Mars Observer Laser Altimeter Research Articles

Outflow channels with deltaic deposits in Ismenius Lacus, Mars

A connected series of outflow channels in the Ismenius Lacus quadrangle are identified for the first time and characterized using High Resolution Stereo Camera images of Mars Express, the Context camera images of Mars Reconnaissance Orbiter and the topography of the Mars Observer Laser Altimeter. These channels (named Okavango Valles), which stretch over >400km south to north and join the northern plains, were identified from braided channels, scour/groove marks, poorly sinuous valleys and depositional landforms. Discharge rates were estimated to 0.1–5×106m3s−1 from analysis of the topography of scour marks. Pathways of channels segments were extracted from topography showing a unique source at a breached crater rim, suggesting overflow from ponded depressions. A series of delta fans are observed inside depressions along the channel pathways. The presence of these deltas formed in former bodies of water is a compelling argument for formation of this outflow channel system by fluvial flows. The similarity of these flows with other outflow channels on Mars proves that volcanically-related outflows cannot explain all such features. In addition, this study also shows that catastrophic floods are able to create fan deltas in transient lakes. This example has to be taken into account in the interpretation of late stage fans associated with poorly branched valleys and single channels.

A swarm of small shield volcanoes on Syria Planum, Mars

This study focuses on the volcanism in Syria Planum, located at the center of the Tharsis bulge at an altitude of 6 to 8 km above Mars datum. Syria Planum was previously recognized as a center for the tectonic activity of Tharsis, but not as a major locus for volcanic activity, despite its centrality over the bulge. Using high‐resolution images from the high resolution stereo camera on Mars Express combined with Mars Observer Laser Altimeter data, we have characterized a volcanic system that reveals a number of very interesting aspects of Mars volcanism. We identified a swarm of tens of coalesced shallow volcanic edifices, typically 10–30 km diameter, 0.1–0.2 km high, and with slopes around 0.5°. These characteristics are similar to those of small shield volcanoes found in Iceland. In addition, an intermediate‐sized volcano, which is the source of lava flows that extend over >200 km, is observed west of this shield swarm. Our study characterizes a previously unrecognized volcanic assemblage on Mars which appears to be much more developed than was documented before, in terms of morphology, inferred origin, and periodicity of eruption. The estimated lava flux of the Syria Planum volcanoes is of the same order as the lava flux of Tharsis Montes. These characteristics suggest that Syria Planum experienced a very specific style of volcanism, which we dated to the Hesperian period.

Glacial, periglacial and glacio-volcanic structures on the Echus Plateau, upper Kasei Valles

An extensive region of low, sinuous ridges occupies the Hesperian plateau above Echus Chasma in the upper Kasei Valles, Mars. The ridges have lengths of up to 270 km, heights of 100 m and widths of 10 km. The total volume of the ridge material is 6×10 11 m 3. In this paper, volcanic flows, depositional and erosional features are discussed using Mars Observer Laser Altimeter (MOLA), THEMIS and Mars Orbiter Camera (MOC) imagery and a chronology that places the ridge formation in the Late Hesperian is developed. The plateau is bounded to the north and west by more recent Late Hesperian and Amazonian lava flows. The plateau floor suddenly changes from being relatively smooth, to elevated, rough, hummocky terrain that extends eastwards to Echus Chasma. This rough terrain is penetrated by 2 km broad, shallow entrant channels that join with the canyons of Echus Chasma. The sinuous ridges appear to control the surface drainage associated with the entrant channels. The sinuous ridges’ size and morphology are similar to those associated with volcanic ridge eruptions. Their degraded structure is reminiscent of Moberg ridges. The rough, hummocky terrain is interpreted as glacial outwash, subsequently eroded by short-lived floods associated with ridge eruptions. The presence of both volcanic and glacial structures on the Echus Plateau raises the possibility that the ridge system arose from subglacial, volcanic events. The resulting jokulhlaups eroded the broad, entrant channels. As surface flow declined, groundwater flows dominated and canyon heads eroded back along the entrant channels, by sapping.

