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Ever since NASA's New Horizons spacecraft flew by Pluto last year, evidence has been mounting that the dwarf planet may have a liquid ocean beneath its icy shell. Now, by modeling the impact dynamics that created a massive crater on Pluto's surface, a team of researchers has made a new estimate of how thick that liquid layer might be.

"Thermal models of Pluto's interior and tectonic evidence found on the surface suggest that an ocean may exist, but it's not easy to infer its size or anything else about it," said Johnson, who is an assistant professor in Brown's Department of Earth, Environmental and Planetary Sciences. "We've been able to put some constraints on its thickness and get some clues about composition."

The research focused on Sputnik Planum, a basin 900 kilometers across that makes up the western lobe the famous heart-shaped feature revealed during the New Horizons flyby. The basin appears to have been created by an impact, likely by an object 200 kilometers across or larger.

The story of how the basin relates to Pluto's putative ocean starts with its position on the planet relative to Pluto's largest moon, Charon. Pluto and Charon are tidally locked with each other, meaning they always show each other the same face as they rotate. Sputnik Planum sits directly on the tidal axis linking the two worlds. That position suggests that the basin has what's called a positive mass anomaly—it has more mass than average for Pluto's icy crust. As Charon's gravity pulls on Pluto, it would pull proportionally more on areas of higher mass, which would tilt the planet until Sputnik Planum became aligned with the tidal axis.

But a positive mass anomaly would make Sputnik Planum a bit of an odd duck as craters go. "An impact crater is basically a hole in the ground," Johnson said. "You're taking a bunch of material and blasting it out, so you expect it to have negative mass anomaly, but that's not what we see with Sputnik Planum. That got people thinking about how you could get this positive mass anomaly."

Part of the answer is that, after it formed, the basin has been partially filled in by nitrogen ice. That ice layer adds some mass to the basin, but it isn't thick enough on its own to make Sputnik Planum have positive mass, Johnson says.

The rest of that mass may be generated by a liquid lurking beneath the surface. Like a bowling ball dropped on a trampoline, a large impact creates a dent on a planet's surface, followed by a rebound. That rebound pulls material upward from deep in the planet's interior. If that upwelled material is denser than what was blasted away by the impact, the crater ends up with the same mass as it had before the impact happened. This is a phenomenon geologists refer to as isostatic compensation.

Water is denser than ice. So if there were a layer of liquid water beneath Pluto's ice shell, it may have welled up following the Sputnik Planum impact, evening out the crater's mass. If the basin started out with neutral mass, then the nitrogen layer deposited later would be enough to create a positive mass anomaly.

"This scenario requires a liquid ocean," Johnson said. "We wanted to run computer models of the impact to see if this is something that would actually happen. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It's also sensitive to how salty the ocean is, because the salt content affects the density of the water."

The models simulated the impact of an object large enough to create a basin of Sputnik Planum's size hitting Pluto at a speed expected for that part in the solar system. The simulation assumed various thicknesses of the water layer beneath the crust, from no water at all to a layer 200 kilometers thick.

The scenario that best reconstructed Sputnik Planum's observed size depth, while also producing a crater with compensated mass, was one in which Pluto has an ocean layer more than 100 kilometers thick, with a salinity of around 30 percent.

"What this tells us is that if Sputnik Planum is indeed a positive mass anomaly —and it appears as though it is—this ocean layer of at least 100 kilometers has to be there," Johnson said. "It's pretty amazing to me that you have this body so far out in the solar system that still may have liquid water."

As researchers continue to look at the data sent by New Horizons, Johnson is hopeful that a clearer picture of Pluto's possible ocean will emerge.

Johnson's co-authors on the paper were Timothy Bowling of the University of Chicago and Alexander Trowbridge and Andrew Freed from Purdue University.

The Daily Galaxy via Brown University

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International teams of astronomers have used the ESO's Atacama Large Millimeter/submillimeter Array (ALMA) to explore the distant corner of the Universe first revealed in the iconic images of the Hubble Ultra Deep Field (HUDF). These new ALMA observations are significantly deeper and sharper than previous surveys at millimeter wavelengths. They clearly show how the rate of star formation in young galaxies is closely related to their total mass in stars. They also trace the previously unknown abundance of star-forming gas at different points in time, providing new insights into the "Golden Age" of galaxy formation approximately 10 billion years ago.

In 2004 the Hubble Ultra Deep Field images, pioneering deep-field observations with the NASA/ESA Hubble Space Telescope, were published. These spectacular pictures probed more deeply than ever before and revealed a menagerie of galaxies stretching back to less than a billion years after the Big Bang. The area was observed several times by Hubble and many other telescopes, resulting in the deepest view of the Universe to date.

