[meteorite-list] Chondrule close up!
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri, 18 Apr 2008 18:26:05 -0500 Message-ID: <011a01c8a1ab$935af370$8250e146_at_ATARIENGINE> Hi, Darren, List, Ceres is not a Chondrule! It's a Clay-drule! The interesting thing about the photo is that it makes Ceres look a little like Mars, doesn't it? Here's what we can say about a surface we can't see yet... until Dawn gets there in February, 2015, less than 7 years to go... So, what's on that surface? 1988: Surface roughness much greater than the Moon: "far in excess of the Lunar value..." Change in physical properties at a depth of 3 cm, is microregolith, gardened regolith, or undifferentiated. "Surface has properties of a finely divided, dry (<20% water) terrestrial clay..." http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1988AJ.....95.1263W&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf Icarus, 2006: Comes down against ammonia magnesium clays, says iron-rich clays and carbonates, dolomite, calcite http://irtfweb.ifa.hawaii.edu/~elv/icarus185.563.pdf "Tomeoka and Buseck (1985) outlined a multi-step aqueous alteration sequence for carbonaceous chondrites. Cronstedtite is the dominant alteration product partway through the sequence, which continues on to make more magnesium-rich phyllosilicates as the iron enters magnetite. Further laboratory studies also found cronstedtite to be more indicative of earlier rather than later stages of alteration (Browning et al., 1996). If Ceres is dominated by iron-rich rather than magnesium-rich clays, and if there really is, or was, a subsurface ocean, the implication is that the surface of Ceres did not experience aqueous alteration to completion, despite the apparent availability of large amounts of water, and that its surface had limited exchange with the interior. This is consistent with the modeling of McCord and Sotin (2005) which predicts ice in the outer 10 km of Ceres would remain frozen, although they note that the frozen crust would be gravitationally unstable and likely overturn, melt, and refreeze. A more detailed consideration of aqueous alteration in the outermost portion of Ceres' crust is beyond the scope of this work." The important point: the surface is fresh and is renewed faster than carbonaceous chondrites equilibrate. It has geologic re-cycling on a short geological time scale. The frozen crust overturns? They interpret this as a lack of geological activity but it could just as easily be a result of continous geological activity! (See below.) 2005: http://www.cosis.net/abstracts/EGU05/10256/EGU05-J-10256.pdf "...the spectrum of Ceres at UV-visible is characterized by an absorption band centered at about 280 nm, with the width of about 100 nm, and about 30% of the reflectance at 555 nm. This absorption feature could be due to a charge transfer or a semi-conductor band, and is present in the spectra of many iron-bearing minerals and salts. The roughness of Ceres is unusually high, and not consistent with the results measured from the ground [4], but consistent with what is inferred by IR observations [7]. Radar observations also indicate a surface that is very rough at scales larger than meters or tens of meters but very smooth at centimeter to decimeter scales [8]. Therefore, if the high roughness is real, the surface of Ceres must be made of very smooth materials, either like the surface of ice or deposited by very fine grained particles, but saturated with craters at the sizes of tens of meters to kilometers, even while it is globally relaxed. Degraded craters with smooth clay "mudflats" with a little snow drifted over it? 2005: The Europeans think it's "blacktopped"! http://www.aanda.org/index.php?option=article&access=standard&Itemid=129&url=/articles/aa/full/2005/24/aa2506-04/aa2506-04.html "The 3 m absorption feature was not detected, implying the absence of OH and/or H2O-bearing minerals on the asteroid surface at the latitude of our observations. The spectrum of 1 Ceres was obtained in the 2.0-4.1 m range and the presence of the 3.06 m absorption feature confirmed. Laboratory measurement of ion-irradiated organics and ices suggest that the 3.06 m feature can be reproduced with a linear mixture of crystalyne ice and residues of ion-irradiated asphaltite." Important to point out here that crystaline ice is an indication of "fresh" ice, otherwise known as SNOW. I don't know what the skiing is like, but it snows, so there must be ice geysers, fog, mist, clouds, something like that, on Ceres. The "asphalite" could be degraded "tholins;" see: http://en.wikipedia.org/wiki/Tholin Since tholins are reddish, they may account for the red tint we see in those images. 1999: http://ntrs.nasa.gov/search.jsp?R=615886&id=3&qs=Ne%3D20%26N%3D4294841293%26Ns%3DHarvestDate%257C0 "Thermal emission spectra of the largest asteroid 1 Ceres obtained from the Kuiper Airborne Observatory display features that may provide information on its surface mineralogy. A plot of the Ceres spectrum (calibrated using alpha Boo as a standard) divided by a standard thermal model (STM) is shown. Also shown is the emissivity spectrum deduced from reflectivity measurements for olivine grains less than 5 microns in diameter. The general shape of the Ceres and the olivine curves agree in essential details, such as the maxima from 8 to 12 microns, the minimum between 12 and 14 microns, the broad peak near 17.5 micron, and the slope beyond 22 micron. (Use of the 10 to 15-micron grain reflectivities provides a better match to the 12- to 14-micron dip. We used a value of unity for beta, the beaming factor associated with small-scale surface roughness in our STM. Adjustment of beta to a lower value raises the long-wavelength side of the Ceres spectrum, providing an even better match to the olivine curve.) The emissivity behavior roughly matches the emission coefficients which were calculated for olivine particles with a particle radius of 3 microns. Their calculations show not only the negative slope from 23 to 25 pm, but a continued decrease past 30 micron. The Ceres emissivity is thus similar to that of small olivine grains from 8 to 30 micron, but olivine's emissivity