Our Solar System: Mars

By Tom Farr   |   January 28, 2021

On July 20, 1976, seven years to the day after humans first walked on the moon, a bunch of us new employees of the Jet Propulsion Laboratory trooped over to Caltech’s Beckman auditorium (the one that looks like a circus tent) to see the first landing of a spacecraft on another planet. Viking 1 was about to touch down on Mars and it didn’t matter that it was three in the morning. We even managed to sneak in a bottle of champagne and waited in the balcony for the big event. 

Down on stage was the eminent Mars scientist Tim Mutch commentating on the preparations for landing, how it had to be autonomous as the time lag for radio commands was too long. We had to wait to find out whether the landing was successful, but the first pictures would come through shortly afterward. No one had seen the surface of Mars, though we had some good photos from orbit. But we couldn’t see enough detail from orbit to know whether Viking 1 would land on a smooth plain or a bunch of rocks, ending the mission before it started. 

Finally, the landing time came and we waited 20 minutes to find out how it went. Success! And then the first pictures came in. As a geologist, I wanted to know whether we could see evidence of the work of water, as the orbital pictures showed. But the slow scan appearing on the screen showed a plain of rocks. That didn’t stop Dr. Mutch from getting so excited he called out “and we can see light rocks, and dark rocks!” It was a punchline we used for years after. 

Viking 2 landed soon after and both landers performed many experiments to help understand Mars’s geology and history. One experiment, designed to see if the soil contained living organisms, seemed to confirm life was present, but later analyses showed that inorganic reactions could produce the same results. 

We wouldn’t land again on Mars for 21 years when Mars Pathfinder and its rover Sojourner landed in Ares Vallis. As its name implies, Pathfinder was more of a tech demo, though it performed its tasks perfectly. It demonstrated for the first time an airbag landing, bouncing along the surface after the parachutes released. The lander platform carried the rover, which was about the size of a toaster oven and weighed only 23 pounds. The rover had to communicate with Earth through the lander platform so it couldn’t rove too far. 

In the years since, over a dozen spacecraft have orbited Mars and 16 have landed (nine successfully, seven crashed: Mars is hard). And in February, three more spacecraft, from NASA, China, and the United Arab Emirates, will arrive at the Red Planet. NASA’s Perseverance and China’s Tianwen will land on the surface while UAE’s Hope will study the planet from orbit.  

So what have we learned about the Red Planet? Well, compared with Earth’s “evil twin” Venus, Mars is downright hospitable. In fact, if you were shown a picture of a Martian landscape and asked where it was taken, you might guess the Mojave Desert, Death Valley, or the Sahara. In fact, many of the geological processes that create and modify our terrestrial desert landscapes also operate on Mars so we planetary geologists spend a lot of time doing fieldwork in those places. But Mars is different too: It’s only about half the size of Earth, and it has some of the most extreme topography anywhere: Hellas impact basin is 23,000 feet deep, while Olympus Mons, the tallest volcano in the solar system, is 88,000 feet high. Valles Marineris is a rift valley as long as the U.S. is wide and four miles deep. Another big difference is that Mars has very little atmosphere: equivalent to about 100,000 feet altitude on Earth. Combined with its distance from the Sun makes Mars a cold place, and although it can get above freezing in the warmer areas, it goes down to about -195° F at the poles, enough to freeze carbon dioxide (dry ice). The low atmospheric pressure also makes for an interesting situation: liquid water can’t exist at the surface because the pressure is too low. A glass of water placed on the surface would either immediately boil away or first freeze then evaporate like dry ice. 

Being farther from the sun than Earth, its year is almost twice that of Earth’s (687 days), but its day is 25 hours, making for a case of continuous jet lag for rover controllers at JPL operating on Mars time. Mars also has a tilt to its axis similar to Earth, so it has seasons – the polar caps come and go as can be seen even in small telescopes. 

The biggest mystery of Mars has been why do we see river valleys and ocean shorelines all over Mars when liquid water can’t flow on its surface. Early on, some scientists proposed that it was liquid hydrocarbons, but finally we realized through further measurements and modeling that Mars lost a thick atmosphere in its first billion years because it had no magnetic field to shield it from the solar wind. The atmosphere literally blew away. In 2004, the rovers Spirit and Opportunity helped cement the notion of an early watery Mars when they found layered outcrops that were set down by flowing water and modified later by percolating fluids. Layered outcrops are the holy grail for geologists as we try to put them together into a stratigraphy defining the history of a place. I like to call the layers ‘pages in the book of Mars’ as we try to piece together the story of the planet. We found a mountain in the center of Gale crater that showed from orbit layers of Mars history from the wet times to the dry times and now the Curiosity rover is slowly climbing Mount Sharp (named after one of my Caltech professors), reading those chapters and pages. 

Now that we’ve established that liquid water existed for a long period on Mars’ surface, the obvious next question is: did life evolve? And if life ever existed, there may be fossils. This has also increased the possibility that life could currently exist in places more hospitable than the surface, like subsurface hot springs. Perseverance is the next step toward answering those questions. It will land on delta deposits (just like the Mississippi Delta) in Jezero crater. The rover includes a microscope to examine rocks for small fossils and an internal chemistry lab to analyze samples for organic molecules. In a first for a Mars mission, Perseverance will select rocks for further analysis back on Earth and cache them for pick up by a Mars Sample Return mission sometime in the future. 

Another first for the Perseverance rover is that it will carry a small helicopter called Ingenuity that will be used to capture aerial HD video as well as help plan rover traverses. While we’ve added lots of smarts to our rovers, we still worry about them driving autonomously too far, so Ingenuity will help move the rover more quickly over the Martian terrain. 

Mars is well positioned in the evening sky these days – you can see its ruddy hue high up in the southern sky, but you’ll have to imagine the small armada approaching their February rendezvous.


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