Paper out tomorrow: http://www.bbc.co.uk/news/science-environment-25849871
Very exciting that we will visit this world soon!

Comet Piazzi ?

paper just out in Nature: Localized sources of water vapour on the dwarf planet (1) Ceres

Thank you for the heads up.

Hydroxyl. OH have previously been found to make up the main part of Ceres extremely rarefied atmosphere.
So finding water vapour is not completely unexpected, it might be the source of the hydroxyl after water molecules have been dissociated by UV-radiation from the Sun.
BBC stated 6 kg per second, and the diagram of the paper suggest it only happens when Ceres is in certain parts of its orbit. So I place my bet that its a slow sublimation, even though -35 C as the warmest summer day isn't exactly balmy.

Neh, don't think so. Mars loses plenty of water vapor to space, too.

Will anything be detectable from the camera at high phase angles, like Cassini and the Enceladean plumes, or will they have to rely on GRaND and VIR? A few kilos a second isn't that much...

Dunno if there will be anything visibly detectable at all; kinda doubt it. 6 kg/sec outflow isn't very much, not much more than an average bathtub faucet flow. If that's a global rate rather than highly localized it's damn near gotta be just sublimation.

Could we get FedEx to deliver a mass spectrometer to Dawn post haste?

6 kg/s sounds uninteresting but it's 518 tons per day. I don't know exact numbers for Enceladus but I saw somewhere number 200 - 250 kg/s.
So maybe with luck, Dawn will be capable of detecting Cerean atmosphere with his camera or VIR spectrometer.

Nprev, the ESA article states:





Super interesting!

Andy

I stand corrected, yet skeptical. Always happy to be proved wrong, of course.

Visibility will depend on the exact nature of the sources to some extent, won't it? I mean that a diffuse source will be much less visible than a concetrated, point-like one?
While this isn't exactly a discovery on par with finding a duplicate Earth hiding behind the Moon, this is a nice little appetite whetter for Ceres. And it does confirm that there is probably a fair bit of water ice there.

As the Ceres orbit has an eccentricity of 0,080 and an orbital period of 4,6 years, is it possible that Dawn spacecraft will be able to stay in Ceres orbit until its perihelion and so, to tentatively observe the surface spots from where this water vapour is originated?

According to the Wikipedia page, Ceres has an axial tilt of about 3 degrees, similar to Jupiter and Venus.

Ceres' last perihelion was September 2013, just four months ago. Aphelion will be at the end of 2015 and the next perihelion in April 2018.

Dawn will arrive there in late March 2015 and (IIUC) currently has a nominal one-year mission. So it will certainly be there through aphelion. It would have to hang around another two years for perihelion. I don't see anything that would prohibit this, though some orbital manipulation may be required -- lower orbits burn fuel faster, as the spacecraft has to make more adjustments.

If the water vapor production really is driven by sublimation -- a reasonable assumption, but who knows -- then we'd expect it to be peaking in the months after perihelion when Ceres' surface is warmest, i.e. right around now. So in that sense Dawn would be arriving at exactly the wrong time. But we really don't know. We'll start getting respectable imaging of Ceres' surface a couple of months before Dawn arrives, i.e. January 2015. So... we wait.


Doug M.

An interesting question I haven't seen addressed: what's happening to the water?

Obviously most of it is being lost to space. But even with Ceres' weak gravity, you'd expect some water to recondense at "cold traps" -- cooler spots on Ceres' surface, i.e. in shadowed crater bottoms and at the poles. The technical term for this is "volatile transport", and we see it in some other places in the Solar System, like Jupiter's moon Callisto. (That's why all the impact craters on Callisto's icy surface look slumped and eroded.) If even 1% of the water vapor were to be recaptured, it would accumulate in the polar regions at a rate of a micron or two per year. That may not sound like much, but over astronomical time you'd see meters of accumulated condensation.

But we don't see that. Ceres is a very dark body (its albedo is around 0.06, almost as dark as fresh asphalt) and water frost is bright. There are lighter patches on Ceres, but there don't seem to be bright ice caps at the poles.

In a little while we'll know more. Patience...


Doug M.

Speaking of behaviors of processes over astronomical timeframes always runs the risk of ignoring the nature of relatively short-term phenomenah which occur in bursts and blurps. (For example, the overall process of accretion over a 4.6-billion-year period resulted in the Earth-Moon system, but the day of the Big Whack created, in mere hours, the conditions resulting in the current system, its angular momentum, Earth's rotational period, etc.)