Martian north polar layered deposits stratigraphy: Implications for accumulation rates and flow

We have used Mars Orbiter Camera (MOC) images and Mars Observer Laser Altimeter (MOLA) data to correlate specific layers within the upper ∼500 m of the Martian north polar layered deposits in many troughs. Using these correlation results, we derive relative accumulation rates across the polar layered deposits and through time. We identify two major layer sequences, one lying stratigraphically above the other, each consistent with an overall curving downward layer structure with shallow slopes but with significant localized variations in layer height. Major disruptions of stratigraphy (such as angular unconformities) do not appear within the correlated layer sequences. On local and broad scales, each layer sequence exhibits a different relative accumulation rate pattern, indicating that this rate has not remained constant through time and has been affected by localized processes. The overall pattern of the lower layer sequence is similar to that of a classic terrestrial ice sheet, with a decreasing relative accumulation rate away from the center of the polar layered deposits southward. The upper layer sequence exhibits no obvious overall trends. We also compare our correlation results to predictions of the effects of simple ice flow on layer structure, finding that mass balance patterns have overprinted any effects of large‐scale ice flow. Flow has not been significant, compared to mass balance, in forming the overall structure of the correlated layers.

Crater serves as terrestrial analog for Martian impact structures

Researchers try to resolve the mysteries of Mars' geomorphological and climatic past by looking for terrestrial analogs and then explaining the processes of their formation. The Haughton impact crater site on Devon Island in Canada's Nunavut Territory has been the focus of Mars analog studies because of the similarity of its geomorphological characteristics to those observed on Mars and its location in a comparable environment [Lee et al., 1998]. The availability of high‐resolution space data of these characteristics is a necessity in such studies.The most detailed topographic survey on Mars used a non‐imaging laser ranging device called the Mars Observer Laser Altimeter (MOLA) aboard the Mars Global Surveyor Mission [Zuber et al., 1992]. The radar imaging technique selected for this study in the Nunavut Territory spaceborne repeat‐pass Interferometric Synthetic Aperture Radar (INSAR), can render subtle, point‐to‐point details on variations in a coherent, three‐dimensional manner of a seemingly featureless surface for which the traditional optical stereo matching techniques will fail to generate accurate topographic information [Rosen, 2001].

Volcanic rifting at Martian grabens

A large fraction of surface extension on Mars occurred at segmented grabens having width/length ratios akin to oceanic rifts on Earth. Association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes. A Martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery, high‐resolution digital elevation models (DEMs), scaled experimental modeling and three‐dimensional boundary element modeling of magmatic and tectonic processes, and a comparison with terrestrial rifts. The DEMs were obtained from Mars Observer Laser Altimeter, Viking Orbiter stereo images, or a combination of both. Comparison of terrestrial rifts included Afar, Iceland, and the East Pacific Rise. The ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly, resulting in emplacement of elongated magma reservoirs along the grabens. Injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of MPa and induces surface collapse. Each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces. The geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width. We conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic Martian grabens on the basis one graben‐one dike. Rather, every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the Icelandic fissure zones.

An empirical approach to studying debris flows: Implications for planetary modeling studies

High‐resolution images acquired by the Mars Global Survey Mars Orbiter Camera show gullies on the walls of impact craters and valley systems on Mars. Depositional aprons associated with these gullies have been interpreted byMalin and Edgett[2000]to be characteristic of debris flow and to indicate the presence of sources of liquid water at shallow depths below the Martian surface. By focusing on a terrestrial debris flow we test the application of Chezy‐type modeling to provide more direct estimates of the dynamics of proposed debris flows on Mars. Traditionally, planetary scientists, constrained by available remotely sensed data, have used a range of flow models to gain insights from deposits into the behavior and rheologic nature of features such as lava flows, debris flows, and gravity‐driven flows. On the basis of these model results, broad interpretations of the physics governing flows of granular materials have been tempting based on limited field‐derived data. This study tests and validates the extent to which the absolute value and relative variations of empirical parameters derived from the Chezy‐type models describe the behavior of the General's slide debris flow (38.24°N, 78.23°W) in Madison County, Virginia. The results indicate that extreme caution must be taken when interpreting model results for turbulent flows of granular materials. We obtained high‐precision topographic profiles, superelevation data, sedimentary facies, and sedimentary textural properties over the debris flow. This study focuses on the empirical parameterC, whose value gauges energy dissipation in a flow, thereby called flow resistance. Using field observations, both variations and absolute values ofCalong the debris flow have been computed using two approaches: (1) assuming constant volumetric flow rateQ, where only topography and high‐resolution images are required to compute channel dimensions, and (2) a variableQ, calculated using field derived data. Assuming near‐Newtonian flow conditions in the debris flow, estimated values ofCrange from 0.035 to 0.099. WhenQis fixed,Cdecreases as a function of distance downstream. WhenQvaries andCis computed from field‐derived flow speeds, its value tends to increase downstream slightly. These opposite results have been compared to the field observations to determine which best describes the behavior of the flow. The variations inCdownstream, obtained using the flow speeds, are most consistent with the geomorphic evidence for erosion of material by the debris flow and the presence of bends in the channel. The average values ofC, 0.036–0.33 for the General's slide, have been compared with computedCvalues for Newtonian and near‐Newtonian flows to assess the rheology of the flow during emplacement. Our terrestrial study demonstrates that advances in understanding of the dynamics of debris flows on the Mars depends on obtaining debris flow speeds in addition to channel topography and flow thickness, rather than just on channel topography, flow thickness, and the assumption thatQis constant. Mars Orbiter Camera (MOC) and Mars Observer Laser Altimeter (MOLA) data can be used to derive channel topography, flow thickness, and possibly flow speeds. Such data will provide more direct estimates of the dynamics of debris flows on Mars than are currently available.

