Prof. Claudius Gros |
On earth, life has come a long way. Eons ago it emerged
from its cradle under a then young sun. Will it find its
destiny on the same planet, scorched to death by a
hugely expanded old sun? Maybe not, if we succeed to
offer life alternative pathways on far away worlds,
which is the central idea of the Genesis project.
Cells are not cells, the difference between
life's two main types, prokaryotes and eukaryotes,
could not be more pronounced. Prokaryotes like
bacteria have no nucleus, no internal organelles and
do not reproduce sexually or coalesce into differentiated
multicellular organisms. All in contrast to eukaryotes,
which are in addition much bigger, typically ten
thousand times in volume.
With eukaryotic cells being so large, it is likely
that the presence of oxygen in the atmosphere
was a precondition for their emergence. Overall, earth
took more than 1.5 billion years to develop eukaryotic
cells, which indicates that the development of organisms
of this level of complexity constitutes a major evolutionary
bottleneck. Regarding the other habitable planets of our
galaxy, it is hence often assumed that exobiological life,
if present, would be mostly in evolutionary stages comparable
to earth's prokaryotic phase and devoid of lifeforms comparable
to terrestrial eukaryotes.
In order to discern life with the naked eye, it has
to be a multicellular organism, like a plant or an animal,
or a macroscopic colony akin to microbial mats. For most
of its history, three billion years and more, little
could have been seen of the life flourishing on our
home planet. This changed however with the advent of the
Cambrian explosion, an intriguing period of only a few
dozen of million years that reshaped earth's face
once and forever.
Animals with legs and eyes would lift their heads out
of churning oceans and conquer barren continents together
with the newly evolved plants. Afterwards, evolution did
equally not stop to surprise. Dinosaurs, birds and mammals
emerged jointly with trees, flowers and grasses. Eukaryotic
cells celebrate a festival of complexity wherever one turns
the head on today's earth.
Maybe we always suspected it, but by now humanity knows
exoplanets and alien solar systems come in sheer endless
varieties. With regard to the host star mass, the radius,
the mineral composition, the water content, the atmosphere
of the planet and the distance to the host star. There is
no one-size-fits-all planetary system blueprint. The
conditions for life to prosper on potentially habitable
exoplanets vary likewise.
Some planets may stay temperate and tectonically active
for several billion years, like earth, others for only a
substantially reduced period. Planets with clement
conditions could also remain utterly lifeless, f.i. when
the primordial atmosphere is oxygen-rich, as for M-dwarf
planets.This condition is likely to prevent life to emerge
in first place, which implies conversely that complex life
would have a chance to evolve on hitherto barren exoplanets if
brought there by a Genesis craft. A mix of unicellular
pro- and eukaryotes synthesized by an on-board gen laboratory
would be enough to lay the foundations for
a pre-cambrian biosphere.
Interstellar payload delivery
Interstellar trips come with several preconditions.
First of all one needs to be modest and settle for a
spacecraft that weighs between a few tons and a few grams,
if aiming for cruising speeds that range from 1000km/s,
which corresponds to c/300, to close to the speed of light.
Next a powerful laser, say of about 100GW, needs to be
directed onto the perfectly reflecting shield of the
spaceprobe. The reflected photons will then transmit
enough momentum to propel the craft.
Difficult as it is to accelerate an interstellar craft,
it is even more demanding to decelerate on arrival. It is
however possible to transfer the momentum of the craft to
the tenuous interstellar plasma using a magnetic sail.
To do the job, install a superconducting loop with a radius
of about 50km, inject a current of the order of hundred
thousand Ampere and wait a few thousand years. Given enough
time, interstellar spacecrafts are capable to deliver payloads
to a distant world.
A payload of eukaryotes could fast forward the evolution of
a target exoplanet by several billion years. The cells could
be synthesized either freshly on arrival or brought from earth
as hibernating germs. Natural selection will take over once
the initial biosphere is in place. Importantly, Genesis is
about enabling life, not about destroying. The craft would
consequently abort whenever complex life is detected from orbit.
Planetary seeding ethics
Regardless whether you are vegetarian, vegan or not,
nearly no one gets moral headaches when eradicating a few
million bacteria while brushing teeth. Our mouth is not
considered a protected habitat. But what about prokaryotic
life on an exoplanet? There is no way to decide whether
the prospect of complex life is worth more than an extant
bacterial film. This is not a setting human ethics has
been developed for.
The possibility that most of the billions of potentially
habitable planets harbor only primitive life, that is
unicellular organisms well below the complexity level
of earth's eukaryotes, may also be factored in. In this
situation, the overall balance will not be altered if
an extant bacterial-level biosphere is superseded by
a Genesis craft with highly developed terrestrial eukaryotes.
Ethical issues become irrelevant altogether for the case of
habitable but sterile exoplanets, the setting that may be
realized for M-dwarf planets.