If non-homogeneous concentrations of volatiles exist within Ceres' outer crust, maybe water ice spurts and sublimates for a few thousand years and then stops, then later methane ices do the same thing over a few thousand years, etc. Each episode of volatile transport would have its own effect on the surface coatings at the poles and in other cold traps, depending on the specific volatiles being transported and how they react to sunlight and radiation over time, etc.

Ceres being so much closer to the Sun than the Jovian and Saturnian moons, it's hard to make direct comparisons, but it's possible that Ceres has (or had) a wider range of volatiles than the moons we've observed, and definitely sees a higher solar constant than do the outer planet moons. These would seem to be important factors, too.

As you say, though, much will become more clear as we approach Ceres with Dawn and get some of the hard data that will let us answer some of these questions.

-the other Doug

@Doug M

Previous to this observation, there had been one observation that found small amounts of water vapour over the north pole.
That lead to a speculation that there might be polar caps of water ice.

These older observations get a mention in the Dawn mission page at NASA even, and also at this AAS meeting quoted here.

So cold traps at the poles is a clear possibility, even actual polar caps - the question is the amount. The Dawn mission page use the phrase "seasonal polar caps" whereas the later paper states "substantial ice deposits on shadowed crater slopes ".

Any chance of telling if these localized sources correlate with the bright spots in Hubble images?

According to the map in the Nature paper, yes they do (scroll down to figures).
http://www.nature.com/nature/journal/v505/...ature12918.html

Map of supposed water vapour spots:

http://www.nature.com/nature/journal/v505/...ature12918.html


http://www.esa.int/Our_Activities/Space_Sc...rf_planet_Ceres

Recent (march 2015) map:

http://www.scientificamerican.com/slidesho...res-slide-show/

Corresponding features:

Click to view attachment

I don't know how to check if the two images use same coordinates system.

Recent paper:
"THE POTENTIAL FOR VOLCANISM ON CERES DUE TO CRUSTAL THICKENING AND PRESSURIZATION OF A SUBSURFACE OCEAN."
http://www.hou.usra.edu/meetings/lpsc2015/pdf/2831.pdf


I can't yet understand where this heat come from. Does "[e.g. 2-5]" mean "references from [2] to [5]"?.



So the model suggests NOT that there's enough heating to create a water geyser (as I supposed), but there's a "squeezing" of the ocean by the ice crust.






but beware of

Here my try:

Assumptions:
Ceres' radius: r = 475 km = 4.75e5 m
Ceres' shape: approximately spherical
Ceres' mass: m = 9.43e20 kg
Ceres' mean surface temperature: T1 = 168 K
Temperature below ice layer (using the melting point of water): T2 = 273 K
Thickness of ice layer: L = 25 km = 25e3 m
Approximated thermal conductivity of ice: k = 2.0 W/(m K)

Simplified assumption for K-40 decay: 1.311 MeV per atom are released on decay to Ca-40 = 1.311e6 * 1.602e-19 J = 2.10e-13 J
Half-life of K-40: t_1_2 = 1.248e9 years = 3.938e16 s
Mass of 1 mole K-40: 0.03996 kg
Ratio of K-40 to K total: 120 ppm = 1.2e-4

Calculations:
Surface area of Ceres: A = 4 pi r² = 4 pi * 475 km = 2.84e6 km² = 2.84e12 m²
Thermal conductance of the ice layer: G = k A / L = 2.0 W/(m K) * 2.84e12 m² / 25e3 m = 227e6 W/K.
Transfered power: P = G * (T2 - T1) = 227e6 W/K * (273 K - 168 K) = 227e6 W/K * 105 K = 23.8e9 W = 23.8 GW

Decaying ratio of K-40 per second: (1 - (1/2)^(1s/t_1_2))/s = (1 - (1/2)^(1s/3.938e16 s))/s = (1 - (1/2)^2.5391e-17)/s = 1.76e-17 / s
Mean power per K-40 atom per second = (1.76e-17 / s) * 2.10e-13 J = 3.696e-30 J/s = 3.696e-30 W

Number of K-40 atoms to provide transferred power: 23.8e9 W / 3.696e-30 W = 6.44e39 = 1.07e16 * 6.022e23 = 1.07e16 mole.
Mass of K-40 to provide transferred power: 1.07e16 * 0.03996 kg = 4.276e14 kg.

