Adapted from a talk given at #CSIROTweetup by Marion Anderson School for Geosciences Monash University. Marion is an incredibly passionate speaker and Australian ambassador for space science. Marion is a geologist, and was involved with site selection for the Curiosity landing.
Our understanding of Mars has changed a lot in the last thirty or so years. From the 1600’s onwards, with astronomers Tycho Brahe and Johannes Keppler, people have peered at Mars through telescopes. In 1877 Schiaparelli proclaimed to see ‘canals’, sparking the interest of Percival Lowell, who drew extensive maps. This sparked the imagination of various writers, such as H.G. Wells, who populated Mars with intelligent creatures. Many people believed these works of fiction. It came therefore as a shock when the Mariner 4 craft flew past Mars in 1965, and sent back the first blurry picture of the planet, and revealed it to be a dead world pockmarked with impact craters.
Personally, I wonder how much the disparity between the hope of finding intelligent life and the apparent reality has contributed to the long-term funk that the space exploration program has suffered. Definitely, I clearly remember that in the 1980’s the prevailing publicly-held opinion was that “there’s nothing out there, so why should we bother looking?” Space exploration was uncool, a waste of money.
Since that time, humanity has been sending its robots to the red planet. You may not quite realise how many of those there were. This page lists them in detail, past and current. In summary, there were flyby missions: Mariner 3-4, Mariner 6-7; orbiter missions: Mariner 8-9, Viking 1-2, Mars Observer, Mars Global Surveyor, Mars Climate Orbiter, 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter; and landers: Viking 1-2, Pathfinder, Polar Lander/Deep Space 2, Mars Exploration Rovers (Spirit and Opportunity), Phoenix, Mars Science Laboratory (Curiosity). That’s a lot of robots.
With all the data they have sent back, a few interesting things have happened to our perception of Mars. In the first place, the planet is no longer solely the domain of astronomers. We have found that the planet has rock like Earth, has quite a lot of water ice, and has an atmosphere. Geologists, hydrologists and atmospheric scientists have become involved in the study of Mars.
We have also found a number of geological features that strongly suggest the past existence of large bodies of water. We have found clay soils, alluvial fans, erosion gullies. We have even seen streaks of what appears to be wet soil form and dry up in the Martian spring. In addition to this, we have detected concentrations of methane in the atmosphere that vary in a seasonal pattern. On Earth, methane comes from two processes: volcanism (which is assumed to have ceased on Mars long ago), and life.
So we seem to have come full circle on Mars. While it is extremely unlikely that we will find anything half as complex as an insect there, but it may not be as dead as we once thought. It certainly isn’t geologically or climatologically dead. With an inclination of 21 degrees, Mars has seasons much like Earth. It also has weather. It may have microbial life. It may have fossil evidence of past life.
The Mars Science Lab, the rover known as Curiosity, has a sophisticated soil analysis ability. It can drill into rocks and take samples and analyse them for composition, including organic compounds.
So, knowing all this, and having to select a place to land a brand-new rover, where would you put it?
Marion was involved, with a few hundred other scientists, in the site selection for Mars Curiosity. At the final meeting, scientists had to select from four sites. What were criteria that swayed it in favour of Gale Crater?
When you look at a map of Mars, you will notice that there is a distinct difference between the northern and the southern hemispheres. The northern hemisphere is low, and very flat. The southern hemisphere is high, and is pockmarked with craters.
Images from the orbiters suggest that substantial parts of the northern hemisphere may once have been under water. The water is still there: frozen, covered under a layer of dust and regolith, because ice on the surface evaporates due to the low surface pressure (only about 1/100th of Earth’s). But you can see structures that have the appearance of a shoreline.
Earliest life on Earth existed in water. It took many, many years before creatures had evolved that possessed the specialised adaptations necessary to live on the land. If life has ever existed or still exists on Mars, it would have evolved in those bodies of water, so if you’re looking for life, the place to look would be below the visible shoreline.
Gale Crater is an impact crater about 154km in diameter, about 3.5 billion years old. A unique feature of it is a large central mound. Central mounds in craters happen when material disturbed by meteorite impact rebounds. The mound, however, is taller than the surrounding crater rim, suggesting that a different process may have been at work here. In volcanic regions on Earth, structures called tufa mounds are formed when water bubbles up from underneath, and deposits calcium and salts to form a growing structure. Since Mars was once volcanic, the central mound in Gale Crater may be such a tufa mound. Seeing as hydrothermal vents on Earth are a rich source of extremophile life, an old tufa mound is a very interesting place to look.
What would be the most awesome result? If Curiosity found in the Gale Crater some structures similar to Earth’s stromatolites.
Image: a picture taken by the Mars Global Surveyor shows the layered structure of the lower part of the central mound in the Gale Crater (photo from NASA JPL).