Introduction to Astronomy and the Search for Life Elsewhere

A computer generated image of a landscape with a mountain in the background

Figure 1: Matijevic Hill Panorama Showing Mars Opportunity Rover[23]

"With a sense of deep appreciation and gratitude," said Thomas Zurbuchen (NASA’s Associate Administrator) on February 13th 2019, "I declare the Opportunity mission as complete."[2]

Opportunity was one of the two exploration rovers sent to Mars in 2003. Along with its twin, Spirit, the rovers had four goals: ‘determine whether there was past life, characterise the climate and geology and prepare for human exploration of Mars’[3].

Mars is thought to be a planet that humans could, in the future, possibly live. The only requirement for life, as scientists understand, is the existence of liquid water on a planet’s surface. Mars was once thought to have rivers of flowing water on its rocky terrain seen by features on its surface (see figure 2) [4]. Now, there is strong evidence of north and south polar caps of water ice underneath solid carbon dioxide on Mars as well as Martian gullies that contain flowing salty water in specific seasons. So what evidence for water on Mars did

Opportunity provide and how significant were its findings?

A close up of a desert with rocks and dirt

Figure 2: Possible Dried Up Rivers on Mars’ Surface[4]

Why Mars?

NASA sent Opportunity to Mars for two reasons. Firstly, evidence from flybys and orbiters portrayed evidence of past life on Mars. Secondly, Mars seems to be the most similar planet to Earth, not in size but in features.

Previous observations show Mars has a gravity of 38% of Earth, a thin atmosphere consisting mostly of carbon dioxide that partially protects from the Sun’s radiation and a day length of 24 hours and 39 minutes[5]. Being part of the four inner planets in the Solar System (Mercury, Venus, Earth and Mars), Mars is characterised as a terrestrial planet, with a rocky surface allowing rovers to land and examine surface composition and weather[6]. The outer four planets (Jupiter, Saturn, Uranus and Neptune) are gas giants and hence can only be investigated through other spacecrafts.

Figure 3: Mars Projection Map and Spacecraft Landings[7]

The Martian surface is complex. Covered in craters similar to Earth’s moon, it has dry lake beds, canyons and volcanoes[7], many of which have been named. Figure 3 shows various spacecrafts sent to study particular areas of Mars. Spirit landed in the impact crater Gusev which was known to have mineral deposits. On the other side of Mars Opportunity landed at Meridiani Planum, what appeared to be a past lake, before proceeding 8.5 years later to Matijevic Hill (see figure 1)[8].

Why Rovers?

Rovers are vehicles specifically designed to move along the surface of a planet and make detailed observations. Other spacecraft such as flybys and orbiters are simpler missions as they only view the planet from orbit. Rovers vary in size and shape but they are approximately the equivalent of a golf cart up to a small SUV.

Interestingly, the first successful rover to reach a celestial body was launched in 1970, more than a year after Neil Armstrong famously walked on the moon[9]. Since then, rovers have been sent to Earth’s Moon and 3 generations to Mars.

The first of these rovers were the two Lunkhods rovers aboard the unmanned Lunar 17 mission to the Moon. These rovers were remotely controlled and had a number of instruments including a laser reflector, X-ray telescope and spectrometer, radiation detectors, an odometer, penetrometer and an imaging system[10]. Lunkhod01 is pictured left. The next rover on the Apollo 15 mission and was used for transporting astronauts and collecting samples to enhance the efficiency and findings of the Apollo 15, 16 and 17 missions[9].

Although the Moon rovers were launched in the 1970s, it wasn’t until 1997 that rovers finally landed on Mars. The first rover sent to Mars was the Sojourner. The Mars rovers were much more difficult to land on the surface than the Moon rovers as Mars has an atmosphere, more gravity and is much further away. Naderi from NASA describes the descent to land on Mars as “six minutes of terror”[11]. Instead of retrorockets used to land rovers on Earth’s Moon, Martian rovers also use airbags with heat shields and parachutes to descend slowly to a stop.

