Viscosity is a fundamental characteristic of liquid and viscoelastic materials. Viscosity measurements can give an insight in the mechanics and chemistry of biotic and geophysical samples; therefore, lander exploration of extra-terrestrial planets will benefit from an integrated viscometer. However, conventional viscometers are unsuitable for space missions. A novel miniaturised ultrasonic sensor is here presented as a rheometer for the in situ analysis of viscoelastic samples in extra-terrestrial lander missions. The measurement accuracy of the ultrasonic sensor was compared against a conventional viscometer for the measurement of aqueous solutions of L- and D-amino acids, with concentrations ranging from 1 to 10% (mass/volume). The instrument measured the viscosity of the aqueous solutions with a precision of 10% in a frequency range of 1.8 to 13.5MHz. The instrument was further used to measure the viscosity of solutions of D-serine and L-serine 10% in water at sub-zero temperatures. Their viscosity was measured at the freezing point, demonstrating that this new sensor can provide innovative means for the study of ice rheology in situ. The compact dimensions of this sensor and the high precision of the measurements make it an ideal tool for in situ mechanical characterisation of biotic and geological samples in alien worlds.