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AIAA-93-2005
† Member AIAA
Copyright © 1993 by Martin Marietta Corp. Published by the American Institute of Aeronautics and Astronautics, Inc. with
permission.
Technological Requirements for Terraforming Mars
Robert M. Zubrin
and Christopher P. McKay
Martin Marietta Astronautics NASA Ames Research Center
PO Box 179, Denver, CO 80201 Moffett Field, CA, 94035
303-971-9299 (ph) 303-977-1893 (FAX) 415-604-6864(ph) 415-604-6779 (FAX)
Abstract
The planet Mars, while cold and arid today, once
possessed a warm and wet climate, as evidenced by
extensive fluvial features observable on its surface. It is
believed that the warm climate of the primitive Mars was
created by a strong greenhouse effect caused by a
thick CO
2
atmosphere. Mars lost its warm climate when
most of the available volatile CO
2
was fixed into the
form of carbonate rock due to the action of cycling
water. It is believed, however, that sufficient CO
2
to
form a 300 to 600 mb atmosphere may still exist in
volatile form, either adsorbed into the regolith or frozen
out at the south pole. This CO
2
may be released by
planetary warming, and as the CO
2
atmosphere
thickens, positive feedback is produced which can
accelerate the warming trend. Thus it is conceivable,
that by taking advantage of the positive feedback
inherent in Mars' atmosphere/regolith CO
2
system,
that engineering efforts can produce drastic changes
in climate and pressure on a planetary scale.
In this paper we propose a mathematical model of the
Martian CO
2
system, and use it to produce analysis
which clarifies the potential of positive feedback to
accelerate planetary engineering efforts. It is shown
that by taking advantage of the feedback, the
requirements for planetary engineering can be
reduced by about 2 orders of magnitude relative to
previous estimates. We examine the potential of
various schemes for producing the initial warming to
drive the process, including the stationing of orbiting
mirrors, the importation of natural volatiles with high
greenhouse capacity from the outer solar system, and
the production of artificial halocarbon greenhouse
gases on the Martian surface through in-situ industry.
If the orbital mirror scheme is adopted, mirrors with
dimension on the order or 100 km radius are required
to vaporize the CO
2
in the south polar cap. If
manufactured of solar sail like material, such mirrors
would have a mass on the order of 200,000 tonnes. If
manufactured in space out of asteroidal or Martian
moon material, about 120 MWe-years of energy would
be needed to produce the required aluminum. This
amount of power can be provided by near-term multi-
megawatt nuclear power units, such as the 5 MWe
modules now under consideration for NEP spacecraft.
Orbital transfer of very massive bodies from the outer
solar system can be accomplished using nuclear
thermal rocket engines using the asteroid's volatile
material as propellant. Using major planets for gravity
assists, the rocket V required to move an outer solar
system asteroid onto a collision trajectory with Mars can
be as little as 300 m/s. If the asteroid is made of NH
3
,
specific impulses of about 400 s can be attained, and
as little as 10% of the asteroid will be required for
propellant. Four 5000 MWt NTR engines would require
a 10 year burn time to push a 10 billion tonne asteroid
through a V of 300 m/s. About 4 such objects would
be sufficient to greenhouse Mars.
Greenhousing Mars via the manufacture of halocarbon
gases on the planet's surface may well be the most
practical option. Total surface power requirements to
drive planetary warming using this method are
calculated and found to be on the order of 1000 MWe,
and the required times scale for climate and
atmosphere modification is on the order of 50 years.
It is concluded that a drastic modification of Martian
conditions can be achieved using 21st century
technology. The Mars so produced will closely
resemble the conditions existing on the primitive Mars.
Humans operating on the surface of such a Mars would
require breathing gear, but pressure suits would be
unnecessary. With outside atmospheric pressures
raised, it will be possible to create large dwelling areas
by means of very large inflatable structures. Average
temperatures could be above the freezing point of
water for significant regions during portions of the year,
enabling the growth of plant life in the open. The
spread of plants could produce enough oxygen to
make Mars habitable for animals in several millennia.
More rapid oxygenation would require engineering
efforts supported by multi-terrawatt power sources. It is
speculated that the desire to speed the terraforming of
Mars will be a driver for developing such technologies,
which in turn will define a leap in human power over
nature as dramatic as that which accompanied the
creation of post-Renaissance industrial civilization.
2
Introduction
Many people can accept the possibility of a
permanently staffed base on Mars, or even the
establishment of large settlements. However the
prospect of drastically changing the planet's
temperature and atmosphere towards more earthlike
conditions, or "terraforming" seems to most people to
be either sheer fantasy or at best a technological
challenge for the far distant future.
But is this pessimistic point of view correct? Despite
the fact that Mars today is a cold, dry, and probably
lifeless planet, it has all the elements required to
support life: water carbon and oxygen (as carbon
dioxide), and nitrogen. The physical aspects of Mars,
its gravity, rotation rate and axial tilt are close enough to
those of Earth to be acceptable and it is not too far from
the Sun to be made habitable.
In fact computational studies utilizing climate models
suggest that it could be possible to make Mars
habitable again with foreseeable technology. The
essence of the situation is that while Mars' CO
2
atmosphere has only about 1% the pressure of the
Earth's at sea level, it is believed that there are
reserves of CO
2
frozen in the south polar cap and
adsorbed within the soil sufficient to thicken the
atmosphere to the point where its pressure would be
about 30% that of Earth. The way to get this gas to
emerge is to heat the planet, and in fact, the warming
and cooling of Mars that occurs each Martian year as
the planet cycles between its nearest and furthest
positions from the Sun in its slightly elliptical orbit
cause the atmospheric pressure on Mars to vary by
plus or minus 25% compared to its average value on a
seasonal basis.
