Category: Earth

Artist’s conception shows Romulus and Remus orbiting the asteroid 87 Sylvia. Image credit: ESO Click to enlarge
Iron meteorites are probably the surviving fragments of the long-lost asteroid-like bodies that formed the Earth and other nearby rocky planets, according to researchers from Southwest Research Institute (SwRI) and Observatoire de la Cote d’Azur in Nice, France. Their findings are described in the Feb.16 issue of Nature.

Iron meteorites, which are composed of iron and nickel alloys, represent some of the earliest material formed in the solar system, with most coming from the cores of small asteroids. According to Dr. William Bottke, an SwRI research scientist and leader of the joint U.S.-French team, iron-meteorite parent bodies probably emerged from the same disk of planetary debris that produced the Earth and other inner solar system planets.

“Small bodies that form quickly in the inner solar system end up melting and differentiating from the decay of short-lived radioactive elements,” explains Bottke. “Iron meteorites came from the molten material that sinks to the center of these objects, cools and solidifies.”

For these meteorites to arrive on Earth, they must have been extracted from their parent bodies and kept around for billions of years. The team’s computer simulations found that any asteroids managing to avoid being gobbled up by the planets were quickly demolished by impacts. Each breakup, however, produces millions of fragments, many in the form of iron meteorites. These leftovers were scattered across the solar system by gravitational interactions with protoplanetary bodies, with some reaching the relative safety of the asteroid belt. Over billions of years, a few of the survivors escaped their captivity in the asteroid belt and were delivered to Earth.

“This means that certain iron meteorites may tell us what the precursor material for the primordial Earth was like, while also helping us unlock several fundamental questions about the Earth’s origins,” says Bottke. “There’s also the possibility that larger versions of this material may still be hiding among the asteroids. The hunt for them is on.”

A new way to look at iron meteorites

A potential problem in using meteorites to understand the formation of Earth and other terrestrial planets Mercury, Venus and Mars is that most come from the distant asteroid belt. This population of interplanetary bodies, ranging from tiny pebbles to Texas-sized objects, is located between the orbits of Mars and Jupiter about 140 million miles from Earth.

Most members of the asteroid belt are assumed to have formed there, so the vast majority of meteorite samples tell us about formation events in that region, not those that took place near Earth. Meteorite compositions are so diverse, however, that it is difficult to reconcile that all came from this one, fairly narrow region of space.

“While tens of thousands of stony meteorites have been collected, most can be traced back to perhaps a few tens of parent asteroids,” says Dr. Alessandro Morbidelli of the Observatorie de la Cote d’Azur. “What is strange is that the iron meteorites, despite their smaller numbers, represent almost two-thirds of all of the unique parent asteroids sampled to date.”

To explain this discrepancy, the team tracked the origin and evolution of iron-meteorite parent bodies using several computer models. They found that while many iron meteorites are likely residing in the asteroid belt today, their precursors probably did not form there. Instead, the simulations indicate that the precursors of most iron meteorites formed in the terrestrial planet region.

To investigate this hypothesis, the researchers first examined the constraints provided by the meteorites themselves. Iron meteorites are unusual in that most come from the disrupted cores of small melted (differentiated) asteroids that formed very early in solar system history. These are precisely the kinds of bodies that computer models predict should have formed near Earth.

“It is hard to produce small differentiated bodies in the asteroid belt without also melting lots of large asteroids,” explains Dr. Robert Grimm, assistant director of the SwRI Space Studies Department. “These events would produce a number of telltale signs that would be easily detected by observers.”

Using computer simulations, the team then tracked how a disk of asteroid-like bodies interacting with a host of protoplanetary objects in the terrestrial planet region might evolve. Simulations showed that some of these asteroid-like bodies could have scattered far enough to take up residence in the asteroid belt.

“While the amount of material reaching the asteroid belt was limited, much of it was placed in regions likely to produce meteorites,” says SwRI Research Scientist Dr. David Nesvorn??bf?. En route to the asteroid belt, the parent bodies of the iron meteorites were repeatedly bashed by other bodies, allowing core fragments from numerous bodies to escape.

“This could explain the many differences seen among iron meteorites,” says Dr. David O’Brien of the Observatoire de la Cote d’Azur.

