“SNOWBALL EARTH”: IMPLICATIONS FOR THE EVOLUTION OF COMPLEX LIFE AND EXOPLANET HABITABILITY

Jonathan Chin1, Sadchla Mathieu2, Jayanthi Ramaswamy3, and Linda Sohl4

1Brooklyn Tech High School, 2Medgar Evers College-CUNY, 3Palisades Park Jr/Sr High School, 4NASA/GISS at Columbia University

Sponsors:National Aeronautics and Space Administration (NASA)NASA Goddard Institute for Space Studies (GISS)NASA New York City Research Initiative (NYCRI)NASA Curriculum Improvement Partnership Award for the Integration of Research into the Undergraduate Curriculum (CIPAIR)

ABSTRACT The Neoproterozoic “Snowball Earth” (750-635 Ma, Ma = million years ago) is an extreme glaciation, or ice age, of interest to both paleobiologists (scientists who study the origin and evolution of life) and astrobiologists (scientists who are interested in the potential for life on exoplanets). The “hard snowball” hypothesis (Hoffman and Schrag, 2001) suggests that the ocean was almost completely frozen over during this time. Here we test the plausability of the hard snowball hypothesis through global climate model (GCM) simulations, and compare the model results to habitat requirements of the complex organisms thought to have evolved shortly before or during the glaciation. Our GCM results do not support a hard snowball scenario, but rather a “soft snowball” or “slushball” scenario that is more in line with habitat requirements of complex life. We also find extensive areas of liquid water despite the fact that the global average surface air temperature is well below freezing (0˚C), so astrobiologists looking for potentially habitable exoplanets need to consider expanding their guidelines for finding worlds with at least some liquid surface water that would permit the existence of life.

I. What is Snowball Earth?

II. Snowball Earth and the Evolution of Complex Life

III. Climate Modeling Tests of the Snowball Earth Hypothesis

IV. Implications of Snowball Earth for Life on Other Worlds

V. Conclusions

References

The Neoproterozoic “Snowball Earth” glaciations (highlighted by the red box above) took place during a time when Earth was very different from today:•The continents were clustered around the equator•The Sun was about 6% less bright than it is now•The only known complex life existed in the oceans, not on land

The fossil record shows that some time during the Snowball Earth interval, life evolved from simpler microscopic organisms (below left) to more complex multicellular life (below right).

We ran a total of 9 global climate model (GCM) simulations, using EdGCM, to see if the model could reproduce “hard snowball” conditions. These runs included the following:

Astrobiologists often use simple climate models to find planets that have liquid surface water, needed for life as we know it. Our snowball Earth runs show that more complex models can expand our view of the range of such planets.

• Based on our climate simulations, we found that it is impossible for the Earth to have been completely frozen over (a “hard snowball”), as all of our runs result in areas of open ocean with sea surface temperatures above freezing.

• The basic habitat requirements of the organisms that lived though the Neoproterozoic Era also refute the “hard snowball Earth” hypothesis.

• Potentially habitable exoplanets may have average surface air temperatures below freezing, thus extending the outer edge of a star’s habitable zone.

• Graham LE, LW Wilcox, ME Cook, PG Gensel 2004 Resistant tissues of marchantioid liverworts resemble enigmatic Early Paleozoic microfossils. Proc Nat Acad Sci USA 101:11025–11029.

• Hoffman PF, and D Schrag 2000 Snowball Earth. Scientific American Jan 2000: 68-75.• Knoll AH, EJ Javaux, P Cohen 2006 Eukaryotic organisms in Proterozoic oceans. Philos Trans R Soc B 361:1023–

1038. • Knoll AH 1994 The early evolution of eukaryotes: a geological perspective. Science 256:622–627.• Peterson et al 2008: The Ediacaran emergence of bilaterians: congruence between the genetic and the geological

fossil records. doi: 10.1098/rstb.2007.2233.

There has been a debate about whether the glaciations resulted in the Earth essentially freezing over (the “hard snowball”) or not (the “soft snowball”, or “slushball”). We wanted to see which scenario was more compatible with life.

This?

Or this?

Open marine water (requires above-freezing temperatures) Access to soft sediment substrate on continental shelves

(requires ice-free continental margins) Photic zone location (i.e., access to sunlight)

Using modern descendants of two animal phyla (Porifera, or sponges, and Cnidaria, e.g. jellyfish), we identify certain habitat conditions we believe necessary for the existence/evolution of complex life during the glaciations:

Even with the extreme forcings listed for Run 4 above, we found that our model simulations could not reproduce a hard snowball scenario.

As we can see from the maps above, these model results are compatible with the minimum habitat requirements we need for our complex organisms belonging to the phyla Porifera and Cnidaria.

Annual avg surface air temperature: -14˚C

Annual avg sea surface temperature: +1.81˚C

Annual avg snow and ice cover: 54%

Energy balance calculation of temperature: 252K, or -21˚C

GCM calculation of temperature range: 237 to

293K, or -36 to +20˚C

Vs.

For solar luminosity of 1304.6 W/m2:

From microscopic organisms

like Acritarchs…

… to megascopic organisms

like Dickinsonia