Rosetta

Rosetta was a space probe launched by the European Space Agency whose mission is to orbit the comet 67P/Churyumov-Gerasimenko (comets are named as a combination of their catalogue number and their discoverers) and study it to learn more about the Solar System's origins.

This space probe consisted of two parts: Rosetta, the main spacecraft, and Philae, the lander that landed on the comet.

Rosetta was originally planned for launched in 2003, to study a totally different comet. But the launch was delayed, and a new trajectory was conceived. Rosetta was launched by the European Space Agency on March 2, 2004, beginning its hugely complicated series of flybys in space. No rocket could propel the spacecraft directly to the comet, so Rosetta's trajectory had to be long and complex. The first encounter was an Earth flyby, about a year after launch on March 4, 2005, where Rosetta tested its instruments and received a gravitational pull which sent it on orbit that would take it close to Mars. On February 25, 2007, Rosetta made a successful flyby of Mars, and it was set towards the Earth once again. As Rosetta approached the Earth for the flyby of November 13, 2007, an astronomer mistook the spacecraft for a small asteroid, about 75 feet in diameter, and predicted that it would pass within only a few thousand miles of the Earth. This caused momentary panic, until its track was recognized as Rosetta's.

After its second Earth flyby, Rosetta flew by by an asteroid, 2867 Steins, but did not use many instruments on it, to save most of its memory for the comet encounter. The asteroid was only about 2.5 miles across and Rosetta approached within 480 miles of the body on September 5, 2008. Although not much information was gathered on the asteroid, its orbit was verified more accurately than ever before.

Rosetta then made its last flyby of Earth on November 13, 2009.

On July 10, 2010, the probe encountered the asteroid 21 Lutetia. At its closest approach Rosetta was only 1960 miles away, and a significant amount of data was gathered. The ongoing analysis of this data will determine the composition of the asteroid. In addition, the exact orbit characteristics, mass, and rotation have been obtained, as well as many photographs.

Rosetta's image of the asteroid 21 Lutetia at closest approach.

After the second asteroid encounter, Rosetta shut off its systems in deep space in order to save its energy for the comet encounter. This stage is known has "Deep-Space Hibernation" and it began in May 2011. In total, this stage lasted for almost three years and ended on January 20, 2014. On this date, Rosetta sent a signal indicating that the hibernation had been successful and the spacecraft's systems were functioning normally. By this time, the comet had passed its apogee and it was once again approaching the Sun, so that Rosetta was close enough to power itself on the star's energy.

Even then, though, both the spacecraft and the comet were traveling relatively slowly. Rosetta began observations in January of that year as it approached the comet. In May, Rosetta began a series of maneuvers to prepare to orbit the comet. At the same time, a halo began to develop around the comet as it approached the Sun. The following image from early May shows this halo, or coma. This coma forms from sublimating ice and the release of gas trapped inside the comet's nucleus. In late June 2014, Rosetta measured that about 2 glasses of water a second were released from the comet in the form of vapor. This rate increased as the comet approached the Sun.

On July 14, Rosetta was close enough to obtain images of the body of the comet, revealing an unusual two-lobed structure (above). Images continued to increase in resolution as Rosetta approached.

Through July and early August 2014, the spacecraft continued to use fuel to decrease its speed relative to 67P/Churyumov-Gerasimenko, since orbit around such a small body ultimately required Rosetta's relative speed to the comet to be only 1 m/s (a typical walking speed)! In addition, Rosetta took its first temperature and surface readings. On August 6, 2014, Rosetta became the first spacecraft to ever enter orbit around a comet.

Rosetta had moved within 60 miles of the comet by this time, and began to map the surface with high-resolution imagery (such as the image above) to identify possible landing sites. By the end of August, 5 possibilities for a landing site had been chosen. During September, Site J, which the white plus sign marks on the image below, was selected for landing. In early November, this site was renamed "Agilkia" after an island on the Nile river.

