Hitoshi Mizutani

Thermal history of comets during residence in the Oort Cloud: Effect of radiogenic heating in combination with the very low thermal conductivity of amorphous ice

The thermal history of cometary nuclei during residence in the Oort cloud is studied with the use of the very low thermal conductivity of amorphous ice recently obtained by Kouchi et al. [1992a]. The heat sources included are (1) radioactive nuclides 40K, 232Th, 235U, and 238U with their chondritic abundances, and (2) latent heat released in transition from amorphous ice to crystalline ice. We model the cometary nucleus as a porous aggregate of grains with each individual grain being composed of a refractory core and an icy mantle. It is assumed that the ice is initially amorphous. The bulk thermal conductivity of a cometary nucleus is assumed to be expressed by the product of the thermal conductivity of individual grains and a reduction factor resulting from the porous structure of the nucleus. Numerical results of the thermal history are presented for various conditions including one case which includes heating by 26Al decay. It is shown that the thermal histories are clearly classified into two distinct types depending mainly on the nucleus thermal conductivity κ. (1) Comets with small κ experience a runaway increase in the internal temperature to higher than 120 K during residence in the Oort cloud, in which case most of the ice in the nucleus crystallizes. (2) Comets with a sufficiently large κ on the other hand, do not exhibit a runaway heating and the temperature is limited to < 100 K so that the initial amorphous ice is almost completely preserved. A criterion of nuclear ice crystallization is presented in an analytic expression derived from the analysis of the physical processes of the crystallization. A brief discussion is given on the implications of the results for the sources of volatile molecules observed in the coma.

The SELENE mission: Goals and status

Abstract SELENE (Selenological and Engineering Explorer) mission is planned in 2005 for lunar science and technology development. The mission will consist of a main orbiting satellite at about 100 km altitude in near-polar circular orbit and two subsatellites in elliptical orbits with apolunes at 2400 km and 800 km. The scientific objectives of the mission are: 1) study of the origin and evolution of the Moon, 2) measurement of the lunar environment, and 3) observation of the solar-terrestrial plasma environment. SELENE will carry 14 scientific instruments for mapping of lunar topography and surface composition, measurement of the magnetic fields, and observation of the lunar and solar-terrestrial plasma environment. The mission period will be one year. If extra fuel is available, the mission will be extended.

LUNAR-A mission : Goals and status

Abstract The Institute of Space and Astronautical Science (ISAS), Japan, plans to launch the LUNAR-A mission in 2004. The scientific objective of the mission is to explore the lunar interior using seismometry and heat-flow measurements. Two penetrators containing two seismometers (horizontal and vertical components) and heat-flow probes will be deployed from a spacecraft onto the lunar surface, one on the nearside and another on the farside of the moon. The seismic observations are expected to provide key data on the size of the lunar core, as well as data on deep lunar mantle structure. The heat flow measurements at two penetrator landing sites will also provide important data on thermal structure and bulk concentrations of heat-generating elements in the moon. Combining these data, we will be able to obtain much stronger geophysical constraints on the origin and evolution of the moon than has ever been obtained.

A new scaling law of the planetary magnetic fields

Abstract A new scaling law for the planetary magnetic field strengths is obtained assuming the magnetostrophic balance. The velocity of the convection current υ c in a planetary core is estimated by the geometric mean of the possible maximum and minimum values to be υ c = c(Ω/4πμσ) 1 2 , where Ω is angular velocity of the core rotation, σ the electric conductivity, c speed of light, and μ the magnetic permeability. Overall agreement of the prediction by the present new scaling law with the data on magnetic field strengths of various planets is superior to those by previously proposed scaling laws. The present scaling law is also found to predict well the Neptunes magnetic field recently determined by the Voyager 2 observation. Success of the present scaling law on the planetary magnetic field strength suggests that the magnetostrophic balance may be a good approximation for planetary dynamo and that toroidal magnetic field is dominant over the poloidal field in the core.

Development of the heat flow measurement system by the LUNAR-A penetrators

Abstract Lunar heat flow experiment is planned by using two LUNAR-A penetrators which will be deployed on the near-side and far-side of the lunar surface in 2000. Each penetrator has seven absolute and eleven relative temperature sensors. Impact experiments for real-size penetrator models onto a lunar-regolith analogue target confirmed that the sensors and electronics used in the Lunar-A Heat Flow Experiment can survive the shock loading expected during penetration of the penetrator in a lunar regolith. The calibration experiment demonstrates that the temperature sensors have a resolution of 0.01 degrees and that the thermal conductivity device have 10 % accuracy. In order to determine the heat flow value, we need a good thermal model and numerical simulation for the penetrator and the regolith which in turn requires accurate measurements of thermal properties of the penetrators components. The current numerical models indicate that we will be able to obtain the lunar heat flow values within 20 to 30 percents in precision with this method.