Saturn

DECCAN HERALD 3

Tuesday, April 25, 2017

Spectrum science

REGULATING BODY HEAT

The sympathetic nervous system, not white blood cells, is vital in the regulation of energy expenditure and thermogenesis, reveals a new report.

Mission to saturn

Discovery of virus sparks debate over tree of life

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a Missing link? Researchers have long been arguing over the origins of viruses. However, a recent discovery of a group of giant viruses may settle the debate, reports sara Reardon

planetaRy gUiDe With several up-close observations, the Cassini-Huygens has deepened our understanding of saturn. as the spacecraft’s journey comes to an end, spoorthy Raman & Rishabh shukla take a look at the intriguing insights it has offered he sci-fi blockbuster Oblivion gave us a glimpse of life in 2077 on Titan, Saturn’s largest moon, after the Earth was devastated by an extraterrestrial attack. But how far from reality is this?

Going by Cassini-Huygens’ findings, perhaps it’s not too far! Cassini-Huygens is the first unmanned spacecraft to orbit Saturn, a spectacular ringed planet in our Solar System. Launched on October 15, 1997, it set out to discover the planet and its satellites, up-close. In its 20 long years, the spacecraft has revealed many fascinating facts about Saturn, its rings and moons, including some clues about the possibility of life. However, Cassini-Huygens will soon draw a close to its eventful life in September 2017.

In the first seven years of its journey, the spacecraft hovered around Venus, flew past Mars, manoeuvred through the asteroid belt and slid past Jupiter, before finally entering Saturn’s orbit on July 1, 2004. Its last leg started on April 22, 2017.

After sailing past Titan, it will criss-cross Saturn’s equator 22 times, and finally sink into Saturn. According to National Aeronautics and Space Administration (NASA), the agency controlling the spacecraft, its thrusters have run out of fuel, forcing the inevitable death.

Scientists at NASA, the European Space Agency and the Italian Space Agency, started the groundwork on Cassini-Huygens back in 1980s. Named after two Italian astronomers, Giovanni Cassini, who studied Saturn’s rings, and Christiaan Huygens, who discovered Titan, the spacecraft has two parts — the Cassini orbiter that orbits Saturn, and the Huygens probe that landed on Titan. Since Saturn is so far away from the Sun, it is infeasible to harness solar rays as a power source. Hence, Cassini-Huygens is powered with limited-period plutonium radioisotope electric generators that generate electricity like a nuclear power plant.

While Cassini was in orbit around Saturn, it detached Huygens to descend on Titan on December 25, 2004. Huygens, after falling freely for 20 days, entered the history books as the first robotic probe to land on a satellite of another planet. It captured every detail of Titan's atmosphere and surface, and sent it to NASA’s Deep Space Network until its batteries drained up.

A closer look Saturn, with its majestic rings and enigmatic moons, is a jewel in our Solar System. It is the second largest planet after Jupiter and is a gas giant made up mostly of hydrogen and helium. Its diameter is approximately nine times that of the Earth, and it is 95 times more massive than the Earth. It is also the only planet in the Solar System that is less dense than water. Until recently, these were the only facts about Saturn that we knew. Cassini-Huygens has tremendously increased our understanding of the planet.

With numerous up-close observations, it has pointed out many earth-like features on Saturn. We now know that Saturn has numerous ‘jet streams’ that spew water vapour and heat into the atmosphere, of which the hexagonal jet stream in the north pole is one. Photographs by Cassini have revealed the magnificent symmetry of this stream and has shown this to be a long-lived phenomenon.

Cassini has captured glimpses of ‘lightnings’ and ‘hurricanes’ in the Saturnian atmosphere, and has studied the seasonal

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DETAILED IMAGING With Cassini (inset), scientists now have data about the size, temperature, composition and distribution of Saturn’s rings. PHOTO CREDIT: NASA

‘Great White Storm’— a once-in-30-years phenomenon (one Saturn year equals 30 Earth years). It has also studied Saturn’s auroras, atmospheric composition, the swelling of its outer atmosphere, and the planet’s rate of rotation (one Saturn day equals 10.5 Earth hours), even correcting previous values by six minutes. By the end of its life, Cassini would have orbited Saturn 293 times.

The system of rings around Saturn is expansive and bright, and hence, easily visible through a telescope. Named alphabetically based on when they were discovered, the rings consist of numerous icy rocks ranging from the size of a grain of salt to the size of a mountain. With Cassini, scientists now have data about the size, temperature, composition and distribution of Saturn’s rings. It has studied the interactions between rings and moons, and has recorded instances of some moons stealing particles from a ring, or throwing particles into a ring.

Saturn’s equinox, when sun rays strike the rings on the edges, provides the perfect opportunity to capture intricate details and features of the rings. Cassini has used this occasion to observe previously unknown or poorly understood phenomena like the ‘spokes’ in the rings, and finger-like radial features that rotate with the rings. Images captured during equinox also show that in places, Saturn’s rings are far less smooth than was thought.