Original Source: NASA Astrobiology

Antarctic ice sheets. Image credit: NASA Click to enlarge
Pamela Conrad, an astrobiologist with NASA’s Jet Propulsion Laboratory, has traveled to the ends of the Earth to study life. Conrad recently appeared in James Cameron’s 3-D documentary “Aliens of the Deep,” where she and several other scientists investigated strange creatures that inhabit the ocean floor.

On June 16, 2005, Conrad gave a lecture entitled, “A Bipolar Year: What We Can Learn About Looking for Life on Other Planets by Working in Cold Deserts.”

In part 1 of this edited transcript, Conrad describes what sort of signs we could look for to see if there is life in an alien environment.

“In the past three years, I’ve been engaged in a project with several of my colleagues that takes us to hot and cold deserts. We want to observe the signatures of life, and see if we can tell the difference between places where life is and where life isn’t. The reason we go to deserts is to cut down on the number of confounding variables that are introduced by all kinds of life. Basically, we don’t want to be scraping away the dog poop to find the bacteria in the dirt.

This past year we were privileged to go to both the Arctic and the Antarctic. So this is my bipolar year, and what we were doing there is relevant to space exploration because, like a desert, the conditions on the surface of other planets are very harsh.

We look at rocks because, if life had been and is already gone – in other words, it’s dead, or it’s so dead it’s been fossilized and altered – you can find that in the rock record.

To detect life anywhere, you need to be able to investigate the environment and find measurable clues. If it’s not something you can define in measurable terms, it’s not science. So by definition, we’re kind of out in the cold, so to speak.

One of the challenges is coming up with measurable terms by which you could define life. The terms have to be universal enough to not miss life on another planet, if it was unlike the life we have here. We have a sample set of one: the biosphere on the Earth. We try to use the knowledge we have about life here to come up with those terms, and so we try to think about life in the most general descriptive terms we can.

We look for life in places that are habitable; places that are capable of supporting life. But habitability is difficult to define, because we only have a vague notion of what makes an environment habitable. At NASA, we’re very big on looking for water as one of the facets of habitability.

Water is as important to life in the desert as it is to us. After a fresh snowfall, when rocks get heated up and melt the ice, you see a bloom of cyanobacteria on the surface of the rock. Yet they are able to maintain a minimal existence when there’s not much precipitation.

One reason metabolism has to slow down in the Antarctic winter is because the water is in a solid phase and it’s not accessible. Living things can only use ice when it melts and becomes a good solvent. Using ice is like using a mineral in the crystal phase — when it’s in the solid form, you’ve got to use some energy to bust up those bonds to do something with it. There are organisms in Antarctica that have antifreeze types of molecules in them, fish that possess molecules called glycoproteins. When an ice crystal forms in the fish, the molecule grabs hold of the ice crystal as it starts to grow, and doesn’t let it grow in the direction that its energetically most easily grown. Because it can’t grow, the ice crystal gives up the ghost and turns back into water.

Besides water, we think that certain kinds of chemical elements are important for life elsewhere. Life on Earth is made of carbon and hydrogen and phosphorus and a few other important things, and we need the oxygen in the air. But there are microbes on Earth that breathe metal, and they don’t care about oxygen.

So habitability is really habitable in the eyes of the beholder. When you’re defining it, you’ve got to think about the broadest set of terms you can in order to encompass any kind of life you might be able to imagine. The ultimate assessment of whether a place is habitable is, of course, to see if it is inhabited.

You ask one set of questions if you want to know, “Can I set up housekeeping here?” You might ask another set of questions if you want to know, “Is anybody home?” But at the heart of it all, whether or want to live there or just see if anyone’s home, you have to know something about the neighborhood. You’ve still got to do all the experiments that tell you about the geophysical, mineralogical, and atmospheric properties of the planet. If you’re looking for life, you’ve got to have some notion about what sort of thing you’re trying to support with that environment.

Erupting about 5 million years ago, from a series of fractures known as the Cerberus Fossae, the water flowed down in a catastrophic flood, collecting in an area 800 x 900 km and was initially an average of 45 meters deep. Click image for larger view.Credit: ESA/Mars Express

So what would constitute proof? If you want to say that something has been proven, you have to achieve a certain level of consensus in the scientific community, otherwise your peers will tear you into little bits and pieces in the literature. Of course, there is never a complete consensus: that’s why we nasty scientists fight with each other endlessly. But we have to at least come up with terms. We can agree or disagree with each other’s theories, but we have to agree on the terms and the measurements.

