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You may have heard of the first interstellar object observed in our solar system, but did you know there's more than one? And speaking of icy rocks, new research suggests the ocean under the icy crust of Enceladus could be more dynamic than we first thought!

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Sources:
https://www.eso.org/public/news/eso2106/
https://press.springernature.com/unusual-polarimetric-properties-for-interstellar-comet-2i-boriso/18966422
https://doi.org/10.1038/s41550-021-01336-w
https://doi.org/10.1038/s41467-021-22000-x
https://www.caltech.edu/about/news/ocean-currents-predicted-on-enceladus
https://www.nature.com/articles/s41561-021-00706-3

Image Sources:
https://hubblesite.org/contents/media/images/2019/53/4578-Image
https://www.nasa.gov/image-feature/jpl/pia20645/protoplanetary-disk/
https://svs.gsfc.nasa.gov/11541
https://commons.wikimedia.org/wiki/File:Comet_Hale-Bopp_1995O1.jpg
https://commons.wikimedia.org/wiki/File:Orion_Watches_over_Paranal_-_potw2009a.jpg
https://solarsystem.nasa.gov/resources/17737/enceladus-plume/
https://www.jpl.nasa.gov/images/bursting-at-the-seams-the-geyser-basin-of-enceladus
https://www.nasa.gov/multimedia/imagegallery/image_feature_1243.html
https://solarsystem.nasa.gov/resources/17646/enceladus-hydrothermal-activity/
This episode of SciShow Space is supported by Skillshare.

The first thousand people to click the link in the description can get a free trial of Skillshare’s Premium Membership. {♫Intro♫}. You might be familiar with our solar system’s first official interstellar tourist ‘Oumuamua,   which was discovered back in 2017.

But it’s not the only one. In 2019, we found another. It’s a comet called Borisov, after the amateur astronomer who first spotted it.

And two papers published this week, in Nature Astronomy and Nature Communications, have revealed a little bit more about it  -- and its possible origins. Through a backyard telescope, Borisov looks like your standard comet. It’s got a nucleus of ice and rock surrounded by a spherical halo of dust and gas called a coma.

And it’s got that characteristic tail. But last year, observations showed that Borisov wasn’t quite the same as the comets we’re used to. Like, it had a lot more carbon monoxide.

And those differences just keep piling up. Both new papers involved a team of astronomers looking at the dust in Borisov’s coma. The one from Nature Astronomy focused on the size of the grains.

Using the ALMA array of radio telescopes in Chile, they determined that most of the “dust” is actually on the scale of millimeters. So, more like tiny pebbles. This agrees with previous results using visible light.

And it differs from what we usually see in our solar system’s comets, where the dust is more fluffy. Yes, that’s the real word astronomers use. These pebbles provide hints about Borisov’s formation.

They tell us it likely formed in the inner part of its star’s protoplanetary disk – the material around baby stars that collapses to form planets and other smaller space rocks. In that region, Borisov could have started out more fluffy, but collision after collision would have compacted its dust into the pebbles we’re seeing. The data also suggest that Borisov’s native system was home to gas giants.

That’s because as Borisov approached our Sun, we observed a change in the ratio of carbon monoxide and water. These ingredients start forming at different distances from a star, so the comet has to have formed in a place where they got all mixed up. And the team says the most likely cause of that mixing is the gravity of gas giants flinging around smaller bodies in a planetary nursery.

Meanwhile, the team behind the Nature Communications paper looked at how the dust polarized light meaning how the dust changed the orientation of the light waves. There are a lot of properties that affect polarization, like what material the light is hitting, as well as the size and shape of that material. Using the European Southern Observatory’s Very Large Telescope, the team found that there was only one body in our solar system with a similar polarization profile.

That was the comet Hale Bopp, which astronomers believe may have only gotten close to our Sun twice. Once in 1997, and the other over two thousand years prior. But Borisov’s polarization was even more uniform than Hale Bopp’s, suggesting it may be the most pristine comet we’ve ever observed.

That means before it came to our solar system, it may have never gotten close enough to a star to degrade and change its surface. Which would make it not only an example of a comet before a star disturbs it, but also a frozen time capsule of its original star system. We’re lucky it paid us a visit!

But comets aren’t the only icy space rocks in the headlines. Last week in Nature Geoscience, astronomers made a new  prediction about the water world Enceladus. Enceladus is one of Saturn’s moons, and it garnered some attention back in 2014.

That’s when the Cassini spacecraft captured evidence of geysers shooting ice and water vapor into space from the moon’s south pole. Cassini even took the time to fly through some of that geyser spray to figure out what it’s made of. And from that data, scientists are all but certain there’s an entire ocean buried beneath kilometers of ice.

Until now, it’s been thought that while there may be some vertical mixing,. Enceladus’s southern ocean is pretty stagnant. But this new research suggests there’s actually a globe-wide current churning the water around like what happens on Earth.

Of course, they’re not identical. There are differences like the source of heat --. Earth’s oceans are heated by the Sun, while Enceladus is heated by its core and that affects how water can move around.

But Enceladus’s and Earth’s oceans are both plenty salty. And salinity can help drive currents here on Earth. Based on that observation, this paper used computer modeling to suggest a similar phenomenon could drive a current on Enceladus.

The models of this hypothetical current are, in part, based on measurements of Enceladus’s ice shell. It’s thicker at the equator, and when salt water freezes, the salts get left out, making the remaining water saltier and heavier. So it sinks.

And up at the poles, the opposite happens -- enough to drive a current loop. Cassini only tested the water at Enceladus’s pole, which this model suggests is the least salty part. So it could be that the whole ocean is actually a lot saltier, which could affect how habitable it might be.

We don’t yet have direct evidence for this current. But astronomers are dreaming up more spacecraft to send to Enceladus including one that could burrow beneath the ice and  swim in this alien ocean. Scientists are always dreaming up new ways to discover new things and we are always dreaming up new ways to bring amazing space news and discoveries to you -- and today’s sponsor, Skillshare, really helps us grow our skills.

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