* Astronomy

Title: Observations of Comets C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3 (WISE), C/2010 S1 (LINEAR), and C/2012 K6 (McNaught) at large heliocentric distances
Author: Oleksandra Ivanova, Lubo Nesluan, Zuzana Seman Kriandova, Jan Svore, Pavlo Korsun, Viktor Afanasiev, Volodymyr Resetnyk, Maxim Andreev

Photometric and spectroscopic observations of five nearly parabolic comets with eccentricity larger than 0.99 at heliocentric distances greater than 4 AU were performed. No molecular emission was observed for any studied comet and the entire cometary activity in all cases was attributed to dust production. Upper limits of the gas production rates for the main neutral molecules in the cometary comae were calculated. The derived values of dust apparent magnitudes were used to estimate the upper limit of the geometric cross-section of cometary nuclei (upper limits of radii range from 2 km to 28 km). Due to the poor sublimation of water ice at these distances from the Sun, other mechanisms triggering activity in comets are discussed.

Read more (1457kb, PDF)

Why Comets Are Like Deep Fried Ice Cream

Astronomers tinkering with ice and organics in the lab may have discovered why comets are encased in a hard, outer crust.
Using an icebox-like instrument nicknamed Himalaya, the researchers show that fluffy ice on the surface of a comet would crystalize and harden as the comet heads toward the sun and warms up. As the water-ice crystals form, becoming denser and more ordered, other molecules containing carbon would be expelled to the comet's surface. The result is a crunchy comet crust sprinkled with organic dust.
Read more

Comets take pole position as water bearers

The tide of an ongoing debate about whether comets or asteroids supplied most of Earth's water has turned back to comets with the discovery that the Hartley 2 comet has a similar ratio of heavy water to ordinary water as Earth.
Paul Hartogh of the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany, and his team measured the comet's ratio of deuterium to hydrogen using the European Space Agency's Herschel Space Observatory.
Read more 

Title: Photophoretic transport of hot minerals in the solar nebula
Authors: A. Moudens, O. Mousis, J.-M. Petit, G. Wurm, D. Cordier and S. Charnoz

Context. Hot temperature minerals have been detected in a large number of comets and were also identified in the samples of Comet Wild 2 that were returned by the Stardust mission. Meanwhile, observations of the distribution of hot minerals in young stellar systems suggest that these materials were produced in the inner part of the primordial nebula and have been transported outward in the formation zone of comets.
Aims. We investigate the possibility that photophoresis provides a viable mechanism to transport high-temperature materials from the inner solar system to the regions in which the comets were forming.
Methods. We use a grid of time-dependent disk models of the solar nebula to quantify the distance range at which hot minerals can be transported from the inner part of the disk toward its outer regions as a function of their size (10^-5 to 10^-1 m) and density (500 and 1000 kgmV^-3). These models will also yield information on the disk properties (radius of the inner gap, initial mass, and lifetime of the disk). The particles considered here are in the form of aggregates that presumably were assembled from hot mineral individual grains ranging down to submicron sizes and formed by condensation within the hottest portion of the solar nebula. Our particle-transport model includes the photophoresis, radiation pressure, and gas drag.
Results. Depending on the postulated disk parameters and the density of particles, 10^-2 to 10^-1 m aggregates can reach heliocentric distances up to  ~35 AU in the primordial nebula over very short timescales (no more than a few hundred thousand years). 10^-3 m particles follow the same trajectory as the larger ones but their maximum migration distance does not exceed  ~26 AU and is reached at later epochs in the disks. On the other hand, 10^-5 to 10^-4 m aggregates are continuously pushed outward during the evolution of the solar nebula. Depending on the adopted disk parameters, these particles can reach the outer edge of the nebula well before its dissipation.
Conclusions. Our simulations suggest that irrespective of the employed solar nebula model, photophoresis is a mechanism that can explain the presence of hot temperature minerals in the formation region of comets. Comets probably had the time to trap the dust transported from the inner solar system either in their interior during accretion or in the form of shells surrounding their surface if they ended their growth before the particles reached their formation location.

Read more