We can not, of course, move Mars to a warmer orbit.
However we do know another way to heat a planet,
through an artificially induced greenhouse effect that
traps the Sun's heat within the atmosphere. Such an
atmospheric greenhouse could be created on Mars in
at least three different ways. One way would be to set
up factories on Mars to produce very powerful artificial
greenhouse gasses such as halocarbons ("CFC's")
and release them into the atmosphere. Another way
would be to use orbital mirrors or other large scale
power sources to warm selected areas of the planet,
such as the south polar cap, to release large reservoirs
of the native greenhouse gas, CO
2
, which may be
trapped their in frozen or adsorbed form. Finally natural
greenhouse gases more powerful than CO
2
(but much
less so than halocarbons) such as ammonia or
methane could be imported to Mars in large quantities
if asteroidal objects rich with such volatiles in frozen
form should prove to exist in the outer solar system.
Each of these methods of planetary warming would be
enhanced by large amounts of CO
2
from polar cap and
the soil that would be released as a result of the
induced temperature rise. This CO
2
would add
massively to the greenhouse effect being created
directly, speeding and multiplying the warming
process.
The Mars atmosphere/regolith greenhouse effect
system is thus one with a built-in positive feedback.
The warmer it gets, the thicker the atmosphere
becomes; and the thicker the atmosphere becomes
the warmer it gets. A method of modeling this system
and the results of calculations based upon it are given
in the sections below.
Equations for Modeling the Martian System
An equation for estimating the mean temperature on
the surface of Mars as a function of the CO2
atmospheric pressure and the solar constant is given
by McKay and Davis
1
as:
T
mean
= S
0.25
T
BB
+ 20(1+S)P
0.5
(1)
where Tmean is the mean planetary temperature in
kelvins,S is the solar constant where the present day
Sun=1, T
BB
, the black body temperature of Mars at
present = 213.5 K, and P is given in bar
Since the atmosphere is an effective means of heat
transport from the equator to the pole,we propose (as
an improvement over equation (1) in reference 2):
T
pole
= T
mean
- T/(1 + 5P) (2)
where T is what the temperature difference between
the mean value and the pole would be in the absence
of an atmosphere (about 75 K for S=1).
For purposes of this analysis it is further assumed
based upon a rough approximation to observed data
that :
T
max
= T
equator
= 1.1T
mean
(3)
and that the global temperature distribution is given by:
T(θ) = T
max
- (T
max
-T
pole
}sin
1.5
θ (4)
3
where θ is the latitude (north or south).
Equations (1) through (4) given the temperature on
Mars as a function of CO
2
pressure. However, as
mentioned above, the CO
2
pressure on Mars is itself a
function of the temperature. There are three reservoirs
of CO
2
on Mars, the atmosphere, the dry ice in the
polar caps, and gas adsorbed in the soil. the interaction
of the polar cap reservoirs with the atmosphere is well
understood and is given simply by the relationship
between the vapor pressure of CO
2
and the
temperature at the poles. This is given by the vapor
pressure curve for CO
2
, which is approximated by:
P = 1.23 x 10
7
{exp(-3168/T
pole
)} (5)
So long as there is CO
2
in both the atmosphere and
the cap, equation (5) gives an exact answer to what the
CO
2
atmospheric pressure will be as a function of polar
temperature. However if the polar temperature should
rise to a point where the vapor pressure is much
greater than that which can be produced by the mass
in the cap reservoir (between 50 and 150 mb) then the
cap will disappear and the atmosphere will be regulated
by the soil reservoir.
The relationship between the soil reservoir, the
atmosphere and the temperature is not known with
precision. an educated guess is given in parametric
form in reference 1 as:
P = {CM
a
exp(T/T
d
)}
1/γ
(6)
where Ma is the amount of gas adsorbed in bar,
γ =0.275, C is a normalization constant set so that with
chosen values of the other variables equation (6) will
reflect known Martian conditions, and T
d
is the
characteristic energy required for release of gas from
the soil. Equation (6) is essentially a variation on Van
Hofft's law for the change in chemical equilibrium with
temperature, and so there is fair confidence that its
general form is correct. However the value of T
d
is
unknown and probably will remain so until after human
exploration of Mars. In reference 2 McKay et al varied
parametrically T
d
from 10 to 60 K and produced curves
using equation (6) with T set equal to either T
pole
or
T
mean
. In this paper we choose T
d
=15 to 40 K (a
reasonable subset of the spectrum slightly on the
optimistic side; the lower the value of T
d
the easier
things are for prospective terraformers.) Because
equation (6) is so strongly temperature dependent,
however, we do not simply set T to the extreme values
of T
mean
or T
pole
and solve equation (6) to get a
global "soil pressure" however, as was done in
reference 2. Rather we use the global temperature
distribution given by equation (4) to integrate equation
(6) over the surface of the planet. This gives a more
accurate quasi 2-Dimensional view of the
atmosphere/regolith equilibrium problem in which most
of the adsorbed CO
2
is distributed to the planet's
colder regions. In this model, regional (in the sense of
latitude) temperature changes, especially in the near-
polar regions, can have as important a bearing on the
atmosphere/regolith interaction as changes in the
planet's mean temperature.
