Saturn’s Moon Titan Sports Earth-Like Features

Using the now-complete Cassini data set, Cornell astronomers have created a new global topographic map of Saturn’s moon Titan that has opened new windows into understanding its liquid flows and terrain. Two new papers, published Dec. 2 in Geophysical Review Letters, describe the map and discoveries arising from it.

Creating the map took about a year, according to doctoral student Paul Corlies, first author on “Titan’s Topography and Shape at the End of the Cassini Mission.” The map combines all of the Titan topography data from multiple sources. Since only about 9 percent of Titan has been observed in relatively high-resolution topography, with 25-30 percent of the topography imaged in lower resolution, the remainder of the moon was mapped using an interpolation algorithm and a global minimization process, which reduced errors such as those arising from spacecraft location.

The map revealed several new features on Titan, including new mountains, none higher than 700 meters. The map also provides a global view of the highs and lows of Titan’s topography, which enabled the scientists to confirm that two locations in the equatorial region of Titan are in fact depressions that could be either ancient, dried seas or cryovolcanic flows.

The map also revealed that Titan is a little bit flatter — more oblate — than was previously known, which suggests there is more variability in the thickness of Titan’s crust than previously thought.

“The main point of the work was to create a map for use by the scientific community,” said Corlies; within 30 minutes of the data set being available online, he began to receive inquiries on how to use it. The data set is downloadable in the form of the data that was observed, as well as that data plus interpolated data that was not observed. The map will be important for those modeling Titan’s climate, studying Titan’s shape and gravity, and testing interior models, as well as for those seeking to understand morphologic land forms on Titan.

Other Cornell authors on the paper are senior author Alex Hayes, assistant professor of astronomy, doctoral candidate Samuel Birch and research associate Valerio Poggiali.

The second paper, “Topographic Constraints on the Evolution and Connectivity of Titan’s Lacustrine Basins,” finds three important results using the new map’s topographical data. The team included Hayes, Corlies, Birch, Poggiali, research associate Marco Mastrogiuseppe and Roger Michaelides ’15.

The first result is that Titan’s three seas share a common equipotential surface, meaning they form a sea level, just as Earth’s oceans do. Either because there’s flow through the subsurface between the seas or because the channels between them allow enough liquid to pass through, the oceans on Titan are all at the same elevation.

“We’re measuring the elevation of a liquid surface on another body 10 astronomical units away from the sun to an accuracy of roughly 40 centimeters. Because we have such amazing accuracy we were able to see that between these two seas the elevation varied smoothly about 11 meters, relative to the center of mass of Titan, consistent with the expected change in the gravitational potential. We are measuring Titan’s geoid. This is the shape that the surface would take under the influence of gravity and rotation alone, which is the same shape that dominates Earth’s oceans,” said Hayes.

The paper’s second result proves a hypothesis that Hayes advanced in his first paper, in graduate school: that Titan’s lakes communicate with each other through the subsurface. Hayes and his team measured the elevation of lakes filled with liquid as well as those that are now dry, and found that lakes exist hundreds of meters above sea level, and that within a watershed, the floors of the empty lakes are all at higher elevations than the filled lakes in their vicinity.

“We don’t see any empty lakes that are below the local filled lakes because, if they did go below that level, they would be filled themselves. This suggests that there’s flow in the subsurface and that they are communicating with each other,” said Hayes. “It’s also telling us that there is liquid hydrocarbon stored on the subsurface of Titan.”

The paper’s final result raises a new mystery for Titan. Researchers found that the vast majority of Titan’s lakes sit in sharp-edged depressions that “literally look like you took a cookie cutter and cut out holes in Titan’s surface,” Hayes said. The lakes are surrounded by high ridges, hundreds of meters high in some places.

The lakes seem to be formed the way karst is on Earth, in places like the Florida Everglades, where underlying material dissolves and the surface collapses, forming holes in the ground. The lakes on Titan, like Earth’s karst, are topographically closed, with no inflow or outflow channels. But Earth karst does not have sharp, raised rims.

The shape of the lakes indicates a process called uniform scarp retreat, where the borders of the lakes are expanding by a constant amount each time. The largest lake in the south, for example, looks like a series of smaller empty lakes that have coalesced or conglomerated into one big feature.

“But if these things do grow outward, does that mean you’re destroying and recreating the rims all the time and that the rims are moving outward with it? Understanding these things is in my opinion the lynchpin to understanding the evolution of the polar basins on Titan,” said Hayes.

The research was supported by grants from NASA and the Italian Space Agency.