Evidence of liquid water in recent debris avalanche on Mars

The identification of landforms associated to liquid water on Mars is a major goal for the understanding of the geologic and climatic evolution of this planet. In the region of Reull Vallis, near Hellas basin, lobate debris aprons are recent landforms due to the creep of water ice mixed with rocks. Among these landforms, one unusually long debris apron has a tongue‐shape and gross morphology that indicates a different mechanism of deposition. This study demonstrates, using topographic data from Viking DEM and MOLA (Mars Observer Laser Altimeter) that the geometric parameters of this feature support the presence of liquid water during its flow. This landform is therefore comparable in size to the most developed wet debris avalanches on Earth. Its late Amazonian age questions the presence of near‐surface liquid water in the recent geological past of Mars.

Tempe Fossae, Mars: A planetary analogon to a terrestrial continental rift?

Tempe Terra, the northeastern part of the Tharsis Region on Mars, is characterized by several extensional structures differing in style and age. The Tempe Fossae, a particularly well‐developed system of graben striking N45°E, have been studied for the first time in detail on the basis of Viking Orbiter imagery and Mars Observer Laser Altimeter (MOLA) topographic data. The graben system appears to be unique in the whole region because of its extent, the remarkable width and depth of the graben, the varying graben pattern, and the associated volcanism. Single graben can be up to 35 km wide and 120 km long, while their depth can reach nearly 3000 m. They typically show an asymmetric architecture, often in halfgraben or step fault style. The graben system widens and changes its style along strike from NE to SW from a single, very deep, and narrow graben to a complex set of several shallower, sinuous graben and halfgraben. Crustal extension was measured in the northeastern part from the observable throw and amounts to 2.5–3.1 km. Volcanic structures can be found at several locations along the graben. In the SW the graben system seems to be connected with the Tempe Terra volcanic province marked by flood basalts and plains volcanism. Craters were counted on volcanic units unaffected by extensional tectonics. Crater statistics indicate an absolute crater model age for the upper end of the extensional deformation of ∼3.5 Ga. The dimensions and the characteristics of the mapped graben system resemble those of terrestrial continental rifts, and a comparison between them and the Kenya Rift reveals striking similarities. Therefore the Tempe Fossae are interpreted as a Martian analog to a continental rift associated with an underlying mantle plume. This hypothesis seems to be supported by recent geophysical models based on topography and gravity data from Mars Global Surveyor which indicate regional uplift for Tempe Terra.

Latent outflow activity for western Tharsis, Mars: Significant flood record exposed

Observations permitted by the newly acquired Mars Observer Laser Altimeter data have revealed a system of gigantic valleys northwest of the huge Martian shield volcano, Arsia Mons, in the western hemisphere of Mars (northwestern slope valleys (NSVs)). These features, which generally correspond spatially to gravity lows, are obscured by veneers of materials including volcanic lava flows, air fall deposits, and eolian materials. Geologic investigations of the Tharsis region suggest that the system of gigantic valleys predates the construction of Arsia Mons and its extensive associated lava flows of mainly late Hesperian and Amazonian age and coincides stratigraphically with the early development of the outflow channels that debouch into Chryse Planitia. Similar to the previously identified outflow channels, which issued tremendous volumes of water into topographic lows such as Chryse Planitia, the NSVs potentially represent flooding of immense magnitude and, as such, a source of water for a northern plains ocean.

The use of laser altimetry in the orbit and attitude determination of Mars Global Surveyor

Altimetry from the Mars Observer Laser Altimeter (MOLA), an instrument on board the Mars Global Surveyor (MGS) spacecraft, has been analyzed for the period of the MGS Science Phasing Orbit‐1 (SPO‐1) mission phase. Altimeter ranges have been used to improve significantly the orbit and attitude knowledge of the spacecraft by the use of crossover constraint equations derived from short passes of the MOLA data. These constraint equations differ from traditional crossover constraints and exploit the small footprint associated with laser altimetry. The rationale for using this technique with laser altimetry over sloping terrain is laid out and evidence of the resulting benefit is presented.