On the days leading up to landing, Rosetta altered its orbit to release trajectory. During the morning of November 12, separation of Rosetta and Philae was confirmed. Seven hours later, the lander successfully touched down at the Agilkia site, but its harpoons did not activate, and the lander bounced twice, ultimately landing about a kilometer away from its original intended site.

Philae took images, such as the above, showing the comet from 40 m above during its initial descent.

Upon its final descent, Philae unfortunately landed on its side, with most of its solar panels facing the ground or in shadow. On November 15, after using its battery for 57 hours, the lander entered hibernation. However, during its 57-hour period of activity, Philae successfully used all 10 of its scientific instruments. This included a drill, which chemically analyzed a sample from the comet's surface and relayed the data to Earth via Rosetta. Notably, Philae did detect the presence of organic molecules on the comet's surface.

Important observations continued to pile up from the Rosetta orbiter over the next several months. During December 2014, Rosetta measured the composition of water vapor released from the comet as it approached the Sun. This vapor had more deuterium (the isotope of hydrogen with one neutron) than the water found on Earth. The measurement suggested that comets such as 67P/Churyumov-Gerasimenko (theoretically originating in the Kuiper Belt near Pluto) may not have been the primary source of water for Earth's oceans, which were likely filled by impacting solar system bodies.

In February 2015, as the comet continued to approach the Sun and grow more active, Rosetta departed its near circular orbit and positioned itself for several close flybys to collect gas released from the comet. The first of these flybys took place on February 14 at a distance of less than 4 miles!

Rosetta took the above image of the two-lobed comet on March 14, 2015. It has been enhanced to indicate the increasing volume of dust and gas streaming from the comet as it approached perihelion.

The same month, Rosetta also completed the first ever detection of molecular nitrogen at a comet. This offered crucial information as to the comet's origin, because molecular nitrogen can only become trapped within a comet's ice at very low temperatures. Therefore, the measurement supported the theory that 67P/Churyumov-Gerasimenko originated in the Kuiper Belt. During the spring, researchers also used data from both Rosetta and Philae to conclude that the comet was not significantly magnetized, the first such measurement for a comet. This was significant because iron was a major component of protoplanetary dust and some models indicated that assembled objects would have been magnetized in the early Solar System. The result gives another way to test theories of the formation of the Solar System.

During June, communication with the lander Philae was sporadically established as its solar panels were illuminated. However, no more investigations were able to be made. Later that month, the ESA officially granted an extension to the Rosetta mission through mid-2016.

The above images show the higher comet activity as it approached perihelion. The comet's closest approach to the Sun did not take it inside Earth's orbit, but it reached a minimum distance of 1.28 AU on August 13, 2015.

The mission continued to yield new results after perihelion had passed. For instance, using data from July 2015, it was able to measure and investigate the size of the diamagnetic cavity surrounding the comet's nucleus. This cavity is a region where the gas streaming off of the comet deflects incoming charged particles from the solar wind. Assuming the comet itself is not magnetized (which Rosetta determined it wasn't), this region should be free of magnetic fields. However, it only becomes appreciable in size near perihelion, when more material is ejected from the comet.

The above chart shows the magnetic field measured by Rosetta during a period of a few hours on July 26, 2015. The blue highlighted region (in which the field is nearly 0) corresponds to when the spacecraft passed through the diamagnetic cavity along its orbit. Rosetta passed into the edges of the cavity many times, allowing for detailed measurements of its size.

Continued analysis of data from near perihelion in August indicated that certain organic substances were present on Rosetta, including glycine, the simplest of the amino acids. Though the discrepancy in isotope ratios discussed above indicates that comets of this exact type did not transport these compounds to Earth in its early history, it supports the general claim that glycine can originate from comets, a conclusion hinted at by earlier study but never unambiguously verified.

