Remember back in 2018 when there was a discovery of a briny \u201clake\u201d underground near the Martian south pole? Pepperidge Farm probably does, and anyone that works there that\u2019s interested in space exploration will be disappointed to hear that, whatever might be causing the radar signal that finding was based on, it\u2019s most likely not a lake. At least according to new data collected by the Mars Reconnaissance Orbiter (MRO) and published recently in Geophysical Research Letters by lead author Gareth Morgan of the Planetary Science Institute and his colleagues. <\/p>\n
Back in 2018, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the Mars Express orbiter found an interesting signal. Underneath the ice sheet at the south pole, it saw something very reflective in its radar frequencies (between 1.8-5 MHz). The investigators on that instrument saw this as a sign of potential liquid water under the surface of Mars, as that best matched the signal they were seeing, and thereby set off a firestorm of speculation about how such an underwater lake could exist.<\/p>\n
There were other orbiters around Mars at the time, including MRO. However, try as the mission operators might, they couldn\u2019t get a clear signal under the ice caps with SHARAD, the shallow radar instrument onboard the mission. That\u2019s likely because it operated at a much higher frequency than MARSIS. That higher frequency, between 15-25 MHz, offers better resolution, but has trouble penetrating farther into the ground, especially when that ground is primarily composed of water ice.<\/p>\n<\/p>\n
Fraser discusses how liquid water can hide on Mars.<\/p>\n
But, after 20 years in service, we recently reported on a massive improvement to MRO\u2019s radar capability. SHARAD had been designed to travel on the \u201cback\u201d of the orbiter, placing the orbiter’s metallic body in between it and the planet it was attempting to survey. This attenuated the signal it SHARAD was able to send and receive by as much at 10 dB – a massive amount given the exponential nature of electromagnetic power scaling. <\/p>\n
Back in June, we reported on the first successful \u201cVery Large Roll\u201d (VLR) made by MRO to turn SHARAD toward the surface, eliminating the interference caused by the spacecraft\u2019s body. This maneuver, which involves turning the spacecraft 120\u00b0 on its axis, was considered too dangerous for much of the mission, but now that MRO has moved into its extended mission phase, the mission operators thought it was time to try something a little more risky.<\/p>\n
Turns out their increased risk tolerance paid off. SHARAD now had a much clearer look at the sub-surface in the south polar region only a few months after the first VLR. But what it found was disappointing – it didn\u2019t see any bright reflection, even though it could scan up to 1.5km more of the ice than it had been able to on previous passes. That lack of signal is a very strong indication that, most likely, there isn\u2019t liquid water located at the site.<\/p>\n<\/p>\n
Video of MRO performing a VLR. Credit – SciTech Daily YouTube Channel<\/p>\n
One of the key findings of the 2018 MARSIS data was the reflection under the surface was significantly higher than that on the surface, indicating that it was made of a \u201cbrighter\u201d material in radar frequencies – typically that\u2019s a feature attributable to liquid water. However, SHARAD found only a signal from the under the surface about .1% as strong as that on top of it. <\/p>\n
To tease out why there would be such a big discrepancy, the MRO engineers tested two theories. First, could the ice have still absorbed the high-frequency signals from SHARAD, not actually allowing it to get a valid signal from low enough to be able to detect the liquid water? When testing the \u201closs\u201d of signal around the supposed \u201clake\u201d compared to other nearby surfaces, they couldn\u2019t find anything to cause such high attenuation that would limit their ability to see where the liquid water was.<\/p>\n
The other theory, and the one that the researchers found the most plausible, was that the lake was actually a patch of unusually smooth dry rock – such as a crater floor filled with sediment. MARSIS would have seen that as something akin to a mirror, with most of the radar signal being reflected back, whereas SHARAD wouldn\u2019t have – accounting for the significantly lowered signal strength.<\/p>\n
This also isn\u2019t the first piece of evidence that the Martian \u201clake\u201d might be illusory. Critics of the theory pointed out that, in order for liquid water to exist there, there would either need to be an extreme amount of geothermal heat, or an absurd amount of salt in the water – neither of which seemed likely. Another study pointed out that frozen clay materials, which do contain water, but aren\u2019t liquid, can be bright enough to reproduce the MARSIS signal strength by themselves.<\/p>\n
For now, this new SHARAD finding, enabled by the dramatic roll to enhance its signal, seems to put the nail in the coffin of the theory of an underground lake on Mars. But there\u2019s still more areas of the planet the MRO operators hope to explore with their newfound improved radar capability. Next on the list is to search for ice near the Martian equator – if they find it there, it would significantly improve the appeal of that much warmer area for preliminary human exploration. Maybe Pepperidge Farm will someday remember when a spunky 20 year old orbiter found the first ever ice deposits exploited by humans on Mars at the equator.<\/p>\n
Learn More:<\/p>\n
NASA \/ JPL – NASA Orbiter Shines New Light on Long-Running Martian Mystery<\/a><\/p>\n G. A. Morgan et al. – High Frequency Radar Perspective of Putative Subglacial Liquid Water on Mars<\/a><\/p>\n UT – Reorienting MRO Massively Increases Its Subsurface Scanning Power<\/a><\/p>\n