So what kind of measurements could we make if we were looking for life? Does a planet look different if life has been there? For example, if you go into my kitchen after I’ve eaten, you might see a plate or a crumb. That’s a clue that I was there. There are clues at the planetary level too. A biomarker – a clue that says life was there – can be anything that was produced by life. The clue can be chemical, because chemicals comprise everything. I am a sack of chemicals, just like this podium is a sack of chemicals. Just what chemicals there are, and in what proportion to each other, and how they’re arranged in 3-D, is what distinguishes me from this. It’s a simple way of distinguishing categories of things.

Chirality is a biomarker as well. What chirality means is that some molecules are mirror images of each other, and the living molecules tend to be a certain handedness. When it comes to amino acids, which are the constituents of the proteins that make up life, living things like to use the left-handed form. And when it comes to the sugars, living things like to use the right-handed form. There are exceptions to these, but that’s a general case.

Isotopes also can be a biomarker. Some molecules come in different isotopic flavors, where some are slightly heavier than others. Living things like the lighter variety, probably because it’s energetically less expensive to process.

Complex polymers also could be biomarkers. Of course, plastic is a complex polymer. The again, we made the plastic. So this whole distinction between natural and unnatural – if humans made it, it’s still biogenic. So think about that. My car is a biosignature. What kind, I’m not sure.

If you’re going to define life in measurable terms, I’d like to keep it really simple. You could define life by what it’s made of, or you could define life by what it does. I like to define life by what it’s made of, because as soon as you say the “does” word, you’re talking about a process. A process is something that happens through time. Then you’ve got to figure out what the sampling rate should be. How often should you look, and how long should the whole experiment take? A process is a little more problematic because it takes time, and you may be wrong about how often to look, or how long you should look for.

Processes – making stuff, reproducing, or evolving – can take place over different time scales. So if you’re only looking at processes, and you have two that are vastly different in their time scales, you won’t be able to do the same experiment to look at them both. So I like to look at life in terms of what it is. Not to say we couldn’t add in a little bit of process-based stuff, but when you look at what life is, it gets simple really fast. It’s unique chemistry, some kind of proportionate chemicals, arranged in some way, and the “arranged in some way” is what I call structure.

If I were looking for life on another planet or a moon, I would look for places where interesting chemistry could happen, so that the ultimate evolution of that chemistry could create a living system. I would think about places like Europa, which has an ocean beneath ice. I would think about other places where ice exists, like comets. I would think about Titan, Saturn’s moon. I would think about all those places where interesting chemistry occurs, because chemistry is clever. You can get all kinds of interesting molecules.

Original Source: NASA Astrobiology

The Earth and its atmosphere today. Image credit: NASA. Click to enlarge.
Washington, D.C. ?CSI-like? techniques, used on minerals, are revealing the steps that led to evolution of the atmosphere on Earth. President of the Mineralogical Society of America, Douglas Rumble, III, of the Carnegie Institution?s Geophysical Laboratory, describes the suite of techniques and studies over the last five years that have led to a growing consensus by the scientific community of what happened to produce the protective ozone layer and atmosphere on our planet. His landmark paper on the subject appears in the May/June American Mineralogist.

?Rocks, fossils, and other natural relics hold clues to ancient environments in the form of different ratios of isotopes?atomic variants of elements with the same number of protons but different numbers of neutrons,? explained Rumble. ?Seawater, rain water, oxygen, and ozone, for instance, all have different ratios, or fingerprints, of the oxygen isotopes 16O, 17O, and 18O. Weathering, ground water, and direct deposition of atmospheric aerosols change the ratios of the isotopes in a rock revealing a lot about the past climate.? Rumble?s paper describes how geochemists, mineralogists, and petrologists are studying anomalies of isotopes of oxygen and sulfur to piece together what happened to our atmosphere from about 3.9 billion years ago, when the crust of our planet was just forming and there was no oxygen in the atmosphere, to a primitive oxygenated world 2.3 billion years ago, and then to the present.