Data from the probe indicated that the comet "breathed out" primordial oxygen near perihelion; that is, heating up the body of 67P released gases that had been trapped there for billions of years. While other mechanisms had been proposed for the molecular oxygen found around the comet, the Rosetta mission verified that a majority must have appeared via this process.

On September 5, 2016, as Rosetta began its final approach toward the comet, it spotted the lander Philae from a distance of 2.7 km after its precise location had not been known for two years! The image below has an astonishing resolution of 5 cm/pixel, and shows the lander in the bottom right, wedged in a crack. This explains why Philae was not able to operate under solar power after its landing.

Late on September 29, 2016, Rosetta's thrusters fired for the last time, sending it on a collision course with the comet. The spacecraft continued to transmit valuable data and images during its descent, taking advantage of its only opportunity to collect information from such a low altitude above the comet. The last image transmitted from Rosetta, shown below, was taken at an altitude of only about 20 meters, just before impact. The final impact occurred on September 30, ending communication with the probe.

The 12-year Rosetta mission provided invaluable knowledge about the structure, composition, and evolution of the comet 67P, and through it a better understanding of the formation of our Solar System billions of years ago. Comets are among the most pristine of time capsules for investigating the beginning of the Solar System as well as possible sources of water and other important molecules on Earth. The Rosetta mission will continue to shape our conception of our origin for years to come.

Sources: (Photos from ESA) ESA-Rosetta, http://en.wikipedia.org/wiki/Rosetta_(spacecraft),http://www.bbc.com/news/science-environment-27498534, http://www.astronomy.com/news/2014/07/the-twofold-comet-comet-67pchuryumov-gerasimenko, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_arrives_at_comet_destination, http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/09/philae_s_primary_landing_site/14819792-1-eng-GB/Philae_s_primary_landing_site.png, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_comet_contains_ingredients_for_lifehttp://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_and_Philae_find_comet_not_magnetised, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_finds_magnetic_field-free_bubble_at_comet,http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_s_comet_contains_ingredients_for_life, http://www.esa.int/Our_Activities/Space_Science/Rosetta/Mission_complete_Rosetta_s_journey_ends_in_daring_descent_to_comet

Tornadoes

This is the second part of a two part post. For the first part see here.

Simple thunderstorms are simply clouds developing at a frontal boundary, and these are the most common, but other, more complicated types also exist.

Although some thunderstorms from exclusively on the frontal boundary, many spawn other thunderstorms miles, or even tens of miles away whenever there is warm, humid air present. This is because thunderstorms cause downdrafts, or cool air to flow down from upper altitudes. When this air hits the ground, it spreads out and flows parallel to the ground. This moving air can help to create new thunderstorms in two different ways. Either the cold air pushes the warm air up, and as we already know, a warm updraft causes thunderstorms, or, a cool wind gust can encounter another one, and they push each other up, creating additional thunderstorms.

In many cases, thunderstorms are composed of individual units, called cells. These cells form and die in minutes, but as one dies, its downdrafts produces another cell, and on. The progression of these cells tend to move because cells form in humid air at the boundary of air masses, and as the boundary moves, the area of formation moves. However, there are cells that form in a more independent way then this.

The most powerful type of cell is known (rather appropriately) as the supercell. Like a normal cell, the supercell has an updraft fueling its development: a mesocyclone. A mesocyclone is a rotating column of air that feeds the storm with the moisture and rotation it needs to be powerful. A mesocylone is typically between 1 and 5 miles in diameter and can extend up to 10 miles in to the sky. It forms when strong high altitude winds and weak ground winds blow past each other, creating a tube of air. An updraft lifts this tube, and it eventually breaks into two tubes, one spinning clockwise, the other counterclockwise. Due to the Coriolis Effect, the clockwise column is neutralized in the northern hemisphere and the counterclockwise column is strengthened, while this is reversed in the southern hemisphere.

