Resembling what the skin on my arms looks like after giving my cat a bath, the surface of Saturn’s moon Tethys is seen above in an extended-color composite from NASA’s Cassini spacecraft showing strange long red streaks. They stretch for long distances across the moon’s surface following the rugged terrain, continuing unbroken over hills and down into craters… and their cause isn’t yet known.
According to a NASA news release, “The origin of the features and their reddish color is currently a mystery to Cassini scientists. Possibilities being studied include ideas that the reddish material is exposed ice with chemical impurities, or the result of outgassing from inside Tethys. The streaks could also be associated with features like fractures that are below the resolution of the available images.”
The images were taken by Cassini during a flyby of the 660-mile-wide (1,062 km) Tethys on April 11, 2015 at a resolution of about 2,300 feet (700 meters) per pixel. They were acquired in visible green, infrared, and ultraviolet light wavelengths and so the composite image reveals colors our eyes can’t directly perceive. The combination of this and the solar illumination needed to image this particular area as the spacecraft passed by are why these features haven’t been seen so well until now.
“The red arcs really popped out when we saw the new images,” said Cassini participating scientist Paul Schenk of the Lunar and Planetary Institute in Houston. “It’s surprising how extensive these features are.”
While the nature of Tethys’ streaks isn’t understood, the observations do indicate a relatively young age compared to the surrounding surface.
“The red arcs must be geologically young because they cut across older features like impact craters, but we don’t know their age in years.” said Paul Helfenstein, a Cassini imaging scientist at Cornell University in Ithaca. “If the stain is only a thin, colored veneer on the icy soil, exposure to the space environment at Tethys’ surface might erase them on relatively short time scales.”
Could these arcs be signs of an underground ocean or reservoir of briny liquid, like Enceladus’ tiger stripes (aka sulcae) or the streaks that crisscross Europa’s ice? Or are they the results of infalling material from one of Saturn’s other moons? More observations with Cassini, now in its eleventh year in orbit at Saturn, are being planned to “study the streaks.”
“We are planning an even closer look at one of the Tethys red arcs in November to see if we can tease out the source and composition of these unusual markings,” said Linda Spilker, Cassini project scientist at JPL.
“The most exciting phrase to hear in science, the one that heralds new discoveries, is not Eureka! (I found it!) but rather, ‘hmm… that’s funny…'” (Isaac Asimov)
A few short years ago, Zooite Hanny van Arkel discovered Hanny’s Voorwerp in an SDSS image of a galaxy (“What’s the blue stuff below? Anyone?”), and a new term entered astronomers’ lexicon (“voorwerpje”).
Very late last year, Zooite mitch too had a ‘that’s funny…’ moment, over a spectrum (yes, you read that right, a spectrum!).
Now neither Hanny nor mitch have PhDs in astronomy …
But I digress; what, exactly, did mitch discover? Judge for yourself; here’s the spectrum of the star in question (it goes by the instantly recognizable name 587739406764540066):
“I asked a couple of white-dwarf aficionados, and neither recalls seeing any star with these features (nor does Jim Kaler, who wrote the book on stellar spectra),” Bill Keel, a Zooite Astronomer known as NGC3314 wrote, kicking off a flurry of forum posts, and a most interesting discussion!
“Can we rule out something along the line of sight, possibly a cold molecular cloud?” EigenState wrote; “If both stars are moving SE (towards the bottom left corner), could Mitch’s star (square) be affected by debris in the trail of the bright red star (triangle)? I am thinking of the trail left by Mira. So the spectrum would be white dwarf shining through cooled red star debris?” said Budgieye. NGC3314 continued “It can’t be like our current Oort Cloud since we don’t see local absorption from our own in front of lots of stars near the ecliptic plane. To show up this strongly, it would then have to be either much denser or physical much smaller. This just in – this may be the most extreme known example of a DZ white dwarf, which have surface metals. White dwarfs aren’t supposed to, because their intense surface gravity will generally sort atmospheric atoms by density, so this has been suggested (with some theoretical backing) to result from accretion either from circumstellar or interstellar material (so it could be at the star’s surface but representing material formerly in a surrounding disk). Watch this space…”
Then, two weeks after mitch’s discovery, Patrick Dufour, of the Université de Montréal, joined in “Hi everyone, I have known this objects for many years. I have done fits almost 5 years ago but just never took time to publish it. Will do it in the next few weeks. Meanwhile, enjoy this preliminary analysis… The abundances are very similar to G165-7, the magnetic DZ, but it’s a bit cooler (explaining the strength of the features).” Patrick, as you might have guessed from this, is an astronomer with specific expertise in white dwarfs; in fact the abstract to his PhD thesis begins with these words “The goal of this thesis is to accurately determine the atmospheric parameters of a large sample of cool helium-rich white dwarfs in order to improve our understanding of the spectral evolution of these objects. Specifically, we study stars showing traces of carbon (DQ spectral type) and metals (DZ spectral type) in their optical spectrum.”
