Máté Ádámkovics

Overview of the coordinated ground-based observations of Titan during the Huygens mission

Coordinated ground-based observations of Titan were performed around or during the Huygens atmospheric probe mission at Titan on 14 January 2005, connecting the momentary in situ observations by the probe with the synoptic coverage provided by continuing ground-based programs. These observations consisted of three different categories: (1) radio telescope tracking of the Huygens signal at 2040 MHz, (2) observations of the atmosphere and surface of Titan, and (3) attempts to observe radiation emitted during the Huygens Probe entry into Titans atmosphere. The Probe radio signal was successfully acquired by a network of terrestrial telescopes, recovering a vertical profile of wind speed in Titans atmosphere from 140 km altitude down to the surface. Ground-based observations brought new information on atmosphere and surface properties of the largest Saturnian moon. No positive detection of phenomena associated with the Probe entry was reported. This paper reviews all these measurements and highlights the achieved results. The ground-based observations, both radio and optical, are of fundamental importance for the interpretation of results from the Huygens mission.

Valley formation and methane precipitation rates on Titan

Branching valley networks near the landing site of the Huygens probe on Titan imply that fluid has eroded the surface. The fluid was most likely methane, which forms several percent of Titans atmosphere and can exist as a liquid at the surface. The morphology of the valley networks and the nature of Titans surface environment are inconsistent with a primary valley formation process involving thermal, chemical, or seepage erosion. The valleys were more likely eroded mechanically by surface runoff associated with methane precipitation. If mechanical erosion did occur, the flows must first have been able to mobilize any sediment accumulated in the valleys. We develop a model that links precipitation, open-channel flow, and sediment transport to calculate the minimum precipitation rate required to mobilize sediment and initiate erosion. Using data from two monitored watersheds in the Alps, we show that the model is able to predict precipitation rates in small drainage basins on Earth. The calculated precipitation rate is most sensitive to the sediment grain size. For a grain diameter of 1–10 cm, a range that brackets the median size observed at the Huygens landing site, the minimum precipitation rate required to mobilize sediment in the nearby branching networks is 0.5–15 mm hr^(−1). We show that this range is reasonable given the abundance of methane in Titans atmosphere. These minimum precipitation rates can be compared with observations of tropospheric cloud activity and estimates of long-term methane precipitation rates to further test the hypothesis that runoff eroded the valleys.

Fluvial features on Titan: Insights from morphology and modeling

Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.

Observations of Rotationally Resolved C3 in Translucent Sight Lines

The rotationally resolved spectrum of the A^1Π_u ← X^1Σ^+_g 000-000 transition of C_3, centered at 4051.6 A, has been observed along 10 translucent lines of sight. To interpret these spectra, a new method for the determination of column densities and analysis of excitation profiles involving the simulation and fitting of observed spectra has been developed. The populations of lower rotational levels (J ≤ 14) in C_3 are best fitted by thermal distributions that are consistent with the kinetic temperatures determined from the excitation profile of C_2. Just as in the case of C_2, higher rotational levels (J > 14) of C_3 show increased nonthermal population distributions in clouds that have been determined to have total gas densities below ~500 cm^(-3).

FORMATION OF ORGANIC MOLECULES AND WATER IN WARM DISK ATMOSPHERES

Observations from Spitzer and ground-based infrared spectroscopy reveal significant diversity in the molecular emission from the inner few AU of T Tauri disks. We explore theoretically the possible origin of this diversity by expanding on our earlier thermal-chemical model of disk atmospheres. We consider how variations in grain settling, X-ray irradiation, accretion-related mechanical heating, and the oxygen-to-carbon ratio can affect the thermal and chemical properties of the atmosphere at 0.25-40 AU. We find that these model parameters can account for many properties of the detected molecular emission. The column density of the warm (200-2000 K) molecular atmosphere is sensitive to grain settling and the efficiency of accretion-related heating, which may account, at least in part, for the large range in molecular emission fluxes that have been observed. The dependence of the atmospheric properties on the model parameters may also help to explain trends that have been reported in the literature between molecular emission strength and mid-infrared color, stellar accretion rate, and disk mass. We discuss whether some of the differences between our model results and the observations (e.g., for water) indicate a role for vertical transport and freezeout in the disk midplane. We also discuss how planetesimal formation in the outer disk (beyond the snowline) may imprint a chemical signature on the inner few AU of the disk and speculate on possible observational tracers of this process.

Titan imagery with Keck adaptive optics during and after probe entry

[1] We present adaptive optics data from the Keck telescope, taken while the Huygens probe descended through Titan’s atmosphere and on the days following touchdown. No probe entry signal was detected. Our observations span a solar phase angle range from 0.05� up to 0.8� , with the Sun in the west. Contrary to expectations, the east side of Titan’s stratosphere was usually brightest. Compiling images obtained with Keck and Gemini over the past few years reveals that the east-west asymmetry can be explained by a combination of the solar phase angle effect and an enhancement in the haze density on Titan’s morning hemisphere. While stratospheric haze was prominent over the northern hemisphere, tropospheric haze dominated the south, from the south pole up to latitudes of � 45� S. At 2.1 mm this haze forms a polar cap, while at 1.22 mm it appears in the form of a collar at 60� S. A few small clouds were usually present near the south pole, at altitudes of 30–40 km. Our narrowband J,H,K images of Titan’s surface compare extremely well with that obtained by Cassini ISS, down to the small-scale features. The surface contrast between dark and bright areas may be larger at 2 mm than at 1.6 and 1.3 mm, which would imply that the dark areas may be covered by a coarser-grained frost than the bright regions and/or that there is additional 2 mm absorption there.

