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Salacia: As big as Ceres, but much farther away

Posted by Emily Lakdawalla

26-06-2012 12:27 CDT

Topics: dwarf planets beyond Neptune, trans-neptunian objects, Hubble Space Telescope, Spitzer Space Telescope, planetary astronomy

I received the June 2012 issue of Icarus in the mail yesterday. This is the peer-reviewed scientific journal most closely associated with the main planetary science professional organization: the Division of Planetary Sciences of the American Astronomical Society. Reading the Table of Contents on the back, I came across "Physical properties of trans-neptunian binaries (120347) Salacia-Actaea and (42355) Typhon-Echidna," by John Stansberry and several coauthors. Stansberry's is one of very few groups of researchers doing the extremely difficult work of following up on the discoveries of distant trans-Neptunian objects by trying to characterize them as more than just moving dots of light.

So I expected an interesting paper about some odd little objects; there's so much variety beyond Neptune that there are pleasing surprises everywhere you look. I didn't expect to discover one of the biggest objects in the Kuiper belt! Salacia, it turns out, is one of the largest known objects out there. How was Salacia's large size not appreciated in the past? It turns out to have a very low albedo, 3 to 4 percent. Stansberry and his coauthors used Hubble optical data and Spitzer infrared data to constrain the size of Salacia. I've described this process before, in a discussion of Orcus and Vanth.

Getting diameters of two members of a binary pair requires a lot of assumptions. Given some reasonable assumptions, Salacia's diameter is 905 ± 103 kilometers. For context, this very similar to the largest asteroid Ceres' diameter of 975 × 909 kilometers. Salacia has a companion, Actaea, that is also good-sized: 303 ± 35 kilometers. That's bigger than Hyperion but smaller than Mimas. The two are separated by about 5600 kilometers, with a period of 5.5 days. The orbit gives you a measurement of the mass of the whole system, about 4.6 · 1020 kilograms, and that gives you a density, about 1.2 grams per cubic centimeter, only slightly denser than water ice.

It's interesting to compare this system to Orcus and Vanth. In terms of size, they're quite similar; the total system mass of Salacia-Actaea is three-quarters that of Orcus and Vanth.  Vanth is likely more than a third Orcus' diameter; the Salacia-Actaea pair is similar. But Salacia and Actaea orbit each other much more closely than Orcus and Vanth, which are separated by 9000 kilometers.

How does Salacia fit in the size ranking of trans-Neptunian objects? Based on what we know now, it's somewhere between seventh and twelfth. The objects that are definitely larger than Salacia are (in order of absolute magnitude) Eris, Pluto, Haumea, Makemake, Sedna, and 2007OR10. Stansberry lists objects whose size is similar (within uncertainty) to Salacia: these include Orcus, Quaoar, 2002MS4, 2003 AZ84, and 2002TC302. There may be more of these that I'm not aware of. (Aside: are there any professionals out there keeping up with TNO size estimates being published in the literature? The only online list I'm aware of is Mike Brown's, but he doesn't seem to have kept up with the zillions of papers published in 2012 already. Someone should start a TNO size wiki.)

Salacia's dark surface is unusual among large trans-Neptunian objects, but not unheard of. Stansberry mentions that 2002MS4 and 2003 AZ84 are similarly sized and also dark, with albedo of 5.1 and 6.5 percent, respectively. (This is dark, but not as dark as Salacia.) Still, most larger objects are also brighter; Orcus has an albedo of 27%, and the biggest ones like Makemake, Haumea, Pluto, Eris, and Triton (which is often lumped among the Kuiper belt objects because it likely originated there and looks like them) are icy white. The bright icy surfaces of the objects bigger than 1000 kilometers in diameter suggest past surface geologic activity. The wide variety of surface properties among intermediate-sized objects 700 to 1000 kilometers in diameter suggests to Stansberry that "evolutionary processes can lead to either high or low albedo, with the outcome depending on the details of the history of individual objects."

In the conclusion, Stansberry and coworkers list a number of ways in which Salacia is exceptional. It's one of the largest trans-Neptunian objects. It's the darkest one in this (intermediate to large) size range. It is also the least dense of the intermediate to large-sized objects whose densities are known (although it should be pointed out that the range of possible densities significantly overlaps the range for Orcus).  After that, they allow themselves to speculate a bit. They suggest that the low density and dark color might mean that Salacia looks more like a primordial trans-Neptunian object, one of the planetesimals from which the solar system's planets were constructed.

