Lecture Notes: Mars Life Search, Biology Fundamentals, and Chemistry Basics

Mars rovers and life-search missions

• NASA has invested heavily in a program to answer whether life exists elsewhere by sending a sequence of Mars rover expeditions.
• Rovers are large, car-sized, mobile landers used to drive around the Martian surface, drill into the ground, and collect soil samples and photos.
• History notes:
  • There have been six successful rover landings on Mars.
  • Five of these landings were by the United States (NASA).
  • Two rovers are still active today: Curiosity (since 2012) and Perseverance (landed in 2021 after launching in 2020).
• Curiosity:
  • Has been on Mars since $2012$.
  • Historically shared updates (even tweeted) from Mars about taking pictures; the speaker fondly recalls following it.
  • Current challenge: its tires are wearing out and may fail in the future; the vehicle has traveled significant distances.
• Perseverance:
  • Launched in $2020$; landed on Mars in $2021$.
  • Has driven across substantial sections of the Martian surface and collected many samples.
  • Has covered about $7000 ext{ km}$ on Mars.
• The core mission aim of rovers: to look for signs of life by analyzing geology, climate, and especially potential bio-signatures.
• The speaker hoped to show an engineering video about Perseverance’s landing and sample collection, but the sound/video failed; still, the takeaway is the rover’s role in demonstrating life-detection capabilities.
• If you can’t watch the video, a quick exercise is to discuss with peers what would count as evidence for life on Mars (evidence can be Earth-centric or more speculative).
• Evidence concepts discussed:
  • Water as a key indicator of habitability.
  • Carbon-based compounds as building blocks for life.
  • The possibility of life forms that might not resemble Earth life.
• The point of the exercise is to realize that our Earth-centric assumptions may bias how we search for life elsewhere.
• A key framing question: what counts as life, when life on Earth is used as a template for defining life elsewhere?

Defining life: Earth-centric definitions and nuance

• Life is a nuanced target; a single simple definition is elusive, especially when extending to potentially non-Earth chemistries.
• Biologists commonly organize life around a set of core features, acknowledging that there are exceptions and gray areas.
• Core features often considered:
  • Growth: Living things increase in size or complexity (e.g., bacteria grow by cell division; multicellular organisms develop tissues and organs).
  • Reproduction: The ability to generate copies of themselves, enabling persistence across generations.
  • Homeostasis: Maintenance of stable internal conditions (e.g., humans maintaining ~$98^ ext{°F}$ internal body temperature despite external changes; endotherms vs ectotherms).
  • Metabolism: Transformation of energy from one form to another to fuel cellular processes (e.g., plants capturing light energy via photosynthesis). Metabolism involves energy transformation and utilization to build tissues and power activities.
  • Response to stimuli: Ability to respond to environmental cues; not only movement, but also processes like plant tropisms (e.g., leaves changing energy allocation toward sunlight).
  • Evolution: Long-term, heritable changes that adapt organisms to changing environments.
• Some examples and thought experiments:
  • Plants respond to light by reallocating energy in leaves, demonstrating response to stimulus without traditional movement.
  • Paramecia and amoebae demonstrate responsiveness to their environment even without a brain or nervous system.
  • Spoon worm reproduction is notably unusual: the female hosts tiny male parasites that live inside her ovaries and fertilize them; this is an extreme example of biological reproduction diversity.
• The field recognizes that biology is a science of exceptions; there are organisms that do not fit neatly into every criterion, illustrating the fluid boundary between life and non-life.
• Example discussions:
  • Mule: a horse-donkey hybrid that is sterile (cannot reproduce) yet clearly alive; thus not every life criterion must be satisfied for something to be considered living.
  • Fire: grows and uses energy but is not composed of cells; it illustrates why cell theory is a strong organizing principle but not the sole criterion for life.
• A foundational framework often invoked is cell theory: all living things are composed of cells, which helps explain most Earth-based life and many exceptions (viruses, prions) force us to refine or rethink definitions.