Planets Around Other Stars Are Like Peas In A Pod

An international research team led by Université de Montréal astrophysicist Lauren Weiss has discovered that exoplanets orbiting the same star tend to have similar sizes and a regular orbital spacing. This pattern, revealed by new W. M. Keck Observatory observations of planetary systems discovered by the Kepler Telescope, could suggest that most planetary systems have a different formation history than the solar system.

Thanks in large part to the NASA Kepler Telescope, launched in 2009, many thousands of exoplanets are now known. This large sample allows researchers to not only study individual systems, but also to draw conclusions on planetary systems in general. Dr. Weiss is part of the California Kepler Survey team, which used the W. M. Keck Observatory on Maunakea in Hawaii, to obtain high-resolution spectra of 1305 stars hosting 2025 transiting planets originally discovered by Kepler. From these spectra, they measured precise sizes of the stars and their planets.

In this new analysis led by Weiss and published in The Astronomical Journal, the team focused on 909 planets belonging to 355 multi-planet systems. These planets are mostly located between 1,000 and 4,000 light-years away from Earth. Using a statistical analysis, the team found two surprising patterns. They found that exoplanets tend to be the same sizes as their neighbors. If one planet is small, the next planet around that same star is very likely to be small as well, and if one planet is big, the next is likely to be big. They also found that planets orbiting the same star tend to have a regular orbital spacing.

“The planets in a system tend to be the same size and regularly spaced, like peas in a pod. These patterns would not occur if the planet sizes or spacings were drawn at random.” explains Weiss.

The similar sizes and orbital spacing of planets have implications for how most planetary systems form. In classic planet formation theory, planets form in the protoplanetary disk that surrounds a newly formed star. The planets might form in compact configurations with similar sizes and a regular orbital spacing, in a manner similar to the newly observed pattern in exoplanetary systems. However, in our solar system, the inner planets have surprisingly large spacing and diverse sizes. Abundant evidence in the solar system suggests that Jupiter and Saturn disrupted our system’s early structure, resulting in the four widely-spaced terrestrial planets we have today. That planets in most systems are still similarly sized and regularly spaced suggests that perhaps they have been mostly undisturbed since their formation.

To test that hypothesis, Weiss is conducting a new study at the Keck Observatory to search for Jupiter analogs around Kepler’s multi-planet systems. The planetary systems studied by Weiss and her team have multiple planets quite close to their star. Because of the limited duration of the Kepler Mission, little is known about what kind of planets, if any, exist at larger orbital distances around these systems. They hope to test how the presence or absence of Jupiter-like planets at large orbital distances relate to patterns in the inner planetary systems.

Regardless of their outer populations, the similarity of planets in the inner regions of extrasolar systems requires an explanation. If the deciding factor for planet sizes can be identified, it might help determine which stars are likely to have terrestrial planets that are suitable for life.

New Study Reveals Strong El Nino Events Cause Large Changes In Antarctic Ice Shelves

A new study published Jan. 8 in the journal Nature Geoscience reveals that strong El Nino events can cause significant ice loss in some Antarctic ice shelves while the opposite may occur during strong La Nina events.

El Niño and La Niña are two distinct phases of the El Niño/Southern Oscillation (ENSO), a naturally occurring phenomenon characterized by how water temperatures in the tropical Pacific periodically oscillate between warmer than average during El Niños and cooler during La Niñas.

The research, funded by NASA and the NASA Earth and Space Science Fellowship, provides new insights into how Antarctic ice shelves respond to variability in global ocean and atmospheric conditions.

The study was led by Fernando Paolo while a PhD graduate student and postdoc at Scripps Institution of Oceanography at the University of California San Diego. Paolo is now a postdoctoral scholar at NASA’s Jet Propulsion Laboratory. Paolo and his colleagues, including Scripps glaciologist Helen Fricker, discovered that a strong El Niño event causes ice shelves in the Amundsen Sea sector of West Antarctica to gain mass at the surface and melt from below at the same time, losing up to five times more ice from basal melting than they gain from increased snowfall. The study used satellite observations of the height of the ice shelves from 1994 to 2017.

“We’ve described for the first time the effect of El Niño/Southern Oscillation on the West Antarctic ice shelves,” Paolo said. “There have been some idealized studies using models, and even some indirect observations off the ice shelves, suggesting that El Niño might significantly affect some of these shelves, but we had no actual ice-shelf observations. Now we have presented a record of 23 years of satellite data on the West Antarctic ice shelves, confirming not only that ENSO affects them at a yearly basis, but also showing how.”