Easier landing and conditions meant early Moon rovers had a much higher expected lifetime than the rovers sent to Mars. The first Moon rover lasted almost a whole Earth year, while the first Mars rover lasted 85 days[12]. However, one particular rover surprised NASA scientists by exceeding its expected lifetime by 60 times. This was the Mars Opportunity rover.

Opportunity and Spirit were the second generation of Mars rovers. These rovers were fitted with the instruments: panoramic camera, microscopic imager, rock abrasion tool, magnets and Mossbauer, alpha particle X-ray and miniature thermal emission spectrometers[1]. Both rovers greatly outlived their estimated lifetime of 90 days. Opportunity lived to over 14 years fitted with “the finest batteries in the Solar System”[13]. As project manager Callas puts it - “we’d all love if our cellphone batteries lasted this long!” [13].

Opportunity continued to break records. In 2015, it had travelled 45.16km, the longest extra-terrestrial travel distance ever previously gone[1]. An artist’s impression of Opportunity can be seen in figure 5. As demonstrated, this rover is much more advanced than Lunkhod01.

The third, current generation of Mars rovers is Curiosity, launched in 2012 and still operational on Mars[14]. It is fitted with over 11 instruments including cameras, spectrometers, radiation detectors, environmental and atmospheric sensors.

Figure 5: Opportunity Rover[15] ; Figure 4: Lunkhod01 – The First Ever Rover[10]

Why is Opportunity special?

Opportunity is classed by NASA as being “one of the most successful and enduring interplanetary missions”[15]. It was significantly heavier than Sojourner, being 1062kg and the size of a golf cart[15]. Opportunity’s landing was incredibly smooth, straight into a crater, and lucky as it almost instantly found evidence of water on Mars – observing numerous little blue circular rocks nicknamed ‘blueberries’. Using Opportunity’s microscopic imager, the rocks were found to be the mineral hematite, an iron oxide[16]. This was significant as hematite generally form in the presence of liquid water which is a strong sign that in Mars’ past, liquid water is likely to have existed.

Figure 6: Blueberries[16]

Although the ‘blueberries’ were found right away, this was not coincidental since Spirit and Opportunity’s landing sites were carefully chosen from orbiter and other mineral evidence. However, it wasn’t until Opportunity landed that scientists could definitively determine the type and abundance of minerals. The first image of hematite blueberries sent to Earth brought excitement and further curiosity. Dr Knoll from Harvard university suggested in 2004 that the entire floor of Meridiani Planum may have blueberries and if so “one might guess that a much larger volume of outcrop once existed and was stripped away by erosion through time”[17].

Opportunity continued its mission to become the first stratigrapher on Mars. Stratigraphy is the study of rock layering. This was significant in finding evidence of the story of Mars over the centuries, much like investigating sedimentary rock layers on Earth for fossils and evidence of evolution. Opportunity’s exceptional travel of 45.16km served to map out Mars and prepare the way for future land missions, such as Curiosity. As Callas said “rovers made Mars familiar to us”[18]. Opportunity went from Endurance crater through Victoria and Endeavour craters to Marathon valley (named after Opportunity completed a marathon’s distance) and finally to Perseverance valley[16]. Despite its success, there were a few issues in Opportunity’s lifetime.

“Mars is a nasty place” says Steve Squyres – principal investigator[18]. Opportunity is powered by solar panels. During the 2006 global dust storm, Opportunity received less than 1% sunlight [18]. Power had to be preserved and used wisely to keep Opportunity ‘alive’. Opportunity’s mission came to an end in Perseverance valley on June 10, 2018, when another, far more intense dust storm covered Mars and Opportunity’s power dropped to zero[2]. As Abigail Fraeman, the deputy project scientist said “we’re gonna operate this vehicle until the day where we can’t – that’s exactly what we did, and I’m really proud” [16].