Not for human benefit
The time scales involved imply that the Genesis project
cannot be justified by utility considerations. It may take
ten thousand years for the craft to arrive and a few
hundred million years for complex multicellular organisms
to emerge as the result of natural selection. No individual and no
organization has the lifespan to wait that long. This
is a problem when sticking to the letter of human ethics
protocols, which state that ethically acceptable is
only what is beneficial for humans. Transcending the
utilitarian perspective one may think of alternative
motivations to carry out the Genesis project, such
as aesthetics. Isn't life as such something to admire?
Selected Media Resonance
Why planetary and exoplanetary protection differ:
The case of long duration Genesis missions to habitable but sterile M-dwarf oxygen planets
Authors: C. Gros
Journal-ref.: Acta Astronautica 157 263 (2019).
Time is arguably the key limiting factor for interstellar exploration. At high
speeds, flyby missions to nearby stars by laser propelled wafersats taking
50-100 years would be feasible. Directed energy launch systems could accelerate
on the other side also crafts weighing several tons to cruising speeds of the
order of 1000\,km/s (c/300). At these speeds, superconducting magnetic sails
would be able to decelerate the craft by transferring kinetic energy to the
protons of the interstellar medium. A tantalizing perspective, which would
allow interstellar probes to stop whenever time is not a limiting factor. Prime
candidates are in this respect Genesis probes, that is missions aiming to offer
terrestrial life new evolutionary pathways on potentially habitable but
hitherto barren exoplanets.
Genesis missions raise important ethical issues, in particular with regard to
planetary protection. Here we argue that exoplanetary and planetary protection
differ qualitatively as a result of the vastly different cruising times for
payload delivering probes, which are of the order of millennia for interstellar
probes, but only of years for solar system bodies. Furthermore we point out
that our galaxy may harbor a large number of habitable exoplanets, M-dwarf
planets, which could be sterile due to the presence of massive primordial
oxygen atmospheres. We believe that the prospect terrestrial life has in our
galaxy would shift on a fundamental level in case that the existence of this
type of habitable but sterile oxygen planets will be corroborated by future
research. It may also explain why our sun is not a M dwarf, the most common
star type, but a medium-sized G-class star.
Universal scaling relation for magnetic sails: momentum braking in the limit of dilute interstellar media
Authors: C. Gros
Journal-ref.: Journal of Physics Communications 1, 045007 (2017).
The recent progress in laser propulsion research has advanced substantially the
prospects to realize interstellar spaceflight within a few decades. Here we
examine passive deceleration via momentum braking from ionized interstellar
media. The very large area to mass relations needed as a consequence of the low
interstellar densities, of the order of 0.1 particles per cm3, or lower, are
potentially realizable with magnetic sails generated by superconducting coils.
Integrating the equations of motion for interstellar protons hitting a Biot
Savart loop we evaluate the effective reflection area A(v) in terms of the
velocity v of the craft. We find that the numerical data is fitted over two
orders of magnitude by the scaling relation
A(v) = 0.081ARlog3(I/(βIc)), where
AR=πR2 is the bare sail area,
I the current and β=v/c. The critical current Ic
is 1.55 106 Ampere. The resulting universal deceleration profile can be
evaluated analytically and mission parameters optimized for a minimal craft
For the case of a sample high speed transit to Proxima Centauri we find
that magnetic momentum braking would involve daunting mass requirements of the
order of 103 tons. A low speed mission to the Trappist-1 system could be
realized on the other side already with a 1.5 ton spacecraft, which would be
furthermore compatible with the specifications of currently envisioned directed
energy launch systems. The extended cruising times of the order of 104 years
imply however that a mission to the Trappist-1 system would be viable only for
mission concepts for which time constrains are not relevant.
Developing Ecospheres on Transiently Habitable Planets: The Genesis Project
Authors: C. Gros
Journal-ref.: Astrophysics and Space Science 361, 324 (2016).
It is often presumed, that life evolves relatively fast on planets with clement
conditions, at least in its basic forms, and that extended periods of
habitability are subsequently needed for the evolution of higher life forms.
Many planets are however expected to be only transiently habitable. On a large
set of otherwise suitable planets life will therefore just not have the time to
develop on its own to a complexity level as it did arise on earth with the
cambrian explosion. The equivalent of a cambrian explosion may however have the
chance to unfold on transiently habitable planets if it would be possible to
fast forward evolution by 3-4 billion years (with respect to terrestrial
timescales). We argue here, that this is indeed possible when seeding the
candidate planet with the microbial lifeforms, bacteria and unicellular
eukaryotes alike, characterizing earth before the cambrian explosion. An
interstellar mission of this kind, denoted the `Genesis project', could be
carried out by a relatively low-cost robotic microcraft equipped with a
on-board gene laboratory for the in situ synthesis of the microbes.
here our current understanding of the processes determining the timescales
shaping the geo-evolution of an earth-like planet, the prospect of finding
Genesis candidate planets and selected issues regarding the mission layout.
Discussing the ethical aspects connected with a Genesis mission, which would be
expressively not for human benefit, we will also touch the risk that a
biosphere incompatibility may arise in the wake of an eventual manned
exploration of a second earth.