Day 15: Defining Life
by Angela Nan
Today, we took a step back from the physics we were doing before and had a discussion about astrobiology. The possibility of life outside of the Earth has always intrigued us, so it’d only be natural for us to touch on the topic in a class about the Solar System. But before talking about where life could be or how we could find life, we had to deal with a more basic problem: what is life in the first place? This was one of those problems that grow as you spend more time on them. We all like to think that we know what life is, but when you actually try to pin it down concretely with a definition, you realize that it’s really hard. In class, we looked at three big definitions of life to see what was good and what wasn’t before coming up with our own versions. Some of the definitions we thought about seemed too vague. Others seemed not general enough. More importantly, we disagreed on a lot of the details of the definitions. It was really easy to pick holes in definitions but not nearly so easy to patch them up right. My group started out by listing a few things we thought amounted to “life”. We thought that life had to be capable of evolution and could actively adapt to the environment. Life should also be self-sustaining and self-replicating to some extent. We also added in a point about being traceable to a natural starting point so sufficiently-advanced robots that fit the other criteria wouldn’t qualify as “alive”. After hearing opinions from other groups and rethinking details, I felt that our definition needed modifications. Overall, the class thought evolution and ability to harness energy to reproduce and locally expel entropy was important. I would also add that something alive would have to have biologically developed through evolution as well. This means that something like a computer program would not qualify as alive while a dog would. I didn’t find the answer we came up with by the end of class that satisfying. It’s probably a sentiment that a lot of other people who think about this issue share. On one hand, it’s difficult to find an appropriate definition for life. On another hand, the only life we know about is what’s happened here on Earth. Statistics make it hard to argue that life hasn’t appeared anywhere else in the Universe, and learning about other life would probably throw everything we think we know about life on its head. This means that even if we find a good definition for life, it probably wouldn’t work for other systems. However, no matter how much we argue the details about what qualifies as life, we will end up knowing more than we did before. I’m very excited about the possibilities.
by Alice Michel
In today’s class we tried to answer the question presented in the video lectures: how do we define life? Everybody seems to come to fairly similar conclusions about what we can call life and what we can’t: a rock isn’t alive, a bacteria cell is, DNA isn’t. There are some things people don’t agree on, of course (this is where people step on other people’s toes), but for the most part life is pretty good at recognizing life. So what characteristic is it about life that makes us know something is alive? That’s what we were trying to figure out in class today. As it turns out, it’s not an easy question to address. But if we want to charge off into space looking for life, it would be helpful to have a working definition such that we will know life when we see it.
Per the usual class structure, we divided up in groups. The first step was to look at some commonly used definitions of life and argue with them. Then all the groups came up with their own definitions and we voted for our favorite. The three definitions we looked at said life (a) avoids decay to equilibrium (Shrödinger), (b) is a self-sustaining system capable of Darwinian evolution, and (c) reproduces, uses energy, and follows instructions embedded in the organism. There are issues with all of these, but I liked the first one the best. It’s overly simplistic, but that leaves room for personal interpretation. When we were looking at the definitions, I kept coming back to all the “what-if” cases, from super-advanced robots to viruses—the fringes of “life.” None of the definitions do a good job excluding robots and viruses.
My group talked a lot about how life seems to be opposed to the 2nd law, at least for a time, because it is based on order whereas everything else in the universe goes to a state of disorder. Atoms aren’t alive, but they make up cells, which can be alive, but are cells within an organism independent life? No...right? Basically, the question gets quite philosophical/meta fast. By the end of class we were pretty sure everything was alive (just kidding).
Our final definition was life is… An ordered system independent of its environment and able to respond, in a “sentient” manner (“active,” be it a brain or just impulses to survive) to its environment. Additionally, it was originally naturally produced by the Universe. (That’s to rule out robots.)
On the search for life in the universe, we paused our extraterrestrial observations to think about life and its definition.
First Professor Brown noted some of the well-known definitions of life.
A) Schroedinger noted that life avoids decay to equilibrium.
B) A team of scientists including Gerald Joyce coined the definition of life as a self-sustaining system capable of Darwinian evolution.
C) Benton Clark narrowed down his 102 observable qualities of life to life reproduces, and life uses energy. These functions follow a set of instructions embedded within the organism.
Next, we had the opportunity to discuss these definitions in small groups. In my group, we decided that Schroedinger’s definition seems incomplete because many other things fall into this category of avoiding decay to equilibrium as well. One example is a crystal that avoids losing its crystal structure. In addition, this doesn’t allow for a clear definition of life. Ideally, we want something that is clearly measurable or definable. This is especially important when we look for new life. One challenge with looking for life outside of earth is that we really do not have a good idea which characteristics of life on earth persists outside.
For definition B, we talked about each part of the two part of the definition. While adaptation is key for life to persist, we, for example, were not sure if Darwinian evolution was needed to persist. Furthermore, it was unclear to us how one would differentiate between things that are self-sustaining or not. Would humans be self-sustaining? Technically we rely on other systems such as our food network to survive.
