When searching for life, scientists first look for an elemental key to sustaining it: fresh water.
Although today’s Martian surface is barren, frozen, and uninhabitable, a trail of evidence points to a once warmer, wetter planet, where water flowed freely. The conundrum of what happened to this water is long standing and unsolved. However, new research published in Nature suggests that this water is now locked in the Martian rocks.
Scientists at Oxford’s Department of Earth Sciences propose that the Martian surface reacted with the water and then absorbed it, increasing the rocks oxidation in the process, making the planet uninhabitable.
Previous research has suggested that the majority of the water was lost to space as a result of the collapse of the planet’s magnetic field, when it was either swept away by high intensity solar winds or locked up as subsurface ice. However, these theories do not explain where all of the water has gone.
Convinced that the planet’s mineralogy held the answer to this puzzling question, a team led by Dr. Jon Wade, NERC Research Fellow in Oxford’s Department of Earth Sciences, applied modeling methods used to understand the composition of Earth rocks to calculate how much water could be removed from the Martian surface through reactions with rock. The team assessed the role that rock temperature, subsurface pressure and general Martian make-up have on the planetary surfaces.
The results revealed that the basalt rocks on Mars can hold approximately 25 percent more water than those on Earth and, as a result, drew the water from the Martian surface into its interior.
Dr. Wade said: ‘people have thought about this question for a long time but never tested the theory of the water’s being absorbed as a result of simple rock reactions. There are pockets of evidence that, together, leads us to believe that a different reaction is needed to oxidize the Martian mantle. For instance, Martian meteorites are chemically reduced compared to the surface rocks and compositionally look very different. One reason for this, and why Mars lost all of its water, could be in its mineralogy.
“The Earth’s current system of plate tectonics prevents drastic changes in surface water levels, with wet rocks efficiently dehydrating before they enter the Earth’s relatively dry mantle. But neither early Earth nor Mars had this system of recycling water. On Mars, water reacting with the freshly erupted lavas that form its basaltic crust, resulted in a sponge-like effect. The planet’s water then reacted with the rocks to form a variety of water-bearing minerals. This water-rock reaction changed the rock mineralogy and caused the planetary surface to dry and become inhospitable to life.”
As to the question of why Earth has never experienced these changes, he said: “Mars is much smaller than Earth, with a different temperature profile and higher iron content of its silicate mantle. These are only subtle distinctions, but they cause significant effects that, over time, add up. They made the surface of Mars more prone to reaction with surface water and able to form minerals that contain water. Because of these factors, the planet’s geological chemistry naturally drags water down into the mantle, whereas on early Earth hydrated rocks tended to float until they dehydrate.”
The overarching message of Dr. Wade’s paper, that planetary composition sets the tone for future habitability, is echoed in new research also published in Nature, examining Earth’s salt levels. Co-written by Professor Chris Ballentine of Oxford’s Department of Earth Sciences, the research reveals that for life to form and be sustainable, the Earth’s halogen levels (Chlorine, Bromine and Iodine) have to be just right. Too much or too little could cause sterilization. Previous studies have suggested that halogen level estimates in meteorites were too high. Compared to samples of the meteorites that formed Earth, the ratio of salt to Earth is just too high.
Many theories have been put forward to explain the mystery; however, the two studies combined elevate the evidence and support a case for further investigation. Dr. Wade said “Broadly speaking the inner planets in the solar system have similar composition, but subtle differences can cause dramatic differences—for example, rock chemistry—the biggest difference being, that Mars has more iron in its mantle rocks, as the planet formed under marginally more oxidizing conditions.”
We know that Mars once had water and the potential to sustain life, but by comparison little is known about the other planets, and the team is keen to change that.
Life in Universe Is Common
A new analysis of the oldest known fossil microorganisms provides strong evidence to support an increasingly widespread understanding that life in the universe is common.
The microorganisms, from Western Australia, are 3.465 billion years old. Scientists from UCLA and the University of Wisconsin–Madison report in the journal Proceedings of the National Academy of Sciences that two of the species they studied appear to have performed a primitive form of photosynthesis; another apparently produced methane gas; and two others appear to have consumed methane and used it to build their cell walls.
The evidence that a diverse group of organisms had already evolved extremely early in Earth’s history—combined with scientists’ knowledge of the vast number of stars in the universe and the growing understanding that planets orbit so many of them—strengthens the case for life existing elsewhere in the universe, because it would be extremely unlikely that life formed quickly on Earth but did not arise anywhere else.
