Bashar Rizk

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.

Sublimation and reformation of icy grains in the primitive solar nebula

Abstract We examine the frictional heating, sublimation, and recondensation of grains free-falling into the solar nebula from a surrounding interstellar cloud. The amount of water ice sublimated varies over a wide range—from over 90% of the grain mass at 30 AU from the nebular center to less than 10% beyond 100 AU. We further conclude that essentially all of the water sublimated eventually recondenses, because the cold nebular gas beyond 10 AU is able to hold only a small fraction as vapor. The expansion of the sublimating gas from the grain surface and abundance of cold grains implies that most of the gas returns to the solid phase near nebular ambient temperatures (∼50 K). Such a process could lead to at least two populations of grains: (1) essentially unaltered interstellar grains which did not sublimate due to drag or accretion shock heating and (2) a component comprised of water ice cocondensed, after heating, with more volatile gases at nebular ambient temperatures, yielding volatile-rich amorphous phases. Component (2) may be by far the most abundant in the portion of the outer solar nebula where Triton and Pluto formed; component (1) may be much more important for comets.

Martian water vapor, 1988-1995

We report new measurements of Martian atmospheric water vapor for the period 1991–1995 and discuss implications of these and earlier measurements from 1988 to 1989. Our measurements indicate abundances (precipitable micrometers (pr μm)) that show some departures from those of the Viking Mars atmospheric water detector (MAWD) experiment and other ground-based measurement programs. Variation of water abundance within Martian season is sometimes as large as a factor of 3 from one year to the next. However, the seasonal shifts and variations between hemispheres show the same trends as observed by MAWD. Column abundances of water vapor varied from barely detectable, <1 (at Ls 320–340) to 36.4 pr μm (Ls 100) at high northern latitudes. Strong latitude variations were observed for all Ls seasons, with late spring and summers wet in both hemispheres. Northern latitudes are up to 5 times wetter than southern latitudes. Equatorial regions (30°S–30°N) show a rather stable abundance of atmospheric water varying between 2 and 20 pr μm, while much larger variations are observed at high latitudes. Southern atmospheric water drops below 10 pr μm rapidly in early autumn and is below our measurement threshold by late autumn. Strong diurnal variations show lowest water column abundance near the evening terminator.

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.

Effects of size‐dependent emissivity on maximum temperatures during micrometeorite entry

We have numerically examined the effects of adopting the absorption efficiencies predicted by the Mie theory for spheres on the maximum temperature of pure olivine and pure iron micrometeorites entering the Earths atmosphere and pure water ice micrometeorites entering the Uranian atmosphere. We found that particles of micron and submicron size with the optical properties that characterize these substances tend not to radiate away their frictional heat of entry as efficiently as classical micrometeorite theory predicts. Consequently, the degree to which these particles are heated and altered during entry is increased. We conclude that micron and submicron-sized particles are more realistically treated as Mie spheres than as blackbodies, when accurate values exist for the imaginary index of refraction at wavelengths of a few microns.

Meridional Martian water abundance profiles during the 1988–1989 season

Abstract In this paper we report on measurements of the latitudinal distribution of atmospheric water vapor column abundance that were collected on 4 nights during spring and summer in the Martian southern hemisphere before and after the 1988 opposition. The profiles for early southern spring and southern autumnal equinox agree with those measured during the same season in 1977 by the Mars Atmospheric Water Detector (MAWD) on the Viking Orbiters, but the profiles during southern mid- and late summer show twice as much water in the southern hemisphere and planetwide as did the Viking MAWD. An equatorial water column abundance maximum was a relatively constant feature of the profiles acquired and 4 × 10 14 g was observed to appear and disappear at latitudes south of −30°. Based on the lack of global dust storms observed during the 1988–1989 spring and summer compared to the 1977 Viking measurements during the same period, the water abundance measurements reported in this paper represent observations in a relatively clear atmosphere.

