Download Mars Atmosphere and Climate

Lecture #21:
The Main Point
• The Martian Climate
•Evidence for climate change
•Did it rain on Mars?
•Can you have a snowball fight on Mars?
•Similarities to variations in Earth's climate...
• Reading:
Today Mars is a cold, dry, barren world.
But there is evidence that Mars 3 to 4
billion years ago may have been a much
more Earth-like planet
– Chapter 10.4
Astro 102/104
1
Astro 102/104
2
Mars ~3 billion years ago?
• Warm
– Thicker atmosphere, more greenhouse effect
• Cold
– Average temperature: -60°C
Mars Today
• Bone Dry
• Wet
– More water, in liquid form?
• Hospitable?
– Equivalent to a global layer of only a few microns
of liquid H2O
– Same ingredients for life as early Earth?
• Lifeless, as far as we can tell
The Climate of Mars has Changed Drastically
– Viking was sensitive to ppb levels of organics
– No ozone layer: Sun's UV gets to surface
Astro 102/104
3
Astro 102/104
This is a THEORY: What's the evidence?
4
1
Evidence for Mars
Climate Change
An Earthlike Mars?
• Can we find evidence that
Mars really had an
Earthlike environment?
• The search takes resources
like money, people, time,
equipment, technology, ...
• How much should we
devote to this search??
• Valley Networks
(a.k.a. "Runoff Channels")
• Heavy erosion of old craters
(degraded rims, no ejecta blankets)
• Presence of surface and subsurface ice
(abundant "stored" water?)
Astro 102/104
5
Martian Channels
• Valley Networks
– Hundreds of km long, tens of km wide
– Contain clear signs of fluid (water ) erosion
– Contain evidence of catastrophic flooding
– Source areas: collapsed terrain
– Formed by rapid melting of subsurface ice?
• How? Volcanism? Impacts?
– 2.5-3.5 b.y.?
– Drainage: S to N
Astro 102/104
6
Martian Channels
• Outflow channels
• OLD
Astro 102/104
Area of
Collapse
Channel
–
–
–
–
a.k.a "runoff channels"
Only tens of km long, a few km wide
Direct evidence of fluid erosion rare
Caused by sapping (undermining)?
• VERY OLD
Streamlined
flow features
Downhill (North)
7
– 3.5 b.y. + ?
– Mostly found in the
ancient S. highlands
Astro 102/104
"stubby" ends
Mars
dendritic
Earth
8
2
Sapping
...but then what are these?
• Valleys grow by headward
undermining of more easily
eroded layers: sapping
• On Earth, underground
liquid water is involved
• On Mars, more likely to be
underground water ice
• Valleys formed by sapping
look very different from
those formed by runoff
Mars Odyssey THEMIS image, near Valles Marineris
We are learning more, as new missions give us
better resolution and coverage of the surface...
• Until recently, there has
been little geologic evidence
for features formed by
rainfall on Mars...
But then where do
these sediments go?
Astro 102/104
9
Still, the state of our ignorance
about Mars is profound
Astro 102/104
Polar Layered Deposits
Layers about 10 m
Thick could be
detected from
initial Mars Global
Surveyor images...
– Accumulation of dark, dusty airfall layers in the ice
• Then, many years of colder conditions
– Less dust accumulation, brighter, icier layers
Astro 102/104
10
Earth
N. Polar layers in Viking image
Trough is about 500 m deep
Each layer is about 50 m thick
• Evidence for cyclical climate change on Mars
• Many years of warmer, dustier conditions
From the thickness of
the layers (tens of
meters) and an
assumption about the
rate of dust
accumulation, we can
estimate how long it
took to form each
layer: 105 to 106 years
Mars
Darker layer
(more dust, less ice)
Brighter layer
(less dust, more ice)
Layers only a few meters thick can
be detected from more recent
Mars Global Surveyor images
etc...
11
Astro 102/104
12
3
Liquid Water on Mars?
• Can liquid water exist on Mars today?
• Probably not, according to the phase diagram of water
• Phase of water (solid, liquid, or vapor) depends on Pressure & Temperature
• Important concept, not well described/discussed in the textbook...
Layering has had
a complex geologic
history!
Vapor
Liquid
T
x
x
MRO/CTX image
March 8, 2007
x
x
x
x
P
Astro 102/104
13
Astro 102/104
x
x
“Triple point”
T=273K (0°C)
P=6.1 mbar
x Solid (ice)
x
If the temperature on Mars
is not > 273 K and the
pressure is not > 6.1 mbar,
14
no liquid water is possible
But then how can we
explain the evidence
for very recent liquid
water on the surface
from these kinds of
MGS images??
• T > 273 K and
P > 6.1 mbar?
• Other processes??
Astro 102/104
15
MGS evidence of recent
Astro 102/104
water flowing on Mars?
Malin et 16
al.
Science, Dec. 2006
4
Mars Rover Discoveries!
• There was liquid water on or very near the surface of Mars
early in the planet’s history and for “long” periods of time...
But:
• Exactly when?
• For how long?
• Implications?
Opportunity in Victoria crater
Spirit and
Opportunity are
still going strong,
still trying to help
us rewrite your
textbook...
Astro 102/104
17
Astro 102/104
Spirit
in salty soils
Mars Odyssey Orbiter Results
more ice
Spirit: 1488 sols
Opportunity: 1465 sols
18
Solid, Liquid, or Gas?
less ice
Evidence for subsurface
Hydrogen! (water ice?)
Astro 102/104
Evidence for variations in
Thermal infrared emission
(different surface minerals?)
19
Astro 102/104
20
5
Digression:
Snowballs on Mars?
Climate Change on Earth and Mars
• Significant and cyclic climate changes have
occurred on Earth (e.g., ice ages; Lecture 12)
• These climate changes are thought to be caused by
variations in Earth's orbital parameters
• Q: Could you have a snowball
fight on Mars?
• A: Sadly, no.
• Compressing snow on Earth turns
some of the snow into liquid
water, which "cements" the
snowball...
• On Mars, the phase remains solid,
whether H2O or CO2
• This also means:
• no skiing on Mars :(
• no ice skating on Mars :(
Astro 102/104
–Earth's polar axis precession: ~26,000 year timescale
–Changes in Earth's tilt: ~ 41,000 year timescale
–Changes in Earth's eccentricity: ~100,000 year timescale
• The same kinds of orbital variations occur for
Mars: Cyclic climate changes there too?
21
Astro 102/104
Summary
Next Lecture...
• There is very good evidence that the Martian climate was
very different 3-4 b.y. ago than today
– Valley networks, suggesting subsurface ice or water
– Outflow channels, indicating huge floods
– Polar layered deposits, suggesting cyclic changes
• Mars climate changes may be caused by the same
astronomical orbit variations that are thought to cause major
changes in Earth's climate
• Amazingly, liquid water may be stable near the Martian
surface today, despite contrary predictions
• Major implications for life on Mars... (Part 4...)
Astro 102/104
22
23
•
•
•
•
•
ININ-CLASS EXAM #2 !
Closed book, independent work
Review the assigned reading
Review your lecture and section notes
Understand any problems you had with homework
assignments or lecture topics
• No electronic devices allowed
• Bathroom breaks are not allowed during the exam
Astro 102/104
24
6