T. Siili

The Martian Slope Winds and the Nocturnal PBL Jet

Abstract The summertime Martian PBL diurnal wind variation, slope winds, and the nocturnal low-level jets were studied using Prandtls theory, a mesoscale numerical model, and Viking lander observations. During moderate prevailing large-scale flow, nocturnal jets were simulated that were rather similar to those on Earth. They were mainly caused by inertial oscillation after sunset with some contribution from the slope wind effects over sloping regions (which are very common in Mars). During weak large-scale flow, shallow nocturnal drainage flows with strong vertical shear developed over the cold Martian slopes. At middle and high latitudes, these katabatic winds tended to turn to flow along the slope by dawn (due to the Coriolis force). For sufficiently steep slopes, near-surface drainage winds could reach considerable speeds. In contrast, the typical afternoon upslope winds were vertically homogeneous up to 2–3 km and weak (only 1–3 m s−1 in magnitude), even over relatively steep large-scale slopes.

The Martian atmospheric boundary layer

The planetary boundary layer (PBL) represents the part of the atmosphere that is strongly influenced by the presence of the underlying surface and mediates the key interactions between the atmosphere and the surface. On Mars, this represents the lowest 10 km of the atmosphere during the daytime. This portion of the atmosphere is extremely important, both scientifically and operationally, because it is the region within which surface lander spacecraft must operate and also determines exchanges of heat, momentum, dust, water, and other tracers between surface and subsurface reservoirs and the free atmosphere. To date, this region of the atmosphere has been studied directly, by instrumented lander spacecraft, and from orbital remote sensing, though not to the extent that is necessary to fully constrain its character and behavior. Current data strongly suggest that as for the Earths PBL, classical Monin-Obukhov similarity theory applies reasonably well to the Martian PBL under most conditions, though with some intriguing differences relating to the lower atmospheric density at the Martian surface and the likely greater role of direct radiative heating of the atmosphere within the PBL itself. Most of the modeling techniques used for the PBL on Earth are also being applied to the Martian PBL, including novel uses of very high resolution large eddy simulation methods. We conclude with those aspects of the PBL that require new measurements in order to constrain models and discuss the extent to which anticipated missions to Mars in the near future will fulfill these requirements.

Network science landers for Mars

Abstract The NetLander Mission will deploy four landers to the Martian surface. Each lander includes a network science payload with instrumentation for studying the interior of Mars, the atmosphere and the subsurface, as well as the ionospheric structure and geodesy. The NetLander Mission is the first planetary mission focusing on investigations of the interior of the planet and the large-scale circulation of the atmosphere. A broad consortium of national space agencies and research laboratories will implement the mission. It is managed by CNES (the French Space Agency), with other major players being FMI (the Finnish Meteorological Institute), DLR (the German Space Agency), and other research institutes. According to current plans, the NetLander Mission will be launched in 2005 by means of an Ariane V launch, together with the Mars Sample Return mission. The landers will be separated from the spacecraft and targeted to their locations on the Martian surface several days prior to the spacecrafts arrival at Mars. The landing system employs parachutes and airbags. During the baseline mission of one Martian year, the network payloads will conduct simultaneous seismological, atmospheric, magnetic, ionospheric, geodetic measurements and ground penetrating radar mapping supported by panoramic images. The payloads also include entry phase measurements of the atmospheric vertical structure. The scientific data could be combined with simultaneous observations of the atmosphere and surface of Mars by the Mars Express Orbiter that is expected to be functional during the NetLander Missions operational phase. Communication between the landers and the Earth would take place via a data relay onboard the Mars Express Orbiter.

Mars Science Laboratory relative humidity observations: Initial results

The Mars Science Laboratory (MSL) made a successful landing at Gale crater early August 2012. MSL has an environmental instrument package called the Rover Environmental Monitoring Station (REMS) as a part of its scientific payload. REMS comprises instrumentation for the observation of atmospheric pressure, temperature of the air, ground temperature, wind speed and direction, relative humidity (REMS-H), and UV measurements. We concentrate on describing the REMS-H measurement performance and initial observations during the first 100 MSL sols as well as constraining the REMS-H results by comparing them with earlier observations and modeling results. The REMS-H device is based on polymeric capacitive humidity sensors developed by Vaisala Inc., and it makes use of transducer electronics section placed in the vicinity of the three humidity sensor heads. The humidity device is mounted on the REMS boom providing ventilation with the ambient atmosphere through a filter protecting the device from airborne dust. The final relative humidity results appear to be convincing and are aligned with earlier indirect observations of the total atmospheric precipitable water content. The water mixing ratio in the atmospheric surface layer appears to vary between 30 and 75 ppm. When assuming uniform mixing, the precipitable water content of the atmosphere is ranging from a few to six precipitable micrometers. Key Points Atmospheric water mixing ratio at Gale crater varies from 30 to 140 ppm MSL relative humidity observation provides good data Highest detected relative humidity reading during first MSL 100 sols is RH75%

Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions

Abstract Towards the end of southern hemisphere winter (Ls ≈ 180°) the Martian southern polar cap extends equatorward to 40°S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57°S and a slope angle of 0.6°, both representative of Hellas southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust (τ = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m⧸s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (fohn, or bora-type of event) with near surface winds exceeding 30 m⧸s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface.

