Origem da Vida pode ter vindo de Marte – Dr. Steven Benner

(Artigo em construcao e pesquisa)

Steven Benner

Steven Benner

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Alien Microbes in Comets? A Tempting Science


Posted by  on 11 September 2013, 4:33 pm


Earth life ‘may have come from Mars’


The first question is, did life start out as RNA? If the answer is no — and some scientists believe that to be the case — then they have to grapple with a different set of challenges to explain the origin of life.

For scientists who do accept an RNA-based origin of life, however, they need to find chemistry to produce it. Dr. Benner has one hypothesis. If scientists don’t like it, then it’s up to them to find an alternative — which other scientists are indeed doing.

And if you accept Dr. Benner’s chemistry, then you have to find a place with oxygen and dry land where it can unfold. If the early Earth doesn’t meet those standards, then we have to look elsewhere.

“That’s the logic that drives you to Mars,” said Dr. Benner.

Dr. Hazen, for one, is taking Dr. Benner up on the challenge to find evidence to test our ideas about the origin of life. He is now studying 3.8-billion-year-old rocks from Greenland, inside which are boron-laden minerals. If the boron once existed as borate in deserts — something Dr. Hazen doubts — then it may have left some clues behind in the rocks.

“I want to prove myself wrong before somebody else does,” said Dr. Hazen.

If Dr. Hazen’s research bears fruit, then Dr. Benner will happily abandon the idea that our ancestors started out on the Red Planet. “Then I have all the deserts I need. I don’t have to go to Mars,” he said.

The atmosphere of early Mars also shows signs of having contained oxygen, enabling molybdate to form. With a supply of both borate and molybdate, Mars might have been a favorable place for RNA to emerge, and for life to start. A giant impact on the Red Planet could then have kicked up microbe-laden rocks, which later fell to Earth.

Just this June, some more evidence emerged that supports this idea. Studying a meteorite from Mars, scientists at the University of Hawaii reported that it contained high levels of boron, a component of borate.




Borates are the name for a large number of boron-containing oxyanions. The term “borates” may also more loosely refer to chemical compounds which contain borate anions. Larger borates are composed of trigonal planar BO3 or tetrahedral BO4 structural units, joined together via shared oxygen atoms[1] and may be cyclic or linear in structure. Boron most often occurs in nature as borates, such as borate minerals and borosilicates.

Under acid conditions boric acid undergoes condensation reactions to form polymeric oxyanions:

Boron is a chemical element with symbol B and atomic number 5 , which is classed as a metalloid, is not found naturally on Earth. Boron is similar to carbon in its capability to form stable covalently bonded molecular networks. Because boron is produced entirely by cosmic ray spallation and not by stellar nucleosynthesis,[8] it is a low-abundance element in both the solar system and the Earth’s crust. Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals. These are mined industrially as evaporites, such as borax andkernite.

Electron shells of boron (2, 3)

An oxyanion or oxoanion is a chemical compound with the generic formula AxOyz (where A represents a chemical element and O represents an oxygen atom). Oxoanions are formed by a large majority of the chemical elements.[1] The formulae of simple oxoanions are determined by the octet rule. The structures of condensed oxoanions can be rationalized in terms of AOnpolyhedral units with sharing of corners or edges between polyhedra. The phosphate and polyphosphate esters AMPADP and ATP are important in biology.

Molybdate – Structure of molybdate

In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, although the latter are only found in the solid state. Molybdate only forms in the presence of oxygen. The atmosphere of the early Earth appears to have been nearly oxygen-free

Molybdenum is a Group 6 chemical element with the symbol Mo and atomic number 42.  Molybdenum does not occur naturally as a free metal on Earth, but rather in various oxidation states in minerals. The free element, which is a silvery metal with a gray cast, has the sixth-highest melting point of any element. Molybdenum is the 54th most abundant element in the Earth’s crust and the 25th most abundant element in the oceans, with an average of 10 parts per billion; it is the 42nd most abundant element in the Universe.