X-ray Ionization of Heavy Elements Applied to Protoplanetary Disks

The consequences of the Auger effect on the population of heavy-element ions are analyzed for the case of relatively cool gas irradiated by keV X-rays with intended applications to the accretion disks of young stellar objects. Highly charged ions are rapidly reduced to the doubly charged state in neutral gas, so the aim here is to derive the production rates for these singly and doubly charged ions and to specify their transformation by recombination, charge transfer, and molecular reactions. The theory is illustrated by calculations of the abundances of 11 of the most cosmically abundant heavy elements in a model of a typical T Tauri star disk. The physical properties of the gas are determined with an X-ray irradiated thermal-chemical model, which shows that the disk atmosphere consists of a hot atmosphere overlaying the mainly cool body of the disk. There is a warm transition layer where hydrogen, carbon, and oxygen are transformed from atomic to molecular species and the ionization drops by several orders of magnitude. The doubly charged ions are then largely confined to the hot outer layers of the disk. The nature of the dominant ions below the transition depends sensitively on the poorly constrained abundances of the heavy elements. Observational consequences and connections with the active layers of the magnetorotational instability are briefly discussed.

DISCOVERY OF FOG AT THE SOUTH POLE OF TITAN

While Saturns moon Titan appears to support an active methane hydrological cycle, no direct evidence for surface-atmosphere exchange has yet appeared. The indirect evidence, while compelling, could be misleading. It is possible, for example, that the identified lake features could be filled with ethane, an involatile long-term residue of atmospheric photolysis; the apparent stream and channel features could be ancient remnants of a previous climate; and the tropospheric methane clouds, while frequent, could cause no rain to reach the surface. We report here the detection of fog at the south pole of Titan during late summer using observations from the VIMS instrument on board the Cassini spacecraft. While terrestrial fog can form from a variety of causes, most of these processes are inoperable on Titan. Fog on Titan can only be caused by evaporation of nearly pure liquid methane; the detection of fog provides the first direct link between surface and atmospheric methane. Based on the detections presented here, liquid methane appears widespread at the south pole of Titan in late southern summer, and the hydrological cycle on Titan is currently active.

Jupiter's Deep Cloud Structure Revealed Using Keck Observations of Spectrally Resolved Line Shapes

Technique: We present a method to determine the pressure at which significant cloud opacity is present between 2 and 6 bars on Jupiter. We use: a) the strength of a Fraunhofer absorption line in a zone to determine the ratio of reflected sunlight to thermal emission, and b) pressure-broadened line profiles of deuterated methane (CH3D) at 4.66 microns to determine the location of clouds. We use radiative transfer models to constrain the altitude region of both the solar and thermal components of Jupiters 5-micron spectrum. Results: For nearly all latitudes on Jupiter the thermal component is large enough to constrain the deep cloud structure even when upper clouds are present. We find that Hot Spots, belts, and high latitudes have broader line profiles than do zones. Radiative transfer models show that Hot Spots in the North and South Equatorial Belts (NEB, SEB) typically do not have opaque clouds at pressures greater than 2 bars. The South Tropical Zone (STZ) at 32 degrees S has an opaque cloud top between 4 and 5 bars. From thermochemical models this must be a water cloud. We measured the variation of the equivalent width of CH3D with latitude for comparison with Jupiters belt-zone structure. We also constrained the vertical profile of water in an SEB Hot Spot and in the STZ. The Hot Spot is very dry for P<4.5 bars and then follows the water profile observed by the Galileo Probe. The STZ has a saturated water profile above its cloud top between 4 and 5 bars.

SHIELDING BY WATER AND OH IN FUV AND X-RAY IRRADIATED PROTOPLANETARY DISKS

We present an integrated thermal-chemical model for the atmosphere of the inner region of a protoplanetary disk that includes irradiation by both far-ultraviolet (FUV) and X-ray radiation. We focus on how the photodissociation of H{sub 2}O and OH affects the abundances of these and related species and how it contributes to the heating of the atmosphere. The dust in the atmosphere plays several important roles, primarily as the site of H{sub 2} formation and by absorbing the FUV. Large amounts of water can be synthesized within the inner 4 AU of a disk around a typical classical T Tauri star. OH is found primarily at the top of a warm region where the gas temperature is T {sub g} ≈ 650-1000 K and H{sub 2}O is found below it, where the temperature is lower, T {sub g} ≈ 250-650 K. The amounts of H{sub 2}O and OH and the temperatures of the regions in which they formed are in agreement with recent Spitzer measurements and support the notion of the in situ production of water in the inner regions of protoplanetary disks. We find that the synthesized water is effective in shielding the disk mid-plane from stellar FUV radiation.