Hebrew U team uncovers origin of the universe’s heavy elements

All the plutonium used on Earth is artificially produced in nuclear reactors. Still, it turns out that it is also produced in nature.

February 21, 2016 01:13
4 minute read.

A spiral galaxy known as NGC 1433 is seen in an undated image captured by the NASA/ESA Hubble Space Telescope. (photo credit: REUTERS)

A solution to the galactic radioactive plutonium puzzle has been offered by a team of scientists from the Hebrew University’s Racah Institute of Physics that discovered rare mergers of binary neutron stars that are a source of radioactive plutonium-244.

In a letter published in the prestigious journal Nature Physics, Dr. Kenta Hotokezaka, Prof. Tsvi Piran and Prof. Michael Paul said: “The origin of heavy elements produced in nature through rapid neutron capture (‘r-process’) by seed nuclei is one of the current nucleosynthesis mysteries.”

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All the plutonium used on Earth is artificially produced in nuclear reactors. Still, it turns out that it is also produced in nature.

Plutonium is a radioactive element, and its longest-lived isotope is plutonium-244, with a lifetime of 120 million years. Detection of plutonium-244 in nature would imply that the element was synthesized in astrophysical phenomena not so long ago (in galactic time scales) and thus, its origin cannot be too far from us.

Several years ago, the physicists said, it was discovered that the early solar system contained a significant amount of plutonium- 244. Considering its short-lived cycle, plutonium- 244 that existed more than four billion years ago when Earth formed has long since decayed, but its daughter elements have been detected.

Recent measurements of the deposition of plutonium-244, including analysis of galactic debris that fell to Earth and settled deep in the sea, suggest that only a very small amount of plutonium has reached Earth from outer space over the past 100 million years. This is in striking contradiction to its presence at the time when the solar system was formed, and that is why the galactic radioactive plutonium remained a puzzle.

The HU team has shown that these contradicting observations can be reconciled if the source of radioactive plutonium (as well as other rare elements, such as gold and uranium) is in mergers of binary neutron stars. These mergers are extremely rare events, but are thought to produce large amounts of heavy elements.

The model implies that such a merger took place accidentally in the vicinity of our solar system less than a hundred million years before it was born. This has led to the relatively large amount of plutonium- 244 observed in the early solar system.

On the other hand, they wrote, the relatively small amount of plutonium-244 reaching Earth from interstellar space today is accounted for by the rarity of these events. Such an event hasn’t occurred in the last 100 million years in the vicinity of our solar system.

ARMS RACE AMONG VENOMOUS ANIMALS? In a new study published in the journal PLOS Genetics, HU scientists have revealed discoveries about how animal venom evolves. Their findings point to a “two-speed” evolution of animal venom, showing for the first time the significant roles played by different forces of natural selection.

Venom is a complex mixture of proteins and other toxic chemicals produced by animals such as snakes and spiders, either to incapacitate their prey or defend against predators.

The influence of positive selection (the process by which a protein changes rapidly over evolutionary time scales) in expanding and diversifying animal venoms is widely recognized.

This process was hypothesized to result from an evolutionary chemical arms race, in which the invention of potent venom in the predatory animals and the evolution of venom resistance in their prey animals, exert reciprocal selection pressures.

In contrast to positive selection, the role of purifying selection (also known as negative selection, which is the selective removal of deleterious genetic changes from a population) has rarely been considered in venom evolution.

Venom research has mostly neglected ancient animal groups in favor of focusing on venomous snakes and cone snails, which are both “young” animal groups that originated only recently in evolutionary timescales, approximately 50 million years ago. It was thus concluded that venom evolution is mostly driven by positive selection.

In the study, Dr. Yehu Moran at HU’s department of ecology, evolution and behavior, and guest scientist Dr. Kartik Sunagar, examined numerous venom genes in different animals in order to unravel the unique evolutionary strategies of toxin gene families. They analyzed and compared the evolutionary patterns of more than 3,500 toxin sequences from 85 gene families that spanned the breadth of the animal kingdom, including ancient venomous groups such as centipedes, scorpions, spiders, coleoids (octopus, cuttlefish and squids) and cnidarians (jellyfish, sea anemones and hydras).

Unexpectedly, despite their long evolutionary histories, ancient animal groups were found to have only accumulated low variation in their toxins.

The analysis also revealed a striking contrast between the evolution of venom in ancient animal groups as compared to evolutionarily “young” animals and highlighted the significant role played by purifying selection in shaping the composition of venoms.

The research showed that while the venoms of ancient lineages evolve more slowly through purifying selection, the venoms in more recent lineages diversify rapidly under the influence of positive selection.”

The proposed “two-speed” mode of venom evolution highlights the fascinating evolutionary dynamics of this complex biochemical cocktail by showing for the first time the significant roles played by different forces of natural selection in shaping animal venoms.

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