Somehow yet another astronomer, Fergal Mullally heard about mitch’s mystery star and joined in too “Many other WDs with strong metal absorption lines are surrounded by a cloud of accretable material. This makes sense because the metals quickly sink below the surface (as mentioned by NGC3314). In some cases, metals are only visible for a few weeks before they are sink too deep to be seen. The disks are exciting, not only because they can be so young, but their composition suggests we might be looking at the remains of an asteroid belt (see http://arxiv.org/abs/0708.0198).” To which Patrick added “Mitch’s Mystery Star is a cool (~4000-5000 K) helium rich white dwarf with traces of metals (abundances similar to G165-7). The metals most probably originate from a tidally disrupted asteroid or minor planet that formed a disk around the star.”
So, mitch’s mystery star is just a rather weird kind of DZ star, and DZs are just unusual white dwarfs?
Yes … and no. “The asymmetrical line near 5000 is almost certainly MgH. As for the one at 6100, I am open to suggestion. I have never seen it anywhere else. For G165-7, the splitting is Zeeman. But the broadening is van der Waals by neutral helium. No splitting is observed in this star (and I took a really good spectra at MMT a few years ago to be sure).” Patrick again; so what is the mysterious asymmetrical line at 6100 Å?
Two more weeks passed, and a possible reason for Fergal’s interest emerges, in a post by NGC3314 “While we wait to see how Patrick’s new calculation shakes out, here’s an interesting new manuscript he was involved with, that points to likewise interesting things about the DZ stars.  Wow. White-dwarf spectra as tombstones for planetary systems… wonder how the system stayed close enough to end up on the white-dwarf atmosphere all through the red-giant phases? The binary systems we can see look awfully far apart to have had helpful dynamical effects for this.” (in case you didn’t read up on Fergal, he’s very keen on exoplanets and ET).
Then, in February, a tweet: “At campus observatory, seeing whether we can measure orbital motion between Mitch’s star and its K-dwarf companion.” The tale is becoming curiouser and curiouser (exoplanets in binary star systems? If life had evolved on a planet in orbit around the star which later went red giant then white dwarf, could it have somehow survived and landed on a planet in orbit around the K-dwarf companion?)
I’ll let NGC3314 have the final word: “This furnishes one more example of how the wide interest in Galaxy Zoo allows things once unthinkable – during the SDSS, the whole analysis plan never conceived that every bright galaxy in the survey, and every one of the million or so spectra would actually be examined by a human being.”
Oh, and the Asimov quote seems to be an urban myth (if any reader knows when, and where, Asimov actually said, or wrote, those words …).
Source: Galaxy Zoo Forum thread Mitch’s Mystery Star
Full caption for image at the top of this article (Credit: Bill Keel): I had a look with the SARA 1m telescope in BVR filters last week, to check for obvious variability. Pending more exact measurements, it’s about as bright as it was in the SDSS images and the older Palomar plates. As SIMBAD shows, this is known as a star of fairly high proper motion (and that’s about all). You can see this when I register the original red-light Palomar photograph to the image from last week, a time span of almost 59 years. The attached picture compares red-light data from the original Palomar Schmidt sky survey in early 1951, the second-epoch Palomar survey around 1990, and SARA on Jan. 7, 2010. You can also see that the bright red star to the southeast has almost exactly the same (large) proper motion.