All instruments passed their reviews with flying colors. The review of the fifth instrument, the framing camera, will take place in September. Much of the work on the camera will now take place at the Max Planck Institut fur Aeronomie (MPAe) in Katlenburg-Lindau.

In conjunction with the new division of labor, Dawn welcomes two new members from MPAe to its science team, Uwe Keller, who will lead the camera development and Uli Christensen, MPAe Director, who will assist with the science planning and analysis.

This summer we also completed testing of the solar cells that will power the Dawn spacecraft and its ion propulsion system. Mission success depends greatly on the efficiency of these cells. When the spacecraft journeys away from the Sun, the illumination drops and the arrays cool.

The cooler arrays are more efficient, compensating somewhat for the drop in illumination, but the combined effects have not previously been well characterized. Since most outer solar system missions have used radioactive thermal generators, the data on this low intensity, low temperature effect (LILT) is sparse. Fortunately, our LILT testing confirmed the efficiencies that were assumed when Dawn was proposed to NASA.

The project opted to add a fifth solar panel to each wing to increase Dawn's power margin at Ceres (at which time the spacecraft could reach 2.9 AU). The project has also been asked to increase its financial reserve, necessitating other mission trades and descopes.

Since the mission is well along in its design phase, there are limited options to do this. Our optimized solution to meet this challenge is to launch on a standard Delta 2925, rather than a heavy, using Mars for a gravity assist.

This mission plan also provides much needed calibration data for the instruments, and an early test of the science operations and data analysis systems. There will be some delay in arrival at Vesta but the very interesting southern polar region will still be illuminated.

After acquiring its Vesta observations, the spacecraft departs for Ceres. This plan requires some rephasing of the budget profile and has not yet been approved by NASA HQ, but we are hoping and expecting it will, and that Dawn will move into the implementation phase late in the calendar year.

Ceres Evolution and Current State: A Summary
by Dawn Co-investigator, Tom McCord Univ. of Hawaii and and Christophe Sotin Laboratory de Planetologie et Geodynamique France

Ceres orbits the sun and is large enough to have experienced many of the processes normally associated with planetary evolution. Therefore, it should be called a planet. Ceres probably survived from the earliest stages of solar system formation, when its sibling objects probably became the major building blocks of the Earth and the other terrestrial inner planets. Thus, Ceres is an extremely important object for understanding the early stages of the solar system as well as basic planetary processes.

Ceres apparently retains considerable volatile material. The latest gross properties indicate that Ceres has a density of about 2100 kilograms per cubic meter, suggesting that the body's composition may be half water. Its density is similar to that of Ganymede (1940 kg/m3) and Callisto (1860 kg/m3).

Observational evidence also points to a wet Ceres. Its reflectance spectrum contains a 3-�m absorption interpreted to be due to OH and perhaps 2O in aqueously altered material such as clays and hydrated salts similar to CI and CM, i.e. primitive, carbonaceous chondrite (CC) meteorites. A 3.1-�m absorption also exists that suggests water ice or, alternatively, HN4-bearing minerals such as saponite reported in aqueous alteration products in CV and CI carbonaceous chondrites. Further, International Ultraviolet Explorer (IUE) spectroscopic observations of a 3080A emission at the northern limb of Ceres suggests the OH molecule, indicating the production of H2O from Ceres is in the range of 1024 to 1025 sec-1, which is a flux that could be sustained over a long period from a subsurface ice layer.

This inference suggests that Ceres may harbor active chemistry that produced evolved materials, considering that it was heated, is still wet and likely started with primitive materials rich in silicates, organics, water and perhaps other volatile materials. Ceres, located between Mars and the Galilean satellites, is the perfect place to study the evolution of volatile rich objects at the interface between our hometown terrestrial planets and colder but volatile-richer outer solar system objects.

We modeled the thermal evolution of Ceres to find out what Dawn might find when it orbits this small planet. We built on the vast literature on the state and evolution of solar nebula material and on meteorites and the earlier thermal modeling work by Hap McSween (a Dawn team member), who explored how the aqueous alteration seen in CC meteorites could occur in the parent bodies early in their histories. In an article submitted to J. Geophys. Res. We describe the probable active thermal evolution of Ceres and its favorable environment for fertile wet chemistry, and the probability that Dawn will find very interesting materials and landforms as evidence of these processes.

Even with a low-energy model - cold accretions, no effect from 26Al or other short-lived radionuclides - and using only energy from long-lived radionuclides with abundance derived from study of carbonaceous chondrite meteorites and a uniform mixture of 75% silicates and 25% water ice, we find that the water ice in Ceres must have quickly melted and continued to circulate, transporting heat by convection and preserving a nearly isothermal mantel approximately 100 km thick. A crust does not melt because it is too conductive. The circulating warm water would alter the silicates, leading to carbonaceous chondrite-like compositions. As heat is lost by conduction through the unmelted crust, water begins to freeze out at the base of the crust [Figure 1]. When the crust reaches a thickness of about 28 km, solid-state convection in the ice-rich crust would become continuous, transporting more heat as well as altered materials to near the surface. Ceres' water layer eventually freezes after about 2 billion years, forming a layered density structure. Additional possible sources of heat, including short-lived radionuclides and exothermal mineralization, enhance the melting of water ice, extend the lifetime of the liquid water mantel, and alter the temperature profiles with depth and time, but they do not produce total melting of the crust or of the silicate core. The frozen crust lid would contain most of the volatiles and altered materials, except that which reached the surface by convection, volcanism and impacts, and preserve evidence of the differ-entiation and the chemistry, perhaps including hydrates, hydroxolates and evolved organics. In addition, melting and freezing plus mineralization would, over time, create topographic features. Thus, present day compositional units and topography on Ceres' and its internal structure, should be of considerable help in constraining Ceres' history and thereby the evolution of the protoplanets in general.

Some specific and intriguing possibilities come to mind. First, NH4-bearing compounds have been suggested to explain the 3.1-�m absorption feature of the surface material. Such materials might have been incorporated at formation. Significant presence would lower the melting point of the liquid water sufficiently to extend the lifetime of the liquid layer even to the present. Second, clathrates could have formed. Considerable carbonaceous material was likely present at formation. Interior thermal processing and mixing with the freezing water could create methane and CO2 clathrates. If these materials were brought near the surface, they would volatilize and could produce explosive release of gases, bringing considerable quantities of altered materials to the surface, and creating surface expressions that Dawn might detect.

Third, Ceres would have shrunk as water formed from ice. Then, as the water froze, Ceres would have expanded, and present day features should be those associated with tension, such as cracks, faults and dropped blocks. Topographic features on the surface could be formed that will be visible to Dawn. Finally, there is the possible evolution of the carbon compounds in addition to formation of methane and CO2. Could more complex compounds have formed?

Ceres appears to be a complicated object in some ways similar to the outer Galilean satellites. With so much water present and the energy to distribute it in liquid form, Ceres probably experienced complex chemistry at least in its interior, perhaps including organic materials. It is further likely that expressions of these processes and materials made it to the surface, at least in places. Dawn may well discover and analyze these. Vesta, the other object on Dawn's itinerary, is much denser, dryer and has extensively melted. Pallas, being denser, also seems to have evolved further and lost more water or it may have had less to start. A major unanswered question is why such different objects as Ceres, Vesta and Pallas, all protoplanets, could have evolved so differently yet so close together in the Solar System. Dawn may give us some answers by visiting these two very different objects.

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