A time-domain electromagnetic sounder for detection and characterization of groundwater on Mars

A prototype time-domain electromagnetic (TDEM) sounder was developed to technical readiness level (TRL) 5 to detect and characterize deep groundwater on Mars. The TDEM method induces eddy currents in the subsurface by abrupt extinction of a steady current in a large, flat-lying loop antenna, and the subsurface response is measured using the same loop or a separate receiver. TDEM has been widely used in terrestrial groundwater exploration and is ideally suited to sense the high electrical conductivity associated with saline groundwater expected on Mars. The inductive regime of TDEM is distinct from ground-penetrating radar: the latter has higher resolution but smaller depth of investigation. Our Mars-prototype TDEM was tested in the laboratory and at a local field site before the principal test was performed on Maui, Hawaii. This location was chosen because of its analogy to Mars in electrical properties: dry, resistive basalt over saline pore water. Results compared favorably to soundings made with a commercial TDEM, clearly detecting the seawater interface at depths of 250 m. We subsequently developed a ballistic deployment system for the loop antenna suitable for robotic missions. Compressed gas launches two projectiles; each consists of two spools on a guide stick. Payout on one spool is back towards the launcher and on the other toward its twin on the other projectile. In this way a triangular loop antenna is formed. The full system was tested twice, successfully achieving a distance of ∼70 m in both. A system capable of deploying a 200 m loop antenna on Mars would have mass <6 kg (including 0.3 kg electronics) and within one sol could detect groundwater at depths up to 5 km. TDEM can probe to depths not possible for radar and answer the question: does groundwater - and a likely subsurface habitable zone - exist on Mars?