The Evolving Planet
“Life is tough.”
Ken Nealson, a geobiologist and the Cecil and Ida Green Senior Fellow, at GL, studies the Earth as a means of advancing the search for life on Mars. His talk, “Energy Flow: A Guide to Life Detection,” focused on looking for life in and around rocks and understanding how energy is processed in a variety of living systems. This information can be used to develop biosignatures—chemical signals indicating the presence of life. Nealson pointed out that some life is nonconformist and shouldn’t be overlooked when looking for life elsewhere. There are microbes, for instance, that eat and breathe rocks. They consume sulfides and iron, ammonia, and manganese, and breathe metal oxides. Nealson presented an energy scale of fuels and oxidants and ranked them in terms of their relative energy potential for life forms. As an example, organic carbon, which is necessary for most life on Earth, has lots of energy. He then gave two examples of the constituents of stratified lakes and basins to show what chemicals make the best biosignatures. Although oxygen topped the list, metals such as manganese and iron oxide are also important life indicators. Nealson closed with “Ken’s Laws of Life Detection”: Know your planet, use non-Earth-centric approaches, and keep an open mind.
What does the core of the Earth have to do with life on the surface?
Erik Hauri of
DTM explained just that with his talk, “The Mantle Convection Connection.”
He began his presentation describing the formation of the Earth. As particles
from the early solar nebula accreted, the volatile elements—hydrogen,
carbon, nitrogen, and sulfur— rose to the surface and partially escaped
while the remaining elements formed the solid Earth. Heat generated by
the early dynamic environment effectively melted the early Earth, making
it an ocean of magma. After the Earth cooled and solidified, the remaining
heat continued (and continues today) to escape from the planet, resulting
in solid-state convection. Convection is a circulatory process resulting
from temperature variations and gravity that brings magma to the surface,
in the same way that steam escapes from boiling water. Volatile elements
thus escape through volcanoes, oceanic hydrothermal vents, and hotspots.
Although geochemists and geophysicists are able to determine much about
the history of Earth from isotopic analysis and 3-D imaging techniques,
Hauri said that the question of where life formed is still open. He speculated
that if the original atmosphere on Earth was made by outgassing and was
therefore tenuous, then life would probably have started in the oceans,
where the elements originating from the interior were plentiful. If, however,
the atmosphere formed from a late bombardment of comets and meteors, then
it is possible that the ingredients necessary for life would have been
available on the surface. In either case, the continual transport of carbon,
nitrogen, and sulfur (all of which may be present in the Earth’s
core) out of the deep planet through volcanoes probably played a crucial
role in allowing life to persist.
(Courtesy Erik Hauri.)
Living under pressure
James Scott, a
microbiologist at GL, talked about the extraordinary experiments he and
Anurag Sharma, a geochemist at the lab, conducted showing that everyday
bacteria can live under pressure that is 16,000 times greater than that
found at sea level—equivalent to 30 kilometers beneath the surface
of the Earth. His talk, “The Effects of Pressure on Microbial Survival
and Evolution,” began with a discussion of organisms on Earth that
are adapted to extreme environments, so-called extremophiles. He then described
the subjects of his experiment—E. coli, found in the human gut,
and Shewanella oneidensis, a metal-reducing bacterium found in lakes.
In the absence of oxygen, these creatures metabolize formate. The scientists
used the diamond-anvil cell from GL’s high-pressure team to subject
the bacteria to extreme pressures. As pressures increased, the two measured
formate oxidation to see if the organisms were alive. After decompression,
some 1% of the bacteria were still viable. The implications for the study
are several: First, even common organisms can adapt to extreme conditions;
second, there may be life in methane ice and water ice on other worlds;
and finally, life can thrive, and may have originated, below the surface
of this planet.James
Scott described experiments subjecting common bacteria to extreme pressures.
Shewanella, shown here, moved to the areas between ice crystals
during compression.
(Courtesy James Scott.)
After
compression  |
“Carbon is the poster child for global ecology.”
It is widely
known that a variety of human-induced activities release carbon into the
atmosphere. Carbon is the best-known heat-trapping greenhouse gas implicated
in the overall warming of the planet. Chris Field, the new director of
the Department of Global Ecology, gave the latest accounting of the global
carbon cycle in his talk, “The Cycling of Carbon.” He itemized
the known man-made and natural sources of carbon and estimated what they
contribute to overall emissions. He also described the natural mechanisms
that trap carbon and take it out of circulation. By current accounts using
new, bottom-up estimates (looking at the small scale and extrapolating
to the bigger picture) Field said that carbon hasn’t increased in
the atmosphere as much as our release of it. One of the challenges to
global ecology now is to figure out what happened to the rest and how
storage or “sinks” on land an in the oceans will change in the
future. To do this, global ecologists are using new techniques to better
determine what all the man-made and natural carbon sources and sinks are
and to understand the processes involved. One thing is clear, Field said.
It is unlikely that unmanaged carbon sinks will increase dramatically,
so it is even more important for people, industries, and countries to
focus on limiting carbon emissions.
(Courtesy Chris Field.)