The opposing effects of El Niño on ice shelves – adding mass from snowfall but taking it away through basal melt – were at first difficult to untangle from the satellite data. “The satellites measure the height of the ice shelves, not the mass, and what we saw at first is that during strong El Niños the height of the ice shelves actually increased,” Paolo said. “I was expecting to see an overall reduction in height as a consequence of mass loss, but it turns out that height increases.”

After further analysis of the data, the scientists found that although a strong El Niño changes wind patterns in West Antarctica in a way that promotes flow of warm ocean waters towards the ice shelves to increase melting from below, it also increases snowfall particularly along the Amundsen Sea sector. The team then needed to determine the contribution of the two effects. Is the atmosphere adding more mass than the ocean is taking away or is it the other way around?

“We found out that the ocean ends up winning in terms of mass. Changes in mass, rather than height, control how the ice shelves and associated glaciers flow into the ocean,” Paolo said. While mass loss by basal melting exceeds mass gain from snowfall during strong El Niño events, the opposite appears to be true during La Niña events.

Over the entire 23-year observation period, the ice shelves in the Amundsen Sea sector of Antarctica had their height reduced by 20 centimeters (8 inches) a year, for a total of 5 meters (16 feet), mostly due to ocean melting. The intense 1997-98 El Nino increased the height of these ice shelves by more than 25 centimeters (10 inches). However, the much lighter snow contains far less water than solid ice does. When the researchers took density of snow into account, they found that ice shelves lost about five times more ice by submarine melting than they gained from new surface snowpack.

“Many people look at this ice-shelf data and will fit a straight line to the data, but we’re looking at all the wiggles that go into that linear fit, and trying to understand the processes causing them,” said Fricker, who was Paolo’s PhD adviser at the time the study was conceived. “These longer satellite records are allowing us to study processes that are driving changes in the ice shelves, improving our understanding on how the grounded ice will change,” Fricker said.

“The ice shelf response to ENSO climate variability can be used as a guide to how longer-term changes in global climate might affect ice shelves around Antarctica,” said co-author Laurie Padman, an oceanographer with Earth & Space Research, a nonprofit research company based in Seattle. “The new data set will allow us to check if our ocean models can correctly represent changes in the flow of warm water under ice shelves,” he added.

Melting of the ice shelves doesn’t directly affect sea level rise, because they’re already floating. What matters for sea-level rise is the addition of ice from land into the ocean, however it’s the ice shelves that hold off the flow of grounded ice toward the ocean.

Understanding what’s causing the changes in the ice shelves “puts us a little bit closer to knowing what’s going to happen to the grounded ice, which is what will ultimately affect sea-level rise,” Fricker said. “The holy grail of all of this work is improving sea-level rise projections,” she added.

Gravitational Waves Measure The Universe

The direct detection of gravitational waves from at least five sources during the past two years offers spectacular confirmation of Einstein’s model of gravity and space-time. Modeling of these events has also provided information on massive star formation, gamma-ray bursts, neutron star characteristics, and (for the first time) verification of theoretical ideas about how the very heavy elements, like gold, are produced.

Astronomers have now used a single gravitational wave event (GW170817) to measure the age of the universe. CfA astronomers Peter Blanchard, Tarreneh Eftekhari, Victoria Villar, and Peter Williams were members of a team of 1314 scientists from around the world who contributed to the detection of gravitational waves from a merging pair of binary neutron stars, followed by the detection of gamma-rays, and then the identification of the origin of the cataclysm in a source in the galaxy NGC4993 spotted in images taken with various time delays at wavelengths from the X-ray to the radio.

An analysis of the gravitational waves from this event infers their intrinsic strength. The observed strength is less, implying (because the strength diminishes with distance from the source) that the source is about 140 million light-years away. NGC4993, its host galaxy, has an outward velocity due to the expansion of the universe that can be measured from its spectral lines. Knowing how far away it is and how fast the galaxy is moving from us allows scientists to calculate the time since the expansion began – the age of the universe: between about 11.9 and 15.7 billion years given the experimental uncertainties.

The age derived from this single event is consistent with estimates from decades of observations relying on statistical methods using two other sources: the cosmic microwave background radiation (CMBR) and the motions of galaxies. The former relies on mapping the very faint distribution of light dating from a time about four hundred thousand years after the big bang; the latter involves a statistical analysis of the distances and motions of tens of thousands of galaxies in relatively recent times. The fact that this one single gravitational-wave event was able to determine an age for the universe is remarkable, and not possible with every gravity wave detection. In this case there was an optical identification of the source (so that a velocity could be measured) and the source was neither too distant or too faint. With a large statistical sample of gravitational wave events of all types, the current range of values for the age will narrow.