METEOROLOGICAL OBSERVATIONS ON MARTIAN SURFACE : MET-PACKAGES OF MARS-96 SMALL STATIONS AND PENETRATORS

The scientific objectives of a meterological experiment on the Martian surface are defined, and the meteorological equipment of the landing elements of the Mars-96 mission are described with emphasis on the applicability for re-use in forthcoming Mars missions. The general strategy for atmospheric surface observations is discussed. Meteorological surface observations are of utmost value in studying the Martian atmosphere. The climatological cycles and atmospheric circulations, as well as the boundary layer phenomena can be understood thoroughly only, if the contribution of in situ surface measurements are amalgamated with the remote observations. The Mars-96 mission had an ambitious goal of deploying four versatile payloads at four Northern hemispheric sites. The observations of pressure, temperature, wind, atmospheric optical thickness and humidity, as well as pressure and temperature measurements during the atmospheric descent were included in the meteorology experiment. Even though the Mars-96 mission was unsuccessful, the objectives and implementation of the meteorology experiment are applicable to any forthcoming landing mission to Mars. This applies both to a mission having a number of observation sites spread all over the surface of Mars, and to a single lander or rover. The main operational objective of this meteorological experiment is to provide a regular time series of the meteorological parameters with accelerated measurement campaigns during dawn and dusk. Such a data set would substantially improve our understanding of the atmospheric structure, dynamics, climatological cycles, and the atmosphere-surface interactions. The implementation of the meteorology instrument features advanced sensor technology and flexible system design. The application on the Mars-96 landing elements was, however, severely constrained by the limited power supply. The usefulness of the system can be substantially enhanced by modest additional resources and with few or no design modifications.

A sophisticated lander for scientific exploration of Mars: scientific objectives and implementation of the Mars-96 Small Station

A mission to Mars including two Small Stations, two Penetrators and an Orbiter was launched at Baikonur, Kazakhstan, on 16 November 1996. This was called the Mars-96 mission. The Small Stations were expected to land in September 1997 (Ls approximately 178 degrees), nominally to Amazonis-Arcadia region on locations (33 N, 169.4 W) and (37.6 N, 161.9 W). The fourth stage of the Mars-96 launcher malfunctioned and hence the mission was lost. However, the state of the art concept of the Small Station can be applied to future Martian lander missions. Also, from the manufacturing and performance point of view, the Mars-96 Small Station could be built as such at low cost, and be fairly easily accommodated on almost any forthcoming Martian mission. This is primarily due to the very simple interface between the Small Station and the spacecraft. The Small Station is a sophisticated piece of equipment. With the total available power of approximately 400 mW the Station successfully supports an ambitious scientific program. The Station accommodates a panoramic camera, an alpha-proton-x-ray spectrometer, a seismometer, a magnetometer, an oxidant instrument, equipment for meteorological observations, and sensors for atmospheric measurement during the descent phase, including images taken by a descent phase camera. The total mass of the Small Station with payload on the Martian surface, including the airbags, is only 32 kg. Lander observations on the surface of Mars combined with data from Orbiter instruments will shed light on the contemporary Mars and its evolution. As in the Mars-96 mission, specific science goals could be exploration of the interior and surface of Mars, investigation of the structure and dynamics of the atmosphere, the role of water and other materials containing volatiles and in situ studies of the atmospheric boundary layer processes. To achieve the scientific goals of the mission the lander should carry a versatile set of instruments. The Small Station accommodates devices for atmospheric measurements, geophysical and geochemical studies of the Martian surface and interior, and cameras for descent phase and panoramic views. These instruments would be able to contribute remarkably to the process of solving some of the scientific puzzles of Mars.

Modeling of albedo and thermal inertia induced mesoscale circulations in the midlatitude summertime Martian atmosphere

