Elisabeth Ann McFarlane

Rain, winds and haze during the Huygens probe's descent to Titan's surface

The irreversible conversion of methane into higher hydrocarbons in Titans stratosphere implies a surface or subsurface methane reservoir. Recent measurements from the cameras aboard the Cassini orbiter fail to see a global reservoir, but the methane and smog in Titans atmosphere impedes the search for hydrocarbons on the surface. Here we report spectra and high-resolution images obtained by the Huygens Probe Descent Imager/Spectral Radiometer instrument in Titans atmosphere. Although these images do not show liquid hydrocarbon pools on the surface, they do reveal the traces of once flowing liquid. Surprisingly like Earth, the brighter highland regions show complex systems draining into flat, dark lowlands. Images taken after landing are of a dry riverbed. The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent. The number density of haze particles increases by a factor of just a few from an altitude of 150 km to the surface, with no clear space below the tropopause. The methane relative humidity near the surface is 50 per cent.

Element partitioning between Mg-perovskite, magnesiowüstite, and silicate melt at conditions of the Earth's mantle

Magnesium-perovskite and magnesiowustite are the dominant phases in the lower mantle of the Earth. We have determined experimentally element partitioning behavior between Mg-perovskite and silicate melt and between magnesiowustite and silicate melt at 24.5 (±0.5) GPa and 2400 (±100)°C using a mantle composition (KLB-1) doped with trace elements. Average Mg-perovskite/melt partition coefficients and 2σ uncertainty estimates from three experiments are: Mg = 1.0 (±0.1), Al = 1.2 (±0.3), Si = 1.17 (±0.05), Ca = 0.3 (±0.3), Ti = 1.3 (±0.3). Average magnesiowustite/melt partition coefficients and 2 σ uncertainty estimates from two experiments are: Mg = 2.2 (±0.2), Al = 0.36 (±0.07), Si = 0.005 (±0.005), Ca = 0.01 (±0.02). Titanium is below detection. Trace element partition coefficients from a sample doped with V, Cr and Mn are: Mg-perovskite—V = 1.14 (±0.06), Cr = 1.1 (±0.1), Mn = 0.62 (±0.08); magnesiowustite—V = 0.45 (±0.01), Cr = 1.27 (±0.03), Mn = 0.77 (±0.06). The ratios of partition coefficients for some of the refractory and moderately refractory lithophile elements listed above deviate significantly from unity. If Mg-perovskite or Mg-perovskite and magnesiowustite segregated from a terrestrial magma ocean, ratios of these elements (Al/Ti, Ca/Ti, Ca/Al, Mg/ Si, Si/Al) would have been imparted upon the primitive mantle of the Earth which are inconsistent with inferred values based on analyses of naturally occurring samples. Olivine addition into the upper mantle can be invoked to eliminate this constraint for some elements (i.e., Mg/Si), but will exacerbate the problem for other elements (i.e., Si/Ca, Si/Ai). If magnesiowustite segregated alone, ratios of these elements (Mg/ Al, Mg/Ca, Si/Ca, Mg/Si, Si/Ai) would be inconsistent with inferred values. There is no surviving evidence of mineral fractionation implying that either (a) the Earth was never substantially molten at the end of accretion; (b) segregation of minerals from magma was suppressed by vigorous convection or some other mechanism; or (c) evidence of fractionation was subsequently destroyed by solid-state mantle convection. The abundances of V, Cr and Mn in the mantles of the Earth and Moon are very similar to each other and differ from all other sampled differentiated planetary bodies. The origin of this similarity is currently unknown, but is not attributable to differentiation of metallic or silicate phases at low pressures. Our results show that this abundance pattern cannot result from high pressure and temperature fractionation of Mg-perovskite and/or magnesiowustite at conditions relevant to the uppermost lower mantle of the Earth.

The Descent Imager/Spectral Radiometer (DISR) Experiment on the Huygens Entry Probe of Titan