Mass ratio of K-40 needed to provide transferred power: 4.276e14 kg / 9.43e20 kg = 4.53e-7.
Mass ratio of K needed to provide transferred power 4.53e-7 / 1.2e-4 = 3.78e-3 = 0.378%

As a comparison: Potassium makes up about 2.6% of the weight of Earth's crust.

Links to data, notions, and formulas:
http://en.wikipedia.org/wiki/Thermal_conductivity
http://en.wikipedia.org/wiki/Thermal_conduction
http://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29
http://en.wikipedia.org/wiki/List_of_thermal_conductivities

http://en.wikipedia.org/wiki/Potassium
http://en.wikipedia.org/wiki/Isotopes_of_potassium
http://en.wikipedia.org/wiki/File:Potassiu...ecay-scheme.svg
http://en.wikipedia.org/wiki/Potassium-40

http://en.wikipedia.org/wiki/Electronvolt
http://en.wikipedia.org/wiki/Mole_(unit)
http://en.wikipedia.org/wiki/Sphere#Surface_area

http://en.wikipedia.org/wiki/Half-life#For...ponential_decay

Nice work there Gerald, really good even.
I really tried to find a mistake there, but you seem to have made a good estimate.
A layer of liquid water might be possible, especially if there's small amounts of other radioactive elements adding to the energy budget as well.
Before reading the paper mcgyver linked, I never did take the pressurisation into account, so I capitulate to the idea that Ceres indeed could have one subsurface ocean - but I still not saying it's there, even though I bet 'some' website featuring space related news quite likely will make a bold statement of 'discovery' any day after this. =)

Thanks a lot for the review! There is always a risk to make a mistake with this lot of numbers.

So you thank me for trying to show you're wrong.
Well serious, it's a good back-of-the-envelope kind of calculation to show the idea is worth considering.

To actually get to the bottom of things (silly pun intended) one have to go quite further to include the pressure of the water down there, some numbers provided in the paper.
That gives how the water might rise in the tube even in the very low gravity of Ceres. It is at that point these guys from the Planetary Science Institute, CIT and JPL adds the fact that there should be heavier material on top of the ice sheet covering Ceres that increase the pressure further. Then adding at least a partial melting of ice, at the highest part the tube.

Here I am lazy and enter my own calculations made for the aquifers in a lime rock environment that keeps flowing trough the winter in sub arctic conditions.
the flow of water here is a magnitude lower in general, and the strongest flow I got just barely is the same ballpark (4 litres/s) as the measurements made by Herschel.

But I still don't get this to work, if water have frozen out and it ended up with one of a handful of pockets with water of very high salinity - perhaps.
Or more heat is needed, meaning the presence of heat produced by the decay of some other long lived elements.
Salts also would prevent freezing in the tube, and even melt some ice of lower salinity in the lower parts.
And at that point I realise that some press releases have been mentioning salts and not ice as one explanation for the bright spots.

They certainly have done the calculations better than me, even so it was one interesting exercise to get a glimpse of how the planetary scientists have been thinking.

Coincidence or not, that paper appears to have been released on March 1, more than two weeks ahead of next week's Lunar and Planetary Science Conference, and a day after I proposed the same volcanism-driving mechanism in post #542, on the 'Dawn approaches Ceres' topic.

They make no mention of salt deposits though, as a consequence of the release of seawater, hence I expect we'll see evidence of that in spectral results from Dawn. Final confirmation of volcanism should occur by May or June, when central pits show up in those bright spots.

"Final confirmation of volcanism should occur by May or June, when central pits show up in those bright spots."

...could occur, if central pits show up...

Phil

It looks like there's actually "something high" under the bright spot!

hipass of the baked dem ( 0 to 360 mapping)


http://www.unmannedspaceflight.com/index.p...st&p=218867

Having read that paper I'll float (he, he, he.. geddit.. no?)an idea that may be rediculously naive: wouldn't a major impact be good way for a pre existing a crack partway through the crust (due to the mechanism proposed in the paper) to be opened up and form a vent? The paper mentions impacts as a source of fracturing sans tensile cracking, why not combine them?