Mesoscale circulations in the Martian atmosphere induced by variations in surface albedo (a) and soil thermal inertia (I) or both have been studied numerically with a two-dimensional primitive equation model. The latitudinal and seasonal parameters correspond to northern midlatitudes and midsummer. The variations in the surface properties induce surface temperature variations, which, in turn, drive horizontal winds and vertical motions. A circulation cell not unlike a terrestrial seabreeze cell is formed at 1300–1400 local time (LT), the vertical motion peaks just prior to sunset and persists until 2100–2300 LT. The horizontal winds accelerate throughout the day and peak around 2100 LT. The acceleration picks up at 1600–1800 LT owing to the collapse of vertical momentum mixing. In the thermal inertia case the surface temperature gradient reverses around 1800 LT, leading to reversed forcing and nocturnal return flow. Owing to the Coriolis effect, the perpendicular wind component (ν) dominates the magnitude of the horizontal flow. The extrema of horizontal winds and vertical motions occurring for variations of a = 0.20–0.30 and I = 250–350 SI units and optical thickness τ = 0.4 can be up to 7–8 m/s and approximately 3–4 cm/s, respectively. The strength of the circulation is sensitive to the amount of suspended dust; the maximum wind speed is reduced by a factor of more than 2 when τ is changed from 0.1 to 1.0. The effects of superimposed thermal inertia and albedo variations depend on their respective magnitudes and signs: if a and I increase in the same direction, the circulations are amplified, and the phases are close to phases of circulations induced by thermal inertia variations alone. If the variations have opposite signs, the circulations are attenuated, and the time of largest forcing can shift from daytime to the night; circulation and surface stress patterns are also shifted in phase by up to several hours. Surface stress τ0 induced by the circulations is discernible as of a few hours after sunrise and peaks in the region of a and I variations. If the forcing is due to a or I variation alone or due to the amplifying combination of the two, τ0 peaks in the early afternoon (typically at 1200–1400 LT) and collapses 2–3 hours prior to sunset, primarily owing to the collapse of vertical momentum mixing. In case of opposing surface property variations, τ0 maximum is shifted in phase and can occur as early as 0800 LT owing to predominantly nocturnal forcing. The circulations studied here appear not to play a significant role in dust raising, as the magnitude of the stress generated is at least an order of magnitude below the estimated dust raising threshold.

Scientific objectives and implementation of the Pressure Profile Instrument (PPI⧹HASI) for the Huygens spacecraft

Abstract The Huygens entry probe will be deployed into the Titan atmosphere by the Cassini spacecraft. During the 3 h descent the Huygens Atmospheric Structure Instrument (HASI) will observe a comprehensive set of variables and phenomena, encompassing pressure, temperature, density and atmospheric electricity. The Titan atmospheric vertical pressure profile will be recorded by the Pressure Profile Instrument (PPI:HASI) provided by the Finnish Meteorological Institute (FMI). The principal sections of the PPI are:• sensor boom extending out of the Huygens main body, • Kiel probe with pitot tube in the end of the sensor boom,• pressure sensors (Barocap ®) inside the Huygens body, and• pressure hose conveying the pressure signal from the Kiel probe to the pressure sensors.The decision to measure total pressure instead of static pressure and the design of the Kiel probe was based on aerodynamic simulations. Simulations were performed for the airflow around the Huygens probe and in the vicinity of the tip of the sensor boom. During the descent the Huygens probe is constantly changing its attitude. Hence a pitot tube alone would not give a reliable pressure reading. By using the Kiel probe the total pressure reading is insensitive to the angle between the streamlines and the Kiel probe up to 45°. The PPI uses pressure sensors with three different sensitivities to cover the pressure range of 0-180 kPa. The sensor technology is a heritage from a concept that has been applyed in earlier space and terrestrial applications. The PPI starts measurements at an altitude of 160 km, producing 28 bits of data per second. Measurements are designed to continue beyond the time of impact on the surface of Titan until Huygens stops operating. The flight unit has been integrated to the Huygens entry probe and tests have been successful. A special balloon test session of PPI and other HASI instruments simulating the actual mission of Huygens was carried out successfully in 1995.

The NetLander atmospheric instrument system (ATMIS): description and performance assessment

Abstract The pointwise meteorological observations of the Viking Lander and Mars Pathfinder as well as the orbital mapping and sounding performed by, e.g., Mariner 9, Viking Orbiters and the Mars Global Surveyor have given a good understanding of the basic behaviour of the Martian atmosphere. However, the more detailed characterisation of the Martian circulation patterns, boundary layer phenomena and climatological cycles requires deployment of meteorological surface networks. The European NetLander concept comprising four well-instrumented landers is being studied for launch in 2005 and operations spanning at least a Martian year in 2006–2008. The landers are to be deployed to areas in both Martian hemispheres from equatorial regions to low mid-latitudes. The NetLander atmospheric instrument system (ATMIS) on board each of the landers is designed to measure atmospheric vertical profiles of density, pressure and temperature during the descent onto the surface, as well as pressure, atmospheric and ground temperatures, wind, atmospheric optical thickness and humidity through a full Martian year, possibly beyond. The main operational objective of this meteorological experiment is to provide a regular time series of the meteorological parameters as well as accelerated measurement campaigns. Such a data set would substantially improve our understanding of the atmospheric structure, dynamics, climatological cycles, and the atmosphere–surface interactions. The ATMIS sensor systems and measurement approaches described here are based on solutions and technologies tested for similar observations on Mars-96, Mars Pathfinder, Huygens, and Mars Polar Lander. Although the number of observation sites only permits characterisation of some components of the general circulation, the NetLander ATMIS will more than double the number of in situ vertical profiles (only three profiles — two from Viking Landers and one from Mars Pathfinder — are currently available and as envisioned at the time of writing, none of the 2001 and 2003 landers’ payloads include entry phase measurements of pressure or temperature), perform the first in situ meteorological observations in the southern low- and mid-latitudes and provide the first simultaneous in situ multi-site observations of the local and general circulation patterns, in a variety of locations and terrains. As such, NetLander ATMIS will be the precursor of more comprehensive meteorological surface networks for future Mars exploration.