Comparison between MARSIS & SHARAD results

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 5Km). MARSIS operates with a very high fractional bandwidth: 1MHz bandwidth allows a vertical resolution of 150m in vacuum which corresponds to 50-100m 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 1.8MHz, 3MHz, 4 MHz and 5MHz. On day side operations, it operates only in 4MHz and 5MHz due to the ionosphere plasma frequencies of Mars cutting of all frequencies lower than 3MHz. 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 20MHz and 10MHz bandwidth. These parameters allow vertical resolution on the order of 10–20m. The carrier frequency of 20MHz 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 2Km; data coming from SHARAD can be processed with focusing algorithm (Chirp Scaling Algorithm), rising a best horizontal resolution of 300m. 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 3Km 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 100–200m 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.