Cell theory and its exceptions: viruses and prions

• Cell theory claims: all living things are composed of cells.
• Exceptions and gray areas discussed:
  • Viruses: do not have cells; cannot grow or reproduce on their own; require a host cell’s machinery to replicate; many scientists debate whether viruses are truly “alive.”
  • Prions: rogue proteins that can misfold and induce other proteins to misfold; no DNA or RNA involved; can self-propagate by changing other proteins; they are not cells and are not organisms in the conventional sense.
• Virus biology highlights two fundamental cycles:
  • Lytic cycle: virus infects a cell, uses cellular machinery to produce new viral copies, then lyses (bursts) the host cell, releasing new viruses.
  • Lysogenic cycle: virus integrates into the host genome and remains dormant while the cell replicates; copies of the virus are inherited by daughter cells; can be activated later.
• The presence of viruses in nearly all life forms (including humans) is emphasized by the fact that a portion of our DNA is of viral origin (ancient viral DNA).
  • In humans, roughly 7 ext{-}9 ext{ ext%} of DNA is viral in origin; these ancient viral elements are typically dormant but can contribute to disease (e.g., cancer) when regulatory control fails.
• Prions, unlike viruses, can self-propagate without nucleic acids; they are rogue proteins that alter the shape of normal proteins, causing widespread dysfunction.
• Where viruses came from: a debated hypothesis called the escapee hypothesis suggests viruses may have originated from transposons (parasitic DNA that replicates itself) that escaped from cells into the surrounding environment; an alternative view suggests viruses may have emerged first and driven the evolution of other genomic elements.
• The “chicken-and-egg” problem exists: did viruses evolve from cellular elements, or did certain cellular elements (transposons) originate from viruses? The exact origins remain an active research topic.
• Takeaway: the boundary between living and nonliving is fuzzy, especially for these entities; this is central to discussions about life beyond Earth and the search for life using Earth-centric criteria.

Viruses in depth: life cycles, origins, and philosophy

• Lytic cycle details:
  • Virus infects a cell, commandeers replication machinery, produces many copies, and causes cell lysis to release new virions.
• Lysogenic cycle details:
  • Virus integrates into the host genome (as a prophage), remains dormant as the host cell divides, and can later reactivate to begin the lytic cycle.
• Significance of dormancy for immune evasion:
  • Dormant viral genomes help viruses dodge immune detection until conditions favor replication.
• Ancient viral DNA and cancer links:
  • Endogenous viral elements exist in our genome; cancer can arise when cellular regulation fails and dormant viral copies become active.
• The escape hypothesis (transposons as ancestral viruses):
  • Parasitic DNA elements (transposons) proliferate within genomes and may have escaped cells to give rise to viral forms.
  • In aquatic environments, DNA can persist in water and potentially seed early viral forms; this is a provocative, not fully established idea.
• Reverse hypothesis (viruses as descendants of transposons) also discussed; the origin story of viruses is not settled.

NASA’s search for life on Mars: instruments and evidence targets

• The Curiosity rover carries an instrument suite including an Organic Molecule Detector nicknamed SANO (name expanded in lecture but not specified).
• SANO’s goal: detect organic molecules and carbon-based compounds; specifically looks for:
  • Carbon-based compounds
  • Water or evidence of past water
• Mars evidence for habitability:
  • Mars has ancient lake beds and deltas, indicating past water activity.
  • Large areas on the Martian surface show dried-up lake basins; some deltas resemble river deltas seen on Earth.
  • This historical water activity supports the possibility that life could have existed or evolved there in the past.
• Current state of water on Mars:
  • Mars has a very thin atmosphere, so most surface water is lost over time (evaporation) or trapped underground; the Earth’s atmospheric water cycle helps Earth retain liquid water; Mars lacks a thick atmosphere to retain water long-term.
• The search logic: if life ever existed on Mars, one would expect to find:
  • Water-related evidence and liquid water history in the geology
  • Carbon-based organic compounds that could serve as building blocks for life
  • Possible biosignatures in minerals or soils that indicate biological activity