Astronomers using ALMA have now surveyed this seemingly unremarkable, but heavily studied, window into the distant Universe for the first time both deeply and sharply in the millimetre range of wavelengths. This allows them to see the faint glow from gas clouds and also the emission from warm dust in galaxies in the early Universe.

Astronomers specifically selected the area of study in the HUDF, a region of space in the faint southern constellation of Fornax (The Furnace), so ground-based telescopes in the southern hemisphere, like ALMA, could probe the region, expanding our knowledge about the very distant Universe. Probing the deep, but optically invisible, Universe was one of the primary science goals for ALMA.

ALMA has observed the HUDF for a total of around 50 hours up to now. This is the largest amount of ALMA observing time spent on one area of the sky so far.

One team led by Jim Dunlop (University of Edinburgh, United Kingdom) used ALMA to obtain the first deep, homogeneous ALMA image of a region as large as the HUDF. This data allowed them to clearly match up the galaxies that they detected with objects already seen with Hubble and other facilities.

This study showed clearly for the first time that the stellar mass of a galaxy is the best predictor of star formation rate in the high redshift Universe. They detected essentially all of the high-mass galaxies and virtually nothing else.

Jim Dunlop, lead author on the deep imaging paper sums up its importance: "This is a breakthrough result. For the first time we are properly connecting the visible and ultraviolet light view of the distant Universe from Hubble and far-infrared/millimetre views of the Universe from ALMA."

The second team, led by Manuel Aravena of the Núcleo de Astronomía, Universidad Diego Portales, Santiago, Chile, and Fabian Walter of the Max Planck Institute for Astronomy in Heidelberg, Germany, conducted a deeper search across about one sixth of the total HUDF.

Some of the new ALMA observations were specifically tailored to detect galaxies that are rich in carbon monoxide, indicating regions primed for star formation. Even though these molecular gas reservoirs give rise to the star formation activity in galaxies, they are often very hard to see with Hubble. ALMA can therefore reveal the "missing half" of the galaxy formation and evolution process.

ALMA's ability to see a completely different portion of the electromagnetic spectrum from Hubble allows astronomers to study a different class of astronomical objects, such as massive star-forming clouds, as well as objects that are otherwise too faint to observe in visible light, but visible at millimetre wavelengths.

"The new ALMA results imply a rapidly rising gas content in galaxies as we look back further in time," adds lead author of two of the papers, Manuel Aravena (Núcleo de Astronomía, Universidad Diego Portales, Santiago, Chile). "This increasing gas content is likely the root cause for the remarkable increase in star formation rates during the peak epoch of galaxy formation, some 10 billion years ago."

The results presented today are just the start of a series of future observations to probe the distant Universe with ALMA. For example, a planned 150-hour observing campaign of the HUDF will further illuminate the star-forming potential history of the Universe.

"By supplementing our understanding of this missing star-forming material, the forthcoming ALMA Large Program will complete our view of the galaxies in the iconic Hubble Ultra Deep Field," concludes Fabian Walter.

The image at the top of the page shows a trove of galaxies, rich in carbon monoxide (indicating star-forming potential) were imaged by ALMA (orange) in the Hubble Ultra Deep Field. The blue features are galaxies imaged by Hubble. This image is based on the very deep ALMA survey by Manuel Aravena, Fabian Walter and colleagues, covering about one sixth of the full HUDF area.

The Daily Galaxy via European Southern Observatory

Image credit: B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble

Credit: B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO); NASA/ESA Hubble

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Our moon is a laboratory: "An archive of solar system history, the signatures of meteorites, comets and the solar wind are written in the dust. A moon village would give scientists the means to explore the body, a lump of the ancient Earth, much as Antarctic bases have opened up the southern continent."

ESA's vision is of a moon village intended to grow incrementally as an open, international effort. "In time," Woerner says, "it would build up the vital infrastructure and practical know-how that humans will need to head more safely into the farther reaches of the solar system."

“The question is what to do after the space station,' says Ian Crawford, professor of planetary science at Birkbeck, University of London. “Either nothing follows and you shut down human space exploration, or you build another space station and it's hard to see the point of that or you go somewhere else, and I strongly believe the moon is the next place to go.”

Before we head to Mars, or any other faraway body, Crawford says, humans must learn how to thrive in dusty, high radiation environments. “To send people to Mars you have to be very confident in all aspects of the technology,” he says. “Going to the moon is risky too, but the advantage of learning and trialling all this stuff on the moon is that if something goes wrong, you can bring people back. The moon is only three days away. Abort options exist.”

“A lunar base isn't a distraction from our desire to visit and explore Mars,” says Katherine Joy, a lunar scientist at Manchester University. “What we learned from Apollo is that touch-and-go-style missions are exciting, and scientifically rewarding, but they don't lead to a sustained human presence on an alien world.”

The Daily Galaxy via  The Guardian: Read more Here

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