried. If the
needed CFCs could be manufactured on Mars, then the scheme could work.
Our calculations (3) indicate that a suite of four existing CFCs could
warm Mars significantly, but to warm it to habitable levels a mixture
of specially developed compounds would be needed. Obviously, on
Earth, CFCs have not been designed to optimize their greenhouse
effects and if some effort were directed this way it could be that a
suitable combination of gases would be found. Solar ultraviolet light
would destroy these molecules and they would need to be reformed
continuously (3). Thus, an oxygen-rich atmosphere aided by the
artificial greenhouse gases could provide an atmosphere on Mars that
is thick and warm and breathable by humans and other animals.
Are there enough of the needed elements on Mars --- carbon dioxide,
water, and nitrogen --- to provide the raw materials for the
construction of either of these hypothetical habitable states? The
published range of estimates as to the amount of these compounds
varies considerably (3,6). Review of these estimates suggest that
there probably is ample carbon dioxide and water on Mars to produce a
habitable state (3). The question of nitrogen is more difficult to
assess, but the upper range of estimates allows for enough of this gas
to construct an Earth-like atmosphere. The question of nitrogen
abundance on Mars is the key issue that must be resolved on future
missions as part of any further assessment of Mars' terraforming
potential.
If habitable states exist and there are sufficient supplies of the
needed resources, how could the present Mars be transformed? We
envision two phases to this process. The first phase would be the
warming of the planet and the release of any carbon dioxide held in
the polar caps and the soil. The second phase would be the conversion
of this carbon dioxide into oxygen and the buildup of nitrogen in the
atmosphere.
The warming of Mars might be accomplished by producing the CFCs
discussed above in the context of warming an oxygen-rich atmosphere.
We have calculated that it would be possible to warm the present Mars
sufficiently with these gases so that any carbon dioxide resident in
the permanent polar caps or in the soil would enter the atmosphere.
This carbon dioxide would enhance the greenhouse effect, further
warming the planet. If there was enough carbon dioxide available to
provide for a surface pressure on Mars of about twice Earth sea level,
then the temperature of the planet would reach habitable limits. The
production of CFCs could stop and Mars would maintain this habitable
state --- the planet would be resuscitated.
If it was decided that restoring the thick primordial carbon dioxide
atmosphere on Mars was only the penultimate goal then transforming the
carbon dioxide into oxygen using plants would begin. There are
difficulties with this proposition. First, in order for there to be a
net production of oxygen, there must be a concomitant sequestering of
reduced carbon or the reaction will merely reverse itself and consume
the oxygen produced. It is not clear how this could be done on Mars
without deep ocean basins and efficient burial processes, which on
Earth are associated with plate tectonics, absent on Mars.
Futher, if a suitable sink for the reduced carbon produced by
photosynthesis on Mars was found, it would take a long time for the
buildup of oxygen to reach significant levels. A simple estimate of
the time required is obtained from the average efficiency with which
ecosystems on Earth convert solar energy into fixed carbon, 0.01% (7).
At this efficiency it would take about 100,000 years of Martian
sunlight to produce an oxygen-rich atmosphere on Mars (3). The
warming of Mars is a more achievable task because the greenhouse
effects involved in warming Mars are fairly efficient at trapping
solar energy and because there is a positive feedback between warming
the planet and releasing carbon dioxide from the polar caps and the
soil. Warming Mars to habitable temperatures may only require a few
centuries (3). It is important to appreciate that solar energy is the
only plausible power source for terraforming Mars. The 10^16 Watts of
solar power on Mars is over 10 thousand times the world electrical
power and exceeds the energy in the combined nuclear weapons of the
world in about 30 mins.
If Mars was made habitable it is likely that the processes that
resulted in the deterioration of its initial habitable state would
again act to destroy the newly salubrious environments. We estimate
that this would take about 100 million years (3,6). Recidivisism on
this timescale may be acceptable.