The detective work involves a pantheon of scientists who have analyzed surface minerals from all over the globe, used rockets and balloons to sample the stratosphere, collected and studied ice cores from Antarctica, conducted lab experiments, and run mathematical models. The synthesis from the different fields and techniques points to ultraviolet (UV) light from the Sun as an important driving force in atmospheric evolution. Solar UV photons drive the production of ozone in the atmosphere and yield ozone that is enriched in 17O and 18O, thereby leaving a tell-tale isotopic signature. The ozone layer began to form as the atmosphere gained oxygen, and has since shielded our planet from harmful solar rays and made life possible on Earth?s surface.

The discovery of isotope anomalies, where none were previously suspected, adds a new tool to research on the relationships between shifts in atmospheric chemistry and climate change. Detailed studies of polar-ice cores and exposed deposits in Antarctic dry valleys may improve our understanding of the history of the ozone hole.

Original Source: Carnegie Institution News Release

The biggest mass extinction in Earth history some 251 million years ago was preceded by elevated extinction rates before the main event and was followed by a delayed recovery that lasted for millions of years. New research by two University of Washington scientists suggests that a sharp decline in atmospheric oxygen levels was likely a major reason for both the elevated extinction rates and the very slow recovery.

Earth’s land at the time was still massed in a supercontinent called Pangea, and most of the land above sea level became uninhabitable because low oxygen made breathing too difficult for most organisms to survive, said Raymond Huey, a UW biology professor.

What’s more, in many cases nearby populations of the same species were cut off from each other because even low-altitude passes had insufficient oxygen to allow animals to cross from one valley to the next. That population fragmentation likely increased the extinction rate and slowed recovery following the mass extinction, Huey said.

“Biologists have previously thought about the physiological consequences of low oxygen levels during the late Permian period, but not about these biogeographical ones,” he said.

Atmospheric oxygen content, about 21 percent today, was a very rich 30 percent in the early Permian period. However, previous carbon-cycle modeling by Robert Berner at Yale University has calculated that atmospheric oxygen began plummeting soon after, reaching about 16 percent at the end of the Permian and bottoming out at less than 12 percent about 10 million years into the Triassic period.

“Oxygen dropped from its highest level to its lowest level ever in only 20 million years, which is quite rapid, and animals that once were able to cross mountain passes quite easily suddenly had their movements severely restricted,” Huey said.

He calculated that when the oxygen level hit 16 percent, breathing at sea level would have been like trying to breathe at the summit of a 9,200-foot mountain today. By the early Triassic period, sea-level oxygen content of less than 12 percent would have been the same as it is today in the thin air at 17,400 feet, higher than any permanent human habitation. That means even animals at sea level would have been oxygen challenged.

Huey and UW paleontologist Peter Ward are authors of a paper detailing the work, published in the April 15 edition of the journal Science. The work was supported by grants from the National Science Foundation and the National Aeronautics and Space Administration’s Astrobiology Institute.

Not only was atmospheric oxygen content dropping at the end of the Permian, the scientists said, but carbon dioxide levels were rising, leading to global climate warming.

“Declining oxygen and warming temperatures would have been doubly stressful for late Permian animals,” Huey said. “As the climate warms, body temperatures and metabolic rates go up. That means oxygen demand is going up, so animals would face an increased oxygen demand and a reduced supply. It would be like forcing athletes to exercise more but giving them less food. They’d be in trouble.”

Ward was lead author of a paper published in Science earlier this year presenting evidence that extinction rates of land vertebrates were elevated throughout the late Permian, likely because of climate change, and culminated in a mass extinction at the end of the Permian. The event, often called “the Great Dying,” was the greatest mass extinction in Earth’s history, killing 90 percent of all marine life and nearly three-quarters of land plants and animals.

Ward said paleontologists had previously assumed that Pangea was not just a supercontinent but also a “superhighway” on which species would have encountered few roadblocks while moving from one place to another.

However, it appears the greatly reduced oxygen actually created impassable barriers that affected the ability of animals to move and survive, he said.

“If this is true, then I think we have to go back and look at oxygen and its role in evolution and how different species developed,” Ward said. “You can go without food for a couple of weeks. You can go without water for a few days. How long can you go without oxygen, a couple of minutes? There’s nothing with a greater evolutionary effect than oxygen.”

Original Source: UW News Release