Publications

The first 18 tracks of laser altimeter data across the northern hemisphere of Mars from the Mars Global Surveyor spacecraft show that the planet at latitudes north of 50° is exceptionally flat; slopes and surface roughness increase toward the equator. The polar layered terrain appears to be a thick ice-rich formation with a non-equilibrium planform indicative of ablation near the periphery. Slope relations suggest that the northern Tharsis province was uplifted in the past. A profile across Ares Vallis channel suggests that the discharge through the channel was much greater than previously estimated. The martian atmosphere shows significant 1-micrometer atmospheric opacities, particularly in low-lying areas such as Valles Marineris.

Laser altimeter sensors are a class of devices, developed for airborne and space-based applications, with a primary role of measuring the topography of the Earth and other planetary surfaces. Laser altimeters are mounted for nadir observation on aircraft or spacecraft platforms and provide a high-resolution, high-accuracy measurement of the elevation and horizontal variability of planetary surfaces. The basis of the altimeter measurement is the timing of the round-trip propagation of short-duration pulses of laser radiation between the platform and the surface. Vertical (i.e. surface elevation) resolution of the altimetry measurement is determined primarily by laser pulsewidth, surface-induced spreading in time of the reflected pulse, and the timing precision of the altimeter electronics. With conventional (~10 nsec wide) gain-switched pulses from solid-state lasers and sub-nsec resolution electronics, sub-meter vertical range resolution is possible from aircraft altitudes (0.5 km - 20 km) to orbital altitudes of several hundred kilometers. Laser divergence angle and altimeter platform height above the surface determine the laser footprint size at the surface, while laser pulse repetition-rate, laser transmitter beam configuration, and altimeter platform velocity determine the spacing between successive laser pulses. Typical sensor footprint diameters for an aircraft altimeter application are 1-10 m and are ~ 100 m for a spacecraft altimeter. Ideally, the pulse-rate is adjusted to make a contiguous profile of topography so that horizontal resolution is equal to the sensor footprint size. Successful operation of the laser altimeter sensor requires additional information on the laser altimeter platform pointing attitude and position 1. Recent major advances in laser gyros, star cameras, and kinematic positioning with the Global Positioning System permit recovery of platform effects to a sufficient level (e.g. 5-500 μrad for pointing angle and 10 cm - 1 m for position) to enable sub-meter laser altimetry for the operating altitudes and velocities of both aircraft and spacecraft platforms.

We suggest that the International Space Station be used to examine the application and validation of low-latency telepresence for surface exploration from space as an alternative, precursor, or potentially as an adjunct to astronaut boots on the ground. To this end, controlled experiments that build upon and complement ground-based analog field studies will be critical for assessing the effects of different latencies (0 to 500 milliseconds), task complexity, and alternate forms of feedback to the operator. These experiments serve as an example of a pathfinder for NASA's roadmap of missions to Mars with low-latency telerobotic exploration as a precursor to astronaut's landing on the surface to conduct geological tasks.

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP$^3$ to measure the surface heat flow of the planet. The package uses temperature sensors that would have been brought to the target depth of 3--5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, by precessing about a point midway along its hull, it carved a 7 cm deep and 5-6 cm wide pit and reached a depth of initially 31 cm. The root cause of the failure - as was determined through an extensive, almost two years long campaign - was a lack of friction in an unexpectedly thick cohesive duricrust. During the campaign -- described in detail in this paper -- the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and by direct support by the latter. The mole finally reached a depth of 40 cm, bringing the mole body 1--2 cm below the surface. The penetration record of the mole and its thermal sensors were used to measure thermal and mechanical soil parameters such as the thermal conductivity and the penetration resistance of the duricrust and its cohesion. The hammerings of the mole were recorded by the seismometer SEIS and the signals could be used to derive a P-wave velocity and a S-wave velocity and elastic moduli representative of the topmost tens of cm of the regolith. The combined data were used to derive a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick unconsolidated layer of sand mixed with dust and above another 10 cm of unconsolidated sand. Underneath the latter, a layer more resistant to penetration and possibly consisting of debris from a small impact crater is inferred.