Once the mesocyclone has formed, if there is a frontal boundary present, a colossal anvil shaped cumulonimbus cloud is formed, often stretching over 60,000 feet into the air! Warm middle layer winds feed the cyclone, while upper level winds spark fierce downdrafts. In addition, the supercell creates such a temperature difference in the air, that its downdrafts create their very own mini-frontal boundary. All this, coupled with the powerful mesocyclone at the center, brings heavy rain or hail for hours, and a supercell can travel over 300 miles before dying out.

As if this wasn't enough, sudden huge downdrafts called microbursts can also form during the duration of a supercell. Often, this downdraft pushes down another coil of air spinning parallel to the ground, again forming two columns. The counterclockwise swirl near the ground is expanded by updrafts to form one huge rotating air mass, from the ground way into the clouds. This is the tornado.



Tornadoes are immensely powerful and are actually, contrary as it is to common belief, invisible. Only when debris is picked up by the tornado does it assume a color and this depends totally upon the terrain. Tornadoes range from virtually harmless to incredibly deadly and can last from seconds to hours. Also, multiple tornadoes can come from one mesocyclone and often orbit around each other within it, assuming the form of a funnel cloud, which appears to be one tornado and can be largely hidden among clouds until it hits.



A funnel cloud hidden in the atmosphere. There is a vague suggestion of a rotation in the cloud structure, but there is no obvious tornado, like there is above.

Although a majority of tornadoes are spawned from supercells, some do not. It is possible for a weak tornado to develop from a regular thunderstorm, but this is uncommon, and a tornado in this situation is usually so weak that it is virtually undetectable. In some cases, tornadoes also from over water, forming what is known as a waterspout. As with land tornadoes, the waterspout assumes the color of water, and in most cases, appears blue. Some waterspouts have been known to develop mysteriously in fair weather, but most, known as tornadic waterspouts, form in thunderstorms. Hurricanes can be a source of tornadoes and waterspouts, when circulations form in their thunderstorm bands, and some hurricanes have caused over 100 tornadoes! Three Atlantic hurricanes have attained this: Hurricane Beulah in 1967, with about 115, Hurricane Frances in 2004, with 103, and Hurricane Ivan in 2004, with 117. Of Frances's tornadoes, 6 were considered significant, or with winds exceeding 110 mph, and Ivan had 19 significant tornadoes.



A waterspout off Florida's coast.

Also, dust devils, or swirling gusts of wind, can form in a dry environment, usually a desert. One last type of rotating mesocyclonic structure is a "fire whirl" or "steam devil". If there is a significantly heated environment, as in a fire, or heated water, the updrafts caused by the heated air can be strong enough to start a mesocyclone. The latter types of rotating mesocyclonic structures are not associated with thunderstorms, but are related since they are similar to tornadoes. They are usually weak in comparison to a real tornado.



A dust devil that formed in Nevada. Note the fair weather conditions.

Until recently, most areas used the Fujita scale to measure tornado strength. However, the U.S. thought the categories and their respective damages educated guesses at best, and the scale was changed into what is now known as the Enhanced Fujita Scale.



A comparison of the original Fujita scale introduced in 1971, and the new Enhanced Fujita Scale, which was first used in 2007. Although the Fujita scale covers a higher range of wind speeds, the Enhanced version gives more accurate damage information, and considers that anything under 65 mph is not a tornado at all, but perhaps a spinning funnel cloud that doesn't reach the ground.

