Timothy Austin Livengood

Alien Maps of an Ocean-Bearing World

When Earth-mass extrasolar planets first become detectable, one challenge will be to determine which of these worlds harbor liquid water, a widely used criterion for habitability. Some of the first observations of these planets will consist of disc-averaged, time-resolved broadband photometry. To simulate such data, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper, we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning 1 day, taken at gibbous phases of 57° and 77°, respectively. As expected, the time-averaged spectrum of Earth is blue at short wavelengths due to Rayleigh scattering, and gray redward of 600 nm due to reflective clouds. The rotation of the planet leads to diurnal albedo variations of 15%-30%, with the largest relative changes occurring at the reddest wavelengths. To characterize these variations in an unbiased manner, we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only two dominant eigencolors. We use the time variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment, we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.

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.

Direct measurement of winds on Titan

We report the first direct measurement of wind velocity in the atmosphere of Titan, one of only two examples in our solar system of a slowly-rotating body with a dense atmosphere and a prime target of the Cassini mission. Zonal wind velocity was determined from Doppler shift of ethane lines emitted from Titans stratosphere (∼0.1–7 mbar) measured by infrared heterodyne spectroscopy near 12 µm (λ/Δ λ ≥ 106). Prograde zonal circulation, in the direction of global rotation, is established with 94% statistical confidence. Results provide information regarding Titan meteorology constraining dynamical theories for slowly-rotating bodies, provide otherwise unobtainable data to optimize the Cassini Huygens Probe investigations, and demonstrate the capability for remotely measuring winds on a small, distant object.

Earth as an Extrasolar Planet: Earth Model Validation Using EPOXI Earth Observations

The EPOXI Discovery Mission of Opportunity reused the Deep Impact flyby spacecraft to obtain spatially and temporally resolved visible photometric and moderate resolution near-infrared (NIR) spectroscopic observations of Earth. These remote observations provide a rigorous validation of whole-disk Earth model simulations used to better understand remotely detectable extrasolar planet characteristics. We have used these data to upgrade, correct, and validate the NASA Astrobiology Institutes Virtual Planetary Laboratory three-dimensional line-by-line, multiple-scattering spectral Earth model. This comprehensive model now includes specular reflectance from the ocean and explicitly includes atmospheric effects such as Rayleigh scattering, gas absorption, and temperature structure. We have used this model to generate spatially and temporally resolved synthetic spectra and images of Earth for the dates of EPOXI observation. Model parameters were varied to yield an optimum fit to the data. We found that a minimum spatial resolution of ∼100 pixels on the visible disk, and four categories of water clouds, which were defined by using observed cloud positions and optical thicknesses, were needed to yield acceptable fits. The validated model provides a simultaneous fit to Earths lightcurve, absolute brightness, and spectral data, with a root-mean-square (RMS) error of typically less than 3% for the multiwavelength lightcurves and residuals of ∼10% for the absolute brightness throughout the visible and NIR spectral range. We have extended our validation into the mid-infrared by comparing the model to high spectral resolution observations of Earth from the Atmospheric Infrared Sounder, obtaining a fit with residuals of ∼7% and brightness temperature errors of less than 1 K in the atmospheric window. For the purpose of understanding the observable characteristics of the distant Earth at arbitrary viewing geometry and observing cadence, our validated forward model can be used to simulate Earths time-dependent brightness and spectral properties for wavelengths from the far ultraviolet to the far infrared. Key Words: Astrobiology-Extrasolar terrestrial planets-Habitability-Planetary science-Radiative transfer. Astrobiology 11, 393-408.

Correlated variations of UV and radio emissions during an outstanding Jovian auroral event

An exceptional Jovian aurora was detected in the FUV on December 21, 1990, by means of Vilspa and Goddard Space Flight Center (GFSC) International Ultraviolet Explorer (IUE) observations. This event included intensification by a factor of three between December 20 and 21, leading to the brightest aurora identified in the IUE data analyzed, and, in the north, to a shift of the emission peak towards larger longitudes (these variations are even more dramatic once the actual source brightness distribution is retrieved from the raw data). The Jovian radio emission simultaneously recorded at decameter wavelengths in Nancay also exhibits significant changes, from a weak and short-duration emission on December 20 to a very intense one, lasting several hours, on December 21. Confirmation of this intense radio event is also found in the observations at the University of Florida on December 21. The emissions are identified as right-handed Io-independent “A” (or “non Io-A”) components from the northern hemisphere. The radio source region deduced from the Nancay observations lies, for both days, close to the UV peak emission, exhibiting in particular a similar shift of the source region toward larger longitudes from one day to the next. A significant broadening of the radio source was also observed and it is shown that on both days, the extent of the radio source closely followed the longitude range for which the UV brightness exceeds a given threshold (∼3 kW m−1). The correlated variations, both in intensity and longitude, strongly suggest that a common cause triggered the variation of the UV and radio emissions during this exceptional event. On one hand, the variation of the UV aurora could possibly be interpreted according to the Prange and Elkhamsi (1991) model of diffuse multicomponent auroral precipitation (electron and ion): it would arise from an increase in the precipitation rate of ions together with an inward shift of their precipitation locus from L ≈ 10 to L ≈ 6. On the other hand, the analysis of Ulysses observations in the upstream solar wind suggests that a significant disturbance in the solar wind, involving the generation of an interplanetary shock and the presence of a CME have interacted with the Jovian magnetosphere at about the time of the auroral event. Both arguments suggest that we may have observed for the first time a magnetic storm-type interaction in an outer planet magnetosphere, affecting simultaneously several auroral processes. Conversely, the observed relationship between the level of UV auroral activity and the detection of decameter emission (DAM), if it were a typical feature, might argue in favour of a more direct and permanent association between the auroral processes leading to UV and radio aurorae, possibly related to “discrete-arc”-like activity and electron precipitation.

