Cassini"s Enceladus Final Flyby

Scientia — In late 2015, NASA’s Cassini spacecraft is making its final three flybys of Enceladus, the little moon that stunned scientists with the revelation it harbors a global ocean under its icy shell, active geysers of water-ice feeding one of Saturn’s rings and the first tantalizing signs of hydrothermal activity beyond Earth. All these discoveries have vaulted Enceladus to one of the top future destinations for exploration and the search for signs of potential life beyond Earth.

• 14 October 2015: Cassini aligned it self to get some of the best shots of Saturn’s Moon Enceladus. Cassini was looking at the moon’s north polar region at an altitude of just 1,142 miles (1,839 kilometers).
  
  

  This high-resolution Cassini image shows a landscape of stark contrasts on Saturn’s moon Enceladus. Thin cracks cross over the pole — the northernmost extent of a global system of such fractures. Credit: NASA/JPL/Space Science Institute – Provided by NASA

  
  
  Scientists expected the north polar region of Enceladus to be heavily cratered, based on low-resolution images from the Voyager mission, but the new high-resolution Cassini images show a landscape of stark contrasts. “The northern regions are crisscrossed by a spidery network of gossamer-thin cracks that slice through the craters,” said Paul Helfenstein, a member of the Cassini imaging team at Cornell University, Ithaca, New York. “These thin cracks are ubiquitous on Enceladus, and now we see that they extend across the northern terrains as well.”
  
  
  
  



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  28 October 2015: Cassini will make a daring flight through the moon’s famous plume only 30 miles (48 kilometers) above Enceladus’ south pole. The flyby is Cassini’s deepest-ever dive through the jets. The encounter will allow Cassini to obtain the most accurate measurements yet of the plume’s composition, and new insights into the ocean world beneath the ice.
  

  NASA’s Cassini spacecraft will sample the ocean of Saturn’s moon Enceladus on Wednesday, Oct. 28, when it flies through the moon’s plume of icy spray.

  
  

  Cassini launched in 1997 and entered orbit around Saturn in 2004. Since then, it has been studying the huge planet, its rings and its magnetic field. Here are some things to know about the mission’s upcoming close flyby of Enceladus:

  
  
  1. Enceladus is an icy moon of Saturn. Early in its mission, Cassini discovered Enceladus has remarkable geologic activity, including a towering plume of ice, water vapor and organic molecules spraying from its south polar region. Cassini later determined the moon has a global ocean and likely hydrothermal activity, meaning it could have the ingredients needed to support simple life.
  
  
  2. The flyby will be Cassini’s deepest-ever dive through the Enceladus plume, which is thought to come from the ocean below. The spacecraft has flown closer to the surface of Enceladus before, but never this low directly through the active plume.
  
  
  3. The flyby is not intended to detect life, but it will provide powerful new insights about how habitable the ocean environment is within Enceladus.
  
  
  4. Cassini scientists are hopeful the flyby will provide insights about how much hydrothermal activity — that is, chemistry involving rock and hot water — is occurring within Enceladus. This activity could have important implications for the potential habitability of the ocean for simple forms of life. The critical measurement for these questions is the detection of molecular hydrogen by the spacecraft.
  
  
  5. Scientists also expect to better understand the chemistry of the plume as a result of the flyby. The low altitude of the encounter is, in part, intended to afford Cassini greater sensitivity to heavier, more massive molecules, including organics, than the spacecraft has observed during previous, higher-altitude passes through the plume.
  
  
  6. The flyby will help solve the mystery of whether the plume is composed of column-like, individual jets, or sinuous, icy curtain eruptions — or a combination of both. The answer would make clearer how material is getting to the surface from the ocean below.
  
  
  7. Researchers are not sure how much icy material the plumes are actually spraying into space. The amount of activity has major implications for how long Enceladus might have been active.
  
  
  
  


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  19 December 2015: Cassini’s final targeted flyby will allow the spacecraft to measure heat flow from the moon’s interior at an altitude of 3,106 miles, or 4,999 kilometers.

  
  
  
  

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– Credit and Resource –

NASA

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Cassini"s Enceladus Final Flyby

Moon"s crust fractured by asteroids

Study finds barrage of small asteroids shattered moon’s upper crust.