The most powerful tornado recorded had peak winds around 300 mph, and wreaked havoc in Oklahoma City in 1999. This tornado also caused the second most damage of any single one, with adjusted damages of $2 billion. However, a tornado in 1896 has the greatest adjusted damage value, at a stunning $2.9 billion. In terms of width, one tornado had a damage path width of 2.5 miles in 2004. In terms of path length, the infamous Tri-State Tornado is the record holder, traveling 219 miles over three states, Missouri, Illinois, and Indiana. It also holds the record for the deadliest U.S. tornado with 695 deaths recorded. The huge F5 tornado wreaked havoc in 1925. In the world, the deadliest tornado probably belongs to the Bangladesh 1989 tornado which took an astounding 1300 lives. Finally, the largest tornado outbreak occurred in 1974, on April 3 and 4 of that year. An extremely powerful system spawned an amazing 148 tornadoes recorded and probably more weak ones going unrecorded. Among these, 23 were F4 and 6 were F5! The number of major tornadoes was simply unheard of, and this outbreak is still, after over 35 years, the largest one in history.



All of the tracks of the tornadoes in the Super Outbreak of 1974. They are numbered in order of development, from 1 to 148.

There have been many exceptional tornado oddities, most involving debris and people being picked up and placed back down unharmed. There have been many claims of this type and some have been disputed. For more information, see here.

Tornadoes are among the most violent weather events on the Earth, and they are the most deadly, there being no accurate way to predict them. Although tornado watches and warnings are put into effect when there is a risk or a sighting of a tornado, these can only be given a few minutes in advance. Climatology wise, tornadoes occur in various parts of the world. As well as the U.S. southern Canada and northern Mexico, much of Europe, South Africa, areas of Argentina, Australia, and Eastern Asia also experience tornadoes regularly.

Currently, Earth is the only celestial body on which tornadoes have been detected. There are rotating systems on other planets, such as Jupiter (the Great Red Spot and various other systems), and many of these may feature mesocyclones, but no real evidence for extraterrestial tornadoes has been collected.

Sources: The Weather Book by Jack Williams, http://www.srh.noaa.gov/mfl/?n=waterspouts, http://www.nssl.noaa.gov/edu/safety/tornadoguide.html, National Hurricane Center

Introduction to Tornadoes: Thunderstorms

Thunderstorms are powerful rain storms that usually have heavy rain, wind, thunder, and lightning associated with them. For a specific post on thunder and lightning, see here.

A thunderstorm is caused by a low pressure system, usually extratropical in nature. An extratropical low is one that is not tropical. A tropical low is usually a tropical storm or hurricane. Furthermore, extratropical lows are usually asymmetrical, and most rainfall with them comes with frontal boundaries. There are two main types of frontal boundaries, cold and warm. For convenience, a brief description is described below.

A cold front is an area of cold air advancing against an area of warm air. The warm air, being lighter, is pushed up. At this boundary, moisture forms, causing clouds. In a cold front, the clouds start low, and billow upward, as the cold air pushes higher up into the sky. The clouds often reach five miles in height and are then called cumulonimbus clouds. Heavy rain or even hail fall form these clouds. Then, as the boundary moves on, the clouds expend all their moisture into rain and dissipate.



A warm front is an area of warm air advancing against an area of cold air. Unlike a cold front, the effects of a warm front start at high altitudes as the warm air pushes the cold from above. The first clouds are usually high-level, such as cirrus or cirrostratus. Then, as the warm front continues to advance, the clouds come at lower altitudes, typically ending in the low altitude nimbostratus cloud. Lighter rain, or, if in winter, snow, can fall from these clouds. Then, as the clouds empty out their moisture, the clouds thin, sometimes ending in a white stratus cloud.



There are other types of fronts as well, that are not as well known. One is the stationary front, which is as boundary of cold and warm air in which neither is advancing. There is usually little storm activity associated with these fronts. Another rarer type of front is the occluded front, where a cold front catches up and combines with a warm front, ending in a boundary of cold and cool air. Since the difference in air temperature is less prominent in these systems, the fronts usually subsequently dissipate.

An area of thunderstorms form in a line, usually along a cold frontal boundary. Along this front, a squall line forms. A squall line is a thin area of heavy rain and wind that trails from a low pressure system, often hundreds of miles away.



A low pressure over northwestern Pennsylvania and an accompanying squall line to the south.