The third definition of life seemed better defined. However, we wondered if mules are not part of life even though they do not reproduce. In addition, do crystals not reproduce by growing, use energy to grow, and are organized this way based on their crystal structures? One thing we remembered from Ge11b was that life influenced and was influenced by its environment. During this discussion, I read up on an article entitled “Defining Life” that actually talked about all three definitions (Mullen 2002). This put the definitions above into context. One interesting idea that was noted was that not every organism had to reproduce but that the community of organisms, called lifeforms had to reproduce.
Next, each student came up with a definition of life. Mine was that life is a collection of organisms that uses energy and follows instructions. Together with organisms that reproduce they respond to outside environments and mutate to persist. Then, combining our ideas, my group came up with a definition as follows:
on an individual level:
• Responds to the environment
• Uses energy to follow instructions
• Has the possibility to mutates
on a collective level:
• Some individuals use blueprints from self or others to reproduce and persist.
After each of the four groups wrote up their definition, we were able to explain and answer questions about our definition. Our main critics were that we could have made our definition simpler and that many innate things also respond to the environment. Therefore, a better definition of responds to the environment is needed. The fourth group addressed this issue by suggesting that the organism responds to the environment in a “sentient” manner.
Their full definition was:
Life is distinguishable from its environment, and can respond to its environment in a “sentient” manner because it has impulses to persist. It must be able to organically arise from the universe.
Group 1’s definition was:
1) Life is a self-sustaining physical system capable of Darwinian evolution. Self-sustaining is defined in the thermodynamic sense.
While Group 2’s definition involved:
• Adapts to environment via Darwinian evolution.
• Sufficient complexity
• Not intentionally designed by other life
In the end, each person got to vote on their favorite of the four definitions. Surprisingly, in the first round of voting our definition tied with that of Group 4. During the second run off, our definition persisted as the favorite. In the end, what I really learned is that defining life is multifaceted and that life is even more complex.
Mullen, Leslie. "Defining Life." Astrobiology Magazine. Astrobiology Magazine, 19 June 2002. Web. 21 May 2014. <http://www.astrobio.net/news-exclusive/defining-life/>.
by Allison Maker
Today’s class went deep into the philosophy of science. Stemming from the video lectures on the possibility of life on other planets, we decided to get down to the heart of the matter and debate on the definition of life itself.
We began by scrutinizing three of the most commonly accepted definitions of life:
1. Schrödinger (or as Mike Brown wrote it, Schrödïngër, because who knows how many umlauts there are) definition: Life “avoids decay to equilibrium.”
2. A “self-sustaining system capable of Darwinian evolution.”
3. “Reproduces, uses energy, follows instructions embedded in the organism.”
We split up into groups to debate these definitions, basically coming to the conclusion that each was both too broad and too narrow at the same time. We also thought of exciting concepts that are not normally deemed life, but fit the definition. The first definition could also be interpreted as life keeping its own entropy as low as possible or creating its own equilibrium, definitions that could include metastable minerals and could leave out the fact that cells go through homeostasis, which keeps an equilibrium.
Several groups agreed that the second definition could work for robots, provided they reproduce imperfectly. We also questioned whether self-modification, or even godlike figures, should they exist, would fit. (Something came up during discussion about an “immortal Obama.”)
The third definition didn’t come under as much scrutiny, partially because of its resemblance to the second, and partially because most groups were so focused on the first two.
After discussing the three main definitions, we came back together to form our own definitions of life. Our group’s idea was to subtly modify the second definition to add that the concept of “self sustaining” should only be by a physical/chemical means—this was done in order to exclude something more conceptual like a computer program. When it came to vote on the best definition, though, our definition was defeated by one that was more lengthy and all encompassing (I actually voted for it over ours). It read, “Life is distinguishable from its environment and can respond to its environment in a “sentient” manner because it has impulses to persist. It must be able to organically arise from the universe.” I thought this was a well-written description that included some qualifiers that were left out of the definitions we went over. It also worked in a wide context that could be applied to life on other planets, which is of course, what we’re really studying.
Day 16: Mission to Mars
Before class, Prof. Mike Brown sent out an email informing the class that we had received a multi-billion dollar budget from NASA to design a mission to Mars to detect signs of ancient life. Each of us were asked to come to class with a proposal of where we wanted to go, what we wanted to send there, what we wanted to do there, and why. The best concept was promised to be launched in 2017.
What a great opportunity! NASA was letting a bunch of Caltech hooligans design the next mission! Once class started, we broke off into committees to refine our mission proposals. Our group wanted to study the polar ice caps on Mars for signs of life. (Actually, a large fraction of our group just wanted to study the ice caps and didn't really care if life was there or not, but the quest for life was written into NASA's budget quite strictly.)
