Do mirages exist on Mars?

By Fred Watson, Australia’s Astronomer-at-Large 30 May 2022
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If you’ve travelled across Australia’s inland plains, you’ve probably noticed light patches in the arid landscape that look like distant stretches of water.

These are mirages caused by heated air near the dry ground bending the sky’s light towards us. It’s a common phenomenon that needs no water.

Could there be mirages on Mars? Perhaps, but it’s completely different from what beguiles the eyes of desert travellers on Earth.

In 2018 results from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) ground-penetrating radar system aboard the European Space Agency’s Mars Express spacecraft led to the exciting conclusion there were large bodies of liquid water below the planet’s southern icecap.

Bright radar reflections from a layer about 1.5km below the ice surface were interpreted as coming from lakes up to 20km long, comparable with Earth’s subglacial Antarctic lakes but smaller. 

Although surface temperature seldom drops below –80°C in Antarctica, it can reach –150°C at Mars’s south pole. So how could Martian lakes stay liquid in such conditions?

Theories included high salt levels acting as natural antifreeze, coupled with a reduction in melting point caused by the pressure exerted by the ice above – and perhaps a geological heat source below. 

Related: Get ready to see the planets align in Australian skies this June

University of Texas planetary scientists examined these proposals and found they didn’t tally with what’s already known of Mars’s polar region. Contemplating an alternative explanation, they examined radar maps of the rest of the planet.

To their surprise they found many volcanic lava flows, which, if they were being observed by radar through ice, would give bright radar reflections such as those at the poles.

The scientists concluded that lava flows are a more plausible explanation of the icecap radar results than subglacial lakes, with all their attendant problems. 

So is liquid water under the icecap a Martian mirage? Not necessarily.

Recent lab work at Southwest Research Institute in Boulder, Colorado, suggests slushy ice-brine mixes at below –100°C have electromagnetic properties that would, indeed, give a strong reflection to the MARSIS radar, restoring the possibility of at least semi-liquid water.

Which theory is correct? For now, we just don’t know.