“By 3.465 billion years ago, life was already diverse on Earth; that’s clear— primitive photosynthesizers, methane producers, methane users,” said J. William Schopf, a professor of paleobiology in the UCLA College and the study’s lead author. “These are the first data that show the very diverse organisms at that time in Earth’s history, and our previous research has shown that there were sulfur users 3.4 billion years ago as well.
“This tells us life had to have begun substantially earlier, and it confirms that it was not difficult for primitive life to form and to evolve into more advanced microorganisms.” Schopf said scientists still do not know how much earlier life might have begun. “But, if the conditions are right, it looks like life in the universe should be widespread,” he said.
The study is the most detailed ever conducted on microorganisms preserved in such ancient fossils. Researchers led by Schopf first described the fossils in the journal Science in 1993, and then they substantiated their biological origin in the journal Nature in 2002. But the new study is the first to establish what kind of biological microbial organisms they are and how advanced or primitive they are.
For the new research, Schopf and his colleagues analyzed the microorganisms with cutting-edge technology called ‘secondary ion mass spectroscopy’, or SIMS, which reveals the ratio of carbon-12 to carbon-13 isotopes—information scientists can use to determine how the microorganisms lived. (Photosynthetic bacteria have different carbon signatures from methane producers and consumers, for example.) In 2000, Schopf became the first scientist to use SIMS to analyze microscopic fossils preserved in rocks; he said the technology will likely be used to study samples brought back from Mars for signs of life.
The Wisconsin researchers, led by geoscience professor John Valley, used a secondary ion mass spectrometer—one of just a few in the world—to separate the carbon from each fossil into its constituent isotopes and determine their ratios.
“The differences in carbon isotope ratios correlate with their shapes,” Valley said. “Their C-13-to-C-12 ratios are characteristic of biology and metabolic function.”
“The fossils were formed at a time when there was very little oxygen in the atmosphere.” Schopf said. He thinks that advanced photosynthesis had not yet evolved and that oxygen first appeared on Earth approximately half a billion years later before its concentration in our atmosphere increased rapidly starting about twobillion years ago.
“Oxygen would have been poisonous to these microorganisms, and would have killed them, he said.
While the study strongly suggests the presence of primitive life forms throughout the universe, Schopf said the presence of more advanced life is very possible but less certain.
Plants Reveal Decision-Making Abilities Under Competition
Biologists from the University of Tübingen in Germany have demonstrated that plants can choose between alternative competitive responses according to the stature and densities of their opponents. A new study by researchers from the Institute of Evolution and Ecology reveals that plants can evaluate the competitive ability of their neighbors and optimally match their responses to them. The results were published in Nature Communications.
Animals facing competition have been shown to optimally choose between different behaviors, including confrontation, avoidance, and tolerance, depending on the competitive ability of their opponents relative to their own. For example, if their competitors are bigger or stronger, animals are expected to “give up the fight” and choose avoidance or tolerance over confrontation.
Plants can detect the presence of other competing plants through various cues, such as the reduction in light quantity or in the ratio of red to far-red wavelengths (R:FR), which occurs when light is filtered through leaves. Such competition cues are known to induce two types of responses: confrontational vertical elongation, by which plants try to outgrow and shade their neighbors, and shade tolerance, which promotes performance under limited light conditions. Some plants, such as clonal plants, can exhibit avoidance behavior as a third response type: they grow away from their neighbors. “These three alternative responses of plants to light competition have been well documented in the literature,” says Michal Gruntman, lead author of the paper. “In our study, we wanted to learn if plants can choose between these responses and match them to the relative size and density of their opponents.”
To answer this question, the researchers used the clonal plant Potentilla reptans in an experimental setup that simulated different light-competition settings. They used vertical stripes of transparent green filters that reduce both light quantity and R:FR and could therefore provide a realistic simulation of light competition. By changing both the height and density of this simulated vegetation, the researchers could present different light-competition scenarios to the plants.
The results demonstrated that Potentilla reptans can indeed choose its response to competition in an optimal way. When the plants were under treatments simulating short-dense neighbors that presented competitors that where too dense to avoid laterally but could be outgrown vertically, Potentilla reptans showed the highest confrontational vertical growth.
The findings of this study reveal that plants can evaluate the density and competitive ability of their neighbors and tailor their responses accordingly.
CAPTION: Twin Peaks photographed by the Mars Pathfinder Mission (NASA/PPL)