The end to Opportunity’s mission, although sad opened up a new era into Mars exploration by rovers through both its findings and challenges. For example, Curiosity was built to harness nuclear power rather than solar to combat this issue of dust storms.

What Were Opportunity’s Most Significant Finds?

Since finding the Martian ‘blueberries’ just a year into its mission, Opportunity continued to gain evidence through images, samples and various spectrographic techniques to support its four mission goals.

Undoubtedly, one of the key attributes leading to Opportunity’s great success was that hypotheses of findings were not accepted right away but as Squyres said “test[ed]…carefully” to “find the one that best fits the observations”[19].

Figure 7: Microbial Community Surrounded by Jarosite in Spain[21] ; Figure 8: Endeavour Crater False-Colour Image[21]

Goal 1: Determine Whether There Was Past Life on Mars.

The main two pieces of evidence for past life on Mars that Opportunity found were the minerals hematite (in the ‘blueberries’) and jarosite. Unlike hematite which can form from volcanic activity[20], jarosite only forms from acidic liquid water[21]. This was a massive discovery as it not only provided concrete evidence that early Mars contained liquid water but also that this water was acidic in nature. As seen in figure 7, particular microbial communities that are acid-lovers or acidophiles grow and flourish these acidic conditions and hence, Mars could have been home to similar acidophiles in its early days.

Goal 2: Characterise the Climate of Mars.

Being on Mars’ surface for several years has allowed Opportunity to examine and provide information of the climate of Mars. Figure 1 shows Opportunity using its panoramic camera which imaged large areas of Mars and demonstrated its desert-like appearance[21]. Furthermore, the panorama shows numerous sand dunes and ripples shaped by high wind currents[21].

Opportunity also discovered clouds in the Martian winter and thin layers of frost on equipment surfaces. Scientists now believe these clouds may contain ice particles[21]. However, in all 14+ years of Opportunity’s exploration, no liquid water was found on Mars’ surface. This is significant as scientists start to understand that if life existed on Mars in the past, it may have looked slightly different to Earth life currently – perhaps in the form of acidophiles.

Goal 3: Characterise the Geology of Mars.

Both Opportunity and its twin provided strong evidence of the planet being rich in sulphate and undergoing several meteorite and volcanic explosions[21]. Using its rock abrasion tool and miniature thermal emission spectrometer, Opportunity was able to take false-colour images by varying the wavelengths of light used to identify rocks and minerals and the processes that formed them[1]. The rover’s trip into Endeavour crater found interesting rock fragments that scientists are still researching (see figure 8).

A frustrating limitation with rovers is that they cannot be fitted with extensive heavy and large machinery due to its inability to land this equipment on Mars. Without using sample-return (where rocks and minerals are taken from a planet and returned to Earth for examination), the next closest way of identifying geology is using panoramic fake-colour imaging which illuminates particular aspects of geological structures.

Goal 4: Prepare for Human Exploration.

Opportunity established safe landing on and mapped out the climate and geology of Mars, as well as possible issues with lack of sunlight, water supply and harsh dust storms. In-depth note keeping of variations in solar radiation, atmospheric dust and temperature as well as types of materials, dust and their relative sizes will be crucial to manufacturing prototypes of human spacesuits for possible future missions[21]. Opportunity’s prolonged survival on Mars is comforting for human missions which could go ahead as soon as 2037 and are planning to last only one year according to NASA[22].

Conclusion

Ultimately, NASA’s exploration rover Opportunity has successfully fulfilled its goals and paved the way for future missions including Curiosity and the possible 2037 human trip to Mars. Opportunity has provided long-term information about Earth’s neighbouring planet and pointed out useful comparisons in possible prior life on Mars’ surface as well as the extreme climate and fascinating geology of the planet. As the future of Mars exploration falls into place I believe that Opportunity’s mission and its place will be invaluable.