Saturn has 62 known moons and numerous moonlets. Though early missions like Voyager and Pioneer provided a rough sketch of Saturn’s moons, it was poorly understood. Cassini has discovered seven new moons and revealed how each moon is unique and sharply different from the others. With high-resolution, close-up images of some of the moons captured during fly-bys, we now know that most of these moons have cratered surfaces, some contain atmos-

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Cassini Huygens’ Mission October 15, 1997: Launch from Cape Canaveral July 1, 2004: Saturn Orbit insertion January 14, 2005:

Huygens’ descent and successful landing onto Titan July 22, 2006: Cassini spots many lakes on Titan March 13, 2008: Cassini detects the presence of an organic brew of gases on Enceladus August 11, 2009: Saturn’s equinox December 5, 2010: Cassini observes the Great White Storm December 4, 2013: Cassini studies Saturn’s hexagon July 28, 2014: Scientists start using data from Cassini. Discover 101 geysers on Enceladus April 22, 2017: The grand finale begins September 15, 2017: The grand finale — Cassini to plunge into Saturn’s atmosphere

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pheres and large amounts of ice, hinting at the possibility of life.

The most remarkable contribution of this spacecraft is the exploration of Titan.

When Cassini flew by Titan, it saw beyond the orange haze of methane clouds, and revealed the presence of huge ‘lakes’ and ‘seas’ of liquid hydrocarbons (like methane and ethane). After Cassini landed on Titan, it revealed that Titan actually has a climate system like Earth, with a ‘water cycle’, where liquid hydrocarbons evaporate to form clouds, and later precipitate as rain. There was also evidence of an internal, liquid ocean beneath Titan’s surface, likely composed of water and ammonia.

Microbial life Cassini also helped answer the reason behind Enceladus being the brightest object in the Solar System. It showed that beneath the icy crust of Enceladus, there may be an underground ocean, from which water vapour and ice particles frequently erupted. Subsequently, it detected water, carbon dioxide and various hydrocarbons, the presence of a unique chemistry and internal heat - the right composition to support life. This excited scientists looking for life, proclaiming Enceladus to be one of the ‘most likely places in the Solar System to host alien microbial life’. This discovery also gave a peek into how our Earth might have been before life originated.

Cassini has provided us with an intimate understanding of Saturn and has opened up a treasure trove of information. It has solved several intriguing mysteries and answered long standing questions about the Saturnian system. But, can life exist on Saturn? Cassini-Huygens’ vital clues have kept scientists pondering over that, while it begins its final journey to become one among Saturn’s specks of dust.

(The authors are with Gubbi Labs, a Bengaluru-based research collective)

volutionary biologists have never known what to make of viruses, arguing over their origins for decades. But a newly discovered group of giant viruses, called Klosneuviruses, could be a ‘missing link’ that helps settle the debate — or provoke even more discord.

In 2003, researchers reported that they had found giant viruses, which they named Mimiviruses, with genes that suggested their ancestors could live outside a host cell. The discovery split researchers into two camps. One group thinks viruses started as self-sufficient organisms that became trapped inside other cells, eventually becoming parasitic and jettisoning genes they no longer needed. Another group views viruses as particles that snatched genetic material from host organisms over hundreds of millions of years.

A study published on April 6 in Science provides evidence for the latter idea, that viruses are made up of a patchwork of stolen parts. But it has already sparked controversy and is unlikely to settle the raucous debate. After the Mimivirus discovery, some researchers developed a theory that put viruses near the root of the evolutionary tree. They proposed that viruses comprised a ‘fourth domain’alongside bacteria, eukaryotes and bacteriasize organisms called archaea. Mimiviruses, which at 400 nanometres across are about half the width of an E. coli cell and can be seen under a microscope, were unique in that they contain DNA encoding of the molecules that translate RNA messages into proteins.

The team that discovered Mimiviruses thought the virus’s ability to make their own proteins suggested that these viral giants descended from an ancient free-living cell type that may no longer exist.

“They reinitiated the debate about the living nature of viruses, and of their relationship with the ‘cellular’ world,” says evolutionary biologist Jean-Michel Claverie of Aix-Marseille University in France, a coauthor of the original Mimivirus paper.

Klosneuviruses may fill this gap. Their genomes contain code for dozens of enzymesandothermolecularmachineryused in making proteins. Some of these parts have never been seen before in any virus, including Mimiviruses. “They’re kind of this missing link we haven’t had before,” says study co-author Tanja Woyke, a microbiologist at the Joint Genome Institute in California, USA. Tanja and her colleagues discovered the Klosneuviruses by accident while studying how bacteria break down sewage at a treatment plant in Austria. They sequenced the genomes in their samples to identify the organisms present, and found four genomes similar to those of Mimiviruses. Using sophisticated software to trace the evolutionary history of their mystery genomes, the researchers found that the translation genes seemed to have been picked up one by one over hundreds of millions of years.

This evidence supports the idea that viruses stole parts of their genomes, they say. It’s possible, however, that Mimiviruses and Klosneuviruses originated in different ways, making both ideas on viral origins possible, says Frederik Schulz, a bioinformatician at the Joint Genome Institute and co-author on the new study.

