Guest “remote astro-geology” by David Middleton

It’s very well established that in the distant past, Mars had enough surface water to erode and deposit a wide range of fluvial sedimentary features, including mudstone (a pretty good indication of past life). In order for Mars to have had liquid water at the surface, it would have had to have had a much thicker and warmer atmosphere. It has long been thought that Mars lost its atmosphere after its magnetic dynamo shut down and it was blown away by the solar wind.

When did the Martian dynamo die?
Posted on November 7, 2012 by rburnham

Current thinking among Mars scientists holds that the Red Planet’s dynamo — the geo-engine in its molten core which generates a global magnetic field — was active soon after the planet formed, but turned off about 4 billion years ago.

Spacecraft in orbit have detected and mapped magnetic fields in parts of the ancient southern highlands and elsewhere. While these show no active global dynamo pattern, they indicate that Mars had an internally generated magnetic field at one time. Yet the field disappeared at some point because the Hellas, Argyre, and Isidis impact basins — about 4 billion years old — contain no magnetic signatures. These would have been printed into the impact-melted rocks if a global field had been present when the basins formed.

But is this ancient age in fact correct? New work by a team of scientists led by Colleen Milbury (Purdue University) and published in the Journal of Geophysical Research suggests the dynamo shutdown happened more recently. If true, this means that Mars kept its magnetic field longer — and this would have protected the atmosphere for longer as well.

A magnetic field strong enough to leave traces in once-molten rocks would deflect most energetic solar radiation and ionizing particles, thus preventing them from eroding the atmosphere. But when the dynamo died, the Martian atmosphere began to die with it. Today’s Mars has no global magnetic field, only a thin atmosphere, and solar radiation and particles can strike the surface unhindered, making conditions hostile for most forms of life.

[…]

Red Planet Report

Researchers at the University of British Columbia have now proposed that the dynamo was active during two geologic time periods, approximately 4.5 and 3.7 billion years ago (Ga).

UBC researchers establish new timeline for ancient magnetic field on Mars

SCIENCE, HEALTH & TECHNOLOGY

May 1, 2020    |   For more information, contact Erik Rolfsen

Mars had a global magnetic field much earlier—and much later—in the planet’s history than scientists have previously known.

A planet’s global magnetic field arises from what scientists call a dynamo: a flow of molten metal within the planet’s core that produces an electrical current. On Earth, the dynamo is what makes compass needles point north. But Mars’ dynamo has been extinct for billions of years.

New findings from UBC researchers working with colleagues in the U.S. and France, published today in Science Advances, bring us closer to knowing the precise timing and duration of Mars’ dynamo.

[…]

The new data for this study come from MAVEN, the Mars Atmosphere and Volatile Evolution satellite. Earlier data about magnetism on Mars had been gathered by the Mars Global Surveyor satellite which orbited the planet between 1999 and 2006, mostly at 400 kilometres above the surface. MAVEN, launched in 2013, operates as close as ~135 kilometres from the surface and picks up weaker signals that MGS could not detect.

MAVEN’s ability to pick up signals from smaller features on and near the surface helps researchers distinguish whether the magnetism is coming from those, or from older rocks buried more deeply in the planet’s crust.

These new insights have researchers wondering what could be revealed if they get even closer. Mittelholz noted that this study focused on two particular features, but craters remain all over Mars with stories to tell. In the future, exploration could progress from satellites to drones or balloons, providing even more detailed data.

University of British Columbia

The second active dynamo phase at 3.7 Ga would be coincident with the Early Hesperian Period:

Pre-Noachian Represents the interval from the accretion and differentiation of the planet about 4.5 billion years ago (Gya) to the formation of the Hellas impact basin, between 4.1 and 3.8 Gya.[10] Most of the geologic record of this interval has been erased by subsequent erosion and high impact rates. The crustal dichotomy is thought to have formed during this time, along with the Argyre and Isidis basins.

Noachian Period (named after Noachis Terra): Formation of the oldest extant surfaces of Mars between 4.1 and about 3.7 billion years ago (Gya). Noachian-aged surfaces are scarred by many large impact craters. The Tharsis bulge is thought to have formed during the Noachian, along with extensive erosion by liquid water producing river valley networks. Large lakes or oceans may have been present.

Hesperian Period (named after Hesperia Planum): 3.7 to approximately 3.0 Gya. The Hesperian Period is marked by the formation of extensive lava plains. The formation of Olympus Mons likely began during this period.[11] Catastrophic releases of water carved extensive outflow channels around Chryse Planitia and elswhere. Ephemeral lakes or seas formed in the northern lowlands.

Amazonian Period (named after Amazonis Planitia): 3.0 Gya to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Lava flows, glacial/periglacial activity, and minor releases of liquid water continued during this period.

The date of the Hesperian/Amazonian boundary is particularly uncertain and could range anywhere from 3.0 to 1.5 Gya.[12] Basically, the Hesperian is thought of as a transitional period between the end of heavy bombardment and the cold, dry Mars seen today.

http://palaeos.com/precambrian/martian_timescale.html

The Yellowknife Bay is thought to have been deposited during the Early Hesperian Period in a “strikingly Earth-like habitable environment.”

At Yellowknife Bay there is no hint of the strongly acidic conditions that have been thought to especially describe the planet’s younger history of aqueous alteration, sedimentation, and habitability (74, 83, 91). The record of aqueous activity at Yellowknife Bay is likely prolonged and complex, involving several stages of diagenesis, including clay formation, that culminate with precipitation of calcium sulfate salts in veins, but without attendant indicators of acidic waters such as iron sul-fates. The simplest interpretation of the sequence of diagenetic events would involve progressive desiccation of mildly saline, pH neutral waters—a very Earth-like scenario (92). Such conditions have been envisaged for the very early history of Mars (91), but it is only recently that they have been considered viable for a younger age (7, 93, 94). The surprising result is that the stratigraphy of Yellowknife Bay may not only preserve evidence of a habitable environment, but one that is relatively young by Martian standards. In the most conservative scenario allowed by geologic mapping, the Yellowknife Bay formation represents part of the older fill of Gale crater, and therefore roughly Early Hesperian in age (6), perhaps overlapping with or post-dating times of bedded sulfate formation elsewhere on Mars. This would indicate that times of sustained surface water, neutral pH, and authigenic clay formation extended later into Mars’ history. The potentially young age of clay formation (and habitability) does not invalidate the general trend that most rocks that interacted with water early in Mars’ history produced clays, and those that interacted later produced sulfates. However, much like Earth’s time-dependent records of iron formation (95, 96) and carbonates (97, 98), it points to the need to understand those special conditions which allow a distinctive aqueous environment to persist for broad spans of geologic time, or to recur when the favorable conditions again emerge. Curiosity’s detection of a relatively young, and strikingly Earth-like habitable environment at Gale crater underscores the biologic potential of relatively young fluvio-lacustrine environments.

Grotzinger et al., 2014

References

Grotzinger, J. P., Sumner, D. Y., Kah, L. C., Stack, K., Gupta, S., Edgar, L., … Yingst, A. (2014). A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale crater, Mars. Science, 343(6169). https://ift.tt/2yz0msF

Mittelholz, A., C. L. Johnson, J. M. Feinberg, B. Langlais, R. J. Phillips. Timing of the martian dynamo: New constraints for a core field 4.5 and 3.7 Ga ago. Science Advances, 2020; 6 (18): eaba0513 DOI: 10.1126/sciadv.aba0513