Electron shells of molybdenum (2, 8, 18, 13, 1)

Biochemistry[edit source | editbeta]

The most important role of the molybdenum in living organisms is as a metal heteroatom at the active site in certain enzymes.[54] In nitrogen fixation in certain bacteria, the nitrogenase enzyme, which is involved in the terminal step of reducing molecular nitrogen, usually contains molybdenum in the active site (though replacement of Mo with iron or vanadium is also known). The structure of the catalytic center of the enzyme is similar to that in iron-sulfur proteins: it incorporates a Fe4S3 and multiple MoFe3S3 clusters.[55]

In 2008, evidence was reported that a scarcity of molybdenum in the Earth’s early oceans was a limiting factor for nearly two billion years in the further evolution of eukaryotic life (which includes all plants and animals) as eukaryotes cannot fix nitrogen, and must therefore acquire most of their oxidized nitrogen suitable for making organic nitrogen compounds, or the organics themselves (like proteins) from prokaryotic bacteria.[56][57][58] The scarcity of molybdenum resulted from the relative lack of oxygen in the early ocean. Most molybdenum compounds have low solubility in water, but the molybdate ion MoO42− is soluble and forms when molybdenum-containing minerals are in contact with oxygen and water. Once oxygen made by early life appeared in seawater, it helped dissolve molybdenum into soluble molybdate from minerals on the sea bottom, making it available for the first time to nitrogen-fixing bacteria, and allowing them to provide more fixed usable nitrogen compounds for higher forms of life. In 2013, Steven Benner suggested it was possible that boron and molybdenum catalyzed the production of RNA on Mars with life being transported to Earth via a meteorite around 3 billion years ago.[59]

Skeletal structure of a molybdopterin with a single molybdenum atom bound to both of the thiolate groups

The molybdenum cofactor (pictured) is composed of a molybdenum-free organic complex called molybdopterin, which has bound an oxidized molybdenum atom through adjacent sulfur (or occasionally selenium) atoms.

Although oxygen once promoted nitrogen fixation via making molybdenum available in water, it also directly poisons nitrogenase enzymes. Thus, in Earth’s ancient history, after oxygen arrived in large quantities in Earth’s air and water, organisms that continued to fix nitrogen in aerobic conditions were required to isolate and protect their nitrogen-fixing enzymes in heterocysts, or similar structures protecting them from too much oxygen. This structural isolation of nitrogen fixation reactions from oxygen in aerobic organisms continues to the present.

Though molybdenum forms compounds with various organic molecules, including carbohydrates and amino acids, it is transported throughout the human body as MoO42−.[60] At least 50 molybdenum-containing enzymes were known by 2002, mostly in bacteria, and their number is increasing with every year;[61][62] those enzymes include aldehyde oxidasesulfite oxidase and xanthine oxidase.[5] In some animals, and in humans, the oxidation ofxanthine to uric acid, a process of purine catabolism, is catalyzed by xanthine oxidase, a molybdenum-containing enzyme. The activity of xanthine oxidase is directly proportional to the amount of molybdenum in the body. However, an extremely high concentration of molybdenum reverses the trend and can act as an inhibitor in both purine catabolism and other processes. Molybdenum concentrations also affect protein synthesismetabolism and growth.[60]

In animals and plants a tricyclic compound called molybdopterin (which, despite the name, contains no molybdenum) is reacted with molybdate to form a complete molybdenum-containing cofactor called molybdenum cofactor. Save for the phylogenetically-ancient molybdenum nitrogenases discussed above, which fix nitrogen in some bacteria and cyanobacteria, all molybdenum-using enzymes so far identified in nature use the molybdenum cofactor.[63] Molybdenum enzymes in plants and animals catalyze the oxidation and sometimes reduction of certain small molecules, as part of the regulation of nitrogensulfur and carbon cycles.[64]

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