Salacia

Jun. 26th, 2012 01:46 pm
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
(Cross-posted from my own journal.)

According to recent research, the trans-Neptunian object Salacia seems to be about 900 km in diameter - that makes it about the same size as Ceres and Orcus and Quaoar. This is a bit of a surprise; it seems that Salacia is actually quite dark, reflecting only about 3 or 4 of the light that falls on it. Previous estimates of its size were based on assumptions that it reflected a higher percentage of light, which led to smaller estimated sizes.

Emily Lakdawalla explains in more detail here.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Mike Brown on the sizes of Eris and Pluto, and why the discovery that Eris is almost exactly the same diameter as Pluto actually tells us that Eris is more different from Pluto than expected.

Basically: We know Eris is a bit more massive than Pluto, so if it also had a bit bigger diameter than Pluto, it could have the same density, and therefore be made more or less of the same stuff. But since it's the same diameter, it must be denser, and therefore made of different stuff.
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
On Friday there were three press releases:
Number one: Subtly Shaded Map of Moon Reveals Titanium Treasure Troves. Certain parts of the moon have surprisingly large amounts of titanium. Maps of the moon in visible and ultraviolet light show which parts of the moon are high in titanium. We don't really understand why the moon has as much titanium as it does, but it does give us some clues about the history of the moon. These visible and ultraviolet light also show some interesting things about space weathering.

Number two: The cause of asteroid Scheila’s outburst. In 2010, the asteroid Scheila started showing comet-like features, including a tail. Material presented at the conference provides evidence that this was caused by an impact from a much smaller asteroid (Scheila is about 110 kilometres in diameter; the impactor is calculated to have been between 60 and 180 metres in diameter, so about a thousandth of Scheila's diameter.)

Number three: Almahata Sitta meteorites come from triple asteroid mash-up. On Oct. 7, 2008, three years to the day before the date of the press release, for the very first time an asteroid which had been detected in space and predicted to hit Earth actually did so. It landed in the Nubian Desert in Sudan, and more than 600 fragments have been retrieved. They're named Almahata Sitta. The varying composition of the fragments suggests that they belonged to a parent body that was created by relatively low-speed collisions between asteroids of different types.

And here's Emily Lakdawalla's post on Day 5 of the conference. Some of the things she talks about: the great storm on Saturn; Mars's moon Phobos is probably a rubble pile; there's good evidence that the grooves on Phobos are caused by stuff ejected by impacts on Mars; a more precise calculation than previously of the mass (and thus density) of the asteroid Lutetia; the asteroids Lutetia and Steins both have more recent large craters than we'd predict, which suggests that we may be underestimating the number of small asteroids.

Now for other bits and pieces and news stories written up by people besides me, from any part of the conference, not just the last day:

Emily Lakdawalla's notes on NASA's Planetary Night, "in which representatives from NASA's Science Mission Directorate speak to the people whose missions and research they fund about their accomplishments and the political and funding climate for the coming years". Most of the news is pretty discouraging, but there's one bright bit: that the production of Plutonium-238 is going to be restarted, so there will be a reliable power source for missions that can't use solar power.

A Whole New World at the Edge of the Solar System. About Kuiper Belt Objects, what we can learn from light curves, what they say about Haumea, what a contact binary is, and, in culmination, how the light curve of a certain Kuiper Belt Object shows that it's a contact binary, and why changes in that light curve over the past few years give an indication that such contact binaries may be common among Kuiper Belt Objects.

The Revelations of Planets' Shadows. About what we've learnt about Makemake and Quaoar from them occulting stars.

Did Earth's oceans come from comets? The water in previously observed comets has about twice as much deuterium (hydrogen with a proton and a neutron rather than just a proton) as Earth's water does. This was a problem for accounts of the origin of Earth's water that said it came from comets. But recent observations of comet Hartley 2 show that it has almost exactly the same amount of deuterium as Earth's water. It was formed in a different part of the solar system from the comets that were found to have more deuterium than Earth's water, and this probably explains why it's different from them.

When Minor Planets Ceres and Vesta Rock Earth Into Chaos. The two largest asteroids, Ceres (which is a dwarf planet as well as an asteroid) and Vesta, gravitationally affect each other; they also gravitationally affect earth, so that we can't reliably predict Earth's orbit farther into the future than about 60 million years; we also can't reconstruct the details of Earth's orbit farther back than about 60 million years.