is lower from 5 to 8 pm." The surface also has small olivine grains. Does that mean erosion of surface rock? Because if there were just olivine grains laying about, the presence of water would rapidly zap'em! So there must be continuous and reasonable rapid erosion of olivine rock... More evidence of active geology... There is a word for very small mineral grains. They're called SAND. So far, we have clay, mud, rock, snow... and sand. For comparison with another ice-rock (or rock-ice) body, but much colder, the radar smoothness of Ganymede: http://adsabs.harvard.edu/abs/1975Sci...188.1211G "Radar cross-section measurements indicate that Ganymede scatters to earth 12 percent of the power expected from a conducting sphere of the same size and distance. This compares with 8 percent for Mars, 12 percent for Venus, 6 percent for Mercury, and about 8 percent for the asteroid Toro. Furthermore, Ganymede is considerably rougher (to the scale of the wavelength used, 12.6 centimeters) than Mars, Venus, or Mercury. Roughness is made evident in this experiment by the presence of echoes away from the center of the disk. A perfectly smooth target would reflect only a glint from the center, whereas a very rough target would reflect power from over the entire disk." and 1979: http://ntrs.nasa.gov/search.jsp?R=904383&id=4&qs=No%3D10%26N%3D4294934111%26Ns%3DHarvestDate%257C0 "Radar observations of asteroid 1 Ceres were made at a 12.6-cm wavelength from the Arecibo Observatory in March- April 1977. The measurements, made with a received circular polarization orthogonal to that transmitted, yield a radar cross section of (0.04 or- 0.01) piR-squared, for R = 510 km. The corresponding radar reflectivity is less than that measured for any other celestial body. Within the accuracy of measurement, no significant variation of cross section with rotational phase is apparent. The shape of the power spectrum suggests that Ceres is rougher at the scale of the observing wavelength than the moon and inner planets, but smoother than the outer three Galilean satellites." Ceres' scatter is 6%, smoother than Mars or the asteroid Toro on the 0.12-meter scale (four inches). General stuff about Ceres: Astronomy magazine, September, 2005: http://www.astronomy.com/asy/default.aspx?c=a&id=3478 A good article on Ceres, it's internal structure, and the shape of the planet in hydrostatic balance. Centauri Dreams: http://www.centauri-dreams.org/?p=1162 Another good article (shorter) on Ceres, April, 2007. McCord and Sotin, 2007: Evolution of Ceres http://www.lpi.usra.edu/meetings/lpsc2007/pdf/2006.pdf The definite (?) paper on the planetary evolution of Ceres July, 2007: Ceres As An Abode of Life: http://www.spacedaily.com/reports/Ceres_As_An_Abode_Of_Life_999.html Now, there's an idea! What's it going to look like? Vostok Lake, Antarctica, in a Synthetic Aperture Radar image. The deep waters are clearly revealed by their effect on the surface topography. Could Ceres look like this? This is an icy surface that has a thermal cycle somewhat similar to Ceres in its range, that is, the climate, as far as temperature goes, at Vostok is roughly the same as Ceres at the equator in the summer. Tropical! http://www.esa.int/images/ASAR_Vostok_lake_13Mar2003_L.jpg Pictures! Big one: http://www.spacedaily.com/images/ceres-hubble-2004-desk-1280.jpg http://www.shatters.net/~impulse/pictures/ceres.jpg http://apod.nasa.gov/apod/image/0608/ceres_hst_big.jpg 2006: <http://cat.inist.fr/?aModele=afficheN&cpsidt=17743823> "The highest resolution (pixel scale 30 km) images of Ceres date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23-29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be 0.087 ? 0.003, 0.056 ? 0.002, and 0.039 ? 0.003, respectively. V-band geometric albedo is calculated to be 0.090 ? 0.003, consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090-1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be 0.070 ? 0.002, 0.046 ? 0.002, and 0.032 ? 0.003, respectively. The photometric roughness of Ceres' surface is found to be about 44? ? 5? from photometric modeling using Hapke's theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I, Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113-133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40-350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites." And, pleasantly, one very strong spectral feature cannot be identified, so there's something else there... Always good. Maybe it's the tholins. Summarize this? Ceres has an active surface with a slow but ongoing hydrological cycle. It shows exposed rock that is eroded to what is (in effect) olivine sand. It snows there. At the equator at noon in the summer, it gets warm enough for liquid water to briefly exist before evaporating; it may have thin clouds. Blocks of icy crust get overturned, then eroded, creating landscape relief, or maybe it's all craters, or maybe it's both. The most of the (solid) surface is clay; there is something else an inch or so down, or a change in density, to mudstones, siltstones, or something else (?). There are organics on the surface with all the potential that implies. My last prediction? It's not going to look like any planetary surface we've already seen. It's going to be a whole new thing. Sterling K. Webb ----------------------------------------------------------------------------------- ----- Original Message ----- From: "Darren Garrison" <cynapse at charter.net> To: <meteorite-list at meteoritecentral.com> Sent: Friday, April 18, 2008 4:17 PM Subject: Re: [meteorite-list] Chondrule close up! On Fri, 18 Apr 2008 23:11:58 +0200, you wrote: >Folks! > >There is only one word to say: WOW!!!! This chondrule is so awesome, if >you don't have seen this fantastic chondrule till now, you should write >Tom a short note. It's a truly must see. Agreed, amazing chondrule-- in fact, it is a chon-drool. Meanwhile, chondrule far away!: http://media.skyandtelescope.com/images/Ceres_f.jpg ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list at meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list Received on Fri 18 Apr 2008 07:26:05 PM PDT |
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