Results of Calculations
In figure 1 we see the results of our model when
applied to the situation at Mars' south polar cap, where
it is believed that enough CO2 may be held frozen as
dry ice to give Mars an atmosphere on the order of 50
to 100 mbar. We have plotted the polar temperature as
a function of the pressure, in accord with equations (1)
and (2), and the vapor pressure as a function of the
polar temperature, in accord with equation (5). There
are two equilibrium points, labeled A and B where the
values of P and T are mutually consistent. However A is
a stable equilibrium, while B is unstable. This can be
seen by examining the dynamics of the system
wherever the two curves do not coincide. Whenever
the temperature curve lies above the vapor pressure
curve, the system will move to the right, i.e. towards
increased temperature and pressure; this would
represent a runaway greenhouse effect. Whenever
the pressure curve lies above the temperature curve,
the system will move to the left, i.e. a temperatures and
pressure will both drop in a runaway icebox icebox
effect. Mars today is at point A, with 6 mbar of pressure
and a temperature of about 147 K at the pole.
Now consider what would happen if someone artificially
increased the temperature of the Martian pole by
several degrees K. As the temperature is increased,
points A and B would move towards each other until
they met. If the temperature increase were
4
1000100101.1
120
140
160
180
200
220
Vapor Pressure
Temp of Pole
Greenhouse Effect of Martian Polar CO2
Pressure (mb)
Temperature at Pole (K)
A
B
Fig. 1 Mars polar cap/atmosphere dynamics. current equilibrium is at point A. Raising polar temperatures by 4 K
would drive equilibria A and B together, causing runaway heating that would lead to the elimination of the cap.
1000100101.1
160
180
200
220
240
260
280
Temp of Regolith
Regolith Pressure
Temperature of Tropic Summer
Greenhouse Effect of Martian Regolith
Pressure (mb)
Temperature (K)
C
Fig. 2 Mars regolith/atmosphere dynamics under conditions of Td=20 with a volatile inventory of 500 mb of CO
2
4 K, the temperature curve would be moved upwards
on the graph sufficiently so that it would lie above the
vapor pressure curve everywhere. The result would be
a runaway greenhouse effect that would cause the
entire pole to evaporate, perhaps in less than a
decade. Once the pressure and temperature have
moved past the current location point B, Mars will be in
a runaway greenhouse condition even without artificial
heating, so if later the heating activity were
discontinued the atmosphere will remain in place.
As the polar cap evaporates, the dynamics of the
greenhouse effect caused by the reserves of CO2
held in the Martian soil come into play. These reserves
exist primarily in the high latitude regions, and by
themselves are estimated to be enough to give Mars a
400 mbar atmosphere. We can't get them all out
however, because as they are forced out of the
ground by warming, the soil becomes an increasingly
effective "dry sponge" acting to hold them back. The
dynamics of this system are shown in fig. 2, in which we
assume Td=20, current polar reserves of 100 mb, and
5
regolith reserves of 394 mb, and graph the pressure
on the planet as a function of T
reg
, where T
reg
is the
weighted average of the temperature given by
integrating the right hand side of equation (6) over the
surface of the planet using the temperature
distribution given by equation (4).
That is
T
reg
= -T
d
ln{
0
S
90
Exp(-T)/T
d
)sinθdθ} (7)
Since T
reg
is a function of the temperature distribution
and T
mean
, it is a function of P, and thus T
reg
(P) can
also be graphed. The result are a set of T(P) curves
and P(T) curves, whose crossing points reflect stable
or unstable equilibrium, just as in the case of the polar
cap analysis.
It can be seen in fig. 2 that the atmosphere soil system
under the chosen assumption of T
d
=20 K has only 1
equilibrium point, which is stable, and which will be
overrun by the pressure generated by the vaporized
polar cap. Thus, by the time the process is brought to a
halt, an atmosphere with a total pressure of about 300
mbar, or 4.4 pounds per square inch, can be brought
into being. Also shown in Fig. 2 is the day-night
average temperature that will result in Mars' tropical
regions (T
max
) during summertime. It can be seen that
the 273 K freezing point of water will be approached.
With the addition of modest ongoing artificial
greenhouse efforts, it can be exceeded.
The assumption of T
d
=20 is optimistic, however, and
the location of the equilibrium convergence point
(point C in fig. 2) is very sensitive to the value chosen
for T
d
. In fig.3 we show what happens if values of
T
d
=25 and T
d
=30 are assumed. In these cases, the
convergence point moves from 300 mb at T
d
=20 to
31 and 16 mb for T
d
=25 and T
d
=30 respectively. (The
value of the T
reg
curve in fig. 3 was calculated under
the assumption of T
d
=25; it varies from this value by a
degree or two for T
d
=20 or 30.) Such extraordinary
sensitivity of the final condition to the unknown value
of Td may appear at first glance to put the entire viability
of the terraforming concept at risk. However in fig 3 we
also show (dotted line) the situation if artificial
greenhouse methods are employed to maintain Treg
at a temperature 10 K above those produced by the
CO
2
outgassing itself. It can be seen that drastic
improvements in the final T and P values are effected
for the T
d
=25 and 30 cases, with all three cases
converging upon final states with Mars possessing
atmospheres with several hundred millibars pressure.