Did Mercury and Uranus Have a Rough Youth After All? About the early history of Mercury and Uranus, and how many major impacts they received; for Mercury, it's about the question of how it got its present density while retaining relatively volatile elements like sulfur and potassium, while for Uranus it's about the idea that several major impacts gave it its present large tilt.

Pea shooter theory aims to build solar system. About a new tentative hypothesis about how the planets of the solar system formed - that rather than forming in about the regions where we now find them, they all formed initially at roughly Earth's distance (well, at least the ones outwards of Earth) and made their way outwards to their present distances.

And finally, one thing I found on Twitter but not in the articles mentioned above. Again, don't take my extractions of stuff from Twitter too seriously.

Satellites of Mars
There was a talk arguing that Phobos and Deimos were created out of the debris from a large impact on Mars - the impact would have created a debris disc around Mars, which then accreted into Phobos and Deimos. This explains their orbits better than the idea that they're captured asteroids - their orbits are too circular and too much in the same plane to be captured asteroids. Mars's rotation rate also implies that Mars got a large impact, and there are several impact basins on Mars that are the right size to account for both the spin rate and the moon formation.

Twitter source:
[twitter.com profile] kat_volk
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
Again, first more information presented by people who know more than I do:

Emily Lakdawalla's post on Day 2 includes:
- a bit about recent flyby data from the comets Hartley 2 and Tempel 1.
- size data on Makemake based on an occultation; also, results from infrared data indicate that Makemake has a surface with mixed really really bright and really really dark patches
- weird results from an occultation about Quaoar's size - it 'looked like someone had taken a bite out of it'
- a few things about Titan

Three official press releases of the day:
One on a discovery made using the Kepler spacecraft on the discovery of a star with three identified planets. The smallest has a mass of 6.9 times that of Earth, while the larger two have mass of 16-17 times that of Earth (i.e., about Neptune's mass). All three are much closer to their star than Mercury is to the sun.

A second on oddities of comet Hartley 2, including that it may be two former comets that got stuck together.

Third, and most exciting to me, is one about a global false-colour map of Titan using data from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). This is really hard to do because of Titan's atmosphere, which is opaque to most wavelengths of light, and even with the wavelengths that can penetrate it, there are still atmospheric effects, e.g. from clouds and mist.

And now for what I've managed to extract from Twitter. As with last time, I could have introduced all kinds of distortions, so take all this with a good dose of grains of salt. There was also plenty more interesting stuff that I saw but couldn't understand/contextualize well enough to put it into a post. (Lots of that was about Titan.)

Vesta
Vesta's pole star is Deneb.

Titan
Titan seems to have seasonal rain in normally dry equatorial deserts.
There's a case made for Titan having dust storms.
Does Titan have ice volcanoes? Maybe, maybe not. Lots of things that looked like volcanoes at first seem not to be on closer inspection, but there's one good candidate: Sotra Facula.

Kuiper Belt Objects
Results from Eris occulting a star were presented, but couldn't be reported on Twitter due to an embargo :-( . They'll be published in Nature on Oct. 26th.

Makemake occulted a star with of magnitude V=18.2 on April 23, 2011. They observed the occultation with 7 telescopes in 5 locations, and found a drop in magnitude of 0.4. They were able to calculate Makemake's albedo as about 0.71, which is greater than Pluto's, and is consistent with the value of 0.81 derived from measurements the infrared telescopes Spitzer and Herschel. They calculate Makemake's size based on the occultation as about 1610 +22/-180 km x 1444+/-9 km. Whether or not the occultation data indicates an atmosphere is still being worked out.

Quaoar occulted a star too, and they observed it from at least 5 sites. Quaoar has is less reflective than was previously thought; this means that it's also larger and less dense. Its diameter is now estimated as 1045-1095 km, and density at 1.95-2.75 grams per cubic centimeter. It might have a methane atmosphere with a pressure of 0.1 ubar [I think ubar is an alternate abbreviation for µbar = microbar, but I'm not certain] at a temperature of 45K.

Extrasolar planets
The Kepler spacecraft's major measurements come from planets transiting their stars, but the measurements are also affected by how the planet and star distort each other tidally. Using these tidal distortions, it's possible figure out the mass ratio between the planet and the star just from observing transits. (Neat! It always amazes me how much information astronomers can squeeze out of a seemingly tiny bit of data.)

Upcoming missions
The European Space Agency's Solar Orbiter mission has been approved. It will orbit the sun in a highly inclined orbit inward of Mercury's orbit, and is expected to launch in 2017.

Other stuff
WISE shows 4 things that might potentially be very low mass brown dwarfs orbiting the sun. But they might also be extragalactic sources of radiation. (That's a big difference!)

