Basking in the ferocious heat of our fiery Star, in the inner regions of our Solar System, the quartet of
rocky terrestrial planets--Mercury, Venus, our Earth, and Mars--are well-lit worlds that are primarily composed of silicate rocks and metals. All terrestrial planets possess solid surfaces, in contrast to the gaseous giant planets in our Solar System's outer realm--Jupiter, Saturn, Uranus, and Neptune--which are mostly made up of some mixture of hydrogen, helium, and water, existing in varying physical states. But, perhaps, no other planet in our Sun's intriguing family has captured our attention more than Mars, the fourth planet from our Sun, as well as our near neighbor in space. This is because it has tickled our imaginations as the world beyond our Earth that is most likely to harbor life. Liquid water is essential for the evolution of life as we know it and, at last, in October 2015, members of the Mars Science Laboratory (MSL) announced their discovery that ancient water once flowed and formed lakes on Mars--and this discovery carries implications about the ancient climate of this bewitching, enchanting, captivating "nearby" world.
The MSL is a robotic space probe mission to Mars that was launched by NASA on November 26, 2011, which successfully landed the rover Curiosity in Gale Crater on August 6. 2012.
For years, planetary scientists studying Mars have heard the intriguing chant: follow the water. In a research paper, published in the October 9, 2015 issue of the journal Science, a team of astronomers from MSL presented the results of their quest. The scientists not only followed the water, but also tried to understand where it had originated, and exactly how long it had lasted, so very long ago, on the surface of the Red Planet.
The ancient Martian story that the astronomers uncovered is a soggy one. Mars apparently had a much more massive atmosphere billions of years ago than it has now, and it also possessed an active hydrosphere capable of storing water in long-lasting lakes. The MSL team has come to the conclusion that this water helped to fill Gale Crater, the MSL rover Curiosity's landing site, with sediment deposited as layers that served as the foundation for the mountain discovered in the middle of the crater today.
Gale Crater is estimated to be between 3.8 billion and 3.6 billion years old. In the middle of September 2014, the heroic little rover reached the foothills of Aeolis Mons, which is a three-mile-high layered mountain that has been nicknamed "Mount Sharp" to honor the memory of the late Caltech geologist Robert Sharp. Curiosity has been exploring the base of this distant mountain ever since.
"Observations from the rover suggest that a series of long-lived streams and lakes existed at some point between 3.8 and 3.6 billion years ago, delivering sediment that slowly built up the lower layers of Mount Sharp. However, this series of long-lived lakes is not predicted by existing models of the ancient climate of Mars, which struggle to get temperatures above freezing," explained Dr. Ashwin Vasavada in an October 8, 2015California Institute of Technology (Caltech) Press Release. Dr. Vasavada is MSL project scientist. Caltech is in Pasadena, California.
Terrestrial Planets
Sometimes alternatively termed telluric planets, terrestrial (Earth-like) planets all have approximately the same structure: a silicate mantle surrounding a central metallic core that is mostly composed of iron. Terrestrial planets can show mountains, craters, volcanoes, and other surface features, depending on tectonic activity and the presence of liquid water. Furthermore, these rocky planets possess secondary atmospheres that were formed as the result of comet impacts or volcanic eruptions. In contrast, the quartet of giant gaseous planets inhabiting our Solar System's outer regions, possess primary atmospheres. It is thought that the four outer planets gravitationally snared their heavy gaseous envelopes directly from the original protoplanetary accretion disk surrounding our neonatal Sun.
Astronomers have spotted a number of protoplanetary accretion disks surrounding young stars. These disks are thought to form at the same time as their stars, within a veiling, opaque shroud that envelops the natal pocket composed of dust and gas. The protoplanetary accretion disk feeds the hungry baby star a nourishing mixture of gas and dust so that it can grow up to achieve true stardom. Accretion disks can whirl around their young stars for as long as 10 million years.
By the time the fiery, hungry, searing-hot stellar toddler has entered the T Tauri stage of its young life, the protoplanetary accretion disk has dramatically thinned out and cooled off. A T Tauri is an extremely young and very active Sun-like star that sports a mass that is about the same--or just a bit less-- than that of our Star today. However, T Tauris show diameters that are several times larger than our Sun, but shrink as they grow older. At this point, less volatile materials have started to condense near the center of the whirling accretion disk. This condensation gives rise to extremely sticky, very tiny particles of dust. These naturally sticky dust grains harbor crystalline sillicates.