We were particularly interested in dating the ice layers on Mars to learn about Mars's past atmospheric composition and temperature. Ice layers that are deposited gradually over time trap tiny bubbles of gas. If we were to go back and take a sample of that ice, we could (hypothetically) determine the atmospheric composition of Mars when that ice layer was originally deposited. Additionally, stable oxygen isotopes fractionate when ice is deposited. This is because the heavier oxygen isotopes would rather become solid ice than the lighter oxygen isotopes. Because this fractionation process is temperature dependent, we hoped to use stable oxygen isotope measurements to determine the temperature of Mars in the past.
Life on Earth has had a dramatic affect on our atmospheric composition. If there was once life on Mars, perhaps that life would have also affected the Martian atmospheric composition. Our proposed experiment would also search for signs of organics trapped in the ice.
We wanted to send our mission to Chasma Boreale at the northern ice cap on Mars. This dramatic canyon boasts canyon walls that are 1,400 m high (4,600 feet). These canyon walls expose many layers of ice and wind-blown sand. Our proposed Mars rover would land at the top of Chasma Boreale. During the landing, a heavy anchor would imbed itself into the top of the canyon. Our Mars rover would then gradually rappel down the face of the cliff. While journeying down the face of the wall, our rover would drill cores of ice samples. These ice samples would be fed into a mass spectrometer to measure composition and isotope fractionation. Our samples would also be analyzed for signs of organics (after accounting for perchlorates, of course.) We would determine the age of the ice by counting ice layers and comparing our observations to a model of Mars's orbital variations.
We were called to present our proposal before a panel of TA-certified judges. The competition was fierce. The other two proposed missions were carefully planned and well presented. After some deliberation, the TA committee informed the class that our mission had been selected to go to Mars! Hooray! But before we could celebrate, Prof. Mike Brown informed us that our mission would never detect life, because the ice layers in Chasma Boreale simply did not go far enough back in time. We would only be able to see a few million years into the past. Our proposal was sadly overruled by the professor. Instead, group #3's rover was chosen to go to group #2's landing site.
However, we learned that a there is a real proposed mission to study the ice layers on Mars by drilling into the ice with a laser. Cool! I found a paper in Applied Spectroscopy by Arp et al. (2004) with more information. Here's a link if you want to learn more:
by Xiaolin Mao
Studies show that there were oceans on Mars in the past. This is easily associated with that there might be life on Mars since ocean initiated life on the Earth. In the class today, we were divided into 3 groups to discuss and come up with 3 multibillion dollar missions to find the answer for the question of whether or not Mars was ever inhabited.
Our group soon noticed that the key point to answer this question is to find remnants of life, which is also the most difficult part. Our plan included:
Firstly, choose a suitable landing site where life was prosperous and the remnants were easily kept in the sediments. By looking at the topography map of Mars, we decided to send our robots to 41oW 10.2oN region. The landing point was close to a delta of the outflow channel on one side, and was connected to the open ocean on the other side. We thought the warm temperature, the mixed materials from both the continent and the ocean and the shallow water depth in this region made it a most likely place to have life in the past.
Secondly, analyze the chemical environments for the sediments around the landing region to get several right drilling points. We need to know the PH values, the water content and the chemical components of the sediment to judge whether it is capable to keep the fossils.
Thirdly, drill several boreholes down to 10-20 meters. Robots will name and store the drill cores in order. At the same, robots take pictures of the drill cores and do some simple measurements, and then send these pictures and tested results back to the Earth. By analyzing all the information, experts might be able to tell whether there is or not any clue for life in the sediments and choose the interested drill cores to be sent back to the Earth.
Finally, do comprehensive measurements and analyses for the sent back drill cores on the Earth.
With our plan, scientists may be able to find some remnants of life or rule out the possibility of life on Mars, and then answer the question of whether or not Mars was ever inhabited.
by Jingyuan Li
After discussing in depth about the definition of life, we now move on to a topic that is currently receiving a lot of attention in the planetary sciences field: how do we find evidence of life outside of Earth? For Thursday's class, we were instructed to come prepared with a paragraph proposing our own mission to find life on Mars, including the location, what we're sending, what we're doing on Mars, and why. We then presented each of our individual ideas in class, and then got into three groups to come up with a detailed mission.
Our group was self-titled the "skeptics," because we weren't sold on the idea of simply going to Mars to find life. Thus, we came up with a mission that, while good for finding evidence of life on Mars, could also give us useful information about Mars even if there was no past life. We chose to go to Chasma Boreale, a canyon in the North Pole of Mars. We proposed an idea similar to ice core drilling on earth - a rover would be sent to climb down the canyon, taking samples along the way. We would also look at the dust layers entrapped in the ice. These layers, upon analysis, can reveal details about Mars' past climate, atmospheric composition, and of course, any possible organics in the dust or ice.
The other two groups chose Jezero Crater to look for microfossils at a region where there is evidence of standing water, and the Northern lowlands, drilling deeply to examine the sediment deposits in a debris field. After we all gave our presentations, the review committee (the TAs) got together and selected our group as the winner. Unfortunately, Mike Brown vetoed the ideas of all three groups, saying that our samples would not be old enough (only a few hundred million years back). Instead, he said the best idea would be to send a mission to Jezero Crater (location of the second group), but do what the third group suggested once there.