The internal structure of the thunderstorm is one of rising air. Usually warm air on the surface rises, cools, forms a few cumulus clouds, and then sinks when it cools to past the temperature of the surrounding air. However, in a cold frontal boundary, abnormally warm air is rising into cold air, and it therefore does not stop. The clouds continue upwards until they reach cumulonimbus stage, where the air finally cools and sinks. A healthy thunderstorm is a series of updrafts and downdrafts balancing each other out. The downdrafts help water particles to condense and become rain. However, although contrary to what would be thought at first glance, hail forms during the warmer seasons, when there are thunderstorms. Hail forms when water droplets form in the part of the cloud where the temperatures are below freezing. The updrafts send the ice higher into the cloud, until the plummet, attach to other droplets in the lower atmosphere, and then are swept up again. This continues until the ice droplets get heavy enough to fall. In strong updraft conditions, hail can reach over half a foot in diameter and weigh over a pound.

Another uncommon occurrence with thunderstorms is thundersnow, which is a thunderstorm at below freezing temperatures. These are rare, because thunderstorms are prevalent in the warm seasons. This is because there is a greater difference between ground temperatures and atmospheric temperatures and air can more easily stay warmer than surrounding air. As a result, cold fronts are generally strong in summer and weak in winter, whereas warm fronts are generally strong in winter and weak in summer.

Most of the time, thunderstorms are relatively simple systems. However, when a tornado forms, the situation is much more complex. Tornadoes are discussed in the next part of this post, see here. For more information on particularly intense thunderstorms called derechos, click here.

DNA and RNA as the Basis of Evolution

DNA is familiar to most people as our genetic code, the stuff that makes us who we are. Although this is true, it does not nearly encompass all that DNA is responsible for.

In humans, you must travel into any organ, such as the heart, find a tissue, such as heart tissue, and find cells within that tissue. The cells, on their own, are as complex as some organisms. But within the cell, we must magnify even further, and find the blob in the center that is known as the nucleus. Inside this nucleus, we encounter DNA wrapped and doubled up upon itself many times to form X-shaped structures called chromosomes. The human body contains 46 of these chromosomes in each cell, and each one has a much longer DNA strand (n fact, uncoiled, the DNA in a chromosome stretches 10,000 times longer). To fully understand of what importance DNA is, one must understand the structure of DNA.

DNA, short for deoxyribonucleic acid, is in the form of a double helix, which resembles a twisted ladder. The sides of this twisted ladder are made of deoxyribose, which is a sugar. The "rungs" of this ladder are called base pairs and there are four types. These types are cytosine, adenine, guanine, and thymine. These are abbreviated C, A, G, and T, respectively. These four are DNA bases. The bases A and T, and the bases C and G, always bond at a molecular level to form each "rung".



A diagram of DNA labeling each of the base pairs.

DNA is present in all known living organisms, which suggests that DNA would be behind the origin of life. However, another component of the cell is needed to aid DNA for life to continue. This component is the all-important enzyme. Most enzymes are proteins, and they are responsible for many of the chemical reactions that occur in the body. Remarkably, the combination, breaking down, and transforming of chemicals within the body is controlled by enzymes. For example, the breaking down of food in the stomach is attributed to acids, but the breaking down of chemicals at a molecular level is handled by the enzymes. In fact, DNA is dependent on enzymes for its very survival! Enzymes help to split DNA and later replicate it and even "check" for errors in the DNA strand, which lowers the number of mutations and changes that occur, which, if left alone, have the potential to cause disease and genetic disorders. In conclusion, enzymes are, along with DNA, a huge success in evolution that has developed remarkably over the last few billion years.

Despite the fame and omnipresence of this remarkable pair, DNA and enzymes are not believed to have been the basis of life on Earth. The most widely believed hypothesis has RNA present before DNA and enzymes, and all the simple life forms that only had RNA have been extinct for several billion years. But first, a brief (or possibly not so brief) word about RNA.