1000100101
160
180
200
220
240
260
P, Td=20
Treg
P, Td=25
P, Td=30
Treg + 10
Atmosphere/Regolith Equilibria for Various Td
Pressure, mb
T regolith, K
Td=30
Td=25
Td=20
Td=30'
Td=25'
Td=20'
Fig. 3 An induced 10 K rise in regolith temperature can counter effect of T
d
variations. Data shown assumes a
planetary volatile inventory of 500 mb CO
2
.
6
In figs 4,5,6, and 7 we show the convergence
condition pressure and maximum seasonal average
temperature in the Martian tropics resulting on either a
"poor" Mars, possessing a total supply of 500 mb of
CO
2
(50 mb of CO
2
in the polar cap and 444 mb in the
regolith), or a "rich" Mars possessing 1000 mb of CO
2
(100 mb in the polar cap and 894 mb in the regolith).
different curves are shown under the assumptions that
either no sustained greenhouse effort is mounted
after the initial polar cap release, or that continued
efforts are employed to maintain the planet's mean
temperature 5, 10 or 20 degrees above the value
produced by the CO
2
atmosphere alone. It can be
seen that if a sustained effort is mounted to keep an
artificial DT of 20 degrees in place, then a tangible
atmosphere and acceptable pressures can be
produced even if T
d
has a pessimistic value of 40 K.
403530252015
0
100
200
300
400
500
Pressure, DT=0
Pressure,DT=5
Pressure, DT=10
Pressure, DT=20
Equilibrium CO2 Pressure on Poor Mars
Regolith Gas Release Energy, Td
Pressure, mb
Fig .4 Equilibrium pressure reached on Mars with a planetary volatile inventory of 500 mb CO
2
after 50 mb polar
cap has been evaporated. DT is artificially imposed sustained temperature rise.
403530252015
230
240
250
260
270
280
290
Tmax, DT=0
Tmax, DT=5
Tmax, DT=10
Tmax, DT=20
Maximum Tropical Temperature on Poor Mars
Regolith Gas Release Energy, Td
Summer Tropical Temperature, K
Fig. 5 Equilibrium maximum seasonal (diurnal average) temperature reached on Mars with a planetary volatile
inventory of 500 mb CO
2
after 50 mb polar cap has been evaporated
7
403530252015
0
200
400
600
800
1000
Pressure, DT=0
Pressure,DT=5
Pressure, DT=10
Pressure, DT=20
Equilibrium CO2 Pressure for Rich Mars
Regolith Gas Release Energy, Td
Pressure, mb
Fig. 6 Equilibrium pressure reached on Mars with a planetary volatile inventory of 1000 mb CO
2
after 100 mb polar
cap has been evaporated
403530252015
230
240
250
260
270
280
290
300
Tmax, DT=0
Tmax, DT=5
Tmax, DT=10
Tmax, DT=20
Rich Mars Maximum Tropical Temperature
Regolith Gas Release Energy, Td
Maximum Tropical Temperature, K
Fig. 7 Equilibrium maximum seasonal temperature (diurnal average) reached on Mars with a planetary volatile
inventory of 1000 mb CO
2
after 100 mb polar cap has been evaporated.
The important conclusion to be drawn from this
analysis is that while the final conditions on a
terraformed Mars may be highly sensitive to the
currently unknown value of the regoliths outgassing
energy, Td, they are even more sensitive to the level
of sustained artificially induced greenhousing ,DT Put
simply, the final conditions of the atmosphere/regolith
system on a terraformed Mars are controllable.
Once significant regions of Mars rise above the
freezing point of water on at least a seasonal basis, the
large amounts of water frozen into the soil as
permafrost would begin to melt, and eventually flow
out into the dry riverbeds of Mars. Water vapor is also a
very effective greenhouse gas, and since the vapor
pressure of water on Mars would rise enormously
under such circumstances, the reappearance of liquid
water on the Martian surface would add to the
avalanche of self accelerating effects all contributing
towards the rapid warming of Mars. The seasonal
availability of liquid water is also the key factor in
8
allowing the establishment of natural ecosystems on
the surface of Mars.
The dynamics of the regolith gas-release process are
only approximately understood, and the total available
reserves of CO
2
won't be known until human explorers
journey to Mars to make a detailed assessment, so
these results are must be regarded as approximate and
uncertain. Nevertheless, it is clear that the positive
feedback generated by the Martian CO
2
greenhouse
system greatly reduces the amount of engineering
effort that would otherwise be required to transform
the Red Planet. In fact, since the amount of a
greenhouse gas needed to heat a planet is roughly
proportional to the square of the temperature change
required, driving Mars into a runaway greenhouse with
an artificial 4 K temperature rise only requires about
1/200th the engineering effort that would be needed if
the entire 55 K rise had to be engineered by brute
force. The question we shall now examine is how such
a 4 K global temperature rise could be induced.
Methods of Accomplishing Global Warming
on Mars
The three most promising options for inducing the
required temperature rise to produce a runaway
greenhouse on Mars appear to be the use of orbital
mirrors to change the heat balance of the south polar
cap (thereby causing its CO
2
reservoir to vaporize), the
importation of ammonia rich objects from the outer
solar system, and the production of artificial halocarbon
("CFC") gases on the Martian surface. We discuss each
of these in turn. It should be noted, however, that
synergistic
3
combination of several such methods may
yield better results than any one of them used alone.