The payload of the Huygens Probe into the atmosphere of Titan includes the Descent Imager/Spectral Radiometer (DISR). This instrument includes an integrated package of several optical instruments built around a silicon charge coupled device (CCD) detector, a pair of linear InGaAs array detectors, and several individual silicon detectors. Fiber optics are used extensively to feed these detectors with light collected from three frame imagers, an upward and downward-looking visible spectrometer, an upward and downward looking near-infrared spectrometer, upward and downward looking violet phtotometers, a four-channel solar aerole camera, and a sun sensor that determines the azimuth and zenith angle of the sun and measures the flux in the direct solar beam at 940 nm. An onboard optical calibration system uses a small lamp and fiber optics to track the relative sensitivity of the different optical instruments relative to each other during the seven year cruise to Titan. A 20 watt lamp and collimator are used to provide spectrally continuous illumination of the surface during the last 100 m of the descent for measurements of the reflection spectrum of the surface. The instrument contains software and hardware data compressors to permit measurements of upward and downward direct and diffuse solar flux between 350 and 1700 nm in some 330 spectral bands at approximately 2 km vertical resolution from an alititude of 160 km to the surface. The solar aureole camera measures the brightness of a 6° wide strip of the sky from 25 to 75° zenith angle near and opposite the azimuth of the sun in two passbands near 500 and 935 nm using vertical and horizontal polarizers in each spectral channel at a similar vertical resolution. The downward-looking spectrometers provide the reflection spectrum of the surface at a total of some 600 locations between 850 and 1700 nm and at more than 3000 locations between 480 and 960 nm. Some 500 individual images of the surface are expected which can be assembled into about a dozen panoramic mosaics covering nadir angles from 6° to 96° at all azimuths. The spatial resolution of the images varies from 300 m at 160 km altitude to some 20 cm in the last frames. The scientific objectives of the experiment fall into four areas including (1) measurement of the solar heating profile for studies of the thermal balance of Titan; (2) imaging and spectral reflection measurements of the surface for studies of the composition, topography, and physical processes which form the surface as well as for direct measurements of the wind profile during the descent; (3) measurements of the brightness and degree of linear polarization of scattered sunlight including the solar aureole together with measurements of the extinction optical depth of the aerosols as a function of wavelength and altitude to study the size, shape, vertical distribution, optical properties, sources and sinks of aerosols in Titans atmosphere; and (4) measurements of the spectrum of downward solar flux to study the composition of the atmosphere, especially the mixing ratio profile of methane throughout the descent. We briefly outline the methods by which the flight instrument was calibrated for absolute response, relative spectral response, and field of view over a very wide temperature range. We also give several examples of data collected in the Earths atmosphere using a spare instrument including images obtained from a helicopter flight program, reflection spectra of various types of terrain, solar aureole measurements including the determination of aerosol size, and measurements of the downward flux of violet, visible, and near infrared sunlight. The extinction optical depths measured as a function of wavelength are compared to models of the Earths atmosphere and are divided into contributions from molecular scattering, aerosol extinction, and molecular absorption. The test observations during simulated descents with mountain and rooftop venues in the Earths atmosphere are very important for driving out problems in the calibration and interpretion of the observations to permit rapid analysis of the observations after Titan entry.

MG‐perovskite/silicate melt and majorite garnet/silicate melt partition coefficients in the SYSTEM CaO ‐ MgO ‐ SiO2 at high temperatures and pressures

We report the results of a Mg-perovskite/silicate melt and a majorite garnet/silicate melt elemental partitioning experiment conducted at high pressures and temperatures in the synthetic system CaO - MgO - SiO2. Partition coefficients for some refractory lithophile elements (e.g., Sc, Sm) deviate significantly from unity. If Mg-perovskite segregated from a terrestrial magma ocean, it would impart nonchondritic refractory lithophile element ratios such as Sc/Sm to the primitive upper mantle of Earth, contrary to inference based on analyses of naturally occurring samples. Either Earth was never substantially molten at the end of accretion, or segregation of minerals from magmas was suppressed by vigorous convection. It is difficult to avoid melting Earth during accretion so the latter alternative seems more plausible.

Descent imager/spectral radiometer (DISR) instrument aboard the Huygens probe of Titan

The Huygens probe of the atmosphere of Saturns moon Titan includes one optical instrument sensitive to the wavelengths of solar radiation. The goals of this investigation fall into four broad areas: 1) the measurement of the profile of solar heating to support an improved understanding of the thermal balance of Titan and the role of the greenhouse effect in maintaining Titans temperature structure; 2) the measurement of the size, vertical distribution, and optical properties of the aerosol and cloud particles in Titans atmosphere to support studies of the origin, chemistry, life cycles, and role in the radiation balance of Titan played by these particles; 3) the composition of the atmosphere, particularly the vertical profile of the mixing ratio of methane, a condensable constituent in Titans atmosphere; and 4) the physical state, composition, topography, and physical processes at work in determining the nature of the surface of Titan and its interaction with Titans atmosphere. In order to accomplish these objectives, the Descent Imager/Spectral Radiometer (DISR) instrument makes extensive use of fiber optics to bring the light from several different sets of foreoptics to a silicon CCD detector, to a pair of InGaAs linear array detectors, and to three silicon photometers. Together these detectors permit DISR to make panoramic images of the clouds and surface of Titan, to measure the spectrum of upward and downward streaming sunlight from 350 to 1700 nm at a resolving power of about 200, to measure the reflection spectrum of >= 3000 locations on the surface, to measure the brightness and polarization of the solar aureole between 4 and 30 degrees from the sun at 500 and 935 nm, to separate the direct and diffuse downward solar flux at each wavelength measured, and to measure the continuous reflection spectrum of the ground between 850 and 1600 nm using an onboard lamp in the last 100 m of the descent.