OSIRIS-REx: Sample Return from Asteroid (101955) Bennu

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

Thermal evolution of Titan's atmosphere☆

Abstract A semianalytical model is constructed of the evolution of Titans atmosphere and surface. The model assumes the presence of a massive (kilometer-deep) ethane-methane ocean at the surface, which serves as a reservoir for atmospheric gases important in determining the thermal balance of the troposphere. As stratospheric photolysis irreversibly converts methane to higher hydrocarbons with time, the ocean becomes progressively enriched in ethane and depleted in methane. The resulting change in the abundance of gases drives the evolution of the atmospheric thermal structure. The solubility in the ocean of nitrogen, argon, and hydrogen is quantified as a function of methane content, which permits the atmospheric composition to be calculated as a function of ocean composition. The surface temperature as a function of atmospheric composition and surface pressure is calculated using a grey atmosphere, two-stream approximation with infrared opacities supplied by collisions among nitrogen, methane, and hydrogen molecules. A suite of evolution models is presented corresponding to different present-day ocean compositions, various atmospheric hydrogen abundances, and different schemes for frequency-averaging the gas opacity for use in the grey atmosphere formalism. Over a plausible range of parameters characterizing present-day ocean and atmospheric compositions, slow evolution over several billion years is obtained. This result is consistent with the original motivation for postulating an ethane-methane ocean, namely, that it provide a means for maintaining the atmosphere observed by Voyager over a reasonable fraction of geologic time. Models of atmospheric evolution in the absence of an ocean are briefly discussed; these are driven by stochastic resupply of methane to the atmosphere by volcanism or impacts. Variation of solar radiation at the surface due to changes in haze column abundance and troposheric clouds are not considered in the present effort. The results show that methane photolysis connects the properties and time history of Titans atmosphere and possible ocean, not only by the production of ethane from methane, but equally importantly through the formation of molecular hydrogen.

Solar heating of the Uranian mesopause by dust of ring origin

Abstract We have estimated the magnitude of an equatorial heat source due to submicrometer dust in the Uranian upper atmosphere under the assumption that the dust collects extra solar energy in the visible while being inhibited from radiating in the infrared. Dust at the orbit of Uranus that possesses the combined bulk optical properties of known materials can attain temperatures of up to 200 K. Thus dust is capable of providing a significant heat source at the mesopause level, where temperatures of from 150 to 200 K have been observed in ground-based occultations. Dust heating may explain heat sources observed only near the equator, for which there is some evidence in both the ground-based and UVS-derived temperature profiles. However, such dust is ruled out as a heat source for the thermospheric temperatures of 500–800 K measured by the Voyager UVS (ultraviolet spectrometer). The influx necessary to provide a significant heat source corresponds to the decomposition into submicrometer dust, averaged over the age of the solar system, of some 102 moons 10 km in diameter of density 1.5 g cm−3. Recent work indicates that this is a realistic average dust flux for the Uranian rings.

OCAMS: The OSIRIS-REx Camera Suite

The OSIRIS-REx Camera Suite (OCAMS) will acquire images essential to collecting a sample from the surface of Bennu. During proximity operations, these images will document the presence of satellites and plumes, record spin state, enable an accurate model of the asteroid’s shape, and identify any surface hazards. They will confirm the presence of sampleable regolith on the surface, observe the sampling event itself, and image the sample head in order to verify its readiness to be stowed. They will document Bennu’s history as an example of early solar system material, as a microgravity body with a planetesimal size-scale, and as a carbonaceous object. OCAMS is fitted with three cameras. The MapCam will record color images of Bennu as a point source on approach to the asteroid in order to connect Bennu’s ground-based point-source observational record to later higher-resolution surface spectral imaging. The SamCam will document the sample site before, during, and after it is disturbed by the sample mechanism. The PolyCam, using its focus mechanism, will observe the sample site at sub-centimeter resolutions, revealing surface texture and morphology. While their imaging requirements divide naturally between the three cameras, they preserve a strong degree of functional overlap. OCAMS and the other spacecraft instruments will allow the OSIRIS-REx mission to collect a sample from a microgravity body on the same visit during which it was first optically acquired from long range, a useful capability as humanity reaches out to explore near-Earth, Main-Belt and Jupiter Trojan asteroids.