The main conclusion from our work (3) briefly summarized above is that
our current knowledge of Mars suggests that it is possible to
transform that planet into one of at least two habitable states using
technologies that we are already demonstrating, probably to our
detriment, on the Earth. Should we do so? I have found it
interesting to pursue this question within the scope of environmental
ethics (8,9,10). We must consider that Mars has no life today (more
on this later) and we are proposing to introduce life there. This is
a situation novel in environmental ethics, which has heretofore only
considered human interactions with systems that were biologically
complete prior to human activity. On Earth the status quo ante and a
living world are the same. Environmental principles that place value
on life do not conflict with environmental principles that attempt to
restrain human activity. For terraforming Mars the situation is the
obverse: there the status quo, nature as humans first found it, is
lifeless and we have the possibility of introducing (or
re-introducing) life there. Faced with this critical difference
between Earth-bound environmental ethics and the problem of Mars it is
not possible to utilize the great body of literature in environmental
ethics directly. One must instead distill this to a set of basic
axioms from which one can then generalize as to the appropriateness of
terraforming Mars. From my own reading of the ethics literature I
have suggested (8) that principles of environmental ethics are based
on some combination of three fundamental axioms: 1) Anti-humanism,
the notion that human action is inevitably harmful; 2) Stewardship, a
requirement that humans must use nature wisely for their own benefit;
and 3) Intrinsic worth, the supposition that some class of objects
have intrinsic worth regardless of their utility to humans. The
translation of the first two axioms to the question of terraforming
Mars is straightforward but becomes more difficult for the third.
Invoking intrinsic worth as a basis for environmental thought is a
fairly recent development (11) and has heretofore focused on living
things and their assemblages as the class of objects with intrinsic
value. In this case, a living Mars is of more intrinsic worth than a
lifeless Mars and restoring life to that planet is ethically
motivated. However, one could conceive of the class of objects with
intrinsic worth as nature prior to any human activity: primoridal
nature. This point of view receives little support from philosophy in
that is assumes that what is can be defended as what ought to be --- a
classic logical error. Assigning intrinsic worth to the present state
of Mars would argue for leaving Mars as it is. Mars' scientific value
is indisputable but it is certainly true that extensive exploration
followed by a careful and studied program of introducing life would
yield an even greater scientific harvest. The scientific knowledge
gained by studying the ways in which a biosphere could be introduced
on Mars may inform us as to the preservation of the one on Earth.
In the previous discussion I have assumed that Mars is presently
lifeless and there is good reason to think that it is. Yet it is
possible that life forms are dormant on Mars within the permafrost or
have found refuge in some cryptic niche. What then for terraforming?
Would it not be appropriate to then alter Mars making conditions on
the planet suitable for that indigenous life so as to allow it to
develop into a diverse and planetary scale biota? Along these lines I
originally proposed a definition of terraforming as the purposeful
alteration of another planetary environment so as to improve the
chances of survival of an indigenous biology or, in the absence of any
native lifeforms, to allow for habitation of most if not all
terrestrial lifeforms.
Will we terraform Mars? It may seem unlikely that Earthly societies
would invest in such a long term project. However, if humans
establish permanent research bases on Mars then orbital mechanics and
travel times will dictate that these bases be as self sufficient as
possible. It may well be the case that terraforming is undertaken by
a group of people who consider Mars their home and for whom it is a
matter of great import. What for us may seem like a flight of fancy
may to them be a legacy for their children.
1. Maddox, J. Can mirrors beat the greenhouse effect? Nature 1990;
346: 311.
2. Joos, F., Sarmieto, J.L. & Siegenthaler, U. Estimates of the
effect of Southern Ocean iron fertilization on atmospheric
CO$_2$ concentrations. Nature 1991; 349: 772-775.
3. McKay, C.P., Toon, O.B. & Kasting, J.F. Making Mars Habitable,
Nature 1991; 352: 489-496.
4. Billings, C.E. Atmosphere. In: Bioastronautics Data Book,
Parker, J.F., Jr. & West, V.R. (ed.). NASA SP-3006: Washington,
DC., 1973, pp 35-64.
5. Lovelock, J.E. & Allaby, M. The Greening of Mars. Warner: New
York, 1984, p. 215.
6. McKay, C.P. & Stoker, C.R. The early environment and its
evolution on Mars: Implications for life. Rev. Geophys. 1989;
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7. Whittaker, R.H. Communities and Ecosystems, second edition,
Macmillan: New York, 1975.
8. McKay, C.P. Does Mars have rights? An approach to the
environmental ethics of planetary engineering. In: Moral
Expertise, MacNiven, D. (ed.). Routledge: London, 1990, pp
184-197.
9. Haynes, R.H. Ecce Ecopoiesis: Playing God on Mars. In: Moral
Expertise, MacNiven, D. (ed.). Routledge: London, 1990, pp
161-183.
10. Haynes, R.H. & McKay, C.P. The implantation of life on Mars:
Feasibility and motivation. Adv. Space Res. 1992; 12: (4)
133-140.
11. Naess, A. The shallow and the deep, long-range ecology
movement: A summary. Inquiry 1973; 16: 95-100.