Some ceres maps I found around can help studying the bright spots:




Near infrared mapping of Ceres surface
http://www.spaceref.com/news/viewpr.html?pid=21032


The composite albedo deviation maps of Ceres through F555W (upper panel), F330W (middle panel), and F220W (lower panel) filters
http://eclipsephase.com/ceres-surface-geography

And my 3d reconstruction of bright-spots crater (4 MB animated gif png):

http://i.imgur.com/c7WRvT7.png

STL file:
http://lc84.altervista.org/vba4.stl

The DEM has got to be considered very preliminary near "feature 5" (ie the brightest bright spot). For one thing we may be seeing more PSF than real structure, which would affect the calculation of elevations. For another, if, as recent reports claim, the brightness of feature 5 varies with time, that could also affect the DEM, since you'd normally assume that to be constant when calculating the elevations.

How is it calculated? And what does PSF stand for?

the first result when you google it
en.m.wikipedia.org/wiki/Point_spread_function

Basically you would combine images from different viewpoints (different frames from the rotation sequences). That contains depth information just like an anaglyph does. But if the details of the bright spot aren't reliable then the elevations you get also aren't reliable.

Well, then the DEM data in that area are totally useless. :-(

Better focusing, then, on tweets about #LPSC2015 in these days: there's a lot of talking on Ceres, and one tweet on the fact that the bright spot is visible above the rim (in unreleased images) and hence is probably a "geyser" ( http://twitter.com/Laurent_Montesi/status/...0697387008?s=02 )

Nature article on Ceres about icy plume over bright spot:
http://www.nature.com/news/bright-spots-on...=TWT_NatureNews

Great find on the DEM -- I'm really surprised to see it released!

If a DEM is calculated from image pairs, I think positional accuracies are no better than some small number of pixels, right? Which is to say it's not really worthwhile to use this one to analyze the detailed rim and floor structure of the crater that the bright feature is in.

I'm not sure how these DEMs were produced, but in principal all the images from the rotation sequence could contribute simultaneously to the DEM, potentially yielding sub-pixel resolution of the topography. Also, the shadowing along the terminator might contribute further. Or it could be all shape-from-shading, in which case take with a grain of salt. Tracking common features in multiple images is tricky and error prone. Anyway, I agree that the bright spots are indeed so poorly resolved that I'd hesitate to make much of their DEMs.

The basic rule is always going to be... don't expect reliable results when you are looking at things close to the resolution of the data. For images, we can't interpret reliably if we have fewer than 4 or 5 pixels across the object of interest - and even more would be better. Stereo DEMs are even more removed from the raw data (the images), so their effective resolution is lower still. Their resolution will be determined by the spacing between common points, the identifiable points matched in the two images so parallax can be measured. If you look at the images you can get a sense of how far apart those points would be.

So please don't read too much into tiny details in images or DEMs.

Phil

the dem was extracted from the vertex plate model
baked the displacement using blender
and compensated for the difference in radi of the mesh and sphere

ftp://naif.jpl.nasa.gov/pub/naif/DAWN/misc/ceres
the README
ftp://naif.jpl.nasa.gov/pub/naif/DAWN/mis...s/aaareadme.txt
a 32 bit isis3 cub is here
https://drive.google.com/file/d/0B6ZYAd08tZ...iew?usp=sharing

To clarify... I was referring to the photogrammetry leading to the creation of the shape (or vertex plate) model. Given that model, the DEM is simply the difference between the shape model and the corresponding ellipsoid fit. Eventually that will be a geoid fit, but these are early days.

From Science Daily website:
https://www.sciencedaily.com/releases/2016/...60316082727.htm

Polar ice found on Ceres, if this ice is trapped there after outgassing of Ceres itself is not clearly stated on this page from Max Planck institute, though I have a hunch it might be the case.

Haze at Occator crater
on dwarf planet Ceres

https://arxiv.org/ftp/arxiv/papers/1701/1701.05812.pdf

p

I examined the original haze claim and found it to be complete nonsense:

https://arxiv.org/abs/1701.08550

Thanks for posting that here. Figure 15 on page 40 of the PDF version you linked to gives the clearest picture I've seen of the central white spot in Occator crater. (For anyone tempted to stop at the abstract or just skim the less technical parts of the conclusions I would say: don't miss the illustrations in that paper!)