Debating domains It’s unclear which eukaryotic organisms donated their genes to the Klosneuvirus group. And because they haven’t identified the host, the researchers can’t grow the virus yet. The viruses do not seem to infect the same type of amoeba as Mimivirus and other known giant viruses. JeanMichel points out that the majority of the Klosneuviruses’ translation machinery does not match that of any other known organism. And he worries that the computational model used to infer the viruses’ ancestry could pick up leftover pieces of DNA in the sample, potentially contaminating the data. “I am waiting to see a real virus isolated with its host in a tube, before I would believe any of their evolutionary interpretations,” he says.

DavidMoreira,anevolutionarybiologist at the University of Paris South, doesn’t think that’s necessary. He says that plenty of evolutionary work can be done on a genome alone, and he is glad to see more paperscomingtotheconclusionthatviruses are not a fourth domain of life.

Filling in the gaps Thequestioncouldberesolvedbycomparing genome sequences from viruses with thoseoftheireukaryotichosts.Mimiviruses contain too few eukaryotic-like genes to perform a statistical analysis that could determine their evolutionary relationships.

The difficulty is compounded by the fact that viral genomes mutate very quickly. The New York Times

UNIQUE VIRUS An illustration of what a Klosneuvirus might look like.

ILLUSTRATION BY ELLA MARU STUDIO

Machine learning predicts the look of stem cells

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o two stem cells are identical, even if they are genetic clones. This stunning diversity was revealed this month in an enormous publicly available online catalogue of 3D stem cell images.

The visuals were produced using deeplearning analyses and cell lines altered with the gene-editing tool CRISPR. And soon the portal will allow researchers to predict variations in cell layouts that may foreshadow cancer and other diseases.

The Allen Cell Explorer, produced by the Allen Institute for Cell Science in Seattle, includes a growing library of more than 6,000 pictures of induced pluripotent stem, or iPS, cells — key components of which glow thanks to fluorescent markers that highlight specific genes.

TheCellExplorercomplementsongoing projects by several groups that chart the uniqueness of single cells at the level of DNA, RNA and proteins. Rick Horwitz, directoroftheAllenInstituteforCellScience, says that the institute’s images may hasten

progress in stem cell research, cancer researchanddrugdevelopment byrevealing unexpected aspects of cellular structure.

“Youcan’tpredicttheoutcomeofafootball game if you know stats on all the players but have never watched a game.”

Looking skin deep The project began about a year ago with adult skin cells that had been reprogrammed into an embryonic-like, undifferentiated state. Rick and his team then used CRISPR–Cas9 to insert tags in genes to make structures within the cells glow.

The genes included those that code for proteins that highlight actin filaments, which help cells to move and maintain their shape. It quickly became clear that the cells, which were all genetic clones from the same parent cell, varied in the placement, shape and number of their components, such as mitochondria and actin fibres.

Computer scientists analysed thou-

FLUORESCENT MARKERS A dividing human stem cell, showing the cell membrane (yellow), DNA (blue) and microtubules (red).

PHOTO CREDIT: ALLEN INSTITUTE FOR CELL SCIENCE

sands of the images using deep-learning programmes and found relationships between the locations of cellular structures.

They then used that information to predict where the structures might be when the programme was given just a couple of clues, such as the position of the nucleus.

The programme ‘learned’ by comparing its predictions to actual cells.

The deep-learning algorithms are similar to those that companies use to predict people’s preferences, Rick says. The 3D interactive tool based on this deep-learning capability should go live later this year.

At the moment, the site shows a preview of how it will work using side-by-side comparisons of predicted and actual images.

Benjamin Freedman, a cell biologist at the University of Washington, USA, looks forward to playing with the Cell Explorer’s predictive function once the Allen Institute team has taught its algorithm to recognise more iPS cells that have been changed genetically or chemically. For example,

Freedman says he could delete a gene related to kidney disease in one of the fluorescently tagged stem cells from the Allen Institute and see how the mutation affects the glowing structure. Then he could use the site’s modelling tool to determine how other cellular components might be altered. “Ultimately,” Benjamin says, “we want to understand processes at the cellular level that cause disease in the kidney as a whole.”

based charity Cancer Research UK is creating interactive virtual reality models of breast cancer cells in tumours. And an international effort called the Human Cell Atlas seeks to define all human cell types in terms of their molecular profiles, including DNA sequences, RNA transcripts and proteins.

Aviv Regev, a computational biologist at the Broad Institute in Cambridge, Massachusetts, who is working on the Human Cell Atlas, says that the Allen Cell Explorer complements her project by focusing on the look of cellular features as opposed to how genes, RNA and proteins interact within the cell. “The community is accepting that there are a lot of differences between cells that we thought were the same until recently,”she says, “so now we’re taking an unbiased approach to learn about pieces in the puzzle we didn’t know existed before.”

Molecular profiles In the coming months, Allen Institute researchers will update the site with images of stem cells at different stages of cell division, and as they transform into distinct cell types, such as heart and kidney cells.

Catching cells at different time points can be crucial to identifying fundamental processes, says Rick. The Allen Institute’s visual emphasis on stem cells dovetails with a number of efforts to catalogue other Amy Maxmen aspects of cells. For example, the London- The New York Times