Sources of tweets that I've used to compile this (again, I hope I haven't left anyone out):
[twitter.com profile] DrFunkySpoon
[twitter.com profile] GovertTweets
[twitter.com profile] AllPlanets
[twitter.com profile] elakdawalla
[twitter.com profile] PlanetDr
[twitter.com profile] spacemandave
steorra: Jupiter's moon Europa (europa)
[personal profile] steorra
I've been following on Twitter some of the things being reported at the joint conference of the American Astronomical Society's Division for Planetary Sciences and the European Planetary Science Congress (DPS-EPSC). I'm sure things will come out in a more refined form at some point, but here are some of the things reported that have caught my attention.

First of all, before my own extractions of stuff from Twitter, some stuff by people who know more than I do.

Emily Lakdawalla has written up a post on first impressions, including stuff from talks on Enceladus and Vesta. Some highlights mentioned in her post are: large salty grains in Enceladus's plumes make it pretty much certain that the plumes have a liquid source; Vesta doesn't have any moons larger than 10 metres diameter; and results for the mass, volume, density, and probable core size of Vesta. Also, Vesta's surprisingly colourful, as solar system objects go.

Three official press releases:
A press release about results from Dawn's mission to Vesta. An additional interesting piece of information from it is that surface temperatures have been measured from 240K to 270K. That's roughly -30° Celsius to 0° Celsius. (I'm assuming these are only dayside measurements, and that temperatures would become much colder during the night, but the press release doesn't actually say that.) There are lots of images and diagrams linked at the bottom of the press release.

And one on 'snow' (actually really really tiny ice particles) on Enceladus.

And a press release on the Kuiper Belt Object 2001 QG298. It's a contact binary - two objects touching each other. This was figured out in 2004 because of changes in its brightness when it rotated; it was rotating nearly edge-on to us, so that one of the two objects sometimes hid the other from view. At first they didn't know if it was rotating in the same plane as its orbit, in which case it would stay nearly edge on, or at an angle to its orbit, in which case the fact that we were seeing it edge on is because we're lucky enough to catch it at the right part of its object. Since it was discovered, the changes in brightness have diminished significantly, indicating that it's rotating nearly at right angles to its orbit, and thus we were just lucky to catch it. This hints that there may actually be a lot of contact binaries out there.

And now for some interesting tidbits from Twitter, sometimes with some context added; it's quite possible that I've gotten some things wrong in processing tweets into what I say below, so it should be taken with a good dose of grains of salt. Among other things, it's often hard to be sure from a tweet whether something is a tentative suggestion or something solidly evidenced.

About Vesta:
There are three kinds of meteorites on earth thought to come from Vesta: howardites, eucrites, and diogenites (grouped together as HED meteorites). Dawn's results so far indicate that the global spectrum of Vesta is more like that of howardites than of diogenites or eucrites. Regionally, the south of Vesta is more like diogenites than the north is. (The Wikipedia article on diogenites indicates that diogenites are thought to be from deep within Vesta. In that case, it makes sense that they would be found in the south, where the giant impact would remove a lot of the overlying surface.) Also, there are east/west differences in how eucrite-like Vesta is.

Spectral variations on Vesta are often associated with geological features, and spectra indicate the presence of pyroxene.

Vesta is estimated to have regolith 1.5-5 km deep; this estimate comes from crater shapes.

Dark material on Vesta occurs in craters, but also elsewhere. Where it comes from is still uncertain. Some possibilities: carbonaceous impactors, or volcanism, or both.

Initial indications are that Vesta has less water than Earth's Moon; this will be better determined once Vesta gets into Low Altitude Mapping Orbit.

About Titan:
The carbon isotope ratio on Titan is similar to that on Earth, according to Cassini data.

Titan's upper atmosphere has lots of heavy, negative ions. No one's sure yet how they get there.

The Huygens probe measured an electrical field in Titan's atmosphere that shows that Titan's crust is 60-80 km thick.

About Kuiper Belt Objects:
On 36% of KBOs, water ice has not been detected.

Varuna's thermal (infrared) lightcurve is correlated with its visible light lightcurve. I'm not entirely sure of the significance of this but I'm guessing that one thing it indicates is that the major variations in Varuna's lightcurve are due to its shape, and not to differences in how bright different parts of it are.

Twitter accounts I used in compiling the above (I hope I haven't forgotten any):
Emily Lakdawalla
asrivkin
David Minton
Franck Marchis
Europlanet Media
Meg Schwamb

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