The tiny motes of sticky dust collide with one another and merge in the crowded environment of the protoplanetary accretion disk, "gluing" themselves to one another, forming ever larger and larger objects--from pebble-size, to mountain-size, to planet-size! The newly formed objects gradually grow, evolving into planetesimals--the primordial building-blocks that collided and then merged together to form major planets.
Some relic planetesimals can even linger around long enough to compose an impressively large population of primitive objects billions of years after the formation of a fully-formed planetary system, such as our own Solar System. In our Solar System, the asteroids are the relic rocky and metallic planetesimals that built up the four, rocky inner planets, while the comets are what is left of the icy planetesimals that contributed to the construction of the giant, gaseous planets of the outer limits. During the formative years of our Sun and its family of objects, there were considerably more rocky, terrestrial planetesimals than there are now.
The Red Planet
Mars is frequently referred to as the "Red Planet" because of the iron oxide that exists abundantly on its surface, giving it a reddish hue. This terrestrial planet is enveloped by only a very thin atmosphere, and it displays surface features that resemble the impact craters of Earth's Moon, as well as the valleys, deserts, volcanoes, and polar ice caps of our own planet. The seasons and rotational period of Mars are also similar to those of Earth, in addition to its tilt that is responsible for the seasonal cycles. Mars also hosts the largest volcano and second-highest mountain in our Solar System--Olympus Mons--as well as Valles Marineris, one of our Solar System's largest canyons. Covering 40% of the Red Planet, the smooth Borealis basin in the northern hemisphere may really be an enormous scar caused by an impacting object long ago.
Mars has a duo of little moons, dubbed Phobos and Deimos, which are thought to be captured asteroids that wandered too close to the gravitational embrace of the major planet that they now circle. Both tiny moons are misshapen objects that resemble large potatoes, lacking the gleaming silvery majesty of Earth's own large, spherical Moon.
Mariner 4 succeeded in making the first successful flyby of Mars back in 1965, and this visit caused many planetary scientists to speculate about the possibility of liquid water existing on its surface. This idea was based on the observed periodic alterations of light and dark regions, most notably in the polar latitudes, which resembled seas and continents--dark, long striations that were interpreted by some scientists as irrigation channels for flowing liquid water. These straight lines, however, were ultimately shown to be only optical illusions. However, geological evidence nonetheless continued to whisper enticing hints that Mars once was covered by water--a large amount of water. Indeed, in 2005, radar data showed the presence of a great quantity of water ice at the poles and at mid-latitudes. The Mars rover Spirit spotted chemical compounds containing water molecules in March 2007, and the Phoenix lander directly detected water ice in shallow soil samples on July 31, 2008.
Mars is currently being visited by seven spacecraft, five of which are in orbit: 2001 Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, MAVEN, and Mars Orbiter Mission. The two missions that are currently exploring the Martian surface are the Mars Exploration Rover Opportunity and the MSL.
Following The Martian Water
A conflict exists between predictions of Mars's ancient climate derived from models designed by paleoclimatologists, and suggestions that the planet had a very soggy past, as proposed by geologists. This conflict bears certain similarities to a century-old scientific puzzle that concerns our own planet's ancient past.
This scientific detective story began when geologists first noticed that the shapes of the continents on Earth matched each other, almost like scattered pieces of a jigsaw puzzle, explained Dr. John Grotzinger in the October 8, 2015 Caltech Press Release. Dr. Grotzinger is Caltech's Fletcher Jones Professor of Geology, chair of the Division of Planetary and Geological Sciences, and lead author of the October 2015 research paper appearing in Science.
"Aside from the shapes of the continents, geologists had paleontological evidence that fossil plants and animals in Africa and South America were closely related, as well as unique volcanic rocks suggestive of a common spatial origin. The problem was that the broad community of earth scientists could not come up with a physical mechanism to explain how the continents could plow their way through Earth's mantle and drift apart. It seemed impossible. The missing component was plate tectonics. In a possibly similar way, we are missing something important about Mars," Dr. Grotzinger went on to explain this scientific conundrum.
Before Curiosity started its historic journey of discovery at Gale Crater, it paused to study numerous regions of particular interest. All targets were imaged, and soil samples were obtained from some; the rocks in certain special places were drilled for samples. These samples were then deposited into the rover's onboard laboratories. Using the treasure trove of information gathered by these instruments, as well as visual imaging obtained from the onboard cameras and spectroscopic analyses, MSL planetary scientists put together an increasingly coherent and intriguing saga about the evolution of this region of the Red Planet.