Mars Observer Laser Altimeter: laser transmitter

The Mars Observer Laser Altimeter utilizes a space-qualified diode-laser-pumped Q-switched Nd:YAG laser transmitter. A simple numerical model of the laser energetics is presented, which predicts the pulse energy and pulse width. Comparisons with the measured data available are made. The temperature dependence of the laser transmitter is also predicted. This dependence prediction is particularly important in determining the operational temperature range of the transmitter. Knowing the operational temperature range is especially important for a passive, thermally controlled laser operating in space.

Optical system design and integration of the Mars Observer Laser Altimeter

The Mars Observer Laser Altimeter, developed for flight on the Mars Observer spacecraft payload in September 1992, is designed to measure the topography of the Martian surface over a 2-year period from a 400-km mapping orbit. A 40 mJ pulse diode-pumped laser together with a 0.5-m-diameter beryllium telescope and a silicon avalanche photodiode are the principal optical subassemblies of this active remote-sensing instrument. Optical design rationale and measured optical performances during assembly and integration are presented.

Multishot laser altimeter: design and performance

The maximum measurement range of a laser altimeter can be extended by averaging the measurements from multiple laser shots at the same target. We present the principles of operation and design of such a multishot laser altimeter, which uses a Si avalanche photodiode detector. As an example, the performance of a spaceborne multishot altimeter containing components similar to those of the single-shot Mars Observer Laser Altimeter are given under operating conditions that would be encountered near Saturn. With 100-shot averages, we show that the multishot laser altimeter is capable of accurate ranging at fly-by distances of 10,000 km from an icy satellite. With 100-shot averages, the minimum optical signal level at a 90% correct-measurement probability under nighttime background is 9.8 detected signal photons per pulse as compared with 76 photons per pulse with a single shot.

The Mars Observer laser altimeter investigation

The primary objective of the Mars Observer laser altimeter (MOLA) investigation is to determine globally the topography of Mars at a level suitable for addressing problems in geology and geophysics. Secondary objectives are to characterize the 1064‐nm wavelength surface reflectivity of Mars to contribute to analyses of global surface mineralogy and seasonal albedo changes, to assist in addressing problems in atmospheric circulation, and to provide geodetic control and topographic context for the assessment of possible future Mars landing sites. The principal components of MOLA are a diode‐pumped, neodymium‐doped yttrium aluminum garnet laser transmitter that emits 1064‐nm wavelength laser pulses, a 0.5‐m‐diameter telescope, a silicon avalanche photodiode detector, and a time interval unit with 10‐ns resolution. MOLA will provide measurements of the topography of Mars within approximately 160‐m footprints and a center‐to‐center along‐track footprint spacing of 300 m along the Mars Observer subspacecraft ground track. The elevation measurements will be quantized with 1.5 m vertical resolution before correction for orbit‐ and pointing‐induced errors. MOLA profiles will be assembled into a global 0.2° × 0.2° grid that will be referenced to Mars' center of mass with an absolute accuracy of approximately 30 m. Other data products will include a global grid of topographic gradients, corrected individual profiles, and a global 0.2° × 0.2° grid of 1064‐nm surface reflectivity.

Radio science investigations with Mars Observer

Mars Observer radio science investigations focus on two major areas of study: the gravity field and the atmosphere of Mars. Measurement accuracies expressed as an equivalent spacecraft velocity are expected to be of the order of 100 μm/s (for both types of investigations) from use of an improved radio transponder for two‐way spacecraft tracking and a highly stable on‐board oscillator for atmospheric occultation measurements. Planned gravity investigations include a combination of classical and modern elements. A spherical harmonic (or equivalent) field model of degree and order in the range 30–50 will be obtained, while interpretation will be in terms of internal stress and density models for the planet, using the topography to be obtained from the Mars Observer laser altimeter. Atmospheric investigations will emphasize precision measurement of the thermal structure and dynamics in the polar regions, which are regularly accessible as a result of the highly inclined orbit. Studies based on the measurements will include polar processes, cycling of the atmosphere between the poles, traveling baroclinic disturbances, small‐scale waves and turbulence, the planetary boundary layer, and (possibly) the variability and altitude of the ionosphere. As the radio occultation is insensitive to dust in the atmosphere per se and measures only the resulting change in thermal structure, it is expected that the radio technique can contribute to understanding of dust storm phenomena. Mutual observations of the atmosphere by means of radio occultation and by the pressure modulator infrared radiometer and the thermal emission spectrometer are expected to strengthen the reliability and accuracy of all three investigations.