Researchers analyzed the gravity signatures of more than 1,200 craters (in yellow) on the far side of the moon.
Courtesy of the researchers

Scientia — Scientists believe that about 4 billion years ago, during a period called the Late Heavy Bombardment, the moon took a severe beating, as an army of asteroids pelted its surface, carving out craters and opening deep fissures in its crust. Such sustained impacts increased the moon’s porosity, opening up a network of large seams beneath the lunar surface.

Now scientists at MIT and elsewhere have identified regions on the far side of the moon, called the lunar highlands, that may have been so heavily bombarded — particularly by small asteroids — that the impacts completely shattered the upper crust, leaving these regions essentially as fractured and porous as they could be. The scientists found that further impacts to these highly porous regions may have then had the opposite effect, sealing up cracks and decreasing porosity.

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The researchers observed this effect in the upper layer of the crust — a layer that scientists refer to as the megaregolith. This layer is dominated by relatively small craters, measuring 30 kilometers or less in diameter. In contrast, it appears that deeper layers of crust, that are affected by larger craters, are not quite as battered, and are less fractured and porous.

Jason Soderblom, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says the evolution of the moon’s porosity can give scientists clues to some of the earliest life-supporting processes taking place in the solar system.

“The whole process of generating pore space within planetary crusts is critically important in understanding how water gets into the subsurface,” Soderblom says. “On Earth, we believe that life may have evolved somewhat in the subsurface, and this is a primary mechanism to create subsurface pockets and void spaces, and really drives a lot of the rates at which these processes happen. The moon is a really ideal place to study this.”

Soderblom and his colleagues, including Maria Zuber, the E.A. Griswold Professor of Geophysics and MIT’s vice president for research, have published their findings in the journal Geophysical Research Letters.

Changing porosity

The team used data obtained by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) — twin spacecraft that orbited the moon throughout 2012, each measuring the push and pull of the other as an indicator of the moon’s gravity.

With the GRAIL data, researchers mapped the gravity field in and around more than 1,200 craters on the far side of the moon. This region, the lunar highlands, makes up the moon’s most ancient, heavily cratered terrain.

They then carried out an analysis called a Bouger correction to subtract the gravitational effect of mountains, valleys, and other topology from the total gravity field. What’s left is the gravity field beneath the surface, within the moon’s crust.

“There’s an assumption we do have to make, which is that there’s no changes in the material itself, and that all of the bumps we’re seeing [in the gravity field] are from changes in the porosity and the amount of air between the rock,” Soderblom explains.

Soderblom calculated the gravity signatures in and around 1,200 craters on the far side of the moon, and compared the gravity within each crater with the gravity of the surrounding terrain, to determine whether an impact increased or decreased the local porosity.

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Origin story

For craters smaller than 30 kilometers in diameter, he found impacts both increased and decreased porosity in the upper layer of the moon’s crust.

“For the smallest craters that we’re looking at, we think we’re starting to see where the moon has gone through so much fracturing that it gets to a point where the porosity of the crust just stays at some constant level,” Soderblom says. “You can keep impacting it and you’ll hit regions where you’ll increase porosity here and decrease it there, but on average it stays constant.”

The researchers found that larger craters, which excavated much deeper into the moon’s crust, only increased porosity in the underlying crust — an indication that these deeper layers have not reached a steady state in porosity, and are not as fractured as the megaregolith.

Soderblom says the gravity signatures of the larger craters in particular may provide insight into just how many impacts the moon, and other terrestrial bodies, sustained during the Late Heavy Bombardment.

“For the smaller craters, it’s like if you’re filling a bucket, eventually your bucket gets full, but if you keep pouring cups of water into the bucket, you can’t tell how many cups of water beyond full you’ve gone,” Soderblom says. “Looking at the larger craters at the subsurface might give us insight, because that ‘bucket’ isn’t full yet.”

Ultimately, tracing the moon’s changing porosity may help scientists track the trajectory of the moon’s impactors 4 billion years ago.

“What we really hope to do is to figure out the number of impacts in the range of 100 kilometers in diameter, and from that, we can extrapolate to the smaller craters, assuming different populations of impactors, and those different assumptions will tell us where the impactors came from,” Soderblom says. “This will help to understand the origin of the Late Heavy Bombardment, and whether it was disrupted material from the asteroid belt, or if it was further out.”

– Credit and Resource –

This research was funded by NASA.

Provided by: Jennifer Chu | MIT News Office

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Moon"s crust fractured by asteroids