Orbiting Mirrors
While the production of a space-based sunlight
reflecting device capable of warming the entire surface
of Mars to terrestrial temperatures is theoretically
possible
5
, the engineering challenges involved in
such a task place such a project well outside the
technological horizon considered in this paper. A
much more practical idea would be to construct a more
modest mirror capable of warming a limited area of Mars
by a few degrees. As shown by the data in fig. 1, a 5
degree K temperature rise imposed at the pole should
be sufficient to cause the evaporation of the CO
2
reservoir in the south polar cap. Based upon the total
amount of solar energy required to raise the black-
body temperature a given area a certain number of
degrees above the polar value of 150 K, we find that a
space-based mirror with a radius of 125 km could
reflect enough sunlight to raise the entire area south of
70 degrees south latitude by 5 K. If made of solar sail
type aluminized mylar material with a density of 4
tonnes/km
2
, such a sail would have a mass of 200,000
tonnes. This is too large to consider launching from
Earth, however if space-based manufacturing
techniques are available, its constructing in space out
of asteroidal or Martian moon material is a serious
option. The total amount of energy required to
process. the materials for such a reflector would be
about 120 MWe-years, which could be readily provided
by a set of 5 MWe nuclear reactors such as are now
being considered for use in piloted nuclear electric
spacecraft. Interestingly, if stationed near Mars, such a
device would not have to orbit the planet. Rather, solar
light pressure could be made to balance the planet's
gravity, allowing it to hover as a "statite"
6
with its power
output trained constantly at the polar region. For the
sail density assumed, the required operating altitude
would be 214,000 km. The statite reflector concept
and the required mirror size to produce a given polar
temperature rise is shown in figs 8 and 9.
Sun
Fig.8 Solar sails of 4 tonnes/km
2
density can be held stationary above Mars by light pressure at an altitude of
214,000 km. Wasting a small amount of light allows shadowing to be avoided.
9
20151050
0
200
400
600
800
1000
Mirror Radius (km)
Mirror Mass (kilotonnes)
Heating Martian Pole with Mirrors
Temperature Increase at Pole (K)
Mirror Radius , Mass
Fig.9. Solar sail mirrors with radii on the order of 100 km and masses of 200,000 tonnes can produce the 5 K
temperature rise required to vaporize the CO
2
in Mars' south polar cap. It may be possible to construct such mirrors
in space
If the value of T
d
is lower than 20 K, then the release of
the polar CO
2
reserves by themselves could be
enough to trigger the release of the regolith's reserves
in a runaway greenhouse effect. If however, as seems
probable, Td is greater than 20 K, then either the
importation or production of strong greenhouse gases
will be required to force a global temperature rise
sufficient to create a tangible atmospheric pressure on
Mars.
Moving Ammonia Asteroids
Ammonia is a powerful greenhouse gas, and it is
possible that nature has stockpiled large amounts of it
in frozen form on asteroidal sized objects orbiting in
the outer solar system. If moving material from such
objects to Mars is envisioned, then such orbits would
be quite convenient, because strange as it may seem,
it is easier to move an asteroid from the outer solar
system to Mars than it is to do so from the Main Belt or
any other inner solar system orbit. This odd result
follows from the laws of orbital mechanics, which cause
an object farther away from the Sun to orbit it slower
than one that is closer in. Because an object in the
outer solar system moves slower, it takes a smaller V
to change its orbit from a circular to an ellipse.
Furthermore, the orbit does not have to be so elliptical
that it stretches from Mars to the outer solar system; it is
sufficient to distort the objects orbit so that it intersects
the path of a major planet, after which a gravity assist
can do the rest. The results are shown in Fig. 10. It can
be seen that moving an asteroid positioned in a circular
orbit at 25 AU, by way of a Uranus gravity assist to Mars,
requires a V of only 0.3 km/s, compared to a 3.0 km/s
V to move an asteroid directly to Mars from a 2.7 AU
position in the Main Belt. the time of flight required for
such transfers is shown in Fig. 11.
Now we don't know for sure if there are numerous
asteroid size objects in the outer solar system, but
there is no reason to believe that there aren't. As of
this writing, only one is known, but that one, Chiron,
orbiting between Saturn and Uranus is rather large
(180 km diameter,), and it may be expected that a lot of
small objects can be found for every big one. In all
probability, the outer solar system contains thousands
of asteroids that we have yet to discover because they
shine so dimly compared to those in the Main Belt (The
brightness of an asteroid as seen from Earth is
inversely proportional to the fourth power of its
distance from the Sun.). Furthermore, because water,
ammonia, and other volatiles freeze so completely in
the outer solar system, it is likely that the asteroids to
be found beyond Saturn are largely composed of
frozen gases (such appears to be the case for Chiron).
This makes it possible for us to move them.
Consider an asteroid made of frozen ammonia with a
mass of 10 billion tonnes orbiting the sun at a distance
of 12 AU. Such an object, if spherical, would have a
diameter of about 2.6 km, and changing its orbit to
intersect Saturn's (where it could get a trans-Mars
10
gravity assist) would require a V of 0.3 km/s. If a
quartet of 5000 MW nuclear thermal rocket engines
powered by either fission or fusion were used to heat
some of its ammonia up to 2200 K (5000 MW fission
NTRs operating at 2500 K were tested in the 1960s),
they would produce an exhaust velocity of 4 km/s,
which would allow them to move the asteroid onto its
required course using only 8% of its material as
propellant. Ten years of steady thrusting would be
required, followed by a about a 20 year coast to impact.