The "history of individual objects" can have a profound effect on their appearance. Just look at similarly sized Io, Europa, Ganymede, and Callisto, all of which formed at the same distance from the Sun from similar materials to see how individual history can make objects unique. How I wish we could visit all these distant moon-sized things beyond Neptune to see their unique faces and learn their individual histories!

Postscript: It seems like every time I mention Pluto, people who are obsessed with the definition of the word "planet" take over the comments. That discussion is not relevant here. Don't be Troll #4. I will delete any comments on that topic.

 
See other posts from June 2012

 

Or read more blog entries about: dwarf planets beyond Neptune, trans-neptunian objects, Hubble Space Telescope, Spitzer Space Telescope, planetary astronomy

Comments:

Chris Radcliff: 06/26/2012 12:48 CDT

Fascinating. You mention that the TNO size lists are few and mostly out-of-date; is the data on these objects still too new to be in browsing-friendly sources like Eyes on the Solar System? I'd love to get a sense of how difficult it might be to do a "TNO grand tour".

Emily: 06/26/2012 01:00 CDT

It's really hard to keep up with the literature. Different methods of estimating diameter produce different results with different uncertainties, and you need to keep track of all of these to determine a likely diameter. For most things we don't have any reliable measurement of diameter; diameters are estimated by assuming an albedo and using the magnitude to calculate the size. But since albedo in the Kuiper belt varies from 3 to 99 percent, this is fraught with peril. And if the object is a binary but you don't know it, the "diameter" is a pseudodiameter of a body that has the same surface area as the two smaller bodies.

Chris Laurel: 06/26/2012 01:39 CDT

Great article! I thought I knew my way around the Solar System pretty well, but I'd never heard of this pair of TNOs before. Does the paper by any chance a complete set of orbital elements for Actaea? The only ones I could find seem outdated, with the semi-major axis estimated at 3440 km. I'm curious when eclipse season falls for Salacia and Actaea.

emily: 06/26/2012 01:55 CDT

Chris: They report two sets, for mirror-image orbits. Here's one of them. period 5.493863 plusmn 0.000095 semimajor axis 5570 plusmn 40 eccentricity 0.0124 plusmn 0.0036 i 124.28 plusmn 0.39 mean long at epoch 323.22 plusmn 0.93 long of ascending node 81.92 plusmn 0.67 long of periapsis 285 plusmn 16 epoch 2454300.0 (2007 July 18 12:00 UT)

Chris Laurel: 06/26/2012 02:15 CDT

Thanks, Emily. That's exactly what I was hoping for, except... what's the reference plane? The ecliptic?

Emily: 06/26/2012 02:45 CDT

In a footnote it says "elements are for secondary relative to primary. The average sky plane residual is 0.5 mas for both orbits." Is that enough information for you?

Chris Laurel: 06/26/2012 05:00 CDT

I'm not sure if I can infer from that footnote that the elements are relative to the sky plane at epoch, or whether it's purely a statement about measurement error. But thanks for sharing the data. I'm trying to make a visualization, and the semi-major axis and eccentricity--both independent of the reference plane--are the most important figures.

Bob Johnston: 06/26/2012 08:26 CDT

I've got an on line list of reported TNO/Centaur diameter estimates here: http://www.johnstonsarchive.net/astro/tnodiam.html By my count there are published (or pre-published) estimates for 96 TNOs and 22 Centaurs. Note that for Pluto and Eris there are stellar occultation constraints on size.

Nigel Gunn: 06/27/2012 08:24 CDT

A very interesting read, thanks. Also a great resource from Bob Johnston.

Les: 06/29/2012 11:03 CDT

As the saying goes, "size isn't everything". As an amateur astronomer, I've been enjoying the challemnge of photographing TNOs (Haumea and Makemake to date). What I'd like to see is a list of APPARENT magnitudes for these objects. My observatory is in the centre of a town (pop. approx. 200,000) but I reckon I can already image down to about magnitude 18.5 (19.0 on a truly clear night). I'd like to know how many TNOs are potentially in range for me.

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