Before Curiosity started to roam around Mars, many planetary scientists already suspected that Gale Crater had been filled with layers of ancient sediments. Some of the proposed theories were "dry", meaning that the sediments had accumulated from dust and sand blowing around in the rushing Martian wind. However, alternative theories suggested that it was possible that sediment layers were deposited in ancient lakes and streams of flowing liquid water. The most recent results obtained from Curiosity suggest that these soggier models were correct for the lower portions of Mount Sharp. Based on the new analysis, the filling of at least the bottom layers of the mountain occurred primarily as the result of ancient rivers and lakes.
"During the traverse of Gale, we have noticed patterns in the geology where we saw evidence of ancient fast-moving streams with coarser gravel as well as places where streams appear to have emptied out into bodies of standing water. The prediction was that we should start seeing water-deposited, fine-grained rocks closer to Mount Sharp. Now that we've arrived, we're seeing finely laminated mudstones in abundance," Dr. Vasavada explained in the October 8, 2015 Caltech Press Release. These silty layers in the strata are believed to be ancient lake deposits.
"These finely laminated mudstones are very similar to those we see on Earth. The scale of lamination--which occurs both at millimeter and centimeter scale--represents the settling of plumes of fine sediment through a standing body of water. This is exactly what we see in rocks that represent ancient lakes on Earth," explained Dr. Woody Fischer in the Caltech Press Release. Dr. Fischer is professor of geobiology at Caltech and coauthor of the paper.
"Paradoxically, where there is a mountain today there was once a basin, and it was sometimes filled with water. Curiosity has measured about 75 meters of sedimentary fill, but based on mapping data from NASA's Mars Reconnaissance Orbiter and images from Curiosity's cameras, it appears that the water-transported sedimentary deposition could have extended at least 150-200 meters above the crater floor, and this equates to a duration of millions of years in which lakes could have been intermittently present within the Gale Crater basin," Dr. Grotzinger explained in the same Press Release. In addition, the total thickness of the sedimentary deposits in Gale Crater that are suggestive of interaction with water could reach up higher still--perhaps as high as 800 meters above the floor of the crater, and possibly representing tens of millions of years.
However, the layers of sediment laid down above that level do not depend on water being the agent of deposition. "Above 800 meters, Mount Sharp shows no evidence of hydrated strata, and that is the bulk of what forms Mount Sharp. We see another 4,000 meters of nothing but dry strata," Dr. Grotzinger noted in the October 8, 2015 Caltech Press Release. He went on to suggest that possibly this era of the crater's history may have been dominated by wind-driven deposition, in a way that some planetary scientists once thought explained the lower part explored by Curiosity. This would have occurred after the wet period that constructed the base of the mountain.
One nagging question that remains involves the original source of the water that drove sediment into the crater. In order for flowing water to have existed on the ancient Martian surface, Mars must have had a thicker atmosphere and toastier climate than what has been suggested for the time frame during which this intense geological activity would have occurred in Gale Crater. Evidence for this ancient, wetter climate does show up in the Martian rock record. Alas, existing models of this paleoclimate--factoring in estimates of the ancient atmosphere's composition, mass, and amount of energy it acquired from the Sun--come up dry. Those models suggest that the atmosphere of Mars could not have sustained the necessary large amounts of liquid water.
However, the rock record discovered at Gale Crater indicates something else. "Whether it was snowfall or rain you have geologic evidence for that moisture accumulating in the highlands of the Gale Crater rim. Right on the other side of Gale's northern rim are the Northern Plains. Some have made the argument that there was a northern ocean sitting out there, and that's one way to get the moisture that you need to match what we are seeing in the rocks," Dr. Grotzinger explained in the Caltech Press Release. However, determining the possible location of an ocean still does not help to explain how that water managed to exist as a liquid for extended periods of time on the Martian surface.
In the case of Gale Crater, at least some of the water originated in the highlands that form the rim of the crater, but groundwater discharge is unlikely in this area--even though it is a standard explanation to reconcile wet geologic observations with dry paleoclimatic predictions.
"We have tended to think of Mars as being simple. We once thought of the Earth as being simple, too. But the more you look into it, questions come up because you're beginning to fathom the real complexity of what we see on Mars. This is a good time to go back to reevaluate all our assumptions," Dr. Grotzinger added.