When the object hit Mars, the energy released would
be about 10 TW-years, enough to melt 1 trillion tonnes
of water (a lake 140 km on a side and 50 meters deep).
In addition, the ammonia released by a single such
object would raise the planet's temperature by about 3
degrees centigrade and form a shield that would
effectively mask the planet's surface from ultraviolet
radiation. As further missions proceeded, the planet's
temperature could be increased globally in accord with
the data shown in Fig. 12. Forty such missions would
double the nitrogen content of Mars' atmosphere by
direct importation, and could produce much more if
some of the asteroids were targeted to hit beds of
nitrates, which they would volatilize into nitrogen and
oxygen upon impact. If one such mission were
launched per year, within half a century or so most of
Mars would have a temperate climate, and enough
water would have been melted to cover a quarter of the
planet with a layer of water 1 m deep.
While attractive in a number of respects, the feasibility
of the asteroidal impact concept is uncertain because
of the lack of data on outer solar system ammonia
objects. Moreover, if T
d
is greater than 20 K, a
sustained greenhousing effort will be required. as the
characteristic lifetime of an ammonia molecule on Mars
is likely to be less than a century, this means that even
after the temperature is raised, ammonia objects would
need to continue to be imported to Mars, albeit at a
reduced rate. As each object will hit Mars with an
energy yield equal to about 70,000 1 megaton
hydrogen bombs, the continuation of such a program
may be incompatible with the objective of making Mars
suitable for human settlement.
A possible improvement to the ammonia asteroidal
impact method is suggested by ideas given in
reference 4, where it is pointed out that bacteria exist
which can metabolize nitrogen and water to produce
ammonia. If an initial greenhouse condition were to be
created by ammonia object importation, it may be
possible that a bacterial ecology could be set up on the
planet's surface that would recycle the nitrogen
resulting from ammonia photolysis back into the
atmosphere as ammonia, thereby maintaining the
system without the need for further impacts. Similar
schemes might also be feasible for cycling methane,
another short-lived natural greenhouse gas which
might be imported to the planet.
50403020100
0
1
2
3
4
5
Mars
Jupiter
Saturn
Uranus
Neptune
Velocity Change Required to Transport Asteroids to Mars
Initial Distance of Asteroid from Sun (AU)
Delta-V Required (km/s)
(Gravity Assist at Intermediate Planets)
Fig.10 Using gravity assists, the V required to propel an outer solar system asteroid onto a collision course with
Mars can be less than 0.5 km/s. Such "falling" objects can release much more energy upon impact than was
required to set them in motion.
11
50403020100
0
50
100
150
200
Mars
Jupiter
Saturn
Uranus
Neptune
Asteroid Flight Time to Mars
Initial Asteroid Distance from Sun (AU)
Flight Time (years)
(Gravity Assist Maneuver at Intermediate Planets)
Fig.11 Ballistic flight times from the outer solar system to Mars are typically between 25 and 50 years.
403020100
1
10
100
1000
# NH3 Missions
NH3 Pressure
Heating Mars with Imported Ammonia
Global Temperature Increase (K)
#Missions, NH3 Pressure
(microbars)
(10 Gt each)
Fig. 12 Importing four 10 billion tonne ammonia asteroids to Mars would impose an 8 K temperature rise, which
after amplification by CO
2
feedback could create drastic changes in global conditions.
Producing Halocarbons on Mars
In Table 1 we show the amount of halocarbon gases
(CFC's) needed in Mars' atmosphere to create a given
temperature rise, and the power that would be needed
on the Martian surface to produce the required CFC'c
over a period of 20 years. If the gases have an
atmospheric lifetime of 100 years, then approximately
1/5th the power levels shown in the table will be
needed to maintain the CFC concentration after it has
been built up. For purposes of comparison, a typical
nuclear power plant used on Earth today has a power
output of about 1000 MW
e
. and provides enough
energy for a medium sized (Denver) American city. The
industrial effort associated with such a power level
would be substantial, producing about a trainload of
refined material every day and requiring the support of
a work crew of several thousand people on the Martian
surface. A total project budget of several hundred
billion dollars might well be required. Nevertheless, all
things considered, such an operation is hardly likely to
be beyond the capabilities of the mid 21st Century.
12
Table
1:
Greenhousing
Mars
with
CFCs
Induced
Heating
CFC
Pressure
CFC
Production
Power
Required
(degrees
K)
(
micro-bar)
(tonnes/hour)
(MW
e)
5 0.012 263 1315
10 0.04 878 4490
20 0.11 2414 12070
30 0.22 4829 24145
40 0.80 17569 87845
In a matter of several decades, using such an
approach Mars could be transformed from its current
dry and frozen state into a warm and slightly moist
planet capable of supporting life. Humans could not
breath the air of the thus transformed Mars, but they
would no longer require space suits and instead
could travel freely in the open wearing ordinary
clothes and a simple SCUBA type breathing gear.
However because the outside atmospheric pressure
will have been raised to human tolerable levels, it will
be possible to have large habitable areas for humans
under huge domelike inflatable tents containing
breathable air. On the other hand, simple hardy
plants could thrive in the CO
2
rich outside
environment, and spread rapidly across the planets
surface. In the course of centuries, these plants
would introduce oxygen into Mars's atmosphere in
increasingly breathable quantities, opening up the
surface to advanced plants and increasing numbers
of animal types. As this occurred, the CO
2
content of
the atmosphere would be reduced, which would
cause the planet to cool unless artificial greenhouse
gases were introduced capable of blocking off those
sections of the infrared spectrum previously
protected by CO
2
. The halocarbon gases employed
would also have to be varieties lacking in chlorine, if
an ultraviolet shielding ozone layer is to be built up.
