Clay Formation Study Leads to New Model for Early Martian Climate. How "Warm" is Warm?

nature astronomy  cover
March 2018 cover of Nature Astronomy magazine View on Nature Astronomy

Last month, new research led by SETI Institute scientist and NASA Astrobiology Institute team member Janice Bishop was published online in Nature Astronomy. Today, that research appears on the cover of Nature Astronomy’s March print edition.

What were the key advances?

New Studies of Clay Formation Provide Clues about Early Martian Climate: A new climate model was developed for Mars that better explains how clays formed on the surface. Short-term warm and wet environments, occurring sporadically in a generally cold early Mars, are proposed to explain formation of surface clays on Mars.

The Approach: Part of this early Martian climate puzzle comes down to how “warm” is warm. Although Mars is currently below freezing, it must have once been warm enough for liquid water to carve out features on the surface. However, cold water is not warm enough for surface clays to form. We realized that in order to better constrain the early Martian climate, we needed to understand the formation conditions of thick outcrops of martian surface clays.

Finding: Clays on Mars belong to three chemical families based on crystal structure. Magnesium-rich, trioctahedral mixed clays tend to form in the subsurface at elevated temperatures, and dominate deeper layers exposed in some craters. Dioctahedral, iron-rich smectite clays tend to form by surface weathering at moderately warm (20-40 °C) and wet conditions. Martian dioctahedral smectite clays occur in a layered configuration characteristic of soil formation. Poorly crystalline aluminum- and silicon-rich clay phases tend to form in colder environments and may record climate change on Mars. Laboratory experiments demonstrate that the thicknesses of surface clay minerals observed on Mars could have formed in a few million years or less, depending on the temperature. The poorly crystalline clay layers overlying the smectite clay minerals record a colder, dryer Mars.

What it means: Understanding the climate on early Mars provides constraints on when liquid water was present on the surface and is essential for determining where on Mars to search for life. Clays are the most abundant hydrated mineral on Mars; thus, defining their formation conditions is a big step towards defining the geochemical environment on Mars.