Though Earth is home to many diverse organisms, the search for forms of novel life has expanded past the planet’s boundaries. Scientists, in addition to investigating oceans and remote islands, are now making efforts to find the prerequisites for life on different planets and celestial bodies even beyond the solar system. These missions all share the same aim: the discovery of liquid water. This deceptively simple substance is the key to life here on Earth. Therefore, liquid water is a rational characteristic to search for in a quest to find different forms of life. Through missions to Mars, ongoing investigation of Pluto, and possible exploration of Jupiter’s moon Europa, the search for liquid water and extraterrestrial life continues.
As the closest accessible planet to Earth, Mars has been the subject of much publicity and exploration. Though Mars is currently barren and unable to retain liquid water due to its incredibly thin atmosphere, it may once have been more hospitable towards life. This claim is supported by many expeditions, including one notable mission involving two landing rovers, Spirit and Opportunity, which were launched from Earth in 2003. Each rover landed on a different side of the Red Planet in 2004, with the specific aim of finding rock formations which would indicate the past presence of liquid water. Both Spirit and Opportunity contained the same five scientific instruments designed to facilitate this search. These include a panoramic camera, a miniature thermal emission spectrometer, a Moessbauer spectrometer, an alpha particle X-ray spectrometer, and a microscopic imager, all of which were intended to augment the rovers’ interaction with the Martian surface. Additionally, each rover had a rock abrasion tool, used to break into and study the interior condition of Martian rocks, rather than their constantly exposed surface.
The mission was designed to take place over the course of 90 days; however, both rovers far exceeded this estimate. Spirit lasted until 2010 and Opportunity continued functioning until 2019, which is a testament to the talent of those who engineered them.
Spirit landed near Mars’ Gustav Crater, a wide basin that may have once been a lake.
Through its movement over the Red Planet, Spirit found evidence that Mars’ surface was shaped by massive impacts and was once home to volcanoes. From Gustav Crater, Spirit eventually reached Columbia Hills, a region of the planet marked with dunes. It discovered an oddly shaped rock, which was dubbed ‘Pot of Gold’, as it surprisingly contained the mineral hematite. This reddish material is an ore of iron and its formation often requires liquid water.
This was an unexpected but exciting find. Over the course of two days, Spirit utilized its rock abrasion tool to break into ‘Pot of Gold’ and survey its contents. The internal state of the rock revealed a heavy salt composition, which is another indication that water was present in the past.
Opportunity began its explorations at Meridiani Planum, a location that contains mineral deposits reminiscent of past water. Like Spirit, Opportunity also discovered hematite, though this time it was found at two craters named ‘Eagle’ and ‘Endurance’. The sand of both craters is characterized by a high sulfate content, which indicates the past presence of water. This high sulfur phenomenon occurs as water evaporates over time for as it disperses as a gas in the atmosphere, water vapor is unable to carry heavier sulfur compounds with it. Consequently, when a large body of water, such as that which can fill a crater, evaporates over a period of time, it leaves behind a great amount of sulfur compounds. This creates a uniquely elevated concentration of sulfur in a location that was once filled with water. The past presence of water in these craters was strengthened by the fact that the hematite found in them took the form of embedded spheres, indicating that the rocks in this area formed under aqueous conditions. Additionally, the hematite mineral is located throughout these rocks, rather than sequestered to particular layers. This implies that water, rich in minerals, was able to soak through these rocks and leave deposits dispersed throughout the formations.
Though the combined mission to Mars carried out by Spirit and Opportunity has formally ended, there are many ongoing explorations occurring within and beyond the solar system. One such undertaking involves the spacecraft New Horizons, which was launched in 2006 and flew by Pluto in 2015. It has since continued to gather and send back information from the Kuiper Belt as well as from regions beyond the solar system. New Horizons was the first spacecraft ever sent with the specific intention of exploring Pluto.
Because Pluto is extremely cold, astronomers did not expect to find liquid water; however, due to Pluto’s proximity to New Horizons, scientists were able to see the surface and composition in novel detail. Pluto’s exterior is so cold that it is able to contain solid nitrogen, methane, and carbon dioxide, which are compounds that exist on Earth as gases.
In addition to these solids, ice was found near Pulfrich Crater, which is located along Virgil Fossae. Though present in quite a significant amount, this ice had never been detected by instruments on Earth.
Another intriguing fact about Pluto is its axial tilt which, like that of Earth, produces yearly seasons. As Pluto approaches the sun, its ice begins to warm and eventually evaporate. Strong winds that whip across this Kuiper Belt object carry these gases and deposit them as solids on darker (and therefore colder) regions of its surface. Additionally, as Pluto turns away from the sun, gases freeze at the surface. Both of these processes facilitate an exchange of materials between Pluto’s atmosphere and its surface, which means that ‘fresh’ ice is continuously being deposited throughout Pluto’s surface.
While many of these discoveries substantiate the belief that Pluto is too cold to support life, there is a possibility that its internal composition might be entirely liquid. Though its surface is far too cold to be inhabitable, its inner depths are likely rather shielded from the extreme conditions of space. Pluto also once had a molten inner core, so regions closer to its center may have once held the capacity to support primitive forms of life. Such ideas will require further exploration to corroborate or disprove, but this demonstrates that even extreme conditions warrant attention in the pursuit for extraterrestrial life.
There are also multiple planned missions that place the search for liquid water at their core. One of these explorations is a mission to Europa, one of the many moons of Juipter. Around 2023 (the date has not yet been determined), Jet Propulsion Labs will launch the Europa Clipper, a spacecraft that will enter orbit around Jupiter in order to conduct intense scrutiny of Europa’s conditions. It will ultimately fly by Europa 45 times. Previous spacecrafts, such as those launched by the space shuttle Atlantis back in 1989, gathered data indicating the presence of a saltwater ocean which spanned the majority of the moon’s surface. While this discovery was unprecedented, the presence of a thick icy surface appeared discouraging for the existence of life.
However, in more recent findings from 2011, astronomers received images from space crafts studying Jupiter, indicating that Europa’s icy surface can mix with the underlying water and create motion. These pictures mostly illustrate the chaotic surface of Europa, whose constant motion is thought to derive from exchanges between its surface of ice and the ocean located below it. Researchers hypothesize that this movement might provide a possible transfer of light and nutrients between the moon’s surface and the depths of its ocean. Such a prospect alleviates some of the harsh conditions present on Europa and is therefore an uplifting discovery for those hoping to find lifeforms in its oceans.
The Europa Clipper is planning to make use of the past 2011 discoveries. Its onboard instrumentation will include both thermal and high resolution cameras and imagers, designed to determine the composition of Europa’s surface and its underlying oceans. A specific ice-penetrating radar will be used to measure the thickness of Europa’s surface ice, while a magnetometer will help determine the depth (and salinity) of its oceans. These measurements will help determine if the conditions on Europa are able to support various forms of life. If they are positive, they could also justify more extensive exploration of the moon in the near future.
Though space exploration is hardly a novel pursuit, there is still much to learn and discover. Some of the data recovered from Spirit and Opportunity is still being processed, and there are also future missions planned for Mars and its possible past life forms. Pluto remains a distant, extreme point of discovery, but the recent data gathered from New Horizons is constantly expanding the boundaries of current scientific knowledge. In a similar fashion, the planned space expedition to Europa has the capability of producing novel results through an in-depth exploration of a chaotic body of water. Thus, as life on Earth continues, the quest for extraterrestrial life is similarly propelled forward.