Providing these matters are attended to, however,
the day would eventually come when the domed
tents would no longer be necessary.
Activating the Hydrosphere
The first steps required in the terraforming of Mars,
warming the planet and thickening its atmosphere,
can be accomplished with surprisingly modest
means using in-situ production of halocarbon gases.
However the oxygen and nitrogen levels in the
atmosphere would be too low for many plants, and if
left in this condition the planet would remain
relatively dry, as the warmer temperatures took
centuries to melt Mars' ice and deeply buried
permafrost. It is in this, the second phase of
terraforming Mars, during which the hydrosphere is
activated, the atmosphere made breathable for
advanced plants and primitive animals, and the
temperature increased further, that either space
based manufacturing of large solar concentrators or
human activity in the outer solar system is likely to
assume an important role.
Activating the Martian hydrosphere in a timely
fashion will require doing some violence to the
planet, and , as discussed above, one way this can
be done is with targeted asteroidal impacts. Each
such impact releases the energy equivalent of 10
TW-yrs. If Plowshare methods of shock treatment for
Mars are desired, then the use of such projectiles is
certainly to be preferred to the alternative option
3
of
detonation of hundreds of thousands of
thermonuclear explosives. After all, even if so much
explosive could be manufactured, its use would
leave the planet unacceptably radioactive.
The use of orbiting mirrors provides an alternative
method for hydrosphere activation. For example, if
the 125 km radius reflector discussed earlier for use
in vaporizing the pole were to concentrate its power
on a smaller region, 27 TW would be available to melt
lakes or volatilize nitrate beds. This is triple the power
available from the impact of 1 10 billion tonne
asteroid per year, and in all probability would be far
more controllable. A single such mirror could drive
vast amounts of water out of the permafrost and into
the nascent Martian ecosystem very quickly. Thus
while the engineering of such mirrors may be
somewhat grandiose, the benefits to terraforming of
being able to wield tens of TW of power in a
controllable way can hardly be overstated.
Oxygenating the Planet
The most technologically challenging aspect of
terraforming Mars will be the creation of sufficient
oxygen in the planet's atmosphere to support animal
life. While primitive plants can survive in an
13
atmosphere without oxygen, advanced plants
require about 1 mb and humans need 120 mb. While
Mars may have super-oxides in its soil or nitrates that
can be pyrolysed to release oxygen (and nitrogen)
gas, the problem is the amount of energy needed:
about 2200 TW-years for every mb produced. Similar
amounts of energy are required for plants to release
oxygen from CO
2
. Plants, however, offer the
advantage that once established they can propagate
themselves. The production of an oxygen
atmosphere on Mars thus breaks down into two
phases. In the first phase, brute force engineering
techniques are employed to produce sufficient
oxygen (about 1 mb) to allow advanced plants to
propagate across Mars. Assuming 3 125 km radius
space mirrors active in supporting such a program
and sufficient supplies of suitable target material on
the ground, such a goal could be achieved in about
25 years. At that point, with a temperate climate, a
thickened CO
2
atmosphere to supply pressure and
greatly reduce the space radiation dose, and a good
deal of water in circulation, plants that have been
genetically engineered to tolerate Martian soils and
to perform photosynthesis at high efficiency could
be released together with their bacterial symbiotes.
Assuming that global coverage could be achieved in
a few decades and that such plants could be
engineered to be 1% efficient (rather high, but not
unheard of among terrestrial plants) then they would
represent an equivalent oxygen producing power
source of about 200 TW. By combining the efforts of
such biological systems with perhaps 90 TW of
space based reflectors and 10 TW of installed power
on the surface (terrestrial civilization today uses
about 12 TW) the required 120 mb of oxygen
needed to support humans and other advanced
animals in the open could be produced in about 900
years. If more powerful artificial energy sources or still
more efficient plants were engineered, then this
schedule could be accelerated accordingly, a fact
which may well prove a driver in bringing such
technologies into being. It may be noted that
thermonuclear fusion power on the scale required
for the acceleration of terraforming also represents
the key technology for enabling piloted interstellar
flight. If terraforming Mars were to produce such a
spinoff, then the ultimate result of the project will be
to confer upon humanity not only one new world for
habitation, but myriads.
Conclusion
We have shown that within broad tolerances of
uncertainty of Martian conditions, that drastic
improvements in the life-sustaining characteristics of
the environment of the Red Planet may be effected
by humans using early to mid 21st century
technologies. While our immediate descendants
cannot expect to use such near-term methods to
"terraform" the planet in the full sense of the word, it
at least should be possible to rejuvenate Mars,
making it again as receptive to life as it once was.
Moreover, in the process of modifying Mars, they are
certain to learn much more about how planets really
function and evolve, enough perhaps to assure wise
management for our native planet.
Beyond such near-term milestones, the tasks
associated with full terraforming become more
daunting and the technologies required more
speculative. Yet who can doubt that if the first steps
are taken, that the developments required to
complete the job will not follow, for what is ultimately
at stake is an infinite universe of habitable worlds.
Seen in such light, the task facing our generation,
that of exploring Mars and learning enough about
the planet and the methods of utilizing its resources
to begin to transform it into a habitable planet, could
not be more urgent, or more noble.