The new result is intriguing for another reason. Although both the CMBR and the galaxy measurements are each quite precise, they seem to disagree with each other at roughly the ten percent level. This disagreement could just be observational error, but some astronomers suspect it might be a real difference reflecting something currently missing from our picture of the cosmic expansion process, perhaps connected with the fact that the CMBR arises from a vastly different epoch of cosmic time than does the galaxy data. This third method, gravitational wave events, may help solve the puzzle.

Chemists Discover Plausible Recipe For Early Life On Earth

Chemists at The Scripps Research Institute (TSRI) have developed a fascinating new theory for how life on Earth may have begun.

Their experiments, described today in the journal Nature Communications, demonstrate that key chemical reactions that support life today could have been carried out with ingredients likely present on the planet four billion years ago.

“This was a black box for us,” said Ramanarayanan Krishnamurthy, PhD, associate professor of chemistry at TSRI and senior author of the new study. “But if you focus on the chemistry, the questions of origins of life become less daunting.”

For the new study, Krishnamurthy and his coauthors, who are all members of the National Science Foundation/National Aeronautics and Space Administration Center for Chemical Evolution, focused on a series of chemical reactions that make up what researchers refer to as the citric acid cycle.

Every aerobic organism, from flamingoes to fungi, relies on the citric acid cycle to release stored energy in cells. In previous studies, researchers imagined early life using the same molecules for the citric acid cycle as life uses today. The problem with that approach, Krishnamurthy explai20ns, is that these biological molecules are fragile and the chemical reactions used in the cycle would not have existed in the first billion years of Earth — the ingredients simply didn’t exist yet.

Leaders of the new study started with the chemical reactions first. They wrote the recipe and then determined which molecules present on early Earth could have worked as ingredients.

The new study outlines how two non-biological cycles — called the HKG cycle and the malonate cycle — could have come together to kick-start a crude version of the citric acid cycle. The two cycles use reactions that perform the same fundamental chemistry of a-ketoacids and b-ketoacids as in the citric acid cycle. These shared reactions include aldol additions, which bring new source molecules into the cycles, as well as beta and oxidative decarboxylations, which release the molecules as carbon dioxide (CO2).

As they ran these reactions, the researchers found they could produce amino acids in addition to CO2, which are also the end products of the citric acid cycle. The researchers think that as biological molecules like enzymes became available, they could have led to the replacement of non-biological molecules in these fundamental reactions to make them more elaborate and efficient.

“The chemistry could have stayed the same over time, it was just the nature of the molecules that changed,” says Krishnamurthy. “The molecules evolved to be more complicated over time based on what biology needed.”

“Modern metabolism has a precursor, a template, that was non-biological,” adds Greg Springsteen, PhD, first author of the new study and associate professor of chemistry at Furman University.

Making these reactions even more plausible is the fact that at the center of these reactions is a molecule called glyoxylate, which studies show could have been available on early Earth and is part of the citric acid cycle today (called the “Glyoxylate shunt or cycle”).

Krishnamurthy says more research needs to be done to see how these chemical reactions could have become as sustainable as the citric acid cycle is today.

Researchers Measure The Inner Structure Of Distant Suns From Their Pulsations

At first glance, it would seem to be impossible to look inside a star. An international team of astronomers, under the leadership of Earl Bellinger and Saskia Hekker of the Max Planck Institute for Solar System Research in Göttingen, has, for the first time, determined the deep inner structure of two stars based on their oscillations.

Our Sun, and most other stars, experience pulsations that spread through the star’s interior as sound waves. The frequencies of these waves are imprinted on the light of the star, and can be later seen by astronomers here on Earth. Similar to how seismologists decipher the inner structure of our planet by analyzing earthquakes, astronomers determine the properties of stars from their pulsations—a field called asteroseismology. Now, for the first time, a detailed analysis of these pulsations has enabled Earl Bellinger, Saskia Hekker and their colleagues to measure the internal structure of two distant stars.

The two stars they analyzed are part of the 16 Cygni system (known as 16 Cyg A and 16 Cyg B) and both are very similar to our own sun. “Due to their small distance of only 70 light years, these stars are relatively bright and thus ideally suited for our analysis,” says lead author Earl Bellinger. “Previously, it was only possible to make models of the stars’ interiors. Now we can measure them.”