RNA is another acronym, standing for ribonucleic acid. This name is the same as deoxyribonucleic acid, except without the "deoxy". As explained previously, the sides of the double stranded DNA ladder are made of deoxyribose. This differs from RNA, because RNA is single stranded, with the strand made of ribose. Due to the absence of a second strand, the base pairs of RNA are just hanging off of it, which makes RNA more unstable, but it also has its advantages. The final major difference is that one of the base pairs, thymine, is absent, and replaced by uracil (U). The new base uracil still bonds with adenine (A) to make a base pair. The unique structure of some types of RNA allows the strand to fold over on itself, forming a double helix shape with a "hairpin" on the end.



In this image, a section of RNA is starting to fold up upon itself.

However, the single stranded nature of the RNA molecule provides many more benefits that to simply fold up upon itself. It can also attach to other RNA strands, and that is where the different types of RNA come in. There are three main types of RNA: messenger, transfer, and ribosomal RNA. Although there are other types, the three above will be sufficient to explain the main purpose of RNA.

Messenger RNA, abbreviated mRNA, contains the genetic code needed for synthesizing chemicals, mainly amino acids, which make up the center of a protein. The genetic code is a sequence of base pairs, just like in DNA. Transfer RNA, abbreviated tRNA, "reads" the genetic code from the mRNA by attaching itself to the mRNA and matching up the base pairs. Together, tRNA and mRNA create the many different types of amino acids using their genetic code to cause chemical reactions. The amino acids are transported by the tRNA, hence their name, to the site of protein formation, the ribosome. The ribosome (one of the main parts of a cell) contains ribosomal RNA, abbreviated rRNA, which develops the "meat" of a protein, the amino acid being a main component. The tRNA carries the amino acid into the rRNA's construction area by attaching to the rRNA, therefore creating a finished protein.

From the above information one can conclude that although DNA and enzymes are better at their individual jobs, RNA is more flexible, capable of both carrying a genetic code in a double helix form and starting chemical reactions that produce important parts of a living thing, amino acids, and completed proteins. Therefore, RNA is hypothesized to be present without DNA and enzymes in the simplest forms of life, some of them, without a cell. We define "living things" as things with cells, which isn't necessarily an accurate definition. Although there is no absolute information from the era of the first living things (which was from 3-4 billion years ago), we can speculate on events that lead to the first single celled organisms, some of which survive today.

The only thing surviving from the era that could be a living thing but isn't considered one is the virus. The virus isn't an organism by our standards because they don't contain cells. They do reproduce, which is another requirement for being a living thing, but it cannot reproduce on its own; it needs a host cell to infect with its genetic material. The encoded genetic message tells the cell to make for viruses, therefore spreading the virus. Viruses, on their own, are mere balls of genetic material, some types have DNA and some types have RNA, surrounded by protein. My personal opinion is that life can exist without cells, and that life, if started on other planets besides ours, could lack cells as well.

The moral of this story is that for all terminology, including "living thing" "mammal" (the main debate here is the platypus, which lays eggs, but is still considered a mammal) and even "planet" (Pluto. The rest is history) does not draw a distinct line classifying things. Things that possess RNA could be considered living, some (not many, but some), consider everything with atoms "living" in a sense. At this point, "living" is not an appropriate word, and should be replaced by "has the potential for living when grouped together with enough of its own kind". Nothing in this realm is clear, and it cannot be solved. By dropping terminology though, we can say that RNA is the basis of the first organisms that have set the stage for all bacteria, protists, plants, fungi, animals, and ultimately, us.

Sources: http://dl.clackamas.edu/ch106-08/enzymes.htm, http://www.uic.edu/classes/phys/phys461/phys450/ANJUM04/RNA_sstrand.jpg (image), http://www.elmhurst.edu/~chm/vchembook/583rnatypes.html, The Ancestor's Tale by Richard Dawkins.