The HABIT instrument is composed of two units, BOTTLE (Brine Observation Transition to Liquid Experiment) and ENVPACK (Environmental Package), which will monitor the formation of brines and the conditions in which it takes place respectively.
BOTTLE can be considered as a first attempt to develop a device to measure the amount of liquid water presumably produced daily on Mars surface through deliquescence of salts. Basically, it consists in a container of salts divided in six different vessels, with a capacity of 9 cm3 each, in which deliquescence process will be monitored as it occurs, providing a first in situ direct observation of liquid water. It includes a set of hygroscopic salts whose presence in the Martian regolith has been reported so far, such as magnesium, sodium and calcium perchlorates, as well as sodium chloride.
They will be exposed to Martian environment mimicking the process that is thought to occur within the soil, so that the water is captured and the change in the conductivity the process causes in the salts is measured by three sensors connected to two pins mounted in each vessel at different heights. These parameters indicate the state of hydration (dry solute, hydrated solute and briny solution), as well as the state change of the brines themselves once formed (liquid to frozen brines). The composition of the mixture of salts on the vessels will be stablished in accordance to the thermal regime of the particular landing site chosen.
The vessels are covered with High Efficiency Particulate Air (HEPA) filters to avoid the deposition of dust over the salts while allowing the air flow through them. Two of them are empty and openly exposed to Martian atmosphere, so that they will be filled little by little with dust along the mission. The conductivity of this dust under the influence of atmospheric water vapour will provide a valuable reference to evaluate the measurements taken from the salt-filled vessels. Each of them mounts a sensor to monitor the salt/brine temperature, and is equipped with a heating system to evaporate the water captured during the night if necessary. This possibility will be very useful in order to achieve a quantification of the total amount of water that could be gathered, regarding the estimation of its availability as a resource to be exploited. This procedure also permits the regeneration of the salts to their initial dehydrated state, assuring their indefinite operational lifespan.
The other unit that forms HABIT is an environmental set of sensors including an ultraviolet sensor (UVS) composed of 6 photodiodes to measure the UV irradiance, a ground temperature sensor (GTS) with an infrared thermopile in the range 8-14 μm, and three air temperature sensors (ATS) to measure the ambient temperature as well as the wind regime in three different directions. All these sensors derive from those already mounted in Curiosity's REMS, so their technology readiness level is 9. Moreover, the operational regime of HABIT will be also the same as REMS one, that is to say, it will take measurements autonomously with each one of its sensors during 5 min per hour, at 1 Hz frequency, plus extended continuous acquisitions of between 1 and 4 hours long that will be scheduled when convenient.