Comparison between MARSIS & SHARAD results

Abstract

MARSIS (Mars advanced Radar for subsurface and ionosphere sounding) is a low frequency nadir looking sounding radar selected by ESA as a payload of the Mars Express mission, whose primary Scientific Objective is to map the distribution of water both solid and liquid, at global scale on the Martin crust. MARSIS is the first instrument to be able to detect what lies beneath the surface of Mars (up to about 5 km). MARSIS operates with a very high fractional bandwidth: 1 MHz bandwidth allows a vertical resolution of 150 m in vacuum which corresponds to 50-100 m in the subsurface, depending on the electromagnetic wave propagation speed in the crust. The center frequency of the pulses transmitted by MARSIS can be set to 1.8 MHz, 3 MHz, 4 MHz and 5 MHz. On day side operations, it operates only in 4 MHz and 5 MHz due to the ionosphere plasma frequencies of Mars cutting of all frequencies lower than 3 MHz. All the four carrier frequencies are available for subsurface sounding on night side. The Mars Shallow Radar Sounder (SHARAD), a facility instrument provided by the Italian Space Agency (ASI), is embarked on board the NASA Mars Reconnaissance Orbiter spacecraft. SHARAD began science operations on October 3rd 2006 : it has been collected data from surface and subsurface. This instrument penetrates to roughly half a kilometer below Mars' to search for information about underground layers of ice, rock and, perhaps, melted water. SHARAD operates with a center frequency of 20 MHz and 10 MHz bandwidth. These parameters allow vertical resolution on the order of 10-20 m. The carrier frequency of 20 MHz guarantees the capability of SHARAD to operate in day side as well as in night side. Both MARSIS and SHARAD use the principle of a Synthetic Aperture Radar (SAR) to achieve a fine along-track resolution. In particular,MARSIS is an un-focused SAR with best along-track resolution of 2 km; data coming from SHARAD can be processed with focusing algorithm (Chirp Scaling Algorithm), rising a best horizontal resolution of 300 m. This paper provides a comparison between MARSIS and SHARAD images in different zones of the Mars' surface. From the preliminary analysis it has been evident that MARSIS detects signals from subsurface interfaces at 3 km of depth, while the signals received by SHARAD in the same zone and at the same depth are much weaker compared with the background noise. However,SHARAD radar-grams show subsurface interfaces at 100-200 m of depth: these interesting targets can not be discriminated by MARSIS because of its coarse vertical resolution. At the same time,SHARAD data add to MARSIS data scientific information about the upper portions of the crust of Mars.