* Astronomy

A new name has been accepted by the IAU for the inner ring, 1986 U 2R, of the Uranian system discovered by the Voyager spaceprobe in 1986
IAUC 8826.

The following names have been approved for rings of Uranus:
Mu (for R/2003 U 1)
Nu (for R/2003 U 2)
Lambda (for 1986 U1R)

Source

Title: Liquid Water Oceans in Ice Giants
Authors: Sloane J. Wiktorowicz, Andrew P. Ingersoll

Aptly named, ice giants such as Uranus and Neptune contain significant amounts of water. While this water cannot be present near the cloud tops, it must be abundant in the deep interior. We investigate the likelihood of a liquid water ocean existing in the hydrogen-rich region between the cloud tops and deep interior. Starting from an assumed temperature at a given upper tropospheric pressure (the photosphere), we follow a moist adiabat downward. The mixing ratio of water to hydrogen in the gas phase is small in the photosphere and increases with depth. The mixing ratio in the condensed phase is near unity in the photosphere and decreases with depth; this gives two possible outcomes. If at some pressure level the mixing ratio of water in the gas phase is equal to that in the deep interior, then that level is the cloud base. Alternately, if the mixing ratio of water in the condensed phase reaches that in the deep interior, then the surface of a liquid ocean will occur. We find that Neptune is both too warm (photospheric temperature too high) and too dry (mixing ratio of water in the deep interior too low) for liquid oceans to exist at present. To have a liquid ocean, Neptune's deep interior water to gas ratio would have to be higher than current models allow, and the density at 19 kbar would have to be ~ 0.8 g/cm^3. Such a high density is inconsistent with gravitational data obtained during the Voyager flyby. As Neptune cools, the probability of a liquid ocean increases. Extrasolar "hot Neptunes," which presumably migrate inward toward their parent stars, cannot harbour liquid water oceans unless they have lost almost all of the hydrogen and helium from their deep interiors.

Source (583kb, PDF)

This NASA Hubble Space Telescope image is a never-before-seen astronomical alignment of a moon traversing the face of Uranus, and its accompanying shadow. The white dot near the centre of Uranus’ blue-green disk is the icy moon Ariel. The 700-mile-diameter satellite is casting a shadow onto the cloud tops of Uranus. To an observer on Uranus, this would appear as a solar eclipse, where the moon briefly blocks out the Sun as its shadow races across Uranus’s cloud tops.


Exand (78kb, 800 x 799)
Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

Though such "transits" by moons across the disks of their parents are commonplace for some other gas giant planets, such as Jupiter, the satellites of Uranus orbit the planet in such a way that they rarely cast shadows on the planet's surface. Uranus is tilted so that its spin axis lies nearly in its orbital plane. The planet is essentially tipped over on its side. The moons of Uranus orbit the planet above the equator, so their paths align edge-on to the Sun only every 42 years. This colour composite image was created from images at three wavelengths in near infrared light obtained with Hubble’s Advanced Camera for Surveys on July 26, 2006.

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