In the post to sci.astro below I suggested that the period of Mars highest
obliquity coinciding with its greatest eccentricity and closest perihelion might have allowed standing bodies of liquid water on Mars.
A report to be presented at the 2002 LPSC conference will argue that such an orbital coincidence would permit both daytime and nightime temperatures to be above zero for days on end. This suggests there could be open pools of water on Mars over several days, if not
open water seas.

NEAR-SURFACE LIQUID WATER ON MARS.
Lunar and Planetary Science XXXIII (2002)
"During Mars' present orbital configuration, night-time temperatures
everywhere on the planet are always well below 273K, which effectively
prevents sub-surface temperatures from reaching the melting point for sustained periods. However, when Mars' obliquity approaches 45°, its eccentricity approaches 0.11, and summer solstice in one of Mars' hemispheres coincides with perihelion, the model results show that
middle and high-latitude surface temperatures during summer can in
fact remain well above the melting temperature throughout the day.
During summers at high-obliquity, poleward-facing slopes at middle
latitudes are warmer at night than flat surfaces or equator-ward facing slopes because they receive direct sunlight at more favorable angles during the late evening and early morning hours."
http://www.lpi.usra.edu/meetings/lpsc2002/pdf/2049.pdf

There is a question of how often this coincidence could occur. The estimates of Mars orbital parameters vary. In the book _The Planet Mars: A History of Observation and Discovery_, William Sheehan gives estimates of the orbital parameters:

"Mars's asymmetric polar caps and cycle of southern hemisphere spring and summer dust storms reflect the current position of its axis and the eccentricity of its orbit. At present, Mars's axial tilt, or
obliquity---25.2° from the perpendicular---is very nearly the same as the Earth's (23.5°). The current agreement, however, is a sheer coincidence. Both Earth and Mars bulge slightly at the equator because of the centrifugal force of their rotation. The gravitational pull of
the Sun on these equatorial bulges causes the axial tilts of both Earth and Mars to vary over time. Earth's axial tilt is largely stabilized by the presence of the Moon, and so ranges through only
four degrees. Mars, which lacks a large and massive satellite, wobbles in a much more extreme fashion---at the current epoch, its axial tilt ranges between extremes of 15° and 35° over a period of 120,000 years, with the present value lying close to the mean.14 The spin axis also wobbles, or precesses, just like a top slowing down, with a period of
173,000 years (compared with 25,800 years for the Earth). This is the effect that on Earth gives rise to the well-known precession of the equinoxes."
Chapter 13: Vikings---and Beyond
http://www.uapress.arizona.edu/online.bks/mars/chap13.htm

From the orbital parameters given on this page, the time for the orbital coincidence could be anywhere from 10's of thousands of years to 10's of millions of years.
Sheehan suggests on this page that the time of Mars greatest obliquity still would not allow liquid water on the surface, but he appears not to be taking into account the possibility of the coincidence of the greatest obliquity, eccentricity and closeness to the Sun at perihelion:

"When the obliquity of Mars reaches its greatest value, the carbon dioxide polar caps may sublimate completely. At such times, Mars's atmosphere would be much more massive than it is now, so that
greenhouse warming might increase the global temperature another 30°C---still not enough to allow liquid water to exist on the surface. And yet, water obviously has flowed on the surface of Mars, probably repeatedly, so some other mechanism must be involved. Recall that the valley networks are found almost entirely in the old, heavily cratered
Noachian terrain, but the outflow channels are younger, being of
Hesperian and Amazonian age. The largest of them debouch onto the wide
northern plains, where ponded sediments, shorelines, and other
features associated with standing water have been tentatively identified. Apparently the northern plains have been periodically inundated by a great ocean, the Oceanus Borealis.17 Since changes in
Mars's atmosphere don't seem to be sufficient to explain these flooding episodes, there can be little doubt that they have been driven by hydrothermal processes---in other words, by heat generated within Mars itself."
Chapter 13: Vikings---and Beyond
http://www.uapress.arizona.edu/online.bks/mars/chap13.htm

Bob Clark

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From: Robert Clark (rgclark@my-deja.com)
Subject: Mars orbital eccentricity and change in climate.
Newsgroups: sci.astro, alt.sci.planetary, alt.astronomy, sci.geo.geology, sci.geo.meteorology
Date: 2001-09-01 07:40:58 PST

The Seasons and the Earth's Orbit - Milankovitch Cycles
"We would not expect the 21,000-year cycle to be very important
climatologically because the Earth's orbit is almost circular - the
distance to the Sun at perihelion is only about 3% less than its
distance at aphelion. That is, whether perihelion occurs in January or
July, it seems unlikely that our seasons would be much affected. At
least, that is the case now; but the eccentricity of the Earth's orbit
(how elliptical it is) also changes over very long periods of time,
from almost zero (circular orbit) to about three times its current
value. The eccentricity of the orbit varies periodically with a time
scale of about 100,000 years. So, it would be reasonable to suppose
that if the 21,000-year perihelion shift cycle were to have any effect
on climate at all, it would only be during the more widely-spaced
epochs when the orbital eccentricity was relatively large. That is,
climatologically, the 100,000-year cycle of eccentricity should
modulate the 21,000-year cycle of perihelion.
"In fact, Mars has an orbit much more eccentric than the Earth's, and
its perihelion cycle (which has a period of 51,000 years) does
apparently have a significant effect on climate and prevailing wind
direction there."
http://aa.usno.navy.mil/faq/docs/seasons_orbit.html

