ELSI Symposium IV:
Early Earth, Venus & Mars: Three Experiments in Biological Origins
Jan. 12 - Jan. 15, 2016
Summary
The symposium comprised a half-day long trio of overview talks, followed by three daysof theme-oriented talks, one theme per day. The themes were: Day 1 - The formation ofVenus, Earth and Mars; Day 2 - Planets as integrated systems; Day 3 - Starting conditions
for prebiotic chemistry. Each day contained a half- to three-quarter-hour discussion ofeach group of talks, and a final two hour long discussion to summarize major points.
The introductory talks enlightened us generally in each thematic area. Our knowledge ofplanetary accretion is changing rapidly due to observational and computational advances.
In particular the roles of orbital migration, pebble accretion, and volatile distribution in
protoplanetary disks are being reexamined. We also learned that small rocky planets up
to five Earth masses are relatively common in exoplanet surveys. The focus on defining awater existence zone might involve processes other than simple insolation, such as
ingassing or burial of water in a planetary interior. Thus systematics based on the distance from the central star may be of questionable value. Finally, we heard that what life
needs is simply a source of enthalpy to scavenge that is in excess of thermal fluctuations.
It could be based on a variety of chemistries, not just redox reactions involving oxygen.
The talks under the theme of the Formation of Venus, Earth and Mars began by informing
us of the limitations to the observations of protoplanetary disks. Spatial resolution islacking to tell the difference between a Venus, Earth or Mars region in an accretion disk.
Pebble accretion allows planets to grow unbelievably fast - to Jupiter mass in a few Keplerian orbital times, solving one problem of classical accretion models. Unfortunately,
the model does not lead to any particular compositional zonation in disks and does not
solve the volatile zonation problem with present knowledge of its character. The Earthappears to have been born dry, with water added at a later stage -- perhaps during the lateveneer or with cometary volatile input.
The Earth’s volatiles contain a component that was inherited early in its accretion history,no more than 100 Ma after the solar system formed. It also started accreting from rather
dry materials, with volatiles added later. Venus superficially appears dry of water, but it
may be buried in the planet’s interior. A magma ocean and an early water-rich atmosphere may have been stripped of hydrogen by greater insolation due to its proximity tothe Sun and the resulting oxygen buried in its magma ocean, whose lifetime is extended
by the hot, opaque atmosphere. Alternatively, the left-over oxygen could have beenburied by impacts that rusted ejecta and buried it in the early-formed lithosphere.
In the theme of Planets as Integrated Systems, exoplanet search criteria for water-bearing
conditions was a key element of discussion. Remote study of exoplanet atmospheres is1

not possible, but study of the atmospheres of the solar system’s planets is. Even as close
as they are, the whole system’s interconnected chemical interactions must be modeled fora successful prediction of observed properties, not simply a few reactions. The composition of exoplanets can perhaps be systematized by thermochemical modelling based on afew key parameters provided by the central star’s composition: Fe/Si and Mg/Si. Outcomes are imperfect, but can be improved if more prior information is available, in particular, the star’s age.
Returning to our own solar system, Venus may have had water in its atmosphere early inits development. Its D/H ration is 150 times the Earth’s suggesting the loss of a few to afew hundreds of meters of planet-covering water depth. A speculation is that perhaps life
originated there and seeded the Earth. Planets can be treated as integrated thermodynamic engines, characterized by their dissipation. Speculatively, life allows a planet toevolve to higher levels of dissipation for a given energy input (chiefly insolation). A way
to characterize planetary evolution is afforded by the study of nitrogen isotopes. OnEarth, they flag the influence of biotic processes at ¾3.2 Ga.
The theme of Starting Conditions and Requirements for Prebiotic Chemistry opened withdiscussion of various forms of chemical disequilibrium. All one needs is a source forchemotrophs, such as a black smoker analogue of a mid-ocean ridge vent system operating on a rocky planet covered with an ice-mantled ocean. On Earth, the early atmosphere
was apparently reducing, which facilitates the production of the amino acids relevant toprebiotic chemistry. Another ingredient required for prebiotic chemistry is tectonic activity. They create the environments in which kinetic barriers to reactions delay the attainment of equilibrium. Life has to operate faster to take advantage of these energy sources;
without kinetic hinderance, life is impossible. One can view life as complexity emerging
as a consequence of dissipative structures, with the required ingredients of redox bifurcations, catalytic metal availability, and pH-driven disequilibria.
Finally, we heard, speculatively, that Mars might have had the right conditions for the origin of life before the Earth, and transport of Martian rocks to the Earth could have seeded
it. The preservation of older rocks on Mars makes it, in a sense, a better laboratory forthe study of early conditions than the Earth is.
Take-home messages from this particular gathering:
There is tremendous energy in the exoplanets community, enabled by the rapid growth ofimaging data and likely to become even stronger in the coming decades. At least one
major emphasis in that community at this time is on diversity and surprise. Our effort tocompose a narrative about chance and necessity in the fates of the inner solar systemplanets seems to have met a response that no well-formed narrative can be posed before
we have come to grips with the necessary and contingent factors that shaped the solar
system as a whole, among the possibilities for planetary systems.

2

A second useful observation, which recommends care going forward, is that part of theenthusiasm for exoplanet studies is as a means to the end of finding extraterrestrial life.
At the same time as this motivation is strong and important with respect to public support(and the interest of many researchers), the connections between what we can realistically
hope to learn about exoplanets for several decades, and the conditions required for life,
are still thin. We should try to avoid sacrificing sound research on exoplanets as objectsof interest in their own right by casting too narrow a field of inquiry. We must also, however, merge the understanding of structures and histories that already exists separately inplanetary studies and in life-science studies, to frame questions of life’s origin as usefully
as possible.
George Helffrich, Eric Smith3