References
1. C. McKay and W. Davis, "Duration of Liquid Water
Habitats on Early Mars," Icarus, 90:.214-221, 1991
2. C. McKay, J. Kastings and O.Toon, "Making Mars
Habitable," Nature 352:489-496, 1991.
3. M. Fogg, "A Synergistic Approach to Terraforming
Mars," Journal of the British Interplanetary Society,
August,1992
4. J. Pollack and C. Sagan, "Planetary Engineering,"
in
Near
Earth
Resources
, J. Lewis and M. Mathews,
eds, Univ. of Arizona press, Tucson, Arizona, 1993.
5. P. Birch,"Terraforming Mars Quickly" Journal of
the British Interplanetary Society, August 1992.
6. R. Forward, "The Statite: A Non-Orbiting
Spacecraft," AIAA 89-2546, AIAA/ASME 25th Joint
Propulsion Conference, Monterey, CA, July 1989.

Discussion

**Regolith** is a superficial layer of loose, heterogeneous deposits (dust, soil, broken rock, etc) covering solid rock. More generally, some possible causes for the thinning of the Martian atmosphere include: - Collision with large bodies - Atmosphere slowly blowing off into space due to Mars’ low gravity (Jeans escape) - Solar wind erosion. About 4 billion years ago Mars’ internal dynamo cooled which meant the lost of the shielding effect of the global magnetic field. This might have increased solar wind erosion of the atmosphere 1000MWe is well within the range of a nuclear power plant. Kashiwazaki-Kariwa in Tokyo is currently the world's largest nuclear power plant and has a capacity of around 8000MWe. NEP stands for **Nuclear Electric Propulsion**. NEP works by converting nuclear thermal energy into electrical energy which then powers a propulsion system. Most of these kinds of spacecraft propulsion systems work by electrically expelling propellant (e.g. positive ions). On average, the temperature on Mars is about -80°F (-60°C). In the winter, near the poles it can get down to -195°F (-125°C), and in the summer near the the equator it can get up to 70°F (20°C). **Halocarbons** are chemicals in which one or more carbon atoms are linked to one or more halogen atoms (fluorine, chlorine, bromine or iodine). Halocarbons are highly effective greenhouse gases. CFCs (chlorofluorocarbons) are an example of halocarbons. ![CFCs](https://upload.wikimedia.org/wikipedia/commons/thumb/4/40/CFCs.svg/530px-CFCs.svg.png) It would indeed be very hard to move Mars to a warmer orbit (at least much harder than all the other terraforming options). That said, the 2 obvious ways to move mars would be: 1. direct application of thrust 2. consistent close passes of a series of massive objects Direct application of thrust would likely be very disruptive. A (slightly) more plausible way of altering Mars’ orbit would be to alter the trajectory of a massive object (say a comet from the Oort Cloud) and have it swing close to Mars on the trailing side. The object would be flung out of the Solar System, and Mars would slow down a little. If you repeat this enough times (and perhaps reuse objects via a Jupiter flyby) you might be able to move Mars. **Statite** is a hypothetical type of artificial satellite that uses a solar sail to continuously adjust its orbit in order to maintain a geostationary orbit (meaning that it appears motionless, at a fixed position to ground observers) that would not be possible with gravity alone. For instance, a regular geostationary satellite of Earth must be positioned at 35,786km above Earth’s equator. With a solar sail you might be able to position the satellite in a different orbit (while maintaining a geostationary property). ![Geostationary Satellite](https://i.imgur.com/wzlpjQ0.gif) *Geostationary Satellite* More recent studies about the amount of $CO_{2}$ available in Mars (e.g. [Inventory of $CO_{2}$ available for terraforming Mars](https://www.nature.com/articles/s41550-018-0529-6.epdf) ) seem to indicate that the Martian reserves of $CO_{2}$ are smaller than the authors predicted. ## Water on Mars ![water on mars](https://upload.wikimedia.org/wikipedia/commons/thumb/9/98/AncientMars.jpg/1920px-AncientMars.jpg) *Artist’s rendition of what mars may have looked like billions of years ago* The notion of the existence of water on Mars goes back to some of the earliest telescopic observers in the 18th century. After first observing white polar caps and clouds they took that as an indication that there was a high likelihood that there could be liquid water on the red planet. It is now believed that before about 3.8 billion years ago, Mars had a denser atmosphere and higher surface temperatures. This might have allowed for vast amounts of liquid water on its surface, possibly even a large ocean (likely in the flatter northern plains). Nowadays most water exists as ice, though it also exists in small quantities as water vapor in the atmosphere and as low-volume liquid brines on the surface. ### Clouds and polar caps ![mars](https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2017/curiositycap.jpg) *Mars, as photographed with the Mars Global Surveyor* ![clouds](https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2017/1-curiositycap.jpg) *Panoramic photo showing clouds in the Martian atmosphere, taken by the Opportunity rover in 2006* ### Fluvial features on Mars ![Martian Rivers](https://www.nasa.gov/sites/default/files/styles/full_width_feature/public/images/53260main_MM_image_feature_98_jwhires.jpg) *Picture from NASA's Mars Global Surveyor that shows a delta like fan* ![Martian River](https://upload.wikimedia.org/wikipedia/commons/c/ce/Scamander_Vallis_from_Mars_Global_Surveyor.jpg) *Meander in Scamander Vallis, as seen by Mars Global Surveyor*