Chemistry: foundations for life detection and matter

• Matter basics:
  • Matter is anything that occupies space and has mass.
  • Elements are pure substances that cannot be reduced to simpler substances by chemical means.
  • Atoms are the smallest units of an element; overall, matter is built from atoms arranged into molecules.
• Atomic structure (models used in teaching):
  • Bohr (planetary) model: a nucleus with protons and neutrons; electrons orbit the nucleus in defined shells.
  • Electron cloud model (more accurate): electrons exist as probabilistic clouds around the nucleus; electrons are not in fixed orbits but occupy regions of space with certain probabilities.
  • Key distinction: the nucleus contains protons (positive charge) and neutrons (neutral); electrons (negative charge) form surrounding electron clouds. Most of the atom’s mass is in the nucleus (due to protons and neutrons) while electrons have negligible mass.
• Reading the periodic table:
  • Periods (rows) indicate the number of electron shells used by the element in its neutral state; for example, elements in period 1 (like hydrogen and helium) have only one electron shell.
  • Groups (columns) reflect similar outer electron configurations (valence electrons) and thus similar chemical properties.
  • Blocks (s-block, p-block, d-block, f-block) describe the sublevels being filled by valence electrons and help predict chemical behavior and bonding patterns.
  • The colors/groups (e.g., noble gases, transition metals, alkaline metals, alkaline earth metals) are common groupings used to describe broad families of elements.
• Atomic number and neutral state:
  • Elements are ordered by the number of protons in the nucleus, the atomic number.
  • In a neutral atom, the number of protons equals the number of electrons, balancing charge.
  • The atomic mass equals the sum of protons and neutrons in the nucleus; electrons contribute negligibly to atomic mass.
• Simple Earth-life examples illustrating atomic concepts:
  • Hydrogen: $1$ proton and $1$ electron; in period 1, group 1.
  • Lithium: same period as hydrogen (one shell) but has $2$ electrons in the outer shell (as discussed in the lecture), illustrating how electron configuration defines grouping.
• How chemistry links to life and interpretation:
  • Chemistry explains how elements combine to form molecules with properties different from the constituent elements (e.g., table salt formed from sodium and chlorine).
  • The interaction of elements and molecules drives metabolism, energy transformation, and the emergence of biomolecules.
• Important caveat from the lecture:
  • The lecture uses some simplified or historical models (Bohr model) to illustrate concepts; actual atomic behavior is more accurately described by quantum mechanical electron clouds.

Synthesis: Connecting biology, chemistry, and astrobiology

• Earth-centric biases in thinking about life can limit how we search for life elsewhere; Mars may host life that looks very different from Earth life.
• The biology section emphasizes core life features but acknowledges exceptions (e.g., viruses, prions) that blur the line between living and non-living.
• The chemistry section provides the toolkit to interpret potential biosignatures: carbon-based compounds, water, and the chemical context in which biomolecules could arise.
• The Mars exploration narrative ties together how rovers like Curiosity (since $2012$) and Perseverance (launched $2020$, landed $2021$) advance our understanding of habitability and the potential for past or present life on Mars.
• Ethical and philosophical implications (implicit in the discussion):
  • Defining life itself has epistemic and practical consequences for how we search for life beyond Earth.
  • The possibility of discovering life (even if microbial or dormant) raises questions about contamination, planetary protection, and how humans interact with other worlds.

Quick reference: key numbers and terms from the lecture

• Rover landings and activity:
  • Total successful rover landings on Mars: $6$
  • Landings by NASA (United States): $5$
  • Rovers still active: Curiosity (since $2012$), Perseverance (since $2021$)
  • Perseverance mission: launched in $2020$, landed in $2021$, distance traveled around Martian surface: $7{,}000 ext{ km}$
• Curiosity notes:
  • On Mars since $2012$; tires wearing out due to long-term driving on rough terrain
• General internal temperatures mentioned:
  • Human internal temperature: $ext{about } 98^ ext{°F}$
  • Room or ambient temperature example: $68^ ext{°F}$ in a typical room
• Viral DNA in genomes:
  • Proportion of DNA of viral origin in humans: 7 ext{-}9 ext{ ext%}
• Mars water context:
  • Mars atmosphere: very thin, contributing to loss of surface liquid water over time
• Chemistry and physics basics:
  • Matter and mass definitions; elements, atoms, nucleus, protons, neutrons, electrons; electron clouds vs. orbital models
  • Atomic number equals the number of protons (and electrons in neutral atoms); atomic mass equals protons plus neutrons
• Miscellaneous terms:
  • SANO: Organic Molecule Detector used in Mars exploration
  • Growth, reproduction, homeostasis, metabolism, response to stimuli, evolution: core life characteristics
  • Spore worm (spoon worm) example in reproduction; paramecium and phagocytosis; lysogenic vs. lytic cycles in viruses
  • Mule vs. fire as examples of exceptions to simple life definitions