To make a model of a star’s interior, astrophysicists vary stellar evolution models until one of them fits to the observed frequency spectrum. However, the pulsations of the theoretical models often differ from those of the stars, most likely due to some stellar physics still being unknown.

Bellinger and Hekker therefore decided to use the inverse method. Here, they derived the local properties of the stellar interior from the observed frequencies. This method depends less on theoretical assumptions, but it requires excellent measurement data quality and is mathematically challenging.

Using the inverse method, the researchers looked more than 500,000 km deep into the stars—and found that the speed of sound in the central regions is greater than predicted by the models. “In the case of 16 Cyg B, these differences can be explained by correcting what we thought to be the mass and the size of the star,” says Bellinger. In the case of 16 Cyg A, however, the cause of the discrepancies could not be identified.

It is possible that as-yet unknown physical phenomena are not sufficiently taken into account by the current evolutionary models. “Elements that were created in the early phases of the star’s evolution may have been transported from the core of the star to its outer layers,” explains Bellinger. “This would change the internal stratification of the star, which then affects how it oscillates.”

This first structural analysis of the two stars will be followed by more. “Ten to 20 additional stars suitable for such an analysis can be found in the data from the Kepler Space Telescope,” says Saskia Hekker, who leads the Stellar Ages and Galactic Evolution (SAGE) Research Group at the Max Planck Institute in Göttingen. In the future, NASA’s TESS mission (Transiting Exoplanet Survey Satellite) and the PLATO (Planetary Transits and Oscillation of Stars) space telescope planned by the European Space Agency (ESA) will collect even more data for this research field.

The inverse method delivers new insights that will help to improve our understanding of the physics that happens in stars. This will lead to better stellar models, which will then improve our ability to predict the future evolution of the sun and other stars in our galaxy.

Special Star Is A Rosetta Stone For Understanding The Sun’s Variability And Climate Effect

The spots on the surface on the Sun come and go with an 11-year periodicity known as the solar cycle. The solar cycle is driven by the solar dynamo, which is an interplay between magnetic fields, convection and rotation. However, our understanding of the physics underlying the solar dynamo is far from complete. One example is the so-called Maunder Minimum, a period in the 17th century, where spots almost disappeared from the surface of the Sun for a period of over 50 years.

A Rossetta Stone for stellar dynamos

Now, a large international team led by Christoffer Karoff from Aarhus University has found a star that can help shed light on the physics underlying the solar dynamo. The star is located 120 light years away in the constellation of Cygnus, and on the surface, the star looks just like the Sun: it has the same mass, radius and age — but inside, the chemical composition of the star is very different. It consists of around twice as many heavy elements as in the Sun. Heavy elements here means elements heavier than hydrogen and helium.

The team has succeeded in combining observations from the Kepler spacecraft with ground-based observations dating as far back as 1978, thereby reconstructing a 7.4-year cycle in this star. “The unique combination of a star almost identical to the Sun, except for the chemical composition, with a cycle that has been observed from both the Kepler spacecraft and from ground makes this star a Rosetta Stone for the study of stellar dynamos.” explains Karoff.

Heavy elements make the star more variable

By combining photometric, spectroscopic and asteroseismic data, the team collected the most detailed set of observations for a solar-like cycle in any star other than the Sun. The observations revealed that the amplitude of the cycle seen in the star’s magnetic field is more than twice as strong as what is seen on the Sun, and the cycle is even stronger in visible light.

This allowed the team to conclude that more heavy elements make a stronger cycle. Based on models of the physics taking place in the deep interior and the atmosphere of the star, the team was also able to propose an explanation of the stronger cycle. Actually, they came up with a two-part explanation. First, the heavy elements make the star more opaque, which changes the energy transport deep inside the star from radiation to convection. This makes the dynamo stronger, affecting both the amplitude of the variability in the magnetic field and the rotation pattern near the surface. The latter effect was also measured. Second, the heavy elements affect the processes on the surface and in the atmosphere of the star. Specifically, the contrast between diffuse bright regions called faculae and the quiet solar background increases as the mix of heavy elements is increased. This makes the cyclic photometric variability of the star stronger.

Can help us understand how the Sun affects our climate

The new study can help us understand how the irradiance of the Sun has changed over time, which is likely to have an effect on our climate. In general special attention is paid to the Maunder Minimum, which coincided with a period of relatively cold climate, especially in Northern Europe. The new measurements offer an important constraint on the models trying to explain the weak activity and possible reduced brightness of the Sun during the Maunder minimum.