Bennu’s penchant for tossing rocks might be explained by temperature changes on the surface, and Enceladus uses “tiger stripes” to flex and vent as it changes temperature due to its orbit around Saturn.
This is the Daily Space for today, Monday, December 9, 2019. I am your host, Dr Pamela Gay, and I am here to put science in your brain.
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This week is a week when we will see a whole lot of planetary science enter the feed. Right now, in San Francisco, California Geologists, Geophysicists, and Planetary Scientists are gathered to discuss the latest science involving all things that orbit our Sun.
This week was also supposed to mark the first launch of Boeing’s StarLiner capsule, and Annie, Susie and I are all in on the Space Coast in Florida to see that launch…. Which got delayed until after we’re slated to fly home… so… we’re going to Disney because there we can at least take off on Space Mountain. It’s a weird week. The audio may sound different. The science will go on, and if you want to follow along on our adventures, we’re posting photos on instagram on my account, starstryder, and when science related on CosmoQuestX.
But we are here to talk to you about Space, and not Space Mountain, and there is some pretty cool news coming out from the OSIRIS-REx mission. As we’ve mentioned before, Bennu is an asteroid that likes to throw rocks. Soon after the mission arrived at its 500m across space rock they noticed particles getting flung into space and reflecting light back to the cameras. These plumes were completely unexpected. Figuring out how this happens has been one of the unexpected research challenges scientists have faced, and a new article in the journal Science offers 3 possible solutions.
The first possibility is thermal effects leading to things getting ejected. As it rotates every 4.3 hours, surface temperatures vary from 400 deg kelvin or about 260 deg F down to 250 degrees Kelvin or -10 Fahrenheit. This kind of a temperature change, from more than enough to boil water to cold enough to kill is the kind of change that can deform and crack rock, and possibly fling materials into space.
The energy needed to eject material from Bennu isn’t that great. According to project scientist Humburto Campins, the energy needed to fling particles off Bennu is similar to the energy needed to break a cracker. If the temperature swings can break rocks, they can definitely throw particles. Additionally, overtime exposure to space can dehydrate the clays on Bennu, and this can also crack rocks, and also send stuff into space.
Finally, the simple act of sticking Bennu with a small rock or very lucky micrometeor could be sufficient to send particles flying. In other words, when gravity is low and there is a transfer of energy, particles can and probably will fly.
Bennu is rocks, on rocks, with boulders, and more rocks, and they are all waiting to just throw debris at us, so… get ready to duck.
Bennu is of particular interest because it is a potentially dangerous asteroid. And while it currently poses no immediate damage, in 150 years or more, the situation could evolve, and we want to know how to deal with it or asteroids like it if they begin to come near Earth. How you deal with an orbiting pile of gravel I’m not ready to say, but this is why we explore, and because of OSIRIS-REx, we will be able to answer these questions.
- Carnegie: How Saturn’s Moon Enceladus Got Its Stripes (CarnagieScience)
From stony Bennu, we now travel out to icy Enceladus, one of the larger of Saturn’s dozens of moons. This ball of water and rock was observed by the Cassini Space Probe to have geysers of ice erupting from a series of long parallel cracks near Enceladus’ southern pole. How Enceladus ended up with these stripes, called Tiger stripes, is a more than a decade old mystery that may finally be solved.
In a new paper in the journal Nature with lead author Doug Hemingway does a straight forward force analysis that is pleasingly easy to follow. The first thing to understand is that because it rotates, the ice at the poles is thinner than the icy at the equator. The next thing to remember is that as ice freezes, it expands. With Enceladus, as it orbits, it regularly changes its distance from Saturn, and this causes distortions that also drive heating and cooling. As it cools, ice expands. At some point, as the ice expanded, something had to give and random chance picked the South pole over the North as the thin place in the ice that would crack. That gets us the first crack. The pressure under the ice builds up cyclically, and drives geysers, and that material rising up settles back down, placing increased weight on the ice in a line that then cracks under the combined force of the internal pressure and external weight. Essentially, the material goes up, the moon rotates a bit, the material comes down in a new place and when that weight gets to be high enough and the pressure gets strong enough, the ice cracks. This series of events repeated twice more, leading to the suite of 4 lines we see today. With 4 stripes, all erupting in fits and starts, they can release enough pressure that no new cracks open.
I love this explanation for its elegance. I’m sure the modeling they had to do was exceedingly complex, and this result is the culmination of years of effort. I can also see future planetary graduate students being assigned homework assignments to recreate this work.
A lot of people think that science is a dry field that requires following specific patterns of behavior and using maths and computers to plug away at solutions. Well, the maths or computers is often true, but what is often needed more than systematic plugging away is wild moments of creativity that allow you to see how ideas can be fit together in a new way. Some people do this when cooking to create new fusion cuisines. Some people do this while building to create symphony halls that look like nothing we’ve ever seen and sound better than anything previously heard. These feats of creativity are necessary in every field including science, and they are what so often advance the field.
And this week, I look forward to seeing many more advances, but for now, as I write on Monday early morning, this is what we have, so far.
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That rounds out our show for today.
Thank you all for listening. The Daily Space is produced by Susie Murph, and is a product of the Planetary Science Institute, a 501(c)3 non profit dedicated to exploring our Solar System and beyond. We are here thanks to the generous contributions of people like you. Want to become a supporter of the show? Check us out at Patreon.com/cosmoquestx
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