A SEARCH FOR ADDITIONAL PLANETS IN THE NASA EPOXI OBSERVATIONS OF THE EXOPLANET SYSTEM GJ 436

We present time series photometry of the M dwarf transiting exoplanet system GJ?436 obtained with the Extrasolar Planet Observation and Characterization (EPOCh) component of the NASA EPOXI mission. We conduct a search of the high-precision time series for additional planets around GJ?436, which could be revealed either directly through their photometric transits or indirectly through the variations these second planets induce on the transits of the previously known planet. In the case of GJ?436, the presence of a second planet is perhaps indicated by the residual orbital eccentricity of the known hot Neptune companion. We find no candidate transits with significance higher than our detection limit. From Monte Carlo tests of the time series, we rule out transiting planets larger than 1.5 R ? interior to GJ?436b with 95% confidence and larger than 1.25 R ? with 80% confidence. Assuming coplanarity of additional planets with the orbit of GJ?436b, we cannot expect that putative planets with orbital periods longer than about 3.4?days will transit. However, if such a planet were to transit, we would rule out planets larger than 2.0 R ? with orbital periods less than 8.5?days with 95% confidence. We also place dynamical constraints on additional bodies in the GJ?436 system, independent of radial velocity measurements. Our analysis should serve as a useful guide for similar analyses of transiting exoplanets for which radial velocity measurements are not available, such as those discovered by the Kepler mission. From the lack of observed secular perturbations, we set upper limits on the mass of a second planet as small as 10 M ? in coplanar orbits and 1 M ? in non-coplanar orbits close to GJ?436b. We present refined estimates of the system parameters for GJ?436. We find P = 2.64389579 ? 0.00000080 d, R ?= 0.437 ? 0.016 R ?, and Rp = 3.880 ? 0.147 R ?. We also report a sinusoidal modulation in the GJ?436 light curve that we attribute to star spots. This signal is best fit by a period of 9.01?days, although the duration of the EPOCh observations may not have been long enough to resolve the full rotation period of the star.

Colors of a Second Earth. II. Effects of Clouds on Photometric Characterization of Earth-like Exoplanets

As a test bed for future investigations of directly imaged terrestrial exoplanets, we present the recovery of the surface components of the Earth from multi-band diurnal light curves obtained with the EPOXI spacecraft. We find that the presence and longitudinal distribution of ocean, soil, and vegetation are reasonably well reproduced by fitting the observed color variations with a simplified model composed of a priori known albedo spectra of ocean, soil, vegetation, snow, and clouds. The effect of atmosphere, including clouds, on light scattered from surface components is modeled using a radiative transfer code. The required noise levels for future observations of exoplanets are also determined. Our model-dependent approach allows us to infer the presence of major elements of the planet (in the case of the Earth, clouds, and ocean) with observations having signal-to-noise ratio (S/N) 10 in most cases and with high confidence if S/N 20. In addition, S/N 100 enables us to detect the presence of components other than ocean and clouds in a fairly model-independent way. Degradation of our inversion procedure produced by cloud cover is also quantified. While cloud cover significantly dilutes the magnitude of color variations compared with the cloudless case, the pattern of color changes remains. Therefore, the possibility of investigating surface features through light-curve fitting remains even for exoplanets with cloud cover similar to Earths.