This discusses the effect of Mars' perihelion cycle, where perihelion
occurs in the year, on its climate change. However, how extreme can
Mars' orbital eccentricity get? At the current level, it results in
40% greater solar illumination at perihelion than at aphelion. Note
also this results in average global temperature of about 0°C:

Hubble finds Mars' climate harsher than thought,
http://rossby.metr.ou.edu/~spark/AMON/v1_n3/news/MARS.html
"Mars has two distinct climates that usually are associated with
periods when it is nearest to the sun and when it is farthest. That
happens every Martian year, which is equal to two Earth years.
"When Mars is nearest to the sun, it gets 40% more sunlight than when
it is farther away. The extra sunlight warms the average surface
temperature by 80 degrees, to about zero, and stirs up
continental-scale dust storms on the surface. The dust is swept high
into the atmosphere, turning the sky pink and blanketing the planet."

This also raises the average pressure over the planet above 6.1 mbars
due to the seasonal melting of the southern polar ice cap.

A recent report suggested changes in Mars' obliquity resulted in
large amounts of surface ice at middle latitudes within the last
100,000 years:

New Images Reveal Martian Ice Age, Signs of Water
"A new global map of Mars has revealed a host of valleys carved from
otherwise smooth areas that might be signs of the Red Planet's most
recent ice age. The features also bolster the case for a vast
reservoir of water ice just below the surface."
http://www.space.com/scienceastronomy/solarsystem/mars_hummocks_010726.html

However, according to the "The Seasons and the Earth's Orbit -
Milankovitch Cycles" page, Earth's climate cycles, I presume measured
by glacial cycles, correlate more strongly with the timing of the
eccentricity cycle:

"The 21,000-year perihelion cycle and the 41,000-year obliquity cycle
do in fact appear to be present in the climatological record. But the
dominant climate cycle that is seen has a period of about 100,000
years. Although this coincides with the period of change in the
eccentricity of the Earth's orbit, the theory outlined above does not
predict that we should see this period directly - the effect of
eccentricity should appear only as a modulation of the 21,000-year
perihelion cycle. The mechanism by which the Earth's orbital
eccentricity could affect the climate in such a direct and important
way is not known, although recent evidence (published in 2000)
indicates that atmospheric carbon dioxide may play a leading role in
amplifying the orbital effect. However, some researchers still have
doubts about the association between the 100,000-year climate cycle
and orbital variations. Thus, many questions remain about long-term
climate variations and their relationship, if any, to astronomical
causes."

Then perhaps the change in eccentricity would have a greater effect
on Mars climate than the change in obliquity. Is it known how close
Mars' gets to the Sun at its most extreme orbital eccentricity?
The argument that the eccentricity cycle should not directly effect
the climate cycle seems strange to me, at least as presented on this
page. Certainly I understand that change in eccentricity should have
an effect on how the perihelion cycle effects climate, but I don't
understand why this means change in eccentricity doesn't itself effect
climate. I'm assuming that at high eccentricity the planet would reach
a closer distance to the Sun at perihelion than at moderate
eccentricities.

Also, interesting on this page is the fact that the closeness of when
perihelion occurs to when the start of Winter occurs, or Summer in the
other hemisphere, has an effect on climate:

"The Earth reaches perihelion - the point in its orbit closest to the
Sun - in early January, only about two weeks after the December
solstice. Thus winter begins in the northern hemisphere at about the
time that the Earth is nearest the Sun. Is this important? Is there a
reason why the times of solstice and perihelion are so close? It turns
out that the proximity of the two dates is a coincidence of the
particular century we live in. The date of perihelion does not remain
fixed, but, over very long periods of time, slowly regresses (moves
later) within the year. There is some evidence that this long-term
change in the date of perihelion influences the Earth's climate."

Currently Mars' perhelion is offset some months from the southern
Summer, occuring in southern Spring. Then it is quite likely that when
perihelion and southern Summer coincide there would be even greater
melting of the southern polar ice cap with an even greater increase in
global atmospheric pressure.
What would be the effect on the climate of Mars when these three
cycles were at their most favorable for temperatures on Mars, when
perihelion coincided with southern summer, obliquity reached its
maximum, and orbital eccentricity was at its highest value? Could it
allow open water seas on the Martian surface?

A Scientific American article that discussed climate change on Mars
is available online:

Global Climatic Change on Mars,
by Jeffrey S. Kargel and Robert G. Strom
"Today a frozen world,
Mars at one time may have had
more temperate conditions,
with flowing rivers, thawing seas,
melting glaciers and, perhaps,
abundant life."
Scientific American, November, 1996,
http://www.sciam.com/1196issue/1196kargel.html

Bob Clark
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