Blinn College Department of Physics

Life in the UniverseChapter 231Cosmic EvolutionWhat is LIFE?Not so easy to answer, especially if we allow for types of life that are not found on EarthThese are some generally agreed-upon characteristics that any life form should have: Ability to react to environment Ability to grow by taking in nourishment and processing it into energy Ability to reproduce, with offspring having some characteristics of parent Ability to evolve

Brief History of Life on Earth4.4 billion years - early oceans form3.5 billion years - cyanobacteria start releasing oxygen2.0 billion years - oxygen begins building up in atmosphere540500 million years - Cambrian Explosion22565 million years - dinosaurs and small mammals (dinosaurs ruled)Few million years - earliest hominids3Necessities for Life As We Know It.A nutrient sourceEnergy (sunlight, chemical reactions, internal heat)Liquid water (or possibly some other liquid)4Applies to life as we know it.

Could there be life on Mars?

Searches for Life on Mars Mars had liquid water in the distant past. Still has subsurface ice; possibly subsurface water near sources of volcanic heat6

In 2004, NASA Spirit and Opportunity rovers sent home new mineral evidence of past liquid water on Mars.7Latest Martian Probe: Curiosity will explore Mars as a potential habitat for life, past or present.Previous twin rovers, Spirit and Opportunity found water on mars.

The Martian Meteorite debateComposition indicates origin on Mars. 1984: meteorite ALH84001 found in Antarctica 13,000 years ago: fell to Earth in Antarctica 16 million years ago: blasted from surface of Mars 4.5 billion years ago: rock formed on Mars9

Does the meteorite contain fossil evidence of life on Mars?Most scientists are not yet convinced.

Meteorite ALH84001Nanobacteria Fossils?Terrestrial Nanobacteria Fossils10

Could there be life on Europa or other jovian moons?11Ganymede, Callisto also show some evidence for subsurface oceans.Relatively little energy available for life, but there still may be enough. Intriguing prospect of THREE potential homes for life around Jupiter alone. GanymedeCallisto

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Titan The surface is too cold for liquid water (but there may be some deep underground). Has lakes of liquid ethane/methane on its surface.13

Enceladus Ice fountains suggest that Enceladus may have a subsurface ocean.

14Are There Habitable Planets Around Other Stars?Definition:A habitable world contains the basic necessities for life as we know it, including liquid water.It does not necessarily have life. 15Constraints on star systems:Old enough to allow time for evolution (rules out high-mass stars - 1%)Need to have stable orbits (might rule out binary/multiple star systems - 50%)Size of habitable zone: region in which a planet of the right size could have liquid water on its surface Even with these constraints, billions of stars in the Milky Way could potentially have habitable worlds. 16The more massive the star, the larger its habitable zone and the higher probability of a planet existing in this zone.

17Search for Extrasolar PlanetsKepler (launched in 2009) will monitor 100,000 stars for transit events for 4 years.105 so farLater: SIM and TPF interferometers may obtain spectra and crude images of Earth-size planets.

Extrasolar: Orbiting stars other than our sun. (The term exoplanet is also used.)18

Spectral Signatures of LifeEarthVenusMarsOxygen/ozone19

Are Earth-like planets rare or common?20

Elements and HabitabilitySome scientists argue that the proportions of heavy elements need to be just right for the formation of habitable planets.

If so, then Earth-like planets are restricted to a galactic habitable zone.21Impacts and HabitabilitySome scientists argue that Jupiter-like planets are necessary to reduce rate of impacts.

If so, then Earth-like planets are restricted to star systems with Jupiter-like planets.

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Climate and HabitabilitySome scientists argue that plate tectonics and/or a large moon are necessary to keep the climate of an Earth-like planet stable enough for life.23The Bottom LineWe dont yet know how important or negligible these concerns are.24

The Search for Extraterrestrial IntelligenceSETI25Factors to consider when calculating the number of technologically advanced civilizations per galaxy:Most of the factors are highly uncertain.Possible results range from 1 communicative civilization within a few dozen light years to us being the only communicative civilization in the Milky Way.0The Drake Equation

SETI experiments look for deliberate signals from extraterrestrials27The Arecibo Message

At dedication of Arecibo Radio Observatory, blocks of 1679 pulses were emitted toward globular cluster M13.

The pulses can be arranged in only two ways: 23 rows of 73 or 73 rows of 23.Resulting 23x73 grid contained basic information about our human society.0

We are also communicatingalthough not deliberatelythrough radio waves emitted by broadcast stations.These have a 24-hour pattern, as different broadcast areas rotate into view.Communication Through Leakage!!29If we were to deliberately broadcast signals that we wished to be found, what would be a good frequency?There is a feature called the water hole around the radio frequencies of hydrogen (21 cm) and the hydroxyl molecule (18 cm). The background is minimal there, and it is where we have been focusing many of our searches.The Water Hole

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How difficult is interstellar travel?Current spacecraft travel at