SYSTEM PARAMETERS, TRANSIT TIMES, AND SECONDARY ECLIPSE CONSTRAINTS OF THE EXOPLANET SYSTEMS HAT-P-4, TrES-2, TrES-3, and WASP-3 FROM THE NASA EPOXI MISSION OF OPPORTUNITY

As part of the NASA EPOXI Mission of Opportunity, we observed seven known transiting extrasolar planet systems in order to construct time series photometry of extremely high phase coverage and precision. Here we present the results for four hot-Jupiter systems with near-solar stars?HAT-P-4, TrES-3, TrES-2, and WASP-3. We observe 10 transits of HAT-P-4, estimating the planet radius Rp = 1.332 ? 0.052 R Jup, the stellar radius R = 1.602 ? 0.061?R ?, the inclination i = 89.67 ? 0.30?deg, and the transit duration from first to fourth contact ? = 255.6 ? 1.9 minutes. For TrES-3, we observe seven transits and find Rp = 1.320 ? 0.057 R Jup, R = 0.817 ? 0.022 R ?, i = 81.99 ? 0.30?deg, and ? = 81.9 ? 1.1 minutes. We also note a long-term variability in the TrES-3 light curve, which may be due to star spots. We observe nine transits of TrES-2 and find Rp = 1.169 ? 0.034 R Jup, R = 0.940 ? 0.026 R ?, i = 84.15 ? 0.16 deg, and ? = 107.3 ? 1.1 minutes. Finally, we observe eight transits of WASP-3, finding Rp = 1.385 ? 0.060 R Jup, R = 1.354 ? 0.056 R ?, i = 84.22 ? 0.81 deg, and ? = 167.3 ? 1.3 minutes. We present refined orbital periods and times of transit for each target. We state 95% confidence upper limits on the secondary eclipse depths in our broadband visible bandpass centered on 650 nm. These limits are 0.073% for HAT-P-4, 0.062% for TrES-3, 0.16% for TrES-2, and 0.11% for WASP-3. We combine the TrES-3 secondary eclipse information with the existing published data and confirm that the atmosphere likely does not have a temperature inversion.

Temporal monitoring of Jupiter's auroral activity with IUE during the Galileo mission. Implications for magnetospheric processes

Abstract In this study, we analyze FUV auroral spectra of Jupiter acquired on a quasi-continuous basis between August 17 and September 25, 1996 as part of the last IUE Key Project. This campaign was coordinated with Galileo measurements. We show that it is possible to define an “auroral activity index” which quantifies the variability of the flux radiated in the H2 bands (1560–1620 A). The activity indices in both hemispheres are generally similar and indicate a strong interhemispheric conjugacy, suggestive of particle precipitation on closed field lines. We identify variability on three different scales: small variations of 10–20% are observed on short time scales (∼a few hours), variations by a factor of 2–4 occur on scales of 5–10 days, and a long-term trend is observed on a scale which exceeds the 6 weeks of observations. The energy output in this spectral range is a direct measurement of the energy losses of the magnetosphere along auroral magnetic field lines. We establish here that its intermediate time-scale variations, the best documented ones in this study, are amazingly well correlated with the state (quiet or disturbed) of the magnetic field measured by the magnetometer on board Galileo, although distances from ∼15 to ∼115RJ and all local times were sampled during that time. This indicates temporal, rather than spatial, variations of the magnetic field, and suggests that some dynamical process, yet to be identified, affects the magnetosphere as a whole and triggers recurrent energy releases along auroral field lines. The recurrence time derived from this study is larger than the 3-day periodicity previously assigned to variabilities in the magnetotail on the basis of Galileo measurements during other time periods. A crude preliminary comparison also indicates correlations with the intensity of the auroral kilometer radio waves and anticorrelations with the local plasma density, both derived from the Galileo PWS experiment. Comparison with other particle and field instruments is underway.

Rotational Variability of Earth's Polar Regions: Implications for Detecting Snowball Planets

We have obtained the first time-resolved, disk-integrated observations of Earths poles with the Deep Impact spacecraft as part of the EPOXI mission of opportunity. These data mimic what we will see when we point next-generation space telescopes at nearby exoplanets. We use principal component analysis (PCA) and rotational light curve inversion to characterize color inhomogeneities and map their spatial distribution from these unusual vantage points, as a complement to the equatorial views presented by Cowan et al. in 2009. We also perform the same PCA on a suite of simulated rotational multi-band light curves from NASAs Virtual Planetary Laboratory three-dimensional spectral Earth model. This numerical experiment allows us to understand what sorts of surface features PCA can robustly identify. We find that the EPOXI polar observations have similar broadband colors as the equatorial Earth, but with 20%-30% greater apparent albedo. This is because the polar observations are most sensitive to mid-latitudes, which tend to be more cloudy than the equatorial latitudes emphasized by the original EPOXI Earth observations. The cloudiness of the mid-latitudes also manifests itself in the form of increased variability at short wavelengths in the polar observations and as a dominant gray eigencolor in the south polar observation. We construct a simple reflectance model for a snowball Earth. By construction, our model has a higher Bond albedo than the modern Earth; its surface albedo is so high that Rayleigh scattering does not noticeably affect its spectrum. The rotational color variations occur at short wavelengths due to the large contrast between glacier ice and bare land in those wavebands. Thus, we find that both the broadband colors and diurnal color variations of such a planet would be easily distinguishable from the modern-day Earth, regardless of viewing angle.