Ian Strangeways and Colin Wilson address the challenges of meteorological measurements on Mars and Venus.
The four planets closest to the Sun – Mercury, Venus, Earth and Mars – are small and rocky with hard surfaces. Beyond them are the gas giants Jupiter, Saturn, Uranus and Neptune, all with no firm surfaces. Mercury, being small and close to the sun, has hardly any atmosphere and so has no ‘weather’. In this article we look at how the weather on Mars and Venus can be measured. Before considering what meteorological sensors might be deployed on these planets, we need to look at the conditions there.
The Martian environment
The most significant characteristics of the atmosphere on Mars, with regards to the operation of meteorological sensors, are its very low atmospheric pressure at the surface (around 6 hPa compared with Earth’s 1000 hPa), its low temperature (from 0°C to -100°C) and the fine dust that permeates the atmosphere.
There are, however, fewer problems than on Earth. There are none of the difficulties associated with water and ice that profoundly affect Earth-based instruments, nor strong and damaging winds. While on Earth there are two extremely different environments – land and sea – on Mars (and Venus) there is but the one, making the same measurements possible globally. And because of the planet’s relatively homogeneous land types, a global average temperature-anomaly could be obtained by using just a few weather stations.
Previous and planned missions to Mars
There have been seven successful missions to Mars in which robotic vehicles have landed and continued working for some time – Viking 1 and 2, Mars Pathfinder, Phoenix Mars Lander and the rovers Spirit, Opportunity and the latest – Curiosity. These have been aimed mostly at looking for evidence of life, past or present.
So far, meteorological variables have been measured only as an incidental function of some of the landers. The two Vikings, Pathfinder, and Phoenix landers had sensors measuring barometric pressure, air temperature and wind. Curiosity’s Rover Environmental Monitoring Station (REMS) measures air temperature, humidity, windspeed and direction (although one of the two wind sensors is damaged), barometric pressure, ground (skin) temperature and UV solar radiation, as shown in Figure 1 (Gómez-Elvira et al , 2012).
The British Beagle 2 lander, which reached Mars on Christmas Day 2003, was also equipped with meteorological sensors to measure air temperature at two heights with thermistors, barometric pressure with a capacitive diaphragm sensor, windspeed and direction by hot film anemometer similar to that on the Viking vehicles, and UV radiation in the 200-400 nm band (Towner et al , 2004). Sadly, Beagle 2 did not deploy correctly and so no measurements are available.
The upcoming European ExoMars ‘EDM’ lander and the American InSight mission, both due to launch in 2016, plan to measure a selection of basic meteorological variables.
What sensors can be used on Mars?
The main meteorological variables that it is useful to measure on Mars are the same as those measured on Earth, less, of course, precipitation; some precipitation has been observed using lidar, but none reaches the surface.
The most commonly used sensor on Earth to measure air temperature electrically is the Platinum Resistance Thermometer (PRT), although thermistors and thermocouples may sometimes be used. All are appropriate for Mars, but their exposure is critical. On Earth, even in its relatively dense atmosphere, care is needed to avoid the sensors becoming heated by solar and terrestrial radiation more than by the air. To protect against this, the sensors are housed in radiation shields.
In the mechanical era these were the familiar wooden Stevenson screens, but since the advent of Automatic Weather Stations (AWSs), these have been in miniature screens (Strangeways, 2003). For precise measurements such as climate monitoring, however, aspirated (force-ventilated) screens are necessary even on Earth. On Mars the problem is much greater due to the very thin atmosphere. The small screens as used on Earth-based AWSs are of no use on Mars, as they would themselves become warmed by the radiation and insufficiently cooled by the rarefied atmosphere. Failure to minimise heating by radiation can result in large errors and various ways of avoiding this are discussed elsewhere (Strangeways, 2014).
Most Mars landers – Viking, Pathfinder, Phoenix – have used thin-wire thermocouples for air temperature sensing, the small diameter of the wires ensuring reduced sensitivity to radiative loads and a response time of around one second. A boom of typically one metre length keeps the sensors away from the heated lander. The Curiosity rover, however, uses thermistors as sensors, housed in a short (35mm) fibreglass boom. This leads to problems both with solar heating and with conduction of heat from the lander itself, which can cause errors of up to 5°C. Correction for these instrumental effects, as well as from the thermal influence of the nuclear-powered rover itself, is not perfect and the earlier stations had better accuracy.
The wet and dry thermometer method was used almost exclusively on Earth for a long time to measure atmospheric relative humidity,until the fairly recent development of the capacitive humidity sensor. This sensor was developed for use on weather balloons (radiosondes), but found spin-off uses on ground-based stations because of its simplicity and convenience, even though the wet and dry technique is more precise and stable (Strangeways 2003).
The capacitive sensors need protection from contamination, achieved by exposing them in a cavity with a cover permeable to water vapour, such as a PTFE film, while excluding particles and dust. These sensors suit Mars well, since the atmosphere at the surface is similar to our stratosphere where the sensors were designed to operate on radiosondes. The first such sensor deployed on Mars is on the Curiosity rover; its performance and calibration are described by Harri et al (2014). The calibration of the sensors may drift over time and it is common practice on Earth AWSs to change the sensors every couple of years. Their long term accuracy when used on Mars is not yet proven.
Cup anemometers are still the most common windspeed sensor on Earthly AWSs. It is probable, however, that they would not be effective in the thin Martian atmosphere since the turning torque would be very low. More usually wind is measured on Mars using hot wire or hot film sensors, deployed in pairs to detect direction as well as speed.
The Curiosity rover’s anemometer uses 24 individual heated elements in order to build up a 3D wind vector to deal with different wind directions. As with the thermistor temperature sensor (mentioned previously), however, the thermal wind sensors are also affected by radiation; their calibration in the rapidly fluctuating air temperatures found in the Martian atmospheric boundary layer is also problematic. Ultrasonic three dimensional wind sensors are now widely used on Earth and these could be effective on Mars, but would require modification to work in the rarefied atmosphere. The fast response times of ultrasonic sensors would allow studies of turbulence and of dust-lifting, and they would have more robust calibrations than the existing thermal sensors.
Incoming solar radiation, reflected solar radiation from the ground (albedo) and terrestrial (long-wave) infrared radiation emitted by all objects in sight of the sensor, are important variables. There are many designs of solar radiation sensor, the most precise being the thermal type in which black surfaces (protected by glass domes), warm under the influence of solar radiation, the temperature increase being measured with a thermopile.
Cheaper, simpler and lighter-weight sensors include semiconductor photodiodes, which can be made sensitive to various frequency bands. The Rover Curiosity measures just the incoming solar ultraviolet spectrum, using six photodiodes sensitive to different bands of the UV spectrum (Gómez-Elvira et al 2012). The main difficulty with any radiation sensor on Mars is the gradual accumulation of dust on them. Active solutions to the dust problem have been examined and suggested by Strangeways (2014).
Atmospheric pressure on Mars reveals many phenomena, from seasonal condensation/sublimation of carbon dioxide into polar caps, to pressure drops of only a few seconds in duration which occur when a dust devil passes over the station. Aneroid capsules with displacement sensors have given way to miniature capacitive diaphragm sensors. These have the same origin as the humidity sensors described earlier, having been developed for use on radiosondes on Earth. The sensor is a single-crystal silicon device, the measurement being based on capacitor plates moved by changes in air pressure.
Their low cost and light weight make them well suited to use on Mars, but for long term stability and higher accuracy the vibrating cylinder type would be preferable. Sensors developed for radiosondes have to operate for a few hours only; their long term performance needs more attention.
Automatic weather stations for Mars
Long term meteorological records were obtained by the Viking 1 and 2 landers, which between them recorded more than five Mars years’ worth of meteorological data. The Viking landers fell silent in 1982, however, and recent missions have been either shorter in duration or have been rover missions, which are not suited for gathering long term meteorological records. There have been many proposals for automatic weather stations (Strangeways 2014 and others) that address the above questions of sensor choice, exposure and protection from dust.
As well as meteorologists, seismologists would also like to see a network of long-lived stations, so combining seismological and meteorological measurements is a pragmatic way of getting such a network to Mars. NASA’s Insight mission has such a payload – although it’s just a single station rather than a network – including a REMS-like meteorological suite. It is scheduled to last at least one Martian year (roughly two Earth years), but hopefully will last much longer.
The Venusian environment
Venus is almost the same size and density as Earth; it is virtually our twin. Its atmosphere, however, is the exact opposite to that of Mars and very unlike Earth’s in most ways, being extremely dense with a surface barometric pressure around 90 times that on Earth, and a surface temperature of almost 500°C. The atmosphere is composed mostly of carbon dioxide, leading to an immensely strong greenhouse warming – explaining why the surface is so hot. High above the surface, atmospheric temperatures decrease enough to permit condensation and cloud layers, although these are composed of sulphuric acid not water. The cloud layers extend from around 45-75km above the surface, and completely envelope the planet.
Robotic missions to Venus
There have been a large number of missions to Venus: 32 by Russia/USSR, 10 by the US, and one by Japan. The Soviet Venera programme, from 1961 to 1985, is one of the largest efforts ever undertaken to study another planet. Successes included three atmospheric probes, 10 landings, four orbiters, 11 flybys or impacts, and even two balloon probes in the clouds. Much of what is known today about Venus was discovered by these missions. The most recent data to be returned from within the atmosphere dates back to 1985. Since then there have been only two successful missions, Magellan and Venus Express, both orbiters. There have been many proposals for future missions, but none have as yet been approved.
Weather stations on the surface of Venus
Because of the very high temperature and barometric pressure at the surface, it is much more difficult to design a robotic rover like Curiosity to survive on Venus. Indeed, it has so far proved impossible to get anything to operate for more than a few hours before failing due to the heat. Silicon-based electronics cannot function at Venus’s ambient surface temperatures. Silicon carbide electronics would survive, but only individual components are currently available, not the full complexity of a flight computer.
So far none of the landers have been cooled, simply insulated, allowing operation until the temperature rises to a point where the systems fail – within an hour or two. But an even greater problem is the power supply. The cloud cover is so thick that only about 1% of the sunlight reaches the surface, making solar panels ineffective. The only alternative is a radioisotope power supply that would also have to power a cooling system; this would be very expensive.
While further missions with just a brief life-time on the surface are under consideration, the simplest method of getting a longer lived mission on Venus is to look upward, where cooler temperatures are to be found.
Balloons for planetary exploration
In contrast to the harsh surface environment, the atmosphere at cloud level is benign. At an altitude of 55km, temperature is 20°C and the pressure 0.5 bar. A human explorer would not need a spacesuit, but would need a tank of breathable gas and a light suit against the sulphuric acid cloud droplets. While human exploration of Venus is an unappealing prospect, the cloud level atmosphere presents an ideal environment for robotic vehicles like balloons and airplanes.
There are two basic types of balloon: zero-pressure and super-pressure. In the former the elastic envelope, generally rubber, is partly filled with a lifting gas – typically helium. At launch the pressure inside and outside the balloon are equal. As the balloon rises it expands, the pressure difference remaining zero. Eventually it can expand no further and bursts, giving it a limited lifetime of just a few hours. This is the type of balloon used with radiosondes on Earth.
The super-pressure balloon has a tough inelastic envelope and is filled with a light gas to a pressure greater than that outside – as shown in Figure 3. It cannot expand and rises to an altitude where lift equals weight and maintains this altitude, drifting with the wind. It has a lifetime dependent mainly on the rate of gas leakage and can operate for months.
There is also the ‘reversible fluid’ balloon, however. This is a more complex design in which the envelope connects to a reservoir containing a fluid that is easily vaporised and condensed. By switching between the two states, the balloon can be made to rise or fall. As noted above, at 55 km altitude the atmosphere of Venus has a temperature and pressure similar to those at Earth’s surface, and so instruments can be operated without difficulty at this altitude.
A super-pressure balloon can be designed that would hover at this height, indeed this technique was used by the Russian Vega balloons in 1984 and has been proposed again recently to both ESA and NASA. At this altitude the winds are strong and the balloon would be carried round the planet rapidly sampling the atmosphere as it travelled, performing a complete circumnavigation in just a week. It does not, however, measure the lower atmosphere or conditions on the surface.
More complex balloon arrangements such as the ‘reversible-fluid’ balloon offer more interesting possibilities. Although not attempted on other planets as yet, these concepts have been described elsewhere (Jones 1995). It is suggested here that with a suitable design of balloon with altitude control, an instrument package could be developed that would be parked normally at around the 55 km altitude, making rapid and multiple vertical excursions of tens of kilometres, perhaps all the way down to the surface, taking readings during its descent and rise back to its parking altitude.
At the surface, measurements could be made of the major meteorological variables, notably temperature, wind speed, barometric pressure, water vapour and solar and terrestrial radiation. Cameras could also obtain images of the surface and landscapes. Remaining on the surface for just a few minutes, all the electronics, batteries and cameras could be protected from high temperature extremes by housing them in an insulated cavity.
A possible design is illustrated in Figure 4. Sensors that can withstand the high air temperature and pressure of the lower atmosphere, and have to be exposed to it directly, are housed on the outside of the shell. Those that cannot survive exposure are housed in the cool cavity inside, with appropriate links to the outside environment. All the electronics (logger, transmitter and six cameras) together with the battery are also housed in the cool inner cavity. The battery would be solar powered (not shown). While the solar cells would survive exposure to the high surface temperature, modifications would be required to eliminate soldered joints. It might be preferable to use a small radioisotope generator.
Rising back to its cool parking altitude, the system would transmit the collected data to an orbiting satellite for relay back to Earth. At this altitude, where the solar radiation is much more intense than below the clouds, the batteries could be recharged by solar cells. Such a mission is undeniably technically challenging and would no doubt be very expensive, but is worth investigating for its ability to provide repeated vertical profiles through this vertically extensive atmosphere.
The weather has been measured on Mars largely as a secondary consideration related to the search for life. Long term monitoring at one location would require stand-alone AWSs (Strangeways 2014). The atmosphere of Venus has been measured at altitudes of around 55km using super-pressure balloons, but measurements at the surface have been limited to a short time only (hours) because of the damaging effects of the high temperature. In contrast, balloon platforms offer the possibility of much longer missions lasting weeks or months, circumnavigating the planet several times; variants on this might allow repeated brief excursions to the lower atmosphere and perhaps even the surface.
Measuring the weather on Venus and Mars in greater detail than has been achieved so far would help us understand more about Earth’s climate by providing three very different, real-worlds which could be measured, modelled and compared; arguably this is more likely to yield results than a single-minded focus on searching for life.
Published: 17th Dec 2014 in AWE International