Martin C. Towner

In situ measurements of the physical characteristics of Titan's environment

On the basis of previous ground-based and fly-by information, we knew that Titans atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained because of uncertainties in the detailed composition. The extent of atmospheric electricity (‘lightning’) was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 ± 0.25 K, and the pressure was 1,467 ± 1 hPa.

A soft solid surface on Titan as revealed by the Huygens Surface Science Package

The surface of Saturns largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion. The Huygens probe entered Titans atmosphere on 14 January 2005 and descended to the surface using a parachute system. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

An anomalous basaltic meteorite from the innermost main belt

The Meteorite Who Fell to Earth Orbital data is available for only a handful of meteorites. Some are found long after they fell to Earth. Others are recovered after they have been observed falling through the atmosphere, but their trajectories are rarely recorded. Bland et al. (p. 1525) used a photographic camera network located in the Australian desert to track a fireball in the sky, find the meteorite, and establish its orbit. The meteorite is a basaltic achondrite; most such rocks have been traced to the major asteroid Vesta. In this case, the meteorites isotopic composition and orbital properties suggest a distinct parent asteroid—a different source of basaltic material residing in the innermost main belt. This meteorite’s composition and orbital properties are such that it cannot be traced to the parent asteroid. Triangulated observations of fireballs allow us to determine orbits and fall positions for meteorites. The great majority of basaltic meteorites are derived from the asteroid 4 Vesta. We report on a recent fall that has orbital properties and an oxygen isotope composition that suggest a distinct parent body. Although its orbit was almost entirely contained within Earth’s orbit, modeling indicates that it originated from the innermost main belt. Because the meteorite parent body would likely be classified as a V-type asteroid, V-type precursors for basaltic meteorites unrelated to Vesta may reside in the inner main belt. This starting location is in agreement with predictions of a planetesimal evolution model that postulates the formation of differentiated asteroids in the terrestrial planet region, with surviving fragments concentrated in the innermost main belt.

Convective vortices on Mars: a reanalysis of Viking Lander 2 meteorological data, sols 1–60

Dust devil data from Mars is limited by a lack of data relating to diurnal dust devil behaviour. Previous work looking at the Viking Lander meteorological data highlighted seasonal changes in temporal occurrence of dust devils and gave an indication of typical dust devil diameter, size, and internal dynamics. The meteorological data from Viking Lander 2 for sols 1 to 60 have been revisited to provide detailed diurnal dust devil statistics. Results of our analysis show that the Viking Lander 2 experienced a possible 38 convective vortices in the first 60 sols of its mission with a higher occurrence in the morning compared to Earth, possibly as a result of turbulence generated by the Lander body. Dust devil events have been categorised by statistical confidence and intensity. Some initial analysis and discussion of the results is also presented. Assuming a similar dust loading to the vortices seen by Mars Pathfinder, it is estimated that the amount of dust lofted in the locality of the Lander is approximately 800 ± 10 kgsol−1km−2.

Analysis of entry accelerometer data: A case study of Mars Pathfinder

Accelerometers are regularly flown on atmosphere-entering spacecraft. Using their measurements, the spacecraft trajectory and the vertical structure of density, pressure, and temperature in the atmosphere through which it descends can be calculated. We review the general procedures for trajectory and atmospheric structure reconstruction and outline them here in detail. We discuss which physical properties are important in atmospheric entry, instead of working exclusively with the dimensionless numbers of fluid dynamics. Integration of the equations of motion governing the spacecraft trajectory is carried out in a novel and general formulation. This does not require an axisymmetric gravitational field or many of the other assumptions that are present in the literature. We discuss four techniques—head-on, drag-only, acceleration ratios, and gyroscopes—for constraining spacecraft attitude, which is the critical issue in the trajectory reconstruction. The head-on technique uses an approximate magnitude and direction for the aerodynamic acceleration, whereas the drag-only technique uses the correct magnitude and an approximate direction. The acceleration ratios technique uses the correct magnitude and an indirect way of finding the correct direction and the gyroscopes technique uses the correct magnitude and a direct way of finding the correct direction. The head-on and drag-only techniques are easy to implement and require little additional information. The acceleration ratios technique requires extensive and expensive aerodynamic modelling. The gyroscopes technique requires additional onboard instrumentation. The effects of errors are briefly addressed. Our implementations of these trajectory reconstruction procedures have been verified on the Mars Pathfinder dataset. We find inconsistencies within the published work of the Pathfinder science team, and in the PDS archive itself, relating to the entry state of the spacecraft. Our atmospheric structure reconstruction, which uses only a simple aerodynamic database, is consistent with the PDS archive to about 4%. Surprisingly accurate profiles of atmospheric temperatures can be derived with no information about the spacecraft aerodynamics. Using no aerodynamic information whatsoever about Pathfinder, our profile of atmospheric temperature is still consistent with the PDS archive to about 8%. As a service to the community, we have placed simplified versions of our trajectory and atmospheric structure computer programmes online for public use.

The indication of Martian gully formation processes by slope–area analysis

Abstract The formation process of recent gullies on Mars is currently under debate. This study aims to discriminate between the proposed formation processes – pure water flow, debris flow and dry mass wasting – through the application of geomorphological indices commonly used in terrestrial geomorphology. High-resolution digital elevation models (DEMs) of Earth and Mars were used to evaluate the drainage characteristics of small slope sections. Data from Earth were used to validate the hillslope, debris-flow and alluvial process domains previously found for large fluvial catchments on Earth, and these domains were applied to gullied and ungullied slopes on Mars. In accordance with other studies, our results indicate that debris flow is one of the main processes forming the Martian gullies that were being examined. The source of the water is predominantly distributed surface melting, not an underground aquifer. Evidence is also presented indicating that other processes may have shaped Martian crater slopes, such as ice-assisted creep and solifluction, in agreement with the proposed recent Martian glacial and periglacial climate. Our results suggest that, within impact craters, different processes are acting on differently oriented slopes, but further work is needed to investigate the potential link between these observations and changes in Martian climate.

The Australian Desert Fireball Network: a new era for planetary science

Through an international collaboration between Imperial College London, the Ondřejov Observatory in the Czech Republic and the Western Australian Museum, the installation of the Australian Desert Fireball Network in the Nullarbor Region of Western Australia was completed in 2007. Currently, the Network, which is the first to be established in the southern hemisphere, comprises four all-sky autonomous observatories providing precise triangulation of fireball records to constrain pre-atmospheric orbits and fall positions of meteorites over an area of approximately 200 000 km2. To date, the Network has led to the successful recovery of two observed meteorite falls. The first recovery was three fragments (174, 150 and 14.9 g) of the same meteorite fall recorded on 20 July 2007 at 19 h 13 m 53.2 s±0.1 s UT that were found within 100 m of the predicted fall line. Named Bunburra Rockhole, the meteorite is a basaltic achondrite with an oxygen isotopic composition (Δ17O = −0.112 ‰) distinguishing it from basaltic meteorites belonging to the Howardite–Eucrite–Diogenite clan thought to be derived from asteroid 4Vesta, and therefore must have come from another differentiated asteroid in the terrestrial planet region. Bunburra Rockhole was delivered to Earth from an Aten-like orbit that was almost entirely contained within the Earths orbit. The second recovered fall was detected by the Network on 13 April 2010 and led to the recovery of a 24.54 g meteorite fragment that is yet to be fully described. To date, the Network has recorded ∼550 fireballs. Records from which precise orbits and trajectories can be determined number ∼150. In addition to the two recovered falls twelve fireballs are considered to have resulted in meteorite falls. Of these, four are probable falls (10s–100 g), and five are certain falls (>100 g). Having proved the potential of the Network, ultimately a large dataset of meteorites with orbits will provide the spatial context for the interpretation of meteorite composition that is currently lacking in planetary science.

Performance characteristics of the PAW instrumentation on Beagle 2 (the astrobiology lander on ESA's Mars Express Mission)

The performance of the PAW instrumentation on the 60kg Beagle 2 lander for ESA’s 2003 Mars Express mission will be described. Beagle 2 will search for organic material on and below the surface of Mars in addition to a study of the inorganic chemistry and mineralogy of the landing site. The lander will utilize acquisition and preparation tools to obtain samples from below the surface, and both under and inside rocks. In situ analysis will include examination of samples with an optical microscope, Mossbauer and fluorescent X-ray spectrometers. Extracted samples will be returned to the lander for analysis, in particular a search for organics and a measurement of their isotopic composition. The PAW experiment performance data will be described along with the status of the project.

Automated crater detection and counting using the hough transform

A manual process for detecting and counting craters on the surface of a planetary body becomes impractical when attempting to survey a large surface area. Similarly, existing automated methods that are effective for specific areas of focus are also impractical for a large data set. We report on the work completed so far in developing a crater detection system to automatically detect craters down to sub-km sizes, across a large portion of a planetary surface. Specifically, we assess the performance of a Hough Transform (HT) for the application and in particular the influence of its preprocessing edge detection phase. Tests are performed on high resolution images of the Martian surface, anticipating a large scale crater counting application for crater chronology on the surface of Mars.

Characterising fireballs for mass determination: Steps toward automating the Australian desert fireball network

Determining the mass of a meteoroid passing through the Earths atmoshphere is essential to determining potential meteorite fall positions. This is only possible if the characteristics of these meteoroids, such as density and shape are in some way constrained. When a meteoroid falls through the atmosphere, it produces a bright fireball. Dedicated camera networks have been established to record these events with the objectives of calculating orbits and recovering meteorites. The Desert Fireball Network (DFN) is one of these programs and will eventually cover ~2 million km2. Automated observatories take high-resolution optical images throughout the night with the aim of tracking and recovering meteorites. From these optical images, the position, mass and velocity of the meteoroid at the end of its visible trajectory is required to predict the path to the ground. The method proposed here is a new aproach which aims to automate the process of mass determination for application to any trajectory dataset, be it optical or radio. Two stages are involved, beginning with a dynamic optimisation of unknown meteoroid characteristics followed by an extended Kalman filter. This second stage estimates meteoroid states (including position, velocity and mass) by applying a prediction and update approach to the raw data and making use of uncertainty models. This method has been applied to the Bunburra Rockhole dataset, and the terminal bright flight mass was determined to be 0.412 ±0.256 kg, which is close to the recovered mass of 338.9 g [1]. The optimal entry mass using this proposed method is 24.36 kg, which is consistent with other work based on the estabished photometric method and with cosmic ray analysis. The new method incorporates the scatter of the raw data as well as any potential fragmentation